Photonics doi: 10.3390/photonics13080755

Authors: Ziming Zhang Qiancheng Zhao

Integrated waveguide photonics has experienced tremendous growth, evolving from discrete components to multi-functional devices and complex architectures for optical power routing and information processing [...]

Photonics doi: 10.3390/photonics13080754

Authors: Manjun Yan Qiuju Wu Long Ma

Haze conditions degrade image quality via atmospheric scattering and absorption, posing challenges for optical imaging applications. In recent years, deep learning has emerged as an effective method for dehazing images. However, data-driven deep learning dehazing methods typically require large amounts of labeled training data and offer limited interpretability. In this paper, we propose a physics-guided dehazing method based on polarization imaging. By integrating polarization imaging with the physical model, the proposed method enables neural network training using only a set of hazy images captured at different polarization angles, thereby reducing the reliance on labeled training data. Experimental results show that the proposed method significantly outperforms commonly used deep learning dehazing methods in terms of contrast and mean gradient, while exhibiting strong generalization and physical interpretability. By integrating physical model and polarization imaging into deep learning, this method overcomes the limitations of traditional deep learning dehazing methods and paves the way for optical imaging in haze conditions.

Photonics doi: 10.3390/photonics13080753

Authors: Liujiang Wang Chenxiao Wang Fan Yang Liaoxin Sun Bimu Yao

For scalable quantum networks and distributed quantum sensing, remote NV color-center spin ensembles require controllable long-range coherent connections. However, the magnetic dipole coupling between NV centers decays rapidly with distance, making direct stable coherent exchange difficult to achieve. Here, we propose an active-microwave-cavity-assisted scheme for long-range synchronization, in which traveling microwave photons establish a remote cavity–spin coupling channel, while saturable gain compensates cavity loss and propagation attenuation. The results show that the system exhibits collective superradiant emission under gain-off and forms a hysteretic synchronized state controlled by the propagation phase, separation distance, and optical cooling rate under gain-on. Moreover, the active cavity can mediate long-range phase synchronization between two remote NV ensembles, with local optical cooling enabling switching between in-phase and out-of-phase synchronized states. This scheme provides a theoretical route toward room-temperature, optically reconfigurable spin networks.

Photonics doi: 10.3390/photonics13080752

Authors: Zbigniew Bielecki Janusz Mikołajczyk

Underwater wireless optical communication (UWOC) has emerged as a key technology for high-speed, low-latency data transmission in aquatic environments, enabling applications in autonomous underwater vehicles (AUVs), remotely operated vehicles (ROVs), subsea sensor networks, and the Internet of Underwater Things (IoUT). This paper reviews photodetector technologies that shape UWOC system performance, covering both mature and emerging detector classes. We discuss the operating principles, key parameters, and practical trade-offs of photomultiplier tubes (PMTs), p-i-n photodiodes (PINs), avalanche photodiodes (APDs), single-photon avalanche diodes (SPADs), and silicon photomultipliers (SiPMs/MPPCs). We also present emerging photodetector technologies, including perovskite-based structures, SiC photoelectrochemical devices, scintillating optical fibers, and photovoltaic solar cells. A comparative analysis of reported UWOC experiments reveals a clear sensitivity–bandwidth trade-off among detector technologies: PIN-based receivers achieve the highest data rates (up to 25 Gbps) but are generally restricted to short-range links, whereas SPAD- and SiPM-based receivers provide sensitivities below −80 dBm and support transmission distances exceeding 200 m, at the cost of moderate data rates. The findings indicate that SiPM/MPPC arrays currently offer the most promising compromise between sensitivity and data rate for long-range UWOC applications.

Photonics doi: 10.3390/photonics13080751

Authors: Mansoor Qadir Muhammad Umar Muhammad Ismail Mohmand Waqas A. Imtiaz Sajjad Aleem

The study examines the application of machine learning algorithms (MLAs) to enhance free space optical (FSO) communication performance by predicting quality-factor (QF) from key system parameters. FSO technology has developed as a promising solution for front-haul links in 5G, beyond the 5G (B5G), and 6G transmission networks. Nevertheless, performance of an FSO communication link is limited by environmental challenges like weather conditions, attenuation and turbulence that can degrade signal quality and affect QF at the receiving end. Using the data collected through simulation analysis in OptiSystem, we trained several MLAs in order to analyze performance of this system through accurate prediction of the QF. Analysis shows that received power serves as an important parameter in translating the overall QF of the received signal. Furthermore, it is shown that the prediction accuracy of the tree-based approach such as random forest and gradient boosting ranges above 97% are reliant on channel conditions and the predicted parameter type.

Photonics doi: 10.3390/photonics13080750

Authors: Bo Li Shenghong Ye Ming Che Naoto Masutomi Yuya Mikami Yuta Ueda Kazutoshi Kato

We propose and experimentally demonstrate an integrated photonic terahertz (THz) frequency-hopping (FH)/dehopping system using electro-optically wavelength-tunable lasers to enhance physical-layer security in terahertz wireless links. The FH THz signal is generated around the 300 GHz band by photomixing a fixed-wavelength laser and an electro-optically tunable laser, while a synchronized FH local oscillator (LO) signal is generated at the receiver for secure photonic dehopping. By utilizing a waveguide-integrated THz combiner and THz detector, the FH LO remains internally coupled and unexposed to potential eavesdroppers. A signal with a 40 GHz hopping span around the 300 GHz band and a 50 Mhops/s FH rate is dehopped to a 5 GHz intermediate frequency. This system simplifies the receiver complexity while enabling secure, high-speed physical-layer terahertz communications.

Photonics doi: 10.3390/photonics13080749

Authors: Chen-Zhi Yuan Xu-Tun Li Si Shen

Landau–Zener–Stückelberg–Majorana (LZSM) interference describes the coherent superposition of nonadiabatic transitions when a quantum system is driven through an avoided crossing, but its classical optical analog remains largely unexplored. This work establishes an optical-resonator-based platform for exploring LZSM interference in atomic polarization dynamics using temporal coupled-mode theory. By exploiting the formal correspondence between the cavity mode and atomic polarization in a weakly driven two-level system (TLS), exact analytical solutions are derived for linear frequency sweeps and sinusoidal modulation. The results reveal that the Landau–Zener transition of atomic polarization occurs even when the input frequency transiently sweeps across resonance. The finite temporal memory inherent in the cavity response gives rise to Stückelberg interference, manifesting as oscillations in transmission and cavity energy. The dependence of the interference pattern on sweep rate, linewidth, and modulation is systematically analyzed, and a non-monotonic behavior of oscillation amplitude versus the cavity linewidth is identified. Furthermore, the conventional critical coupling concept is reexamined in the nonadiabatic regime, where counterintuitively the deepest transmission dip occurs under over-coupling rather than critical coupling. Finally, an inverse-design approach engineering both the amplitude and frequency of the input frequency is introduced to tailor the LZSM interference pattern. Our results provide a new platform to all-optical simulation of complex quantum interference.

Photonics doi: 10.3390/photonics13080748

Authors: Ruiqi Wang Ziquan Wang Ling Guan Zikai Zhang

Detecting naval vessels at extreme distances using infrared search and track (IRST) systems presents severe physical challenges, notably the complete loss of geometric texture and the non-linear submersion of weak target signals within high-dynamic-range sea clutter. Traditional pure data-driven convolutional neural networks (CNNs) rely heavily on visual appearances and suffer from critical feature blind spots under such extreme physical degradation. To overcome this, we propose a Physics-Aware Deep Coupling Network that shifts the detection paradigm from appearance-based feature extraction to physics-guided attribute recognition. Our method deconstructs the degraded infrared signal into three complementary physical domains: an adaptive radiation energy mapping, corresponding to the energy domain, to rescue weak targets; a bio-inspired spatial saliency filtering mechanism, corresponding to the frequency domain, to maximize the signal-to-clutter ratio; and a PSF-coherent gradient topology framework, corresponding to the gradient domain, to discriminate genuine point targets from chaotic sun glints and island edges. These processed priors, alongside the raw image, are integrated into a 4-channel tensor and fused via a Cross-Domain Attention Module, ensuring deep network coupling. To evaluate this architecture, we conduct extensive experiments on the real-world Maritime-SIRST dataset. Since the original dataset provides only pixel-level segmentation masks, we generate axis-aligned bounding-box detection labels from these masks and retrain both the proposed method and a suite of state-of-the-art YOLO detectors under a unified detection paradigm. Extensive benchmarking demonstrates that our physics-aware methodology consistently outperforms these detectors, achieving a mAP50 of 0.923 and an F1 score of 89.92%, thus providing a highly interpretable and robust solution for maritime domain awareness under extreme physical constraints.

Photonics doi: 10.3390/photonics13080747

Authors: Jindong Zheng Dongyang Wei Ming Chen Jia Rui Pengfei Cao Huaping Wang Ping Xiang

Carbon fiber-reinforced polymer (CFRP) composites are increasingly used in aerospace, rail transportation, and energy engineering owing to their high specific strength and corrosion resistance. However, their complex and interacting damage mechanisms, including delamination and matrix cracking, present significant challenges for reliable structural health monitoring. Fiber Bragg grating (FBG) sensors offer distinct advantages for monitoring composite structures because of their compact size, immunity to electromagnetic interference, embeddability, and capability for distributed strain measurement. Nevertheless, the effectiveness of an FBG sensing network depends strongly on the spatial distribution of the sensing points. This study proposes a finite-element-assisted framework for evaluating and improving FBG sensor layouts for strain-field reconstruction and structural feature characterization of composite plates. The framework first reconstructs the spatial strain field from limited sensing data using interpolation and least-squares fitting methods, and then evaluates the performance of existing and candidate sensor layouts based on reconstruction errors and spatial coverage of structurally important regions. A strain-gradient-informed heuristic strategy is subsequently developed to improve sensor placement by combining high-gradient region identification, spatially uniform coverage, minimum-distance constraints, and predefined support-region monitoring requirements. The Fourier least-squares fitting method provides the lowest reconstruction error among the investigated approaches and is therefore adopted for subsequent layout evaluation and improvement. Finite-element simulations and experimental measurements are used to assess the reconstruction performance and identify the advantages and limitations of different sensor layouts under static and dynamic loading conditions. The results demonstrate that the proposed framework can effectively evaluate existing FBG layouts and provide a systematic basis for their improvement, while also revealing the trade-off between local strain-gradient resolution and global spatial coverage. The proposed framework provides practical guidance for the performance-oriented design and improvement of FBG sensor networks for structural health monitoring of composite structures.

Photonics doi: 10.3390/photonics13080746

Authors: Giannis Poulopoulos Panagiotis Toumasis Hercules Avramopoulos

Stimulated Brillouin scattering (SBS) constrains the launch peak power in long Erbium-Doped Fiber Amplifier (EDFA)-amplified pulsed fiber-optic sensing links, but its dependence on duty cycle is not determined by peak power alone. When the repetition rate is fixed and the pulse width is varied, the duty cycle changes both the pulse-limited Brillouin interaction length in the fiber and the pulse peak power delivered by the EDFA. Here, we treat duty cycle as a coupled SBS operating margin parameter and examine this dependence in a 25 km standard single-mode fiber link operated at 4 kHz. The analysis compares the duty-cycle-dependent SBS threshold, estimated from the pulse-limited interaction length, with the launched on-state pulse power extracted from Amplified Spontaneous Emission (ASE)-filtered signal band average power measurements. A Continuous-Wave (CW) sweep gave a reference SBS threshold of 5.5 dBm. In pulsed operation, the extracted launched pulse peak power decreased by 23.8 dB across the investigated duty cycle range, whereas the measured backward optical power increased by 20.3 dB up to its maximum and then decreased by 7.1 dB at higher duty cycle. Variable Optical Attenuator (VOA)-controlled launch power sweeps showed no 3 dB onset at duty cycle (D) of 0.5% within the measured range, while onsets between 1.76 dBm and 3.49 dBm were extracted for duty cycles from 1% to 50%. These results show that SBS risk in EDFA-assisted pulsed sensing links cannot be assessed only from total EDFA output power. Signal band launch power calibration after ASE rejection is required to estimate the pulse peak power relevant to SBS onset. The validation is limited to power-meter-based backward power measurements and does not spectrally resolve the Brillouin Stokes component.

Photonics doi: 10.3390/photonics13080745

Authors: Haotian Tang Yiyang Deng Shuyi Xu Caoyuan Pan Yingqi Chen Haoyan Peng Zewei Cai Hailong Chen

Highly reflective surfaces often cause intensity saturation in captured fringe images, leading to phase errors and inaccurate 3D reconstruction. High-dynamic-range (HDR) fringe projection profilometry is an effective solution to this problem, but existing methods usually require additional image acquisition or auxiliary calibration, which limits their applicability to high-speed online inspection. In this work, we propose a phase-optimization-guided progressive adaptive fringe projection method for HDR 3D measurement. First, quality-guided RGB phase fusion is used to fuse reliable phase information from RGB channels, reducing unreliable phase regions and identifying residual overexposed areas. Second, local phase repair restores pixels that cannot be directly mapped in overexposed regions, thereby establishing stable camera–projector correspondence. Third, an adaptive optimal projection intensity is estimated through local intensity fitting to adjust the brightness of highly reflective regions. These steps are embedded into the frequency-varying phase-unwrapping process of conventional digital fringe projection, progressively suppressing saturation-induced phase distortion without additional projected/captured images or calibration operations. Experiments on a highly reflective blade and a white plaster bust demonstrate that the method suppresses saturation without extra image acquisition. Comparative results show high measurement accuracy and low missing ratios with fewer images.

Photonics doi: 10.3390/photonics13080744

Authors: Jihad Zallat Yoshitate Takakura Christian Heinrich Romain Attal Laurent Schwartz

The degree of polarization is usually obtained from the coherency matrix or, equivalently, from the mean Stokes vector of a partially polarized optical field. Here, we adopt a complementary geometric viewpoint by representing normalized local Stokes vectors as random directions on the Poincaré sphere. When these directions are described by an effective unimodal von Mises–Fisher distribution, the concentration parameter gives a direct one-to-one description of the degree of polarization through the mean resultant length. This formulation does not define a new independent polarization observable. Instead, it gives the degree of polarization a rotation-invariant information-theoretic meaning, expressed in terms of directional concentration and angular disorder. Within this framework, we derive closed-form expressions for the differential entropy of the von Mises–Fisher distribution and for the Kullback–Leibler divergence between two directional polarization states. The symmetrized divergence further incorporates both differences in concentration and relative orientation on the Poincaré sphere. We also discuss the assumptions, range of validity, and limitations of the single-vMF model, particularly in relation to Gaussian field statistics and more general directional models needed for anisotropic or multimodal polarization fluctuations. Overall, this formalism establishes a model-based theoretical framework for entropy and divergence descriptors of unimodal directional polarization and suggests natural extensions toward mixtures of vMF, Bingham, or Kent distributions.

Photonics doi: 10.3390/photonics13080743

Authors: Saki Ota Kengo Yamaguchi Yuki Mando Kota Nomura Nobuyuki Kamihara Yoshiaki Takeuchi

Laser-based Optical Wireless Power Transmission (OWPT) enables highly directional long-distance energy delivery. However, atmospheric turbulence near the ground significantly degrades transmission efficiency, particularly during daytime when beam scintillation becomes more severe. In this study, a diffractive optical element (DOE) was employed at the transmitter for beam shaping, while receiver-side disturbance mitigation was achieved using a transmissive diffuser-based homogenizer, a reflector-based optical confinement structure, and a bypass-capacitor-based smoothing circuit. Photovoltaic (PV) cells fabricated by laser cutting commercially available crystalline silicon solar cells were connected in series to construct a 600 mm × 600 mm PV panel. Without receiver-side disturbance mitigation, the output power decreased by more than 50% as the atmospheric structure constant (Cn2) increased from 10−14 to 10−13 m−2⁄3. In contrast, the proposed receiver-side techniques effectively suppressed turbulence-induced performance degradation and maintained nearly constant output power. Furthermore, the combination of the homogenizer and the smoothing circuit increased the receiver output power by approximately a factor of 2.2 under a turbulence condition of Cn2 ≈ 3 × 10−14 m−2⁄3. Using these techniques, 150 W of electrical power was generated over a 1 km outdoor optical link with a 1035 W, 1070 nm near-infrared laser. These results demonstrate that receiver-side disturbance mitigation is an effective approach for improving the efficiency and stability of practical long-distance OWPT systems.

Photonics doi: 10.3390/photonics13080742

Authors: Jijuan Jiang Yachen Gao Jia Liu Pengfei Hui Baocheng Zhang Wenlong Yang

Nonlinear absorption (NLA) in silver nanoparticles (Ag NPs) is strongly affected by plasmonic field enhancement. However, its wavelength- and energy-dependent evolution on the red side of the surface plasmon resonance (SPR) band remains insufficiently understood. Here, we systematically investigate the NLA properties of Ag NPs using open-aperture (OA) Z-scan measurements combined with femtosecond transient absorption (TA) spectroscopy. The results show that Ag NPs exhibit saturable absorption (SA) under low excitation energies, whereas higher excitation energies induce a mixed SA/reverse saturable absorption (RSA) response. A pronounced wavelength dependence is also observed as the excitation approaches the SPR band, where nonlinear absorption gradually evolves with spectral detuning. TA measurements revealed ultrafast carrier dynamics characterized by a fast electron–phonon relaxation process (~3.3 ps) followed by a slower lattice cooling process (~46 ps). These results demonstrate that the nonlinear response is governed by the interplay between plasmonic resonance and ultrafast carrier relaxation, providing insights for the design of plasmonic nanomaterials for ultrafast nonlinear photonic applications.

Photonics doi: 10.3390/photonics13080741

Authors: Rafael G. González-Acuña

We present a comprehensive vector diffraction analysis of stigmatic pairs of mirrors derived from closed-form equations that describe all possible aplanatic two-mirror systems. Using a vectorial generalized version of Richards-Wolf integral, we evaluate the focal field distributions, accounting for polarization effects, high numerical apertures, and deviations from the scalar approximation. Numerical simulations based on the derived mirror profiles demonstrate diffraction-limited performance free of spherical aberration, with agreement to geometric ray-tracing results.

Photonics doi: 10.3390/photonics13080740

Authors: Linkun Zhang Shangshang Cui Mengfei Li Xin Cai Wenjing Fang Xinye Fan Xiaowei Yang Xueli Geng

We proposed an all-dielectric stochastically encoded metasurface operating in the 1310 nm near-infrared band, which enables multifunctional optical field manipulation across six independent polarization channels. Utilizing a shared-aperture stochastic matrix-encoding strategy combined with cooperative propagation and geometric-phase decoupling modulation, we investigated stochastically encoded phase distributions, vortex beam profiles, near-diffraction-limited focusing profiles, and the characterization of complex-amplitude multifocal focusing under six independent polarizations. Under left- and right-circularly polarized (LCP/RCP) illumination, vortex beams with topological charges of 1 and 2 have demonstrated substantial enhancement in spatial edge contrast, achieving high edge contrasts of 20 dB and 7.45 dB, respectively, enabling high-fidelity extraction of fine structural boundaries for edge-enhanced imaging. In contrast, near-diffraction-limited focusing under x- and y-polarized illumination has been achieved with numerical apertures (NA) of 0.66 and 0.59, facilitating the realization of bright-field imaging. Significantly, parallel imaging based on complex-amplitude multifocal spot arrays exhibits markedly improved channel isolation, achieving high power ratios of 88.8% and 94.5% under 45° and 135° linearly polarized excitation, respectively. The compact shared-aperture architecture integrates all polarization-controlled functionalities without mechanical tuning, enabling polarization-switchable bright-field imaging, edge detection, and parallel optical manipulation in the near-infrared band.

Photonics doi: 10.3390/photonics13080739

Authors: Jau Tang

Recent weak-measurement experiments performed by the University of Toronto reported an apparent negative transmission delay associated with photons propagating through a cold rubidium atomic cloud, raising fundamental questions concerning the physical interpretation of negative interaction time. In this work, we develop a second-quantized synchronization framework to describe resonant photon transmission through a collective atomic ensemble. Both the incident photon and the collective atomic excitation are treated as coupled quantum resonators interacting through an effective synchronization Hamiltonian with finite lifetime broadening. An analytical expression for the transmission amplitude and its phase response is derived, demonstrating that the apparent transmission delay naturally becomes negative on the resonance wings owing to coherent interference between direct transmission and resonant absorption–reemission pathways. In contrast, the microscopic excitation time, defined by the time integral of the atomic excitation-number operator, is rigorously proven to remain nonnegative. Numerical simulations using representative parameters for the Toronto cold-rubidium experiment reproduce the experimentally observed order of magnitude of the negative weak delay without introducing empirical fitting parameters. The calculations further show how the apparent delay depends systematically on frequency detuning and the effective collective coupling strength, providing experimentally testable predictions beyond the existing weak-value interpretation. The present synchronization framework therefore resolves the apparent paradox of negative transmission time by demonstrating that the observed temporal advance originates from phase synchronization and quantum interference rather than from a negative microscopic interaction time.

Photonics doi: 10.3390/photonics13080738

Authors: Yifan Ren Dewang Yang Shibo Wang Zihan Wang Yuee Chen

Raman spectroscopy has emerged as a powerful tool for gas detection due to its label-free operation, molecular specificity, and multi-component analysis capabilities. However, its widespread application is hindered by limited sensitivity, particularly for trace gas analysis. To overcome this challenge, this study introduced an effective Raman spectroscopy detection system that synergistically combines a 532 nm high-power laser with a near-concentric multipass cell (MPC), enabling dual enhancement of both Raman excitation and signal collection. We simultaneously determined three critical performance metrics, including gas Raman signal intensity, signal-to-noise ratio (SNR), and limit of detection (LOD). Under the optimized experimental conditions, the CO2 Raman signal reached an SNR of approximately 58 for laboratory air containing 916 ppm CO2, corresponding to a concentration-equivalent detection limit of 48 ppm according to the 3σ criterion. Notably, the intensity of the generated Raman scattering signal is proportional to the average power. The intensity of Raman signals at different wave numbers increases at different rates with the increase in the average excitation power. The system achieves a remarkable LOD of 48 ppm for CO2, representing an advancement over conventional Raman gas sensors. This work validates high-power near-concentric cavity-enhanced Raman spectroscopy as a reliable method for trace gas detection, with potential implications for multi-component gas analyzers in environmental monitoring, industrial safety, and medical diagnostics.

Photonics doi: 10.3390/photonics13080737

Authors: Talia Tene Nelly Andrade Mejía Cristina Estefanía Ramos Araujo Elfahem Sakher Houssem Eddine Doghmane Nozha El Ahlem Doghmane Cristian Vacacela Gomez

Cortisol monitoring is relevant for stress assessment and endocrine-related disorders, but conventional assays often require labeled reagents, multistep protocols, or laboratory instrumentation. This work numerically evaluates a black phosphorus (BP)-based surface plasmon resonance (SPR) platform for label-free optical detection of cortisol-related refractive-index changes using angular interrogation at λ = 633 nm. The proposed structure consists of a SiO2 prism, an aluminum plasmonic layer, a TiO2 dielectric layer, a BP monolayer, and the sensing medium. The optical response was calculated using the transfer matrix method under TM polarization, and the platform was assessed through resonance-angle shift, sensitivity, full width at half maximum, detection accuracy, quality factor, figure of merit, theoretical refractive-index detection limit, and combined sensitivity factor. Sequential optimization identified SiO2 as the prism material, 70 nm Al as the plasmonic layer, and 21 nm TiO2 as the dielectric layer. BP was retained as the interfacial 2D material within the intended SiO2/Al/TiO2/BP architecture and exhibited the lowest imaginary refractive-index component among the evaluated 2D materials at 633 nm. For cortisol-related refractive-index changes, the resonance angle shifted from 87.08° to 87.42° as the concentration increased from 0.72 to 4.5 ng/mL. The maximum sensitivity was 480.00/RIU at 0.72 ng/mL, whereas 1.8 ng/mL yielded the highest composite performance according to the adopted CSF definition. These results support SiO2/Al/TiO2/BP as a sensitivity-oriented SPR transduction platform, with future validation requiring fabrication, calibration, and cortisol-selective surface functionalization.

Photonics doi: 10.3390/photonics13080736

Authors: Zhi-Yuan Zheng

Unidirectional guided resonances (UGRs) provide a compact route to highly directional radiation without metallic reflectors, but their realization usually relies on precise structural tuning and is often restricted to isolated points in momentum or parameter space. Here, we propose a Ge2Sb2Se4Te1 (GSST)-based phase-change metasurface that enables nonvolatile spectral tuning of UGR-like resonances by controlling the momentum-space evolution of paired circular-polarization singularities. Instead of relying on an exact C-point merging condition at a single phase state, the proposed structure controls the approach and departure of a symmetry-related C-point pair relative to the ky=0 line, with the closest sampled approach occurring near an intermediate crystallization state. This near-merging evolution selectively suppresses air-side radiation, giving rise to continuously tunable UGR-like resonances with persistent high directionality. At the high-directionality operating point selected for each of the crystallization states considered, the eigenwavelength shifts by 202.2nm, exceeding three times the largest eigenmode linewidth. Meanwhile, the downward-to-upward radiation asymmetry ratio η consistently exceeds 102 over the investigated phase states and reaches approximately 2.4×104 near the optimal crystallization state. A self-consistent fixed-angle analysis further shows that a fixed substrate-side direction of 16.52∘, equivalent to 22.93∘ in air, retains 119.6≤η≤191.3 throughout all 11 states considered while preserving a 186.6nm tuning range. Despite increased crystalline-state absorption, the intrinsic substrate-side modal branching fraction remains at least 75.7%. These results establish a nonvolatile strategy for combining spectrally resolvable tuning, persistent directional radiation, and substantial dominant-side modal out-coupling in a single phase-change metasurface.

Photonics doi: 10.3390/photonics13080735

Authors: Yikai Zhang Bing Xie Yuliang Ye Kangcheng Wu Xin Xiong

As core components for ultra-precision positioning, grating encoders have technologically evolved toward higher accuracy, faster speeds, and multi-degree-of-freedom (multi-DOF) integration. This review systematically traces the technological progression of commercial grating encoders from imaging scanning to interferential scanning, from incremental to absolute encoding, and from single axis to multi-DOF integration. It compares the distinct technical strategies adopted by Heidenhain and Renishaw for robustness enhancement and absolute encoding within the imaging scanning paradigm. It also uncovers the physical mechanism of optical subdivision that allows interferential scanning encoders to surpass the sub-nanometer resolution barrier. Building upon this foundation, recent academic advances in multi-DOF interferometric measurement systems and Fizeau interferometer-based grating self-calibration methodologies are also reviewed.

Photonics doi: 10.3390/photonics13080734

Authors: Xu Wang Guanghao Rui

The chiral interaction between light and matter is intrinsically weak at the nanoscale, which leads to a sensitivity bottleneck in the detection of chiral samples by photoinduced force microscopy. To address this issue, we propose an enhancement strategy that combines a semiconductor tip with a superchiral optical field to improve the optical force response in nanoscale chiral detection. Compared with a conventional metallic tip, a silicon tip can generate a larger optical force difference under left- and right-handed circularly polarized illumination. This enhancement mainly arises from the more pronounced magnetic response of the silicon tip, which increases the contribution of the magnetic dipole term to the total optical force. Furthermore, when the incident field is changed from a circularly polarized field to a superchiral field, the optical force difference acting on the same tip can be further increased by more than one order of magnitude. Relative to the reference case of a gold tip excited by circularly polarized light, the combination of a superchiral field and a silicon tip enhances the optical force difference by nearly two orders of magnitude. Analysis based on the dipole approximation shows that this enhancement originates from the synergistic effect of two factors: the high-refractive-index silicon tip provides a stronger magnetic response, while the superchiral field further strengthens the coupling between the localized chiral field and the tip and effectively excites the magnetic dipole and electromagnetic coupling terms, thereby jointly amplifying the chiral optical force signal. This work provides a new route for the highly sensitive detection of chiral materials at the nanoscale and may further extend the applications of photoinduced force microscopy in the characterization of chirality and optomagnetic interactions.

Photonics doi: 10.3390/photonics13080733

Authors: Helena Cristina Vasconcelos Maria Gabriela Meirelles

Lanthanide co-doped photonic materials are commonly interpreted through selected resonant transitions between assigned 4f multiplet states. Here, we introduce an energy-gap topology framework that compares Ln3+ ion pairs from the complete set of internal separations within their 4f manifolds. Reported multiplet-centre energies for a common LaF3 spectroscopic reference were reduced to energy-only manifolds, from which normalized gap distributions were constructed. A symmetric descriptor, CA−B, quantifies global similarity between complete gap distributions, whereas a directional descriptor, DA→B, measures the inclusion of the gaps of ion A within the occupied support of ion B. When applied to 11 trivalent lanthanide ions, this reveals distinct coupling architectures. Tb3+–Ho3+, Sm3+–Dy3+, Dy3+–Ho3+, Nd3+–Ho3+, and Er3+–Ho3+ exhibit high symmetric overlap, defining a broad manifold–manifold topology signature. In contrast, Yb3+-containing pairs show low global compatibility but high directional inclusion of the Yb3+ gap within the landscapes of candidate acceptor ions, such as Er3+, Tm3+, and Nd3+, consistent with sparse-to-rich sensitizer-like architectures. The pairwise organization remains stable for bin widths between 250 and 1000 cm−1. The descriptors provide a pre-spectroscopic screening map of energetic architecture; they do not predict the transfer efficiency, dominant mechanism, or final optical performance and should not be interpreted as host-independent constants.

Photonics doi: 10.3390/photonics13080732

Authors: Wenjie Lai Fanyu Zeng Xiao Hu Shaowei He Ziji Liu Huiling Tai Yadong Jiang

Single-image depth from defocus is limited by blur-radius ambiguity: two object distances on different sides of the focal plane can produce similar point spread functions (PSFs). We study single-shot dual-focus imaging (DFI) with a polarizer-free liquid crystal (LC) lens, where the ordinary-ray component remains unmodulated while the extraordinary-ray component is refocused. Unlike conventional multi-capture DFD, the proposed system records a dual-focus superposition in one exposure and selects the optical setting before network training using the ambiguity-interval length A and PSF correlation Cr. We show that DFI does not reduce ambiguity unconditionally: its benefit depends on the LC-lens power. A deblurring-based depth estimation network with a physics-calibrated Wiener bank translates the selected dual-focus cue into quantitative depth. On model-matched synthetic DFI data with an 8 m focus setting, the proposed configuration reduces RMS error from 0.227±0.001 m to 0.190±0.001 m (mean ± s.d. over four independent runs) relative to single-focus imaging. A 116-pair indoor prototype dataset provides feasibility evidence under the tested configuration, but is not used to claim broad physical generalization.

Photonics doi: 10.3390/photonics13080731

Authors: Philip Lichtenegger Philipp Hurdax Georg Spernbauer Bernhard Lamprecht

Femtosecond laser writing (FLW) enables mask-free three-dimensional photonic integration in transparent materials and offers a flexible route for prototyping glass-based photonic circuits. Here, we present a monolithic fabrication approach that combines femtosecond laser waveguide writing, femtosecond laser-induced chemical etching (FLICE), laser ablation, and metallization to realize reconfigurable photonic circuits in borosilicate glass. The process is implemented in a CAD-to-device workflow that allows optical, mechanical, and electrical structures to be co-designed and fabricated within the same substrate. A stress-assisted waveguide-writing regime is developed in borosilicate glass, enabling single-scan fabrication of optical waveguides, directional couplers, and Mach–Zehnder interferometers at writing speeds of 30 mm/s. The fabricated devices demonstrate stable guiding, directional coupling, and interferometric operation, providing a practical basis for implementing reconfigurable photonic building blocks in this material platform. FLICE is then used to fabricate suspended glass microbridges incorporating femtosecond-laser-written waveguides and integrated resistive microheaters. These structures act as thermally isolated thermo-optic phase shifters and enable a full 2π phase shift with an electrical power consumption of approximately 17 mW. The results demonstrate the feasibility of combining FLW and FLICE within a single borosilicate glass substrate to monolithically integrate passive photonic circuits with actively tunable thermo-optic phase shifters. This work establishes a laser-based fabrication route for reconfigurable three-dimensional photonic circuits in glass and provides a basis for future optimization toward larger programmable photonic systems.

Photonics doi: 10.3390/photonics13080730

Authors: Shuyang Lan Hancong Wang Liang Wu Jingyi Xie Jianhua Wang

Surface plasmons are collective oscillations of electrons that can confine light within extremely tiny nanoscale spaces, acting to enhance the spectral signals (such as Raman and circular dichroism) of molecules. Borrowed from circular dichroism in chemistry, plasmonic local circular dichroism (CD) refers to the difference in electric field enhancement excited by left-handed (LCP) and right-handed (RCP) circularly polarized light in a nanogap. Nanosphere trimers have been employed to realize local CD. However, spherical nanoparticles exhibit a small gap area and poor structural stability. In this article, we propose asymmetric nanocylinder trimers on SiO2/Si substrate to realize a strong local CD signal. In contrast to nanospheres, nanocylinder sidewalls increase effective gap areas and can be obtained by nanofabrication. We find that the asymmetric nanocylinder trimer achieves a local CD parameter 0.94 under the excitation of 782 nm. This strong local CD within the gaps mainly originates from the near-field interference between different modes generated by circularly polarized light. Furthermore, the gap distance and symmetry have a significant influence on the local CD. A giant local CD can be obtained for small gaps (1 nm) and right-angle trimers. These results provide a physical basis for the development of plasmonic CD and chirality, and they are of interest for the field of nanosensors, nanoantennas, solar energy conversion, and polarization-dependent photochemistry.

Photonics doi: 10.3390/photonics13080729

Authors: Serguei P. Murzin

Ultrafast laser micro- and nanofabrication has emerged as a powerful platform for precision manufacturing due to the unique capability of femtosecond pulses to provide highly localized energy deposition and controlled laser–matter interactions. However, conventional scanning-based processing approaches remain limited by a fundamental trade-off between spatial resolution and fabrication throughput. Recent advances in ultrafast optical field engineering provide new strategies for overcoming these limitations through coordinated control of temporal, spatial, and spatiotemporal characteristics of ultrashort laser fields. This review presents recent developments in ultrafast optical field engineering for laser micro- and nanofabrication, covering programmable pulse shaping, spatiotemporal control, spatial light modulation, structured light approaches, holographic methods, and hybrid optical architectures. The operating principles of these technologies are discussed together with their influence on energy deposition, processing accuracy, scalability, and manufacturing efficiency. Particular attention is given to applications in high-throughput surface structuring, parallel microfabrication, three-dimensional processing, photonic device fabrication, and functional material modification. Different optical architectures are compared in terms of flexibility, optical efficiency, power-handling capability, and industrial applicability. The review highlights the transition from conventional single-spot processing toward adaptive, parallel, and programmable optical manufacturing systems, emphasizing integrated control of ultrafast optical fields as a key direction for future laser fabrication.

Photonics doi: 10.3390/photonics13080728

Authors: Wei Jiang

Three-dimensional display is fundamentally a problem of reconstructing view-dependent optical rays rather than presenting planar images. Multilayer light field display provides an effective panel-based route for 3D scene reconstruction, but it remains constrained by two coupled bottlenecks: additional attenuation layers can improve angular reconstruction while reducing transmitted light, and higher spatio-angular density increases the cost of inverse synthesis. This study proposes a dynamic-backlight-enabled multilayer framework in which a pixel-addressable emission plane participates in light-field synthesis, thereby replacing one purely passive illumination stage with a controllable optical degree of freedom. The emitted rays of conventional- and dynamic-backlight configurations are represented by a unified non-negative tensor model, and layer synthesis is formulated as a weighted non-negative tensor factorization (NTF) problem. A physics-consistent convolutional neural network is further used to predict display-layer patterns through the same optical forward model and is extended from 5 × 5 to 10 × 10 viewpoint inputs. The quantitative evidence reported in this paper comprises NTF runtime and test-set PSNR, whereas the prototype photographs provide qualitative verification of viewpoint reconstruction. Because the conventional and dynamic prototypes use different LCD and OLED panels that were not luminance- or color-matched, their apparent brightness and contrast differences are not interpreted as calibrated measurements of architectural gain. Within this evidence boundary, the results support proof-of-concept feasibility of jointly designing the emission plane, attenuation layers, and inverse solver, rather than complete validation of a practical electronically addressable multilayer display.

Photonics doi: 10.3390/photonics13080727

Authors: Yangjian Xu Ziyang Xiao Wenju Feng Ruixiang Fan Tao Yang Wei Chen

Erbium-doped fibers (EDFs) suffer from radiation-induced absorption (RIA) and radiation-induced gain variation (RIGV) under ionizing radiation, which limit their applications in space optical communication systems. To address this issue, a novel core–cladding Ce co-doped fiber is proposed, in which core Ce suppresses radiation-induced defect formation while cladding Ce reduces localized energy deposition in the fiber core. GEANT4 Monte Carlo simulations were performed to optimize the Ce-doping configuration, and three kinds of fiber samples, namely conventional erbium-doped fiber (EDF), core Ce co-doped EDF (CEDF1), and core–cladding Ce co-doped EDF (CEDF2), were fabricated for experimental validation. At a total dose of 1200 Gy, CEDF2 exhibits a 40.5% reduction in RIA and a 58.1% reduction in RIGV compared with the conventional EDF. The experimental results demonstrate that the proposed core–cladding Ce co-doped structure effectively enhances the radiation resistance of erbium-doped fibers and provides a practical design strategy for radiation-hardened active optical fibers.

Photonics doi: 10.3390/photonics13080726

Authors: Wenhao Xu Yunlan Zuo Shiyang Zhang Qianze Li Lan Xu

We propose a photonic spin Hall spatial differentiator based on a natural hexagonal boron nitride (hBN) interface in the mid-infrared Reststrahlen band. The optical-axis orientation is set by the crystal cut and lies in the incidence plane, while the operating mode is selected through the incidence angle, input polarization, and analyzer orientation. Near the ENZ-assisted Brewster-like condition, co-polarized reflection is strongly suppressed, enabling a high-contrast X-differentiation channel, whereas the Y-differentiation channel originates from first-order angular-spectrum spin–orbit coupling and remains background-free. At ν=824.8cm−1 and fixed orientation ϕ=20∘, the effective Brewster-like minimum occurs at θBeff≈69.28∘ with |rp|2≈2.8×10−4. Numerical simulations demonstrate switching among intensity-like imaging, X differentiation, and background-free Y differentiation. These results identify fixed-orientation hBN as a natural-crystal platform for polarization-selective mid-infrared optical spatial differentiation without subwavelength patterning.

Photonics doi: 10.3390/photonics13080725

Authors: Ke-Dong Liu Tai-Shuang Yin Aixi Chen

We explore the generation of photon blockade effect in a Laguerre–Gaussian optorotational system where a Gaussian beam exchanges orbital angular momentum with a rotating spiral phase mirror. In addition to the typical optorotational coupling, we consider the existence of cross-Kerr nonlinearity between the cavity mode and the rotating mirror. We investigate the statistical characteristics of photons by numerically and analytically calculating the second-order correlation function. In particular, we find that the antibunching effect of photons is dominated by the cooperative operation between the optorotational coupling and the cross-Kerr coupling instead of any individual part. The optimal single photon blockade can be achieved in a moderate coupling regime and enhanced due to the presence of cross-Kerr nonlinearity. The dependence of photon blockade effect on the different system parameters is discussed in detail. Our work provides an alternative way to manipulate the photon quantum behaviors in Laguerre–Gaussian optorotational systems, which may find potential applications in quantum information processing and optical communication utilizing the optical orbital angular momentum.

Photonics doi: 10.3390/photonics13080723

Authors: Jaewon Choi Minhyuk Kim

We present an evolutionary optimization framework for designing neural architectures for holographic phase prediction in optical tweezer arrays. The framework combines Fourier-transform-based network structures with genetic optimization of layer composition and connectivity, enabling optimization of prediction quality under an inference-time constraint. The optimized architecture achieves phase prediction quality comparable to several iterations of the Gerchberg–Saxton (GS) algorithm while significantly reducing the inference time. The resulting model generalizes to larger array sizes beyond the training set and can be directly implemented on a spatial light modulator. We present this as a proof-of-concept demonstration of the architecture-search methodology and of the physical realizability of the predicted holograms; a fully benchmarked model with large-scale statistical evaluation and real experimental validation is left for future work. Defect-free single-atom array assembly for neutral-atom quantum computation and quantum simulation provides the broader motivation for this line of work.

Photonics doi: 10.3390/photonics13080724

Authors: Nan Huo Xinyue Lv Jinze Wu Jing Wang

Nonlinear interferometers offer a robust platform for quantum sensing beyond the standard quantum limit. Specifically, the generation of high-quality intensity-difference-squeezed (IDS) states provides a distinct advantage by circumventing the need for complex phase-coherent local oscillators. However, practical imperfections such as asymmetric inter-stage transmission and unbalanced detection losses severely degrade the generation of these states. In this work, we analyze IDS state generation within fiber nonlinear interferometers utilizing cascaded parametric amplifiers. Using a single-mode theoretical framework and experimental demonstration, we derive an optimal electronic gain ratio for differential detection to compensate for asymmetric detection-induced noise mismatches, extending achievable squeezing into the high-gain regime. We further analyze system tolerance to inter-stage losses and path delays. Experiments demonstrate that while higher initial parametric gain improves quantum correlations, it amplifies intensity noise and accelerates gain saturation, resulting in optimal squeezing at balanced low-gain configurations. These findings establish a robust framework for the design and optimization of all-fiber quantum sensors.

Photonics doi: 10.3390/photonics13080722

Authors: Yuxi Lin Yu Liu Yuxuan Yang Xiaoyan Zhou Bo Ni Lifu Wu

Structure colors play a key role in display devices, imaging security certification, optical data storage, and so on. However, relatively little work has been done on controlling color brightness, and the reported structures are unable to maintain high saturation at the same time. We demonstrate a symmetry-breaking-driven titanium dioxide (TiO2) dimer metasurface. By precisely inducing quasi-BIC (q-BIC), this structure achieves a spectral response in the visible light band with a reflectance approaching 1 and a full width at half maximum (FWHM) of less than 7.2 nm. Utilizing global size scaling enables support for an ultra-wide color gamut display covering approximately 149.9% of the sRGB color space. By perturbing the structure on one side, we have achieved independent control over the brightness of structural color whilst maintaining high saturation. Furthermore, it is found that this metasurface structure exhibits good robustness against geometric distortions. The structure proposed in this paper opens the way for a wide range of 3D nanoprinting and modern advanced displays in both scientific fields and practical applications.

Photonics doi: 10.3390/photonics13080721

Authors: Vedran Vujnović Nenad Kralj Marin Karuza

We develop a versatile description of Fabry–Pérot resonators comprising internal dielectric structures, based on transfer matrices and interpreted from a non-Hermitian one-pole self-energy viewpoint. For a two-mirror cavity containing an internal slab of arbitrary thickness and refractive index, we derive closed-form expressions for transmission and identify cavity poles as zeros of the reduced denominator in the complex-frequency plane. For a weak-reflector, we obtain leading-order expressions for the pole shifts of individual modes and show that the resulting mode pulling and linewidth change are, respectively, governed by the imaginary and real parts of a single complex quantity formed by the coherent sum of two mirror-side scattering paths. These expressions provide placement criteria for dispersive or dissipative operation and support practical workflows for extracting weak-reflector parameters from measured resonance traces, predicting slab-modified cavity spectra from the empty cavity calibration, and designing doubly resonant cavities with fine constraints on slab position. We extend the consideration to multiple reflectors, with emphasis on two-membrane and three-mirror geometries, relevant to coupled filter cavities and membrane-in-the-middle architectures. The coupled-pole parametrization relates these configurations, providing a compact framework for the analysis and design of composite Fabry–Pérot elements for precision filtering and quantum-noise shaping in advanced interferometric experiments.

Photonics doi: 10.3390/photonics13080720

Authors: Bin Shao Jinxin Hui Wu Zhang Xiaojun Cheng Deyu Du Peng Zhang Xiaohua Lei Qinggui Tan

To eliminate the sensitivity of frequency-swept interferometric ranging to Doppler-induced errors and laser frequency-sweeping nonlinearities, as well as to overcome the limitations of existing suppression strategies—such as strict synchronization requirements in dual-sweep configurations and increased system complexity in single-sweep schemes with auxiliary optical paths—a microwave photonic frequency-swept interferometric method based on dual-sideband modulation is proposed. In the proposed approach, an electro-optic modulator is utilized to generate a pair of frequency-symmetric optical sidebands with opposite chirp characteristics. Owing to the bidirectional symmetry of the frequency sweeps, Doppler-induced and frequency-sweeping nonlinearity induced errors in the two sidebands exhibit equal magnitudes but opposite signs, enabling intrinsic cancellation of Doppler effects as well as compensation of system-nonlinearity induced errors. Experimental validation demonstrates that, compared with conventional frequency-swept interferometric techniques, the proposed method achieves approximately 35% improvement in measurement accuracy under dynamic conditions, while preserving high resolution and strong robustness against environmental perturbations. The results indicate that the proposed scheme offers a simple, effective, and practical solution for high-precision dynamic ranging in high-speed and complex environments.

Photonics doi: 10.3390/photonics13080719

Authors: Weiwei Cao Bo Wang Yang Yang Bingli Zhu Peng Xu Xiaohong Bai Junjun Qin Yongsheng Gou Xiaogang Tong Jingping Zhu Yonglin Bai

This work presents a laboratory prototype of an Electron-Bombarded Active Pixel Sensor (EBAPS) to investigate the effects of passivation film thickness on electron energy loss and bombardment gain. Through combined experimental characterization and numerical simulations, we systematically examine the correlations among accelerating voltage, passivation layer thickness, electron gain, and dead-layer energy dissipation. Experimental results show that the fabricated EBAPS achieves a spatial resolution of 18 lp/mm at an accelerating voltage of 8000 V. Reducing the passivation layer thickness from 70 nm to 30 nm decreases dead-layer energy loss from 2000 eV to 1000 eV. An optimized Monte Carlo model is developed to simulate electron penetration behaviors under different thicknesses and voltages, and its predictions are in good agreement with experimental data. This study confirms that thinning the passivation layer effectively lowers the required bombardment voltage and improves the long-term operational reliability of EBAPS devices. These findings offer both experimental evidence and theoretical guidance for substrate thinning and surface modification strategies in EBAPS development.

Photonics doi: 10.3390/photonics13080718

Authors: Huanhuan Qin Xizheng Ke

With advances in emerging material technologies, intelligent reflecting surface (IRS)-assisted vehicular networks have been gaining growing interest. By adaptively shaping the wireless propagation environment, IRSs can improve vehicular network quality of service (QoS). However, most IRS-assisted vehicular network studies are limited to individual RF or VLC frameworks, while only a few investigate IRS-assisted aggregated VLC-RF vehicular networks that combine wide RF coverage with high VLC data rates. In this paper, aggregated VLC-RF vehicular networks are supported by both optical IRSs (OIRSs) and RF IRSs, and a resource allocation scheme is developed to improve the total achievable rate. First, we establish a system model for IRS-assisted aggregated VLC-RF vehicular networks, and then formulate a problem to maximize the total achievable rate. Furthermore, we decompose the maximization of the total achievable rate into five subproblems and solve them iteratively via an efficient alternating optimization scheme based on block coordinate descent (BCD). Moreover, simulation results validate the convergence and efficiency of our algorithm, while highlighting the effects of crucial parameters on system performance, providing valuable insights for resource allocation in IRS-assisted aggregated VLC–RF vehicular networks.

Photonics doi: 10.3390/photonics13080716

Authors: Shiqi Yang Daohong Xiao Xiangjie Luo Kui Wang Haishan Tian Leyong Jiang

Optical bistability (OB) with low threshold and high tunability is crucial for advanced photonic devices. This work theoretically investigates low-threshold and tunable OB in a one-dimensional photonic crystal Fabry-Pérot (FP) cavity embedded with a Weyl semimetal (WSM) layer. By combining the local field enhancement of the cavity and the large third-order nonlinear refractive index of the WSM, we achieve OB in the terahertz regime with a low threshold of ~105 V/m. Furthermore, the OB threshold and hysteresis width can be flexibly manipulated via the Fermi energy of WSM, incident angle, photonic crystal period, the refractive index of the dielectric inside the cavity, and the position of the WSM inside the FP cavity. Moreover, based on the high sensitivity of switching thresholds to refractive index and displacement variations, we also propose a dual-parameter sensing scheme. This exhibits excellent sensing performance for gas sensing (sensitivity up to 96.48 × 105 V/m·RIU) and high sensitivity for displacement sensing (0.91 × 106 V/m·μm). We believe that the above results can provide reference schemes for nonlinear photonic devices.

Photonics doi: 10.3390/photonics13080717

Authors: Alisher Rajabov Obid Sabirov Bogibek Urinov Gaetano Assanto Usman Sapaev

We numerically investigate high-efficiency third-harmonic generation via cascaded χ(2) processes in orientation-patterned Gallium Arsenide waveguides comprising two quasi-phase-matched sections. The first section is phase-matched for second-harmonic generation (ω→2ω) and the second for sum-frequency generation (ω+2ω→3ω). Assuming a continuous-wave pump at λ=10.61μm, the structure converts the fundamental to the third harmonic at λ3ω≈3.54μm, within the atmospheric transmission window. By solving the coupled-wave equations, we optimize the number of domains in each section and estimate THG conversion efficiencies exceeding 90% at a pump intensity of ∼100MW/cm2 in a total crystal length ≈1.6cm. Using temperature-dependent parameters from the Sellmeier equations for GaAs, we also address thermal tuning of quasi-phase-matching over the range 22–300 °C and pinpoint robustness against fabrication tolerances of ±10 domains and ±2% in domain length. Our results indicate two-section orientation-patterned GaAs as a suitable platform for high-efficiency frequency tripling from mid-infrared.

Photonics doi: 10.3390/photonics13080715

Authors: Hanwen Zhao

With the growing need for high-density data and real-time communication, multimode optical fibers (MMFs) have emerged as a vital technology due to their ability to transmit multiple optical modes simultaneously through spatial division multiplexing (SDM). However, issues such as mode coupling and dispersion continue to compromise signal clarity. This study presents a quantitative analysis of machine learning applications for enhancing mode conversion and holographic reconstruction in MMF systems. We demonstrate that a shallow artificial neural network can predict reconstruction accuracy, information transfer capacity, and adaptive system efficiency based on mode coupling and dispersion inputs, achieving prediction errors below 4% on case-study data. A convolutional neural network achieves 93.7% accuracy for mode separation, with a 62% reduction in mode error rate compared to conventional reconstruction methods. Hybrid physics-informed learning approaches incorporating energy conservation and phase stability constraints are explored, showing promise for improved generalization. Our results indicate that machine learning significantly enhances reconstruction precision and information transmission in MMFs, providing a foundation for next-generation communication and imaging systems. Despite certain training challenges, these models effectively process complex data, demonstrating their reliability and predictive capabilities.

Photonics doi: 10.3390/photonics13080714

Authors: Aijun Liu Hanlin Zhang Tengfei Li Kaixiang Yang Jinghua Han

Efficient laser cleaning of glazed ceramic tiles requires the ink layer to be removed without damaging the brittle glaze. We investigated the removal of acrylic ink using a 1064 nm, 10 ns Nd:YAG laser operating at 1 Hz. The lens-to-sample working distance L, pulse energy, and pulse number were varied, and the cleaned regions were evaluated by optical microscopy (OM), scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM–EDS), theoretical analysis, and COMSOL simulation. At L = 20 cm, the sample was close to the nominal focal plane, and the high local fluence removed the ink rapidly, but it also produced whitening, depressions, micro-pits, and glaze damage. Increasing L to 25–30 cm enlarged the measured spot diameter, lowered the average fluence, and widened the controllable cleaning range. Two low-damage conditions were identified at L = 30 cm: 30.80 J/cm2 with 3 pulses and 41.00 J/cm2 with 2 pulses. SEM–EDS showed that cleaning quality cannot be judged from exposed area or carbon content alone; morphology, preservation of the native glaze microstructure, the C/O ratio, and recovery of substrate-related elements must be considered together. Under the stated model assumptions, the calculated local temperature exceeded the acrylic decomposition temperature, and the thermoelastic stress reached tens to hundreds of MPa. These results make thermal decomposition and stress-assisted interfacial separation physically plausible. However, the present data do not separate thermoelastic stress from pressure-wave loading. Likewise, visible air breakdown and non-monotonic cleaning at high pulse energy only suggest possible plasma-related attenuation because plasma density and transmitted laser energy were not measured. The reported combinations should therefore be regarded as a system-specific process window rather than a universal optimum.

Photonics doi: 10.3390/photonics13080713

Authors: Mingliang Yao Yinnian Liu Pengfei Zhao Chen Zhu Youlong Ke

The convex blazed grating is a key dispersive component in high-performance spectrometers, offering advantages such as a broad operating wavelength range, uniform dispersion, high diffraction efficiency, and the ability to achieve a large field of view. With the popularization of spectral detection technology and the ever-increasing demand for specialization, its design and fabrication technologies have drawn considerable attention in the field. This paper systematically reviews the development history of convex blazed grating design theory, from early scalar diffraction theory to the current mainstream rigorous vector methods, including rigorous coupled-wave analysis (RCWA), the finite-difference time-domain (FDTD) method, and commercial software such as Gsolver and PCGrate, and summarizes the applicable scenarios and limitations of each method. In terms of fabrication techniques, we comprehensively survey three typical technology routes—mechanical ruling, holographic ion beam etching, and electron beam lithography—covering their principles and progress, and analyze their respective merits and drawbacks in terms of precision, operating waveband, groove profile flexibility, and production capacity through comparative analysis. On this basis, we highlight recent breakthroughs achieved via electron beam lithography in blaze angle control and high-aspect-ratio etching for convex blazed gratings spanning from the ultraviolet to the very-long-wave infrared band; the diffraction efficiency has exceeded 80%, and such gratings have been successfully applied in aerospace engineering projects. Finally, this paper summarizes the current challenges facing convex blazed grating technology and provides an outlook on future development trends, including fabrication uniformity on curved substrates, large-area high-precision manufacturing, and design–process co-optimization, with the aim of offering a systematic reference for researchers and engineers in related fields.

Photonics doi: 10.3390/photonics13080712

Authors: Sanfeng Hao Donghao Zheng Yonghong Zong Shuai Liu Xiyu Li Xin Gao

Segmented primary mirror technology overcomes the manufacturing limitations of large-aperture monolithic mirrors by assembling the primary mirror from multiple off-axis aspheric sub-mirrors. However, null testing of such off-axis aspheric surfaces with a compensator introduces mapping distortion, which prevents the interferometric data from directly guiding high-precision deterministic figuring. This paper proposes a mapping distortion correction method for sector-shaped off-axis aspheric sub-mirrors tested with a null lens. The method exploits the fact that one-dimensional radial distortion symmetry of the null lens about its distortion center. First, the distortion center coordinates are determined from the radial boundary data of the distorted interferogram of the sector-shaped mirror. Then, an error function is constructed based on the deviation of the measured outer-to-inner radius ratio of the sector-shaped mirror from its nominal value, and the radial distortion coefficients are iteratively solved using a radial distortion model refined by a sparse set of fiducial marks. Finally, the mapping distortion is corrected through an inverse mapping with solved distortion center coordinates and radial distortion coefficients. The proposed method was validated on a sector-shaped sub-mirror with outer and inner radii of 357.5 mm and 75.5 mm, reducing the outer-to-inner radius ratio error from 0.3833 to 0.0094, with a maximum fiducial coordinate deviation of 0.91 mm. Based on the distortion-corrected map, a single deterministic figuring run reduced the surface figure RMS from 0.034λ to 0.025λ (λ = 632.8 nm), demonstrating the effectiveness of the method.

Photonics doi: 10.3390/photonics13080711

Authors: Tianyu Dai Jiagang Hou Hanjie Wang Guangmang Cui Shangda Han Jufeng Zhao

Optical image-based texture and material surface recognition is important for computational imaging, intelligent inspection, and material-reference retrieval, where a system often needs not only a closed-set label but also inspectable evidence and an expandable gallery. Scale-varying micro-textures, repeated surface structures, global layout cues, and visually ambiguous categories make this task difficult under non-controlled imaging conditions. This paper presents Reliability-Aware Global-Local Metric Retrieval (RAGL-MR), which maps images and patches into a shared ArcFace metric space, retrieves multi-scale patch neighbors as local surface evidence, compresses whole-image reference galleries using Multi-Prototype Global Distribution Modeling (MP-GDM), and combines branch scores through validation-selected reliability-aware fusion. The framework is evaluated on public texture/material surface image benchmarks used as proxies for surface image analysis rather than as controlled photonic experiments. On the first five official DTD splits, the final fusion reaches 74.06 ± 0.11 Top-1, 92.61 ± 0.61 Top-5, and 96.11 ± 0.23 Top-10; on MINC-2500, it reaches 85.84 ± 0.25 Top-1, 97.63 ± 0.28 Top-5, and 99.15 ± 0.10 Top-10, with additional checks on FMD and KTH-TIPS2-b. Additional perturbation, calibration, large-gallery, and industrial surface-defect checks further examine robustness, reliability, and practical retrieval behavior. These results indicate that RAGL-MR provides an evidence-aware and gallery-extensible retrieval complement for optical material surface image analysis.

Photonics doi: 10.3390/photonics13080710

Authors: Mengyu Li Yuxin Chen Chenglong Zheng Yiming Wang Quanping Fan Lai Wei Shaoyi Wang Huaping Zang Leifeng Cao

Composite vortex beams (CVBs) with multiple spatial singularities and orbital angular momentum (OAM) are widely used in various applications including multiple optical traps and optical communication. Here, based on the equal-angle segmentation and radial displacement modulation approach, we propose an innovative scheme for generating CVBs with controllable OAM spectrum by proposing a simple and compact optical element termed as single-helicity annulus-sector spiral zone plates (SASZPs). Theoretical analysis reveals that by modulating the structural parameters of the SASZPs, such as the topological charge, the radial misalignment parameter and the number of annulus-sector primitives, an attractive intensity pattern consisting of petal-like structures can not only be produced but also the mode purity of CVBs can also be flexibly controlled. In addition, by adopting the high-quality and low-defect diamond substrate we have synthesized, based on the electron beam lithography technology and dry etching technology, the SASZP samples with different parameters have been fabricated and the focusing properties of such optics in the visible light region have been carried out and verified. These findings direct a new avenue for improving the performance of ultra-compact solar-blind UV imaging, optical communication and integrated optics.

Photonics doi: 10.3390/photonics13080709

Authors: Muhammad Asad Khan Asadullah Dawood M Hisham Alnasir Muhammad Junaid Ambreen Ayub Abdul Wahab Ajmal Salem AlFaify Mahmoud Fahmy Youssef Shalaby

The growing volume of plastic waste produced has made the need for efficient identification and quantification strategies to detect and identify harmful contaminants, particularly heavy-metals, in industrial plastic-waste streams more urgent than ever. Traditional chemical digestion methods (acid digestion, ICP-MS, or AAS) provide accurate quantification and are time-consuming, costly, and not high-throughput-sorting. Laser-induced breakdown spectroscopy (LIBS) offers several advantages, including rapid real-time analysis, minimal sample preparation, multi-element detection, and efficient elemental characterization. The current review aims to investigate the possibility of using the LIBS technique to detect and quantify some heavy-metals like lead (Pb), cadmium (Cd), chromium (Cr), and arsenic (As) in heterogeneous industrial plastic wastes. The calibration strategies (reference standards) as well as optimization of the signal response and algorithms for data processing, including chemometric models, are evaluated to reduce the limit of detection and enhance analytical accuracy. The results indicate that LIBS can detect heavy-metal contamination at concentrations relevant to regulatory limits and can be scaled for online quality-control monitoring in plastic recycling facilities. In this review, the results have been summarized to support the use of LIBS for waste management towards environmental compliance, reduced use of hazardous materials in recycling products, and reduced health problems due to heavy-metal pollution.

Photonics doi: 10.3390/photonics13080708

Authors: Lifeng Ma Yi Huang Ting Zheng Jun Chang Huilin Jiang

Conventional snapshot-type polarization devices often suffer from inherent ohmic losses caused by the metal structure, resulting in low utilization of system light energy. This research proposes a dual-layer sandwich architecture metasurface that integrates polarization control and high light transmittance for the mid-wave infrared 3~5 μm band. The top metal polarization-selective structures and the bottom dielectric hemispherical anti-reflection (AR) array are integrated monolithically on the same substrate. Specifically, the numerical simulations predict a peak transmittance of 95% at 4.4 and 4.8 μm, while maintaining extinction ratios ranging from 81.8 dB to 84.3 dB. This enables high extinction ratio polarization splitting while significantly broadening the transmittance flux of the device. It breaks the inherent trade-off between “high extinction ratio” and “high transmittance” in polarization devices, providing a high signal-to-noise ratio hardware foundation for high temporal resolution detection.

Photonics doi: 10.3390/photonics13080707

Authors: Tao Zhang Hairui Wang Rui Zhao Qiang Fu

The complex morphology and optical anisotropy of anisotropic sea-fog particles make it difficult to accurately describe the multiple scattering behavior of polarized light. To overcome the insufficient coupling between polarization evolution and scattering direction in conventional Monte Carlo models, as well as their limited representation of particle parameters, this study proposes a polarization transmission model combining an improved Discrete Dipole Approximation with a direction-adaptive Monte Carlo method. In this model, the probability density function of scattering direction is constructed as a functional of the photon Stokes vector, enabling tight coupling between polarization state updates and photon propagation. Particle structures are refined by incorporating size, aspect ratio, Euler angles, morphological perturbations, and complex refractive indices. Meanwhile, an equivalent scattering kernel containing higher-order statistical moments is introduced to balance computational efficiency and physical fidelity. An indoor sea-fog polarization transmission platform was established to measure six incident polarization states at 450, 532, 671, and 808 nm. The results show over 80% agreement with model predictions and a root mean square error below 0.1. The study further indicates that circular polarization retains polarization better than linear polarization under high optical thickness, while longer wavelengths provide more stable polarization transmission. This framework offers theoretical support for polarization imaging and optical communication in sea-fog environments.

Photonics doi: 10.3390/photonics13080706

Authors: Haojin Wang Jiayue Ren Zekuo Zhang Ruixiang Chen Chong Geng Shu Xu

White LEDs are widely used in fields such as lighting and display. However, existing white light devices suffer from excessively broad emission spectra full width at half maximum (FWHM) and severe leakage of pump blue light. Hence, this study proposes a theoretical design for an ultra-narrow-band white LEDs based on an asymmetric Fabry-Pérot (F-P) microcavity. The microcavity employs a dual-DBR configuration with asymmetric optical responses: the bottom DBRs utilize a ZnS/MgF2 (dL/2-dH-dL/2)5 stack, providing a low reflectance of ~20% at 457 nm for efficient blue light transmission and quantum dot excitation; conversely, the final device incorporating top DBRs with a TiO2/MgF2 (dH/2-dL-dH/2)4-stacked structure achieved a high reflectivity of approximately 89% at 457 nm, effectively decreasing the excessive blue light in the output spectrum. The intermediate emissive layer uses polymethyl methacrylate (PMMA) as the quantum dot host. By precisely tuning the cavity thickness, the resonant modes of the F-P microcavity—specifically the fourth-order (626 nm) and fifth-order (534 nm)—are aligned with the emission peaks of the red and green quantum dots, respectively. Simulation results demonstrate that the structure leverages the cavity filtering effect to compress the FWHM of the red and green emissions from initial values of 36.5 nm and 29.8 nm down to 2.2 nm and 1.9 nm, respectively, representing an order-of-magnitude improvement in color purity. Through the co-optimization of the top-DBR’s central wavelength, cavity optical thickness, and the doping ratio of red/green quantum dots, standard white light emission with CIE coordinates of (0.32, 0.33) was achieved, accompanied by emission efficiencies of 6.3% (green) and 22.3% (red) for the QDs. However, strict manufacturing tolerances and narrow observation angles limit the applicability of the device. This work provides theoretical research for developing white light sources.

Photonics doi: 10.3390/photonics13080705

Authors: Pengju Kuang Changsong Wang Chengwei Xian Ning Fu Yang Zhang Rudi Zhou Guangjun Wen Yongjun Huang

To prevent the collapse of strip waveguides caused by complete undercut during hydrofluoric acid (HF) release in SOI-based cavity optomechanical accelerometers, we propose using rib waveguides as the on-chip optical transmission medium. Based on a 250-nm-thick SOI wafer, we systematically analyze the photonic crystal (PhC) microcavity, rib waveguide transmission, edge coupling, mode conversion to the PhC waveguide, and evanescent coupling. The PhC microcavity has a quality factor of 2.26 × 105 at 1549.15 nm. The optimized rib waveguide (rib width 500 nm, rib height 220 nm) shows a transmission loss of 0.16 dB over 5000 μm. The rib-to-PhC waveguide coupling efficiency is 94.3% (0.25 dB loss), and a 90-μm-long tapered edge coupler achieves 65% efficiency (1.9 dB loss). The total optical path loss (including two edge couplers, rib waveguide transmission, and rib-to-PhC taper) is 4.21 dB. While maintaining optical performance comparable to strip waveguides, the rib waveguide design significantly improves post-release structural integrity at the design level. This work provides a viable optical circuit design foundation for reliable monolithically integrated cavity optomechanical accelerometers, with device fabrication and full system-level characterization planned as future work.

Photonics doi: 10.3390/photonics13080704

Authors: Justas Berškys Klemensas Laurinavičius Sergej Orlov

We investigate a lensless Airy-type optical beam generated by direct Fresnel propagation of a Gaussian apodized real space cubic phase. Starting from the scalar Fresnel diffraction integral, we derive a closed-form expression showing that the propagated field can be written in terms of an Airy function, but with an argument different from that of the conventional finite energy Airy beam. This difference leads to distinct free space dynamics: the conventional beam is already formed at the input plane and follows a parabolic trajectory, whereas the lensless beam forms during propagation and undergoes stronger transverse reshaping and broadening. The two fields are also compared under atmospheric turbulence using a phase screen approach and multiple performance metrics. The comparison shows that no single beam performs best for all detection scenarios: the lensless field gives lower scintillation at the beam maximum, the two beams provide comparable aperture collected power, and the conventional Airy beam gives higher coherent fiber coupling efficiency.

Photonics doi: 10.3390/photonics13080703

Authors: Jing Chen Huizu Wu Weiqing Cheng

Photoelectric intelligent sensor chips have become a key technology for next-generation intelligent sensing by integrating photonic devices, electronic circuits, and artificial intelligence algorithms. Recent advances in silicon photonics, two-dimensional materials, heterogeneous integration, and intelligent signal processing have significantly improved their sensitivity, response speed, and integration capability. This review presents a comprehensive overview of photoelectric intelligent sensor chips from fundamental principles to system-level applications. The operating mechanisms, device architectures, fabrication technologies, and photonic integration strategies are summarized, followed by recent progress in industrial, medical, and intelligent sensing applications. Current technical challenges, including material quality, heterogeneous integration, power consumption, and intelligent data processing, are also discussed. Finally, future trends toward highly integrated, low-power, and AI-enabled sensing systems are highlighted. This review provides a concise reference for the development of next-generation photoelectric intelligent sensor chips and their practical applications.

Photonics doi: 10.3390/photonics13080702

Authors: Xiaorui Wang Ziran Zhan

With the rapid development of space laser communication and deep-space exploration, Deep Space Optical Sensor Networks (DSOSN) have become a vital support for deep-space information transmission. Conventional Directed Diffusion (DD) adopts fixed single-path reinforcement and blind flooding forwarding, which cannot adapt to high-dynamic topology and random link outages in deep space, resulting in high delay, unstable links and heavy network overhead. This paper puts forward a DSODD routing algorithm dedicated to DSOSN. It realizes selective packet forwarding in interest propagation and data return to reduce redundant relay nodes and transmission hops, thus prolonging network lifetime. Meanwhile, a gradient-based path reinforcement mechanism is adopted to strengthen reliable transmission paths. Under the simulation condition of solar scintillation and coronal fading interference, compared with DD, DSODD reduces end-to-end delay by 28% and delay jitter by 32%, and greatly improves link stability with 65% longer average link lifetime and 50% fewer route switches. Simulation results validate that DSODD achieves better comprehensive routing performance for deep-space optical sensor networks.

Photonics doi: 10.3390/photonics13080701

Authors: Jijuan Jiang Xiaohua Xie Guan Wang Yang Jia Qi Chu Shuo Chen Pengfei Hui

We propose an absorber composed of graphene and vanadium dioxide (VO2). The phase transition of VO2 enables dynamic switching of the absorption response between a single broadband state and three narrowband states. The results indicate that when VO2 changes to the metallic state, the absorber exhibits triple narrow band absorption. When VO2 is in the dielectric phase, the absorber demonstrates single broadband absorption. The absorption can be regulated through the Fermi energy level (EF) of graphene. Furthermore, the proposed absorber exhibits polarization-independent characteristics and maintains stable absorption performance under wide-angle oblique incidence. The absorber also has high refractive index sensitivity. Impedance matching theory is employed to investigate the physical mechanism governing the absorber. The tuning principle of the device was analyzed and verified successfully by using an equivalent circuit model (ECM).

Photonics doi: 10.3390/photonics13080700

Authors: Jing Yang Yuli Han Jun Xie Hengjia Liu Shuhua Zhang Jiawei Li Lai Feng Chong Chen Dongsong Sun Tingdi Chen Xianghui Xue

Stratospheric airship navigation requires accurate wind field measurements at a ~20 km altitude, where low pressure and density limit the effectiveness of conventional wind sensors. To address this, we present a compact direct-detection Rayleigh Doppler wind lidar based on the molecular double-edge technique. The system utilizes a 532 nm fiber-coupled pulsed laser (0.5 W, 5 ns) and a fixed-cavity dual-channel Fabry–Perot etalon as the frequency discriminator. A liquid crystal variable retarder (LCVR) combined with a polarization beam splitter (PBS) enables non-mechanical, high-speed beam switching between two orthogonal line-of-sight (LOS) directions for horizontal wind measurement. Systematic tests are performed in controlled wind fields within Mie-dominated and Rayleigh-dominated regimes. The lidar effectively captures the sharp radial velocity profiles at wind speeds up to 7.6 m/s. Comparative experiments with a reference anemometer show that the system delivers reliable performance at 0.48 m range resolution, with measurement uncertainty below 0.34 m/s. With its compact, lightweight, and high-precision design, the developed lidar demonstrates reliable wind measurement capability under laboratory conditions, indicating its potential for future deployment on stratospheric airships.

Photonics doi: 10.3390/photonics13080699

Authors: Yongge Wang Jingfeng Yao Ying Wang Chengxun Yuan Zhongxiang Zhou

The conventional description of time-varying media assumes that electromagnetic fields evolve according to fixed continuity conditions during parameter jumps. However, recent discussions have made it clear that such continuity conditions are not physical constraints, but are determined by the microscopic processes that underlie the modulation. However, a unified theoretical framework capable of systematically describing different continuity conditions is still lacking. By treating continuity rules as tunable parameters and incorporating them into a unified time-varying theoretical framework, the scope of time-varying metamaterials is expanded to encompass non-resonant reflectionless wave amplification without momentum bandgaps, reversible conversion between propagating waves and static fields, etc. Hence, in this work, wave phenomena previously considered impossible become attainable, opening a new dimension for controlling light–matter interactions through time-varying media.

Photonics doi: 10.3390/photonics13080698

Authors: Bian Wang Bo Zhang Qi Lu Chengkun Dong

Conventional quasi-bound state in the continuum (quasi-BIC) terahertz (THz) metasurfaces usually operate at fixed resonance frequencies and lack electrically controlled dynamic tunability. To address this limitation, we propose two schemes for realizing liquid-crystal-integrated quasi-BIC THz metasurfaces by exploiting the electrically tunable refractive index of liquid crystals in the THz regime. A microfluidic multilayer architecture combining a liquid-crystal functional layer with a silicon (Si)-based metasurface is constructed, in which the transition from an ideal BIC to a quasi-BIC is realized through two distinct mechanisms: pixelated control of the liquid-crystal orientation and geometric symmetry breaking of the Si resonators. Cartesian multipole decomposition reveals that the resonances in both configurations are dominated by magnetic dipole modes. The effects of the liquid-crystal orientation angle and key geometric parameters on the resonance frequency, peak absorptance, and quality factor are systematically investigated. Numerical results demonstrate continuous tuning of the terahertz resonance through variation in the liquid-crystal orientation angle, corresponding to electrically driven liquid-crystal reorientation in practical devices. The sensing performance of the optimized Si-resonator-based configuration is further evaluated, achieving a refractive index sensitivity of 117 GHz RIU−1 and a figure of merit (FOM) of 146.25. This work overcomes the fixed-frequency limitation of conventional static quasi-BIC metasurfaces and provides a feasible route toward electrically reconfigurable terahertz microfluidic sensing devices with tunable operating frequencies.

Photonics doi: 10.3390/photonics13080697

Authors: Rongbing Yang Shirui Liu Zhang Zhang Wei Xu Kaishuai Yang Yawei Kuang Zhida Han Zijie Dai Kejiang Yan Yi Liu Shuai Yin Tianyue Yao Jun Yang Feiyang Zhang Ziyan Zhou Chenchen Zhao Wenjuan Han Guohui Tu Longhai Liu Lanju Liang Jianquan Yao

Topological photonics provides a robust framework for controlling light, with edge states offering immunity to disorder. However, utilizing this stability for practical sensing remains a challenge, as conventional high-sensitivity photonic crystal sensors suffer common imperfections. Here, we numerically design a topological photonic crystal sensor for THz refractive-index detection. The silicon-based structural slab is engineered, showing a wide photonic bandgap (PBG). By breaking the inversion symmetry, two structures with trivial and nontrivial topological phases are constructed. Due to their opposite valley Chern numbers, the topologically protected edge states can be formed at the interface. Such edge states were further integrated with the resonant cavities, so that the resonant frequencies were observed in transmittance property, forming a stable cavity–edge states coupling channel for refractive-index sensing. Numerical results demonstrate strong suppression of transmission distortion induced by boundary defects, verifying favorable topological robustness of the proposed architecture. The structure exhibits linear refractive-index response with a simulated sensitivity of 1.9 THz/RIU, outperforming conventional photonic crystal sensors in numerical comparison. This work merges topological stability of edge states with high sensitivity response of resonant frequency, offering a theoretical candidate for robust biosensing and chemical detection.

Photonics doi: 10.3390/photonics13080696

Authors: Kaicheng Wu Mohammad Kabir Bangzhi Liu Shizhuo Yin

In this paper, we report a novel self-smoothed gradient index cladding waveguide for low-loss, high-density photonic integrated circuits (PICs). In conventional PIC waveguides, there is a fundamental trade-off between propagation loss and bending loss. High-index-contrast waveguides can provide strong mode confinement and small bending radius, but they are highly sensitive to sidewall roughness and therefore exhibit increased scattering loss. In contrast, low-index-contrast waveguides can reduce propagation loss, but they require a much larger bending radius and are not suitable for dense photonic integration. To overcome this limitation, we propose a self-smoothed double-cladding waveguide architecture composed of a high-index core, a gradient index first cladding layer, and a low-index second cladding layer. The first cladding layer can be formed by advanced conformal coating processes, such as non-uniformly cycled atomic layer deposition and gradient index dip coating, which provide both a gradual refractive index transition and a self-smoothing effect on the rough sidewall. We perform quantitative analyses of the propagation loss, bending loss, and mode field distribution of the proposed structure. The numerical results confirm that the propagation loss can be reduced by approximately two orders of magnitude while maintaining low bending loss and mode confinement comparable to that of a conventional rib-shaped waveguide. We also experimentally verify the self-smoothing effect by using atomic layer deposition (ALD)-grown non-uniformly cycled nanolaminates and coating a rough sapphire bar with a high-refractive-index polymer, which significantly reduces the measured surface roughness and improves optical transparency. These results confirm that the proposed self-smoothed gradient index cladding waveguide can be an effective platform for realizing low-loss, high-density PICs.

Photonics doi: 10.3390/photonics13080695

Authors: Dokhyl AlQahtani Fady I. El-Nahal

We present a short-preamble digital signal processing (DSP) architecture for downstream Alamouti coherent-lite passive optical network (PON) reception using a simplified optical network unit (ONU) receiver. A four-branch gear-shifted least-mean-square (LMS) equalizer uses a large step during quadrature phase-shift keying (QPSK) preamble training and a small step during decision-directed payload tracking. In 50 GBaud 16-ary quadrature amplitude modulation (16-QAM) simulations over 20 km single-mode fiber, a 2048-symbol preamble is the first cold-start point whose mean bit-error rate (BER) falls below the selected uncoded BER target of 10−2. A 20-realization-per-point sweep gives a sub-1 dB penalty for 2048 symbols and a mean offline mean-square-error (MSE) settling time below 100 ns, with low-gain locking events retained. However, frame-level reliability improves with longer training: in an indicative 20-frame Monte Carlo test, 13 of 20 frames met the BER target with a 2048-symbol preamble, compared with 17 of 20 using 4096 symbols. For repeated frames to the same ONU after initial acquisition, warm-start tap reuse allows frames 4–5 in the tested sequence to fall below target with 512-symbol preambles, corresponding to a later-frame nominal coded payload rate of 163 Gb/s instead of 123 Gb/s. The results quantify the preamble/reliability/payload-rate trade-off and indicate that downstream Alamouti coherent-lite reception can support short-training equalizer acquisition after preamble synchronization with one balanced detector and one analog-to-digital converter.

Photonics doi: 10.3390/photonics13080694

Authors: Pauline Conigliaro Marianna Portaccio Alain Moréac Maria Lepore Ines Delfino

Surface-enhanced Raman spectroscopy (SERS) is a powerful technique for detecting, identifying, and quantifying analytes of interest in both environmental and clinical contexts. A key factor in SERS is the choice of substrate, which directly influences the enhancement factor and measurement reproducibility. A wide range of commercial SERS substrates is currently available, featuring tailored nanostructures and surface patterns designed to optimize signal enhancement. Recently, SERS has also been applied to the development of detection strategies for phenolic compounds. Within this framework, we aimed to evaluate several commercial SERS substrates and one homemade SERS substrate using catechol as a molecular probe. Each substrate was initially assessed by acquiring spectra of the bare substrate using the laser excitation wavelengths recommended by the manufacturers. Raman spectra of catechol solutions at relatively high concentrations were also acquired using the same wavelengths. These preliminary measurements guided the selection of experimental conditions for subsequent substrate performance evaluations. Hyperbola and linear function fitting were performed to quantitatively characterize the tested substrates in catechol detection. The proposed approach allowed for the identification of a parameter that can be used for estimating a substrate’s overall efficiency, along with the main sensing figures of merit.

Photonics doi: 10.3390/photonics13070693

Authors: Bingcai Sun Xingcheng Zhao Mosong Li Zhaoheng Liu Quan Li

Distributed acoustic sensing (DAS) is increasingly used for third-party intrusion (TPI) detection in oil and gas pipeline monitoring, but labeled DAS data are often scarce, leading to overfitting, poor generalization, and increased false alarms and missed detections. Conventional data augmentation, GAN-based synthesis, and transfer learning may generate physically implausible samples or fail to cover the event feature space. To address this, we propose a physics-guided large language model (LLM) prompt-engineering framework for DAS data augmentation and pipeline intrusion detection. The framework establishes a physically grounded feature-indicator framework for DAS disturbance-event classification by mapping primary event mechanisms to measurable signal indicators, and then uses a standardized four-module prompt template to guide LLM-based synthesis-script generation. A two-stage iterative verification procedure is further introduced to constrain the generated samples in terms of physical-mechanism compliance and feature-parameter consistency. Synthetic data are combined with real data to train a lightweight PatchTransformer model for TPI detection, while an additional CNN is used to assess cross-architecture applicability. Using the public DAS1K benchmark with five-fold stratified cross-validation and a univariate controlled experiment (0–800 synthetic samples per category), the results show that the use of synthetic data improves detection performance overall. The configuration with 600 synthetic samples per category achieves 92.27% accuracy and 92.38% macro-F1, outperforming the conventional augmentation baseline by 4.74 and 4.86 percentage points, respectively. An additional CNN experiment also showed consistent performance gains across the tested augmentation settings, indicating that the benefit of the proposed synthetic data was not restricted to the PatchTransformer architecture. These findings indicate that LLM-assisted data augmentation can effectively improve the generalization of DAS-based pipeline intrusion detection when field-labeled samples are scarce.

Photonics doi: 10.3390/photonics13070692

Authors: Anyi Jiao Jiaze Wu Yu Ding Xiaotao Yang Xiaoming Duan

Lasers operating in the 2 µm spectral region have significant application value and development potential in fields such as lidar, medical surgery, and mid-infrared nonlinear optics because they are located in the atmospheric absorption window and coincide with a strong absorption peak of water molecules. This article first briefly introduces the energy level characteristics and commonly used crystal matrices of Tm3+, Ho3+ and Tm3+/Ho3+ codoped systems, and then reviews the research progress of 2 µm side-pumped solid-state lasers based on these ions and matrices. This review summarizes the research progress of 2 µm side-pumped solid-state lasers, categorized by operating mode and gain medium. Particular attention is given to the mature advantages of Tm-doped garnet lasers in high average power output and the potential of Tm/Ho-codoped fluoride lasers in high-energy, high-beam-quality pulse output. Finally, this paper further reviews the development of side-pumped laser structures and offers a prospective outlook on the future development of 2 µm region side-pumped lasers.

Photonics doi: 10.3390/photonics13070691

Authors: Silvia Paukovčeková Peter Tatar

Biological aerosols represent a significant challenge for modern CBRN defense due to their potential for long-range dispersion and the need for rapid threat assessment. Current stand-off detection systems are effective in recognizing anomalous aerosol clouds but often lack the molecular specificity required for reliable agent identification. This review examines the role of mid-infrared (MIR) spectroscopy as an emerging approach for chemically resolved stand-off bioaerosol sensing. The physical principles of MIR detection are discussed, including molecular vibrational fingerprints, differential scattering (DISC), and circular intensity differential scattering (CIDS), together with their relationship to aerosol optical properties and Mie resonance effects. Existing and emerging sensing architectures are reviewed, ranging from operational CO2 laser-based DISC systems to semiconductor-based platforms utilizing tunable differential absorption lidar (DIAL), Quantum Cascade Lasers (QCLs), and dual-comb spectroscopy. The analysis highlights the ability of MIR sensing to access biomolecular signatures associated with proteins, lipids, nucleic acids, and bacterial spores, while also addressing challenges related to atmospheric attenuation, biological variability, and signal interpretation. The reviewed literature indicates that MIR spectroscopy offers a promising pathway toward improved stand-off identification of hazardous bioaerosols, supporting early threat detection and enhanced situational awareness in applications including CBRN defense, critical infrastructure protection, environmental monitoring, public health surveillance, and emergency response.

Photonics doi: 10.3390/photonics13070690

Authors: Zhi-Chao Han Yu-Bo Liang Ming-Jie Liao Zi-Jian Lin Shuai-Ling Wang Jing-Ping Xu Jabir Hakami Ya-Ping Yang

In this study, we design a new magnetic field measurement model. Specifically, we use a single two-level atom coupled to two cavities to construct a parity–time-symmetric system supporting a third-order exceptional point. If a perturbation is applied to the atomic transition frequency, the eigenvalue of the system will change, and the eigenvalue change is proportional to the cubic root of the perturbation. If the perturbation comes from the magnetic field, a sensitive magnetic field measurement device is formed. By introducing gain and loss via the input–output field, we realize a third-order EP in the non-Hermitian Hamiltonian. Our analysis shows that the system exhibits a nonlinear response to magnetic field perturbations, leading to enhanced spectral sensitivity compared to conventional linear detectors. This equivalent EP based on a cavity-QED sensing scheme breaks the limitation of passive EP sensors and provides a new theoretical idea for the design of sensitive magnetic field measurement devices.

Photonics doi: 10.3390/photonics13070689

Authors: Weijun Wu Xufei Zhuang

Object detection in real-world scenarios is often challenged by adverse visual conditions, such as low illumination, strong glare, and dense fog, which severely degrade visible-spectrum features and lead to missed detections, inaccurate localization, and reduced detection accuracy. To address these issues, this paper proposes AQFS-Net, a dual-modal fusion detection network for visible-infrared object detection. Built upon YOLOv13, AQFS-Net adopts a symmetric dual-branch backbone by incorporating infrared images, thereby exploiting the complementary information between the visible and infrared modalities. To alleviate the negative transfer caused by conventional static fusion strategies, an Adaptive Quality-Aware Fusion Module (AQFM) is designed to dynamically enhance informative features and suppress degraded information according to the modality-specific reliability of different regions. In addition, a Foreground-Aware Saliency Guidance (FASG) branch is introduced to guide the network to focus on target regions through foreground supervision, reducing interference from complex backgrounds. Experimental results on the public LLVIP and M3FD datasets show that the proposed method improves mAP@0.5 by 6.8 and 3.1 percentage points, respectively, compared with the baseline using only visible images. These results demonstrate the effectiveness of AQFS-Net in improving dual-modal fusion quality and detection performance under challenging visual conditions, providing a practical reference for visible-infrared object detection in complex illumination scenarios.

Photonics doi: 10.3390/photonics13070687

Authors: Jiajun Wang Xiuye Liu

Optical inverse design faces a dilemma: neural surrogates enable fast, differentiable search but can yield physically unreliable pseudo-optima, whereas solver-in-the-loop optimization is reliable yet costly. Most surrogate methods also trust the surrogate throughout the search, train separate models for performance prediction and structure optimization, and remain largely black-box. We propose Physics-Verified Spectral Dreaming (PVSD), a unified framework for forward prediction, inverse design, and physical interpretability: a frozen differentiable spectral surrogate “dreams” structural mutations by input-gradient ascent to explore the design space, while a physical solver adjudicates every accepted update—the surrogate proposes, physics decides. We instantiate it as PVSD-TMM for one-dimensional multilayer radiative coolers. The forward predictor attains R2=0.9936/0.9964/0.9828 for net cooling power, solar reflectance, and primary-window emissivity; neural dreaming lifts the population-mean net cooling power of 1000 random seeds from −466.7 to 65.8 W m−2 (91.4% reaching net cooling), and continuous-thickness refinement with 5 nm rounding yields a 14-layer manufacturable final design. Independent COMSOL finite-element and analytic TMM cross-validation converge to Pcool≈172 W m−2, Rsolar≈0.970, and εwin=0.9252. This is a full-spectrum radiative-balance result for an idealized radiative-only case (hconv=0), not a window-emittance-only metric; PVSD thus achieves high simulated broadband radiative-cooling performance under the stated assumptions, without claiming global optimality.

Photonics doi: 10.3390/photonics13070688

Authors: Yunfei Niu Luning Qian Chunchun Bei

Conventional Pancharatnam–Berry (PB) phase metasurfaces operate within the two-dimensional SU(2) polarization space of the electric field, fundamentally limiting holographic multiplexing to two independent channels. Here, we propose and numerically demonstrate a four-channel holographic metasurface exploiting the recently discovered Riemann–Silberstein (RS) geometric phase arising from SU(4) polarization evolution in the full electromagnetic field space. The RS vector Ψ = E + icB unifies electric and magnetic fields into a four-dimensional polarization state space. By engineering bianisotropic Huygens meta-atoms with independently controllable electric-dipole orientation angle α and magnetic-dipole orientation angle ψ, four geometric-phase channels—labeled by the joint spin eigenstates |σ,κ⟩∈{|+,+⟩,|+,−⟩,|−,+⟩,|−,−⟩}—are simultaneously addressed from a single aperture. We develop the complete SU(4) transfer-matrix formalism and optimize four quasi-independent phase profiles using an extended Gerchberg–Saxton algorithm with a three-parameter (α,ψ,h) design library, where the pillar height h serves as a third degree of freedom to overcome the linear phase constraint inherent to the two-angle parameterization. Numerical simulations at 0.8 THz demonstrate simultaneous projection of four independent holographic images with mean diffraction efficiency 60.4% and inter-channel crosstalk below 3.2%, doubling the information capacity of conventional dual-channel PB holograms. An intrinsic ~24× common-mode noise suppression arising from electromagnetic duality symmetry is also demonstrated. This work establishes a direct link between fundamental electromagnetic symmetry and high-capacity wavefront engineering.

Photonics doi: 10.3390/photonics13070686

Authors: HeeJeong Yoo Hoon Yoo

LiDAR sensors provide reliable physical distance measurements using laser signals, enabling accurate acquisition of 3D information for various optical systems. However, they are costly, require significant weight and space, and their reliability and accuracy degrade under adverse environmental and weather conditions. In contrast, thermal cameras operating in the infrared spectrum can capture stable visual information even in challenging scenarios such as nighttime, low-light, and rain. However, they cannot directly provide the physical 3D depth information that LiDAR offers. To design efficient optical systems, there is a growing need for techniques that transform thermal image data into LiDAR-like depth information. While deep learning models can theoretically learn direct mappings between thermal and LiDAR modalities, the scarcity of acquiring paired thermal–LiDAR datasets and the difficulty of acquiring them make this task challenging. In this paper, we propose a thermal image-to-LiDAR depth transformation framework. Our method leverages large-scale pretrained visual models for depth estimation to generate initial depth predictions from thermal inputs. Since pretrained RGB-based models face a modality gap when applied to thermal data, we introduce a two-stage depth refinement. Stage 1 corrects global scale inconsistencies, and Stage 2 refines local structural details. Experiments on the MS2 dataset demonstrate that the proposed framework consistently improves the initial DepthPro outputs across day, night, and rainy conditions. Both quantitative metrics and qualitative comparisons show that RGB-pretrained depth predictions can provide useful structural cues for thermal depth estimation when their global scale and local structural errors are explicitly refined.

Photonics doi: 10.3390/photonics13070685

Authors: Yingmeng Wang Wei Jiang Yashu Zang Shilin Liu Wenyu Kang Jun Yin Junyong Kang

Total internal reflection (TIR) loss is a critical bottleneck limiting light extraction in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs), primarily due to the large refractive-index contrast at the light-emitting interface. Here, pyramid-shaped dielectric metasurfaces are designed and fabricated at the sapphire/air interface of flip-chip AlGaN-based DUV LEDs using a scalable nanoimprinting process. The metasurface functions as a light outcoupling layer that modifies the interfacial momentum-matching condition and redistributes photon propagation directions. Experimental results and theoretical simulations show that metasurfaces with different feature sizes enhance light extraction through distinct mechanisms. The subwavelength pyramid nanoarray perturbs the local optical field and provides additional in-plane momentum components, facilitating the coupling of high-angle photons into radiative channels, whereas the larger pyramid void structure mainly promotes photon extraction through geometrical redirection, tilted output interfaces, and dry-etching-induced rough surface scattering. As a result, an average light output power (LOP) enhancement of over 8% is achieved for AlGaN-based DUV LEDs emitting at approximately 275 nm. This work demonstrates a low-cost, scalable, and effective strategy for enhancing the LEE of DUV LEDs, with promising potential for high-efficiency ultraviolet optoelectronic application.

Photonics doi: 10.3390/photonics13070684

Authors: Thawatchai Mayteevarunyoo Boris A. Malomed

We consider quasi-one-dimensional (Q1D) continuous waves (CWs) in the two-dimensional (2D) optical system with the cubic–quintic (CQ) nonlinearity and a Q1D potential trough. In the case of a smooth trough profile, we confirm the known modulational instability (MI) of Q1D CWs with the transverse structure corresponding to the 1D ground state (GS) in the potential trough, and demonstrate the MI of CWs with the dipole mode (DM) transverse structure, corresponding to the lowest 1D excited state in the potential trough. The CWs of both GS and DM types remain nearly stable close to the edges of their existence regions. Stable stationary states in the form of periodic chains of 2D solitons, trapped in the potential trough, are produced in a numerical form. The dynamics of the soliton chains excited by a localized kick is studied too. For the potential trough with the singular delta-functional profile, we find two species of exact analytical solutions for CWs. One of them, composed of the free-space CQ solitons, remains unstable, while the other solution, which is composed of solutions that are singular in the free space, is completely stable.

Photonics doi: 10.3390/photonics13070683

Authors: Nino Rozić Petar Bašić Zvonimir Šipuš Elis Sutlović

Vibration measurements are essential for the early detection of faults in rotating machinery and are particularly important for hydrogenerators in hydro power plants. Industrial applications of vibration measurements typically rely on displacement, velocity, and acceleration sensors, each offering distinct advantages and limitations. This paper discusses and proposes a cost-effective, contactless optical vibration sensing system based on standard fiber-optic telecommunication components, enabling its integration into existing fiber-optic networks. The proposed system utilizes interferometric sensing principles, providing inherent immunity to electromagnetic interference and galvanic effects while achieving micrometer-scale resolution. The key advancement of the proposed sensor lies in the relatively simple and cost-effective configuration—the realization of the Michelson interferometer. It facilitates a combination of standard optical communication hardware, including a 3 × 3 fiber-optic coupler and two photodetectors for reliable discrimination of vibration displacement directions. The associated signal processing platform is based on a cross-correlation algorithm by which both the direction and the magnitude of displacement are determined. The optical sensor was experimentally validated using a realistic-scenario laboratory setup, which demonstrates the feasibility and performance of the proposed approach.

Photonics doi: 10.3390/photonics13070682

Authors: Haiyi Bian Yingzhou Zhu Yeman Liu Lei Liu Xiaoteng Yan Zhongjie Cheng Yanrong Wang

Optical coherence tomography (OCT) is widely used in biomedical imaging and ophthalmology. However, OCT images are frequently corrupted by speckle noise from coherent light interference. This degradation hampers clinical diagnosis of retinal lesions and identification of tissue layers. We propose a speckle noise reduction algorithm based on a Multi-Scale Self-Attention Generative Adversarial Network combined with a Siamese network (SA-Siamese-GAN). To address the limited receptive fields of traditional convolutional neural networks (CNNs), which can cause broken or blurred retinal layers, a self-attention mechanism is integrated into the bottleneck layer of the generator to capture global pixel dependencies. A Siamese network enforces structural consistency, and a joint loss function combining Wasserstein distance, perceptual loss, and structural similarity (SSIM) is used. Experiments on 11,103 pairs of clinical OCT retinal images show that SA-Siamese-GAN outperforms BM3D, NLM, Wavelet, and GAN-ResNet. It removes speckle noise while preserving retinal layer structure and fine textures, and achieves the highest Peak Signal-to-Noise Ratio (PSNR) and Structural Similarity Index (SSIM).

Photonics doi: 10.3390/photonics13070681

Authors: Maoto Suzuki Tetsuo Sasaki Saroj R. Tripathi

Terahertz time-domain spectroscopy (THz-TDS) is a widely used technique for characterizing a broad range of materials in the terahertz frequency region. Conventional THz-TDS requires both reference and sample signals to extract optical parameters such as refractive index and absorption coefficient. Determining optical anisotropy, specifically birefringence and linear dichroism, typically requires separate measurements of the optical parameters of the sample parallel and perpendicular to the terahertz electric field. This process increases measurement time and depends heavily on a stable reference scan. In this work, we present a simple and accurate method to directly obtain birefringence and linear dichroism without the need for a reference measurement. The proposed approach extracts anisotropic parameters solely from the sample signals by analyzing the differential phase delay and amplitude attenuation between orthogonally polarized terahertz electric field components. We validate this method experimentally using optically anisotropic materials such as TiO2 and bamboo samples and confirm that the results agree closely with those from conventional reference-based THz-TDS. This technique offers a practical route to measure the optical anisotropy of materials, particularly in situations where acquiring a reference signal is challenging.

Photonics doi: 10.3390/photonics13070680

Authors: Jingbo Duan Tong Zhang Xue Dong Pingli Han Fei Liu

Multi-target imaging beyond the optical memory effect (OME) is fundamentally challenged by the incoherent superposition of speckles originating from different OME regions, which renders object separation highly ill-posed. To address this challenge, we introduce a polarization-guided blind unmixing framework that exploits the distinct polarization responses of different targets as an additional discrimination cue for speckle separation. Polarized speckles are first used to construct a dense polarization-response representation through Stokes-based synthesis. The resulting polarization diversity is leveraged to estimate the number of targets without prior knowledge and to identify structurally representative autocorrelation endmembers associated with different objects. An energy-constrained non-negative matrix factorization is then developed to separate individual target autocorrelations, followed by phase retrieval for object reconstruction. Experimental results demonstrate the reconstruction of up to five targets located beyond the OME range, achieving target separations of up to 2.6× the measured OME limit and peak signal-to-noise ratio (PSNR) values above 25 dB, without requiring prior knowledge of target locations, structures, polarization states, or target number.

Photonics doi: 10.3390/photonics13070679

Authors: Qi Zhang Congming Dai Heli Wei

To address the bias in radiative simulation caused by the horizontal inhomogeneity of atmospheric parameters under large scan angles for the HIRAS-II (Hyperspectral Infrared Atmospheric Sounder-II) onboard the Fengyun-3E satellite, this study proposes a method for constructing slant-path atmospheric parameter profiles along the satellite’s line-of-sight (Exp.2). In contrast to the conventional method (Exp.1) based on the assumption of horizontally homogeneous vertical atmospheric profiles, this method accurately calculates the intersection points between the satellite’s line-of-sight and the various altitude layers of the ECMWF reanalysis data version-5 (ERA5). It employs a hybrid interpolation algorithm combining inverse distance weighting and spline interpolation to obtain a continuous distribution of atmospheric parameters along the slant path, thereby accounting for the actual observation geometry of the satellite. The results show that when the satellite zenith angle exceeds 30°, the simulation differences between Exp.2 and Exp.1 increase significantly. Specifically, in the CO2 absorption band, the differences between the two methods are mainly concentrated between −0.1 K and 0.1 K, whereas in the water vapor absorption band, this range expands to between −0.5 K and 0.5 K. Notably, biases are concentrated near the scan edges and exhibit a strong latitudinal dependence, with high-latitude areas showing more prominent deviations due to steeper atmospheric parameter gradients and asymmetric orbital geometry. Moreover, a comparison with the measured satellite-observed brightness temperature further demonstrates that Exp.2 effectively reduces simulation biases near the scan edge. The mean bias is reduced by up to 0.1 K in water vapor absorption channels, a more significant improvement compared to the 0.03 K reduction in CO2 absorption channels. These results indicate that the proposed slant-path profile construction method significantly enhances the accuracy and reliability of infrared hyperspectral radiative transfer forward simulations under complex observation geometries by providing a more realistic representation of the three-dimensional slant-path radiative transfer process. This advancement holds important implications for improving the atmospheric correction of remote sensing data.

Photonics doi: 10.3390/photonics13070678

Authors: Shengyao Yan Bokai Hou Zhe Feng Zhiwu Chen Zengyi Xu Zijian Zhou Suning Guan Nan Chi

We demonstrate an underwater visible light communication system using a circularly polarized 520 nm laser transmitter, 32APSK modulation, and a polarization-diverse dual-aperture receiver. An end-to-end post-equalization network, DirectDemodNet, directly maps dual-polarization received waveforms to 32APSK symbol logits, replacing conventional Least Mean Square (LMS) + Volterra equalization. By combining waveform-difference features, dual-scale dilated temporal convolutions, and multi-period positional encoding, DirectDemodNet improves nonlinear compensation and branch fusion. Extensive evaluations are conducted across data rates from 7.5 to 13.75 Gbps over a 1.2 m static underwater channel. Experiments show that DirectDemodNet broadens the forward error correction compliant operating range and provides a maximum net transmission rate gain of 4.095 Gbps over LMS + Volterra at the 7% Hard-decision Forward Error Correction (HD-FEC) threshold.

Photonics doi: 10.3390/photonics13070677

Authors: Runqi Guan Kexin Li

To narrow spectral linewidth, stabilize longitudinal mode and improve output performance of a 1064 nm distributed feedback (DFB) semiconductor laser, we design and fabricate a laser module adopting fiber Bragg grating (FBG) external-cavity feedback and a butterfly packaging structure. The butterfly package effectively enhances heat dissipation and optical coupling reliability. Based on the classic Schawlow–Townes theory, we elaborate on how the narrowband filtering of FBG and the extended external cavity suppress mode hopping and reduce laser linewidth. A delayed self-heterodyne testing system is built to evaluate the photoelectric characteristics, spectral features and linewidth performance under varying driving currents and ambient temperatures. Experimental results show that the laser has a threshold current of 22.54 mA and a slope efficiency of 0.18 W/A, and its maximum output power reaches 80.8 mW at 480 mA. The side-mode suppression ratio (SMSR) reaches 58.2 dB at a temperature of 25 °C and driving current of 150 mA. Benefiting from FBG feedback, the laser linewidth is compressed from 485 kHz to 115 kHz, with lower noise and excellent wavelength stability. This compact all-fiber laser is well-suited for fiber sensing, coherent detection and LiDAR systems.

Photonics doi: 10.3390/photonics13070676

Authors: Lincheng Guo Xiaodan Zhao Dong Li Min Wu Yibiao Yang

Terahertz metasurfaces capable of dynamic modulation face a fundamental trade-off: high-quality-factor resonances require structural symmetry, but active tuning typically relies on symmetry-breaking that inevitably introduces radiative losses. This study presents a theoretical analysis of tunable terahertz metasurfaces utilizing vanadium dioxide (VO2) as an active component that circumvents this trade-off through current-loop-mediated symmetry control. Based on the current-loop-mediated coupled-mode theory, we establish a quantitative relationship between VO2’s conductivity and resonant mode characteristics through the symmetry-breaking parameter α(σ)∝∮(r×K(σ))⋅dl. The proposed theoretical framework describes the conversion mechanism between non-radiative bound states in the continuum (BIC) and radiative quasi-BIC (qBIC), where VO2’s metal–insulator phase transition modulates surface current distributions and activates radiation channels. Through systematic analysis of multiple metasurface designs, we demonstrate that conductivity-dependent current-loop formation governs the switching between high-Q dark modes and radiative states, achieving modulation depths up to 243%—providing a robust approach for dynamic terahertz wave manipulation with explicit physical linkage between materials parameters and modal properties.

Photonics doi: 10.3390/photonics13070675

Authors: Xu-Li Yan Rui-Ping Jia Zhou-Xiang Wang Miao Yan Jia-Qi Lü

The high-precision estimation of spatial parameters, such as transverse displacement, beam tilt and angular rotation, is of great importance in precision measurement, optical imaging and quantum sensing. Structured light, with controllable spatial modes and special momentum degrees of freedom, provides new opportunities to promote the encoding and readout of quantum metrology for spatial parameters. Here, we review recent progress in structured-light-enabled quantum metrology for spatial parameter estimation. The theoretical foundations of parameter estimation are introduced, based on which the modal-encoding mechanisms of higher-order structured light modes in spatial parameter measurements are summarized. Representative studies in which structured light is combined with nonclassical resources and the mode-matched readout strategies to achieve quantum enhancement are also demonstrated. In addition, current technical bottlenecks and future directions are discussed.

Photonics doi: 10.3390/photonics13070673

Authors: Hyoseop Shin Dongkun Shin

As semiconductor design rules continue to shrink, random nanoscale particle contamination on non-patterned wafers has become a critical source of yield loss and process instability. This study presents a simulation-guided workflow for improving the practical detectability of 30 nm particles in an optical wafer inspection system without replacing the installed platform. Three controllable optical parameters—illumination polarization, wavelength, and incidence angle—were investigated through defect simulation and then validated on a production-relevant non-pattern inspection tool. The simulation and experimental results showed that p-polarized illumination generated stronger defect-relevant scattering contrast than S-polarization, 266 nm illumination provided the best practical detection performance among the evaluated wavelength conditions, and oblique illumination produced more favorable defect visibility than vertical incidence. Guided by these findings, an integrated inspection recipe using P-polarization, 266 nm illumination, and an oblique incidence angle of 20–25° was implemented on the inspection tool. Under the optimized condition, 30 nm particles were detected, whereas the legacy condition failed to provide equivalent sensitivity. The results demonstrate that defect simulation can be used as a practical engineering instrument for recipe screening, sensitivity enhancement, and faster deployment of inspection improvements in high-volume semiconductor manufacturing.

Photonics doi: 10.3390/photonics13070674

Authors: Wei Gao Ang Liu Shuqin Lou Yuying Guo Xin Wang Zhenggang Lian

An enhanced genetic algorithm (EGA) is proposed to optimize our previously reported seven-tube single-ring anti-resonant hollow-core fiber (SR-ARF) with a record-low loss of 4.30 dB/km at 1080 nm. Taking advantage of an improved roulette wheel selection and a threshold elimination mechanism, the EGA effectively prevents premature convergence and enhances optimization efficiency. By adopting the proposed EGA to optimize fiber structural parameters, the confinement loss of the seven-tube SR-ARF is further reduced to 2.79 dB/km, and the higher-order mode extinction ratio reaches 204 at 1080 nm, confirming robust single-mode operation. The single-mode operation bandwidth reaches up to 240 nm, covering 920 to 1160 nm. According to the optimized structural parameters, we experimentally fabricated a seven-tube SR-ARF, of which the transmission loss is reduced to 3.29 dB/km at 1052 nm and 3.90 dB/km at 1080 nm, while maintaining near-diffraction-limited single-mode guidance with an M2 of 1.07/1.05. The proposed EGA model is of great significance for the structural parameter optimization of hollow-core fibers.

Photonics doi: 10.3390/photonics13070672

Authors: Lovre Kardum Grgur Premec Neven Šantić Damir Aumiler

The potential to precisely control both the linear and nonlinear index of refraction through optical manipulation of the atomic states has recently pushed warm alkali vapors to the forefront of research in the field of quantum sensors, quantum memories, and quantum fluids of light. Rubidium (Rb) vapor in centimeter-scale glass cells or millimeter-scale micro-electro-mechanical system (MEMS) cells has proven to be a very promising platform for these applications, yet only a handful of research works have been dedicated to the investigation of the (non)linear refractive index of Rb vapor. We present results of theoretical calculations of the (non)linear refractive index of warm Rb vapor, based on the optical Bloch equations for 6-level Rb atoms interacting with a probe laser. They are compared to the experimental results obtained using an interferometric technique, showing very good quantitative agreement. A Kerr nonlinear refractive index n2 of up to −1.2×10−4 cm2/W is obtained. Python scripts for all theoretical calculations presented in this work are provided, including the refractive index calculation, that can readily be used in practical implementations for simulating the (non)linear refractive index of Rb vapor including the effects of Doppler broadening, transit time broadening, pressure broadening, saturation, optical pumping, and spin-exchange collisions.

Photonics doi: 10.3390/photonics13070671

Authors: Jiahong Dong Hongshuai Qin Xingru Huang Zhiwen Zheng Lihuan Shao Huiyu Qi Xiaoshuai Zhang Jin Liu

Reliable three-dimensional (3D) reconstruction from endoscopic video is essential for endoscopic digital twins, scene review, and minimally invasive visual analysis. However, endoscopic images are not clean observations of intrinsic tissue appearance. Depth-dependent blur, shallow mucosal color diffusion, wet-surface specular reflection, and frame-wise color variation are often coupled with the captured signal. When such observation-dependent effects are directly optimized as Gaussian colors, conventional 3D Gaussian Splatting may encode transient imaging artifacts as persistent tissue appearance, leading to blurred textures, color drift, specular residues, and unstable novel-view synthesis. This paper presents EndoDGS (Endoscopic Degradation-Decoupled Gaussian Splatting), a degradation-decoupled Gaussian Splatting framework for endoscopic novel-view reconstruction. The core idea is to keep stable geometry and base tissue appearance in the Gaussian representation, while modeling endoscope-induced degradations separately in a bounded render-space compensation pipeline. EndoDGS combines lightweight appearance modulation for frame-wise color stabilization with sequential degradation compensation for optical blur, mucosal color transport, and wet-surface specular response. This design reduces the entanglement between persistent tissue appearance and transient imaging degradations without changing the underlying Gaussian geometry and visibility ordering. Experiments on synthetic colonoscopy and real endoscopic/laparoscopic datasets covering 38 scenes show that EndoDGS consistently improves reconstruction quality over representative implicit and explicit reconstruction baselines. The results demonstrate that separating stable tissue representation from observation-dependent endoscopic degradations provides a more faithful, stable, and interpretable foundation for endoscopic 3D reconstruction.

Photonics doi: 10.3390/photonics13070670

Authors: Md Ghulam Saber Zhiping Jiang

Hollow-core fiber (HCF) is often presented as an incrementally better transmission medium that can be slotted into networks designed around solid-core silica. We argue instead that recent progress—most visibly reported as attenuations below 0.1 dB/km and now approaching 0.05 dB/km, together with a broad low-loss window, reduced propagation delay and very low optical nonlinearity—makes it worth asking which long-standing design conventions are intrinsic to optical communication and which are artifacts of silica. Reviewing physical-layer, transceiver and network architecture implications, we suggest that the most durable gains may come not from treating HCF as a drop-in replacement, but from cross-layer co-design, and we outline the studies and demonstrations needed to test where that advantage is real.

Photonics doi: 10.3390/photonics13070669

Authors: Ke Yin Zheng Wen Yuan Liao Shubin Liu Xiyang Zhi Guangzhen Bao

Conventional imaging systems often suffer from a coupled limitation of narrow field of view, single modality, and insufficient effective resolution. Wide-angle imaging preserves scene context but compresses distant or small-scale targets into limited pixels, while narrow-field imaging improves details at the cost of global perception. Moreover, single-modal visible imaging is sensitive to illumination and contrast variations, whereas infrared imaging lacks fine spatial texture. To increase information at the imaging source, we propose coupled spectral–spatial fusion-enabled multi-scale panoramic imaging, a dual-field-of-view (FOV) visible–infrared framework for wide-field high-resolution perception. Two imaging units acquire paired visible and infrared images from adjacent overlapping views. For each view, a visible–infrared fusion super-resolution model integrates visible structural details with infrared radiative cues to reconstruct a high-resolution fused image. A multi-scale stitching algorithm then extracts robust features, estimates cross-view correspondences, and merges the two fused images into a large-FOV panoramic result. Outdoor experiments demonstrate that the proposed method improves local contrast, suppresses pixelation artifacts, enhances readable fine details, and expands the observable field of view, providing an effective route toward multimodal panoramic imaging.

Photonics doi: 10.3390/photonics13070668

Authors: Li Wang Ke Chen Xueliang Kang Bai Zhang Hongmei Zheng

The polygonal scanner is a key component of a laser scanning system. The reflection point drift of a polygonal scanner is one of the primary sources of error that limits improvements in system measurement accuracy, significantly affecting the application of laser scanning systems in high-precision scenarios. Based on a geometric optics model, this article quantitatively analyzes the magnitude of the reflection point drift and the direction of the reflected ray caused by the rotation of the polygonal scanner, as well as the impact of this drift on measurement accuracy. The research shows that, compared to the ideal state, the reflection point drift causes the system’s absolute distortion to increase by nearly 20 times and relative distortion by nearly 5 times, indicating a great effect on measurement accuracy. Future research can directly incorporate this quantitative analysis model into the design of the relevant f-theta lens, effectively and conveniently improving lens design accuracy and thereby enhancing the overall measurement accuracy of the system. This study holds significant theoretical importance and engineering value in advancing the application of laser scanning technology in high-precision scenarios.

Photonics doi: 10.3390/photonics13070667

Authors: Jing Li Fanjiang Xu Si Chen Shudan Tan Lei Duan Xiongxin Tang

High-energy, ultrafast pulse generation from mid-infrared (MIR) fiber lasers is often limited by pulse splitting caused by excessive nonlinear phase accumulation, especially in anomalous-dispersion cavities. Dissipative soliton resonance (DSR) offers a route to energy scaling by accommodating additional gain through temporal pulse broadening under peak-power clamping. Here, we numerically demonstrate DSR operation in a net-normal-dispersion Er3+: ZBLAN mode-locked fiber laser at 2.8 µm, using an As2S3 fiber for dispersion and nonlinear management. Systematic parameter sweeps show that the gain saturation energy primarily governs pulse-energy scaling, whereas the output-coupling ratio controls peak-power extraction and the operation regime. The spectral filter bandwidth and saturable absorber parameters define the stability window and mediate transitions among dissipative solitons, DSR pulses, noise-like pulses, multi-pulse states, and unstable operation. After coordinated cavity optimization, a stable rectangular DSR pulse with a maximum energy of 408.52 nJ is obtained under a 55 nm filter bandwidth, relaxing the narrow-filtering requirement reported in previous MIR DSR designs. A high-peak-power DSR state with 3112.1 W peak power and 316.58 nJ energy is also achieved at a 95% output-coupling ratio. By tuning the As2S3 fiber length, DSR dynamics are also accessed near the zero-dispersion boundary and in the anomalous-dispersion regime, where spike-on-pedestal temporal profiles and dual-peak spectra emerge. This work advances the understanding of MIR DSR dynamics and offers design guidance for compact, high-energy ultrafast sources at 2.8 µm.

Photonics doi: 10.3390/photonics13070666

Authors: Igor Glukhov Sergey Moiseev Sergey Sukhov

The linear transformation of optical fields during propagation poses a fundamental limitation for neuromorphic computing applications. In this paper, a realization of a nonlinear operator based on the diffraction of partially coherent light by an aperture partially covered with a binary phase grating is proposed. The dependence of the effective aperture width on the spatial coherence of the incident light produces a nonlinear behavior that formally introduces a nonlinear integration kernel into the propagation integral. The proposed concept is validated through numerical experiments performed using the PyWolf framework for partially coherent light propagation modeling. This coherence-induced nonlinearity, which effectively implements a nonlinear propagation kernel with an input-dependent transfer function, offers a viable pathway to overcome the limitations of purely linear diffractive networks and can be leveraged for constructing multilayer architectures.

Photonics doi: 10.3390/photonics13070665

Authors: Xiwen Dou Zhengyang Zhou Aixi Chen

Magnonic systems present a compelling platform for quantum technology, owing to their strong capacity to form hybrid quantum systems via diverse couplings. To unlock the full potential of these systems, the engineering of flexible coupling between multiple magnon modes is essential. Here, we propose a method to realize switchable dissipative Ising coupling in magnon systems, leveraging the three-body coupling among photon, phonon, and magnon. This type of dissipative coupling is a critical component for constructing Ising machines designed to solve complex combinatorial optimization problems. By dynamically tuning the phase of a nonlinear mechanical pump, we demonstrate the realization of both ferromagnetic and antiferromagnetic dissipative interactions. The validity of the scheme is confirmed by numerical simulations, which also demonstrate its robustness against a strong uncontrollable part of dissipation. Our work provides a versatile tool that can facilitate the implementation of magnon-based quantum computing and the exploration of many-body magnon physics.

Photonics doi: 10.3390/photonics13070664

Authors: Cong Wang Meng Gan

Optical coherence tomography (OCT) is an established imaging modality for esophageal disease assessment, enabling high-resolution visualization of tissue layers. However, automated segmentation of esophageal OCT images remains challenging due to low contrast, complex tissue structures, in vivo confounders, and scarce labeled clinical data. To address these issues, we propose a self-prompting segment anything model (SAM)-based one-shot segmentation framework tailored for esophageal OCT images. The framework integrates a pre-trained SAM encoder for general feature extraction and a self-pretrained prompt encoder to capture domain-specific features from unlabeled OCT data. Experiments on a self-collected mouse dataset and a public human dataset yielded Dice similarity coefficient (DSC) values exceeding 85%, with the highest mean DSC among the evaluated methods under the one-shot protocol, although greater performance dispersion was observed in the heterogeneous human cohort. These findings demonstrate the potential of combining unlabeled OCT pretraining with one labeled B-scan for annotation-efficient esophageal OCT segmentation.

Photonics doi: 10.3390/photonics13070663

Authors: Shuchen Zuo Yu Wang Richard Fields Olivier Allegre

This study presents a phase-characterised optical set-up for shaped-beam femtosecond laser processing of double-sided lithium-ion battery anodes. The phase response was found to be power-independent, with consistent phase scaling across applied powers and a 2π phase shift completed at approximately 220–225 grey levels (GL). The measured modulation visibility stayed high. The generated beam profiles were in good agreement with MATLAB simulations, confirming reliable wavefront control. Under identical processing conditions, wavefronts carrying different orbital angular momentum (OAM) produced distinct kerf morphologies: at 30 overscans, m=0 gave a deeper cut with a taper angle of 16.5±1.1°, while m=1 gave a shallower groove with a less steep angle of 26.3±1.9°, indicating different Cu-layer interaction and ejecta behaviour. When the overscan number was increased to 45 at the same average power, both m=0 and m=1 achieved through-cuts. However, the m=1 condition produced a cleaner cut edge, a more vertical kerf wall, and reduced graphite delamination and heat-affected damage compared with the Gaussian beam. These results demonstrate the potential of wavefront engineering for laser processing of layered battery anodes, where improved cut confinement and edge quality can be achieved through beam shaping under relatively low-energy, moderate-overscan, and tight-focusing conditions.

Photonics doi: 10.3390/photonics13070662

Authors: Jianxin Zhang Gaoben Shi Xiaoyun Liu Haizhu Sun Xinmin Fan Pingping Wang Yan Wang Fuyong Qin Zaifa Du

Semiconductor lasers are key devices in optical communication, optical storage and other fields. Achieving single-mode, high-power and high-beam-quality output is an important research goal in laser technology. Conventional slab-coupled optical waveguide lasers can effectively suppress higher-order modes, but suffer from a low optical confinement factor and limited ridge width. This paper proposes a 1064 nm deep-etched wide-ridge waveguide slab-coupled photonic crystal semiconductor laser. A photonic crystal structure is introduced into the conventional slab-coupled optical waveguide. The optical field and mode characteristics are analyzed by numerical simulation, verifying the feasibility of the structure in improving the optical confinement factor and realizing single-mode operation with a deep-etched wide-ridge waveguide. Different from traditional photonic crystal lasers, this design provides a new route for high-performance photonic crystal semiconductor laser design.

Photonics doi: 10.3390/photonics13070661

Authors: Zexiang Cheng Chen Xu Bo Peng Jinjin Chen

Laser scanning systems are adopted for diverse applications in imaging, printing, and material processing, and there is a growing trend toward volume miniaturization without compromising optical performance. In this work, a highly compact optical configuration is proposed by combining catadioptric architecture with freeform optics. A multi-stage design method, from starting point acquisition to fine-tuning, is proposed, with particular attention to concurrently controlling physical layout and optical performance. Two systems were designed, demonstrating an average RMS wavefront error below 0.006λ and a linearity error under 0.3% over a 216 mm scanning width. The spot size deviation is below ±9%, and the position deviation is kept within half the diffraction-limited spot diameter over a ±2 mm depth of field. Compared to a conventional freeform lens design, the proposed systems reduce the total length by over 37.7%. The significant reduction in size achieved paves the way for next-generation portable and embedded laser scanning devices.

Photonics doi: 10.3390/photonics13070660

Authors: Alejandro Arratia Pavat Pablo Palacios Játiva María Camila Reyes Muhammad Ijaz Cesar Azurdia Meza David Zabala-Blanco Iván Sanchez

Visible light communication (VLC) can reduce radio-frequency (RF) congestion in indoor industrial monitoring, but a short transmitted payload does not by itself prove that visual or task-relevant information has been preserved. This study therefore frames the proposed method as an autoencoder-based compact latent-payload transmission scheme and explicitly distinguishes it from channel-aware joint source-channel coding (JSCC). Raw red–green–blue (RGB), lossless Huffman, and autoencoder latent payloads are first compared under the same VLC model using bit error rate (BER), calculated/model-derived VLC transmission energy, reconstruction quality, and task utility. The 128-component latent representation contains 4096 bits, corresponding to a 294-fold payload-size reduction relative to an uncompressed 224×224, 24-bit RGB image; this ratio is used only as a raw-payload reference and not as a general codec-compression claim. An independent industrial-domain audit is conducted on 30 Northeastern University (NEU) steel-surface images using four-fold out-of-fold evaluation, peak signal-to-noise ratio (PSNR), structural similarity index measure (SSIM), and a fixed defect-class proxy. An NEU architecture-and-resolution comparison shows that changing from Compact-NEU to the full 224×224 model increases PSNR from 14.12 dB to 15.61 dB at the same 4096-bit bottleneck, while a regularized full model gives the highest SSIM of 0.471. Because input resolution and network capacity both change in this comparison, the result is interpreted as evidence that the architecture/resolution setting contributes to the industrial-domain gap, not as a strict isolation of capacity alone. Finally, an end-to-end JSCC-VLC baseline with 4096 nonnegative optical channel uses is trained through a differentiable intensity channel. It obtains 23.82 dB/0.568 SSIM in the clean case and 22.18 dB/0.519 SSIM at 5 dB SNR, showing more channel-aware behavior and more graceful degradation than the separated serialized-latent pipeline. Overall, the results support the modeled energy and active-time benefits of compact latent payloads while showing that robust industrial visual transmission requires architecture/resolution controls, practical codec baselines, and channel-aware JSCC comparisons.

Photonics doi: 10.3390/photonics13070659

Authors: Yunfeng Bai Mingliang Xiong Liangrong Sun Jinsong Kang Changsheng Li Qingwen Liu Xin Wang

With the development of the Internet of Things (IoT), demands of power and data for IoT devices increase drastically. In order to resolve the supply–demand contradiction, simultaneous wireless information and power transfer (SWIPT) has been envisioned as an enabling technology by providing high-power energy transfer and high-rate data delivery concurrently. In this paper, we analyze and design a high-efficiency resonant beam (RB) charging and communication scheme. The scheme is based on semiconductor materials for the gain medium, which provide a better energy absorption capacity compared with the traditional solid-state one. Moreover, the telescope internal modulator (TIM), which can concentrate beams to match the gain size, is adopted in the scheme, reducing the transmission loss. To evaluate the scheme’s SWIPT performance, we establish an analytical model and study the influence factors of its beam transmission, energy conversion, output power, and spectral efficiency. Numerical results show that the proposed RB system can realize 16 W electric power output with 11% end-to-end conversion efficiency, and it can support 18 bit/s/Hz spectral efficiency for communication.

Photonics doi: 10.3390/photonics13070658

Authors: Helena Cristina Vasconcelos Maria Meirelles

The electronic spectra of trivalent lanthanide ions constitute one of the most extensively characterised manifestations of partially filled 4f shells. Conventional rare-earth spectroscopy provides a detailed microscopic description based on assigned multiplets, angular-momentum structure, crystal-field effects and transition probabilities. Here, we examine a complementary question: whether information associated with the underlying 4f electronic architecture remains detectable after the spectroscopic labels normally used to identify it are deliberately removed. Using the multiplet-centre energies of Ln3+ ions in LaF3, each ion was represented as an energy-only spectral manifold, Mn={Ei}, obtained after removing term identity, angular-momentum labels and transition assignments. The resulting manifolds were analysed through a controlled information-reduction sequence involving spectral occupancy and four complementary descriptors: spectral entropy S, compactness C, mean manifold energy μE and spectral dispersion σE. The results show that the reduced manifolds do not collapse into featureless collections of energies. Instead, they retain structured organisation across the lanthanide series, with distinctive behaviour associated with the shell-edge configurations Ce3+ (4f1) and Yb3+ (4f13), and with the half-filled-shell configuration Gd3+ (4f7). A blind reconstruction test provides the strongest internal validation: when Gd3+ is excluded from the reference evolution and subsequently reintroduced, it emerges as the dominant positive deviation of the mean manifold-energy coordinate. This demonstrates that the 4f7 signature is not encoded only in explicit term labels or selected multiplets but leaves a measurable fingerprint in the collective energy distribution. The comparison of normalised descriptor responses further indicates that electronic shell information is distributed across state occupation, compactness, energetic displacement and spectral spreading. These results suggest that lanthanide spectra can be analysed not only as collections of assigned transitions, but also as collective spectral manifolds whose global organisation preserves physically meaningful information about shell filling. This approach does not replace conventional rare-earth spectroscopy; rather, it provides a complementary framework for testing which aspects of electronic structure remain accessible after microscopic spectroscopic identity has been removed.

Photonics doi: 10.3390/photonics13070657

Authors: Minhan Yang Zinan Liu Guoqi Zhan Qin Zhong

Low-light enhancement is a crucial task in computer vision; it can improve either the subjective experience of viewers or the usability of computer vision systems designed for normal-light images. In this paper, a variational Retinex model in the image domain is developed for low-light enhancement, which infuses classical/fractional differentiation of the input image into the illumination/reflectance component by means of a structure/texture-aware map (SAM/TAM). Firstly, the SAM (TAM) is generated by the inverse square of classical (fractional) differentiation of the input image. Secondly, the regularization term of illumination (reflectance) is defined by utilizing the SAM (TAM) as a weighted matrix, and an illumination guidance term is incorporated into the objective function. The illumination guidance term encourages the estimated illumination to encompass more structural information by penalizing deviation of illumination from the illumination pre-estimated by a dark channel prior to a guided image filtering. Finally, an alternative algorithm is employed to solve the minimization problem involved in the model. The performance of the proposed method is evaluated on three datasets for low-light enhancement and compared with eight state-of-the-art Retinex methods, qualitatively and quantitatively. Evaluation results show that the proposed method generally achieves higher performance in terms of low-light enhancement.

Photonics doi: 10.3390/photonics13070656

Authors: Jiawei Lin Xiaoyan Wang Songjie Luo Ziyang Chen Xiaoyan Wu Jixiong Pu

Infrared small target detection (ISTD) remains challenging in complex cluttered environments because targets usually occupy only a few pixels and exhibit weak thermal radiation with limited texture information. The problem becomes more severe in high-resolution infrared imaging systems, where sliding-window inference is commonly adopted under memory and computational constraints. However, the truncated field of view may lead to contextual information loss and increased false alarms in cluttered regions. To address these issues, we propose the Selective Contextual Uplift and Attention Network (SCUA-Net). The proposed network adopts a U-Net++-style densely nested encoder–decoder architecture to enhance multi-scale feature interaction and preserve fine-grained weak-target features. In addition, a Global-Context Calibration Coordinate Attention (GCC-CA) module is introduced to inject window-level contextual statistics into coordinate attention, thereby improving clutter suppression and localization robustness under sliding-window inference. During training, a joint optimization strategy combining Online Hard Example Mining (OHEM) and Dice Loss is employed to alleviate severe foreground–background imbalance. During inference, Gaussian-weighted fusion is adopted to reduce stitching artifacts between adjacent windows. Experimental results on NUDT-SIRST and IRSTD-1k validate the effectiveness of the proposed method. SCUA-Net achieves 99.15% Pd, 0.558 × 10−6 Fa, and 0.9570 IoU on NUDT-SIRST, while maintaining competitive performance on IRSTD-1k at 161.6 FPS on an NVIDIA RTX 4090 platform, demonstrating favorable accuracy, robustness, and real-time performance in complex infrared scenarios.