Photonics

29 pages, 1180 KB

Open AccessArticle

Nonlinear Band-Correlation Guided Deep Reinforcement Learning for Multispectral Autofocus Optimization

by Zhenzhen Chen, Sen Wang, Chao Ma, Shaowen Jing, Jiayu Huang and Mingkun Zhang

Photonics 2026, 13(9), 837; https://doi.org/10.3390/photonics13090837 - 1 Sep 2026

Multispectral imaging systems often exhibit wavelength-dependent focus responses because of chromatic aberration, nonuniform spectral sensitivity, and scene-dependent reflectance, making single-band autofocus inadequate for optimizing the complete spectral image cube. This study proposes a nonlinear band-correlation guided deep reinforcement learning framework for autofocus optimization [...] Read more.

Multispectral imaging systems often exhibit wavelength-dependent focus responses because of chromatic aberration, nonuniform spectral sensitivity, and scene-dependent reflectance, making single-band autofocus inadequate for optimizing the complete spectral image cube. This study proposes a nonlinear band-correlation guided deep reinforcement learning framework for autofocus optimization in a self-developed 31-band multispectral imaging system covering 360–980 nm. Through-focus response curves are first extracted from all spectral channels, and a nonlinear inter-band dependency matrix is constructed to characterize complementary and redundant focusing information across wavelengths. A compact subset of representative bands is then selected and embedded into the reinforcement-learning state to guide closed-loop motor control. The agent jointly determines focusing direction, displacement, and stopping time while maximizing a global multispectral focus objective that considers mean sharpness, inter-band consistency, and worst-band degradation. During online autofocus, only the representative bands are acquired at intermediate positions, whereas the complete 31-band cube is captured after convergence. The framework is evaluated using standardized optical targets and spectrally heterogeneous scenes through focal-position error, MTF, full-band focus retention, acquisition time, and robustness. The proposed design provides an efficient computational-photonics solution for broadband multispectral autofocus with reduced band-acquisition and motor-search overhead. Full article

(This article belongs to the Section New Applications Enabled by Photonics Technologies and Systems)

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22 pages, 4058 KB

Open AccessArticle

Fiber Laser Induced Breakdown Spectroscopy Combined with Surface Enhancement and Spark Discharge for Sensitive Detection of Heavy Metals in Hair Dye Creams

by Shudi Zhang, Jianyun Lin, Jingru Huang, Zhisen Liang, Fangfang Chen and Guihong Wang

Photonics 2026, 13(9), 836; https://doi.org/10.3390/photonics13090836 - 1 Sep 2026

Abstract

The unauthorized addition of silver nitrate and excessive heavy metals in hair dye creams poses serious health risks, creating an urgent need for rapid and sensitive detection methods. In this study, a spark discharge-enhanced fiber laser-induced breakdown spectroscopy (SD-LIBS) system was developed for [...] Read more.

The unauthorized addition of silver nitrate and excessive heavy metals in hair dye creams poses serious health risks, creating an urgent need for rapid and sensitive detection methods. In this study, a spark discharge-enhanced fiber laser-induced breakdown spectroscopy (SD-LIBS) system was developed for the simultaneous detection of five heavy metal ions (Ag, Pb, Cd, As, Hg) in hair dye creams. The system combines three signal enhancement strategies: high-voltage spark discharge, Au nanoparticle-assisted enhancement, and analyte enrichment on a superhydrophobic–hydrophilic patterned graphite substrate. Hydrophilic micro-pits were fabricated by laser scanning on the superhydrophobic graphite surface to confine and concentrate analyte residues upon droplet drying. Under optimized discharge conditions of 22 nF and 2000 V, the spark discharge achieved a signal-to-noise ratio enhancement of approximately 12 times. Au nanoparticle enhancement further increased the signal intensity by 49.3%. The optimal pit diameter and sample loading volume were 0.1 mm and 30 μL, respectively. The calibration curves for all five elements showed good linearity with R² values close to 0.99, and the lowest detection limits reached 1.9 μg/L for Ag. The average spiked recovery was 82.07%. The method was applied to four commercial hair dye cream samples, and the Ag detection results agreed well with ICP-OES reference values with an average error of 16.41%. This work demonstrates that the proposed SD-LIBS system offers a low-cost, rapid, and sensitive approach for on-site screening of heavy metals in cosmetics and other consumer products. Full article

(This article belongs to the Section Lasers, Light Sources and Sensors)

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12 pages, 3125 KB

Open AccessArticle

Tunable Optical Bistability and High-Sensitivity Terahertz Refractive-Index Sensing Based on a Fabry-Pérot Cavity Incorporating Weyl Semimetals

by Shiqi Yang, Daohong Xiao, Jiaqi Ma, Bowen Wei, Kui Wang, Haishan Tian and Leyong Jiang

Photonics 2026, 13(9), 835; https://doi.org/10.3390/photonics13090835 - 1 Sep 2026

Abstract

Optical bistability (OB) is a fundamental nonlinear effect enabling all-optical switching, logic, memory, and sensing, yet its practical implementation is often hindered by high power thresholds and limited tunability. Here, we theoretically demonstrate low-threshold and widely tunable OB in a Fabry-Pérot (FP) cavity [...] Read more.

Optical bistability (OB) is a fundamental nonlinear effect enabling all-optical switching, logic, memory, and sensing, yet its practical implementation is often hindered by high power thresholds and limited tunability. Here, we theoretically demonstrate low-threshold and widely tunable OB in a Fabry-Pérot (FP) cavity integrated with a Weyl semimetal (WSM) layer. By combining the strong local-field enhancement of the FP cavity with the large third-order nonlinear conductivity of the WSM, we achieve pronounced bistable hysteresis loops in both the transmitted field amplitude and transmittance at terahertz frequencies, with incident field switching thresholds on the order of 10⁶ V/m. The bistability thresholds and hysteresis width can be flexibly controlled by tuning the WSM Fermi energy, the position of the WSM layer inside the cavity, and the mirror transmittance. Moreover, exploiting the strong dependence of the down-switching threshold on small changes in the refractive index of the cavity medium, we propose a terahertz refractive-index sensor with a maximum sensitivity of 296.8 GW·m⁻²·RIU⁻¹. These results provide a theoretical basis for tunable nonlinear photonic devices and threshold-based terahertz refractive-index sensors using topological semimetals. Full article

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14 pages, 17990 KB

Open AccessArticle

A Broadband Interferometric Fiber-Optic Hydrophone Enabled by a PPSU Mandrel and a Metamaterial Liner

by Yongchao Zou, Minzheng Sun, Kang Lou, Pan Xu, Zhengliang Hu and Min Zhu

Photonics 2026, 13(9), 834; https://doi.org/10.3390/photonics13090834 - 1 Sep 2026

Abstract

Interferometric fiber-optic hydrophones are widely deployed in passive underwater acoustic detection systems due to their high sensitivity, broad dynamic range, and immunity to electromagnetic interference. However, their adoption in active acoustic systems has been largely constrained by the limited operational bandwidth of conventional [...] Read more.

Interferometric fiber-optic hydrophones are widely deployed in passive underwater acoustic detection systems due to their high sensitivity, broad dynamic range, and immunity to electromagnetic interference. However, their adoption in active acoustic systems has been largely constrained by the limited operational bandwidth of conventional architectures. Through coupled acoustic–structural simulations, this work identifies the operational bandwidth bottleneck as arising from low-order mechanical resonance of high-modulus mandrel structures combined with acoustic cavity resonance and near-field scattering within the enclosed cylindrical geometry. To address these limitations, an optimized push–pull mandrel structure, featuring a miniaturized profile of φ16 mm × 16 mm and a low-acoustic-impedance material, is implemented to eliminate mechanical resonance within the operational bandwidth. Additionally, an integrated acoustic metamaterial liner is incorporated into the inner tube to mitigate scattering and reverberation effects. Experimental characterization of the prototype demonstrates an average phase sensitivity of −132 dB re 1 rad/μPa, with fluctuations below ±1.5 dB across 20 Hz to 31.5 kHz. Utilizing a custom-built demodulation system, the hydrophone achieves a minimum detectable pressure of 42 dB re 1 µPa/√Hz (126 µPa/√Hz) at 20 Hz and 27 dB re 1 µPa/√Hz (22 µPa/√Hz) above 2 kHz, remaining over 28 dB below the deep-sea state zero ambient noise floor at 20 Hz, and hence confirms its passive acoustic monitoring capability. In the high-frequency regime above 2 kHz, the hydrophone maintains ±1.5 dB sensitivity flatness with a horizontal directivity variation of only ±1.51 dB at 30 kHz, providing a robust platform for broadband active acoustic applications, including active sonar, underwater acoustic imaging and acoustic communications. These results establish a viable pathway toward next-generation dual-mode underwater acoustic systems requiring both high-fidelity passive listening and broadband active detection. Full article

(This article belongs to the Special Issue Advanced Optical Fiber Sensing Technologies in Harsh Environments)

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14 pages, 1138 KB

Open AccessArticle

Narrowband Metamaterial Absorber Based on Plasmonic Square Nanoring for Optical Sensing and Amplification

by Bin Wang, Yue Liu, Kaifeng Zheng, Xiaoyan Shi, Bing Liu, Siyao Ma, Shuo Yang, Jinguang Lv, Yanxiu Jiang and Enzhu Hou

Photonics 2026, 13(9), 833; https://doi.org/10.3390/photonics13090833 - 31 Aug 2026

Abstract

A plasmonic metamaterial absorber (MA) exhibits unique capabilities of electric field enhancement and spectral modulation and plays a critical role in biosensing applications. In this work, a narrowband MA structure based on a plasmonic square nanoring is designed and numerically investigated. The proposed [...] Read more.

A plasmonic metamaterial absorber (MA) exhibits unique capabilities of electric field enhancement and spectral modulation and plays a critical role in biosensing applications. In this work, a narrowband MA structure based on a plasmonic square nanoring is designed and numerically investigated. The proposed MA exhibits a high narrowband absorption of 99.77% at a 1216 nm wavelength with a 10 nm full width at half maximum (FWHM). Furthermore, the MA has sensitive absorption properties to the refractive index (RI) of the environment and shows a sensitivity of 186 nm/RIU with a stable value of quality factor around 120 in the RI ranging from 1.0 to 1.4. The sensing and recognition ability of the MA on actual biological cells is also numerically verified. Meanwhile, the proposed MA exhibits excellent capability of fluorescence amplification, achieving 3438–fold fluorescence enhancement at a 1216 nm wavelength with a high directivity of ~590. These properties make the proposed MA structure an excellent candidate for application in an optical sensing system. Full article

(This article belongs to the Special Issue Plasmonic Sensors: Advances and Applications)

13 pages, 848 KB

Open AccessArticle

Single-Photon Transmission, Quantum Key Distribution, Multimode Fiber, and Higher-Order Poincaré Spheres

by Daniel A. Nolan

Photonics 2026, 13(9), 832; https://doi.org/10.3390/photonics13090832 - 31 Aug 2026

Abstract

In this study, we simulate the propagation of a single photon propagating within a modal group, traversing a multimode optical fiber in the presence of mode coupling. We illustrate the propagation graphically on a group of higher-order Poincaré spheres. The spheres display the [...] Read more.

In this study, we simulate the propagation of a single photon propagating within a modal group, traversing a multimode optical fiber in the presence of mode coupling. We illustrate the propagation graphically on a group of higher-order Poincaré spheres. The spheres display the propagation of the light within the modal group, including polarization in time and in distance as the transmission proceeds. Thus, the amplitudes and the relative phases within the group can be visualized throughout the transmission, which is novel and very useful for understanding the propagation. At the fiber output, we show how to recover the input of a classical state using the simulated propagation information displayed on such multiple spheres. Once a classical modal path is established from end to end, one can transmit quantum states, for example, as a spatial–time binned QKD (quantum key distribution) code. A quantum state follows this classical path. Hence, it can include binary or qudit information. Accessing a photon output state is important for many quantum network applications, including quantum key distribution, routing, and entanglement swapping. In addition, one can use other modal groups within the same fiber to multiplex other quantum channels as well as multiplex within a modal group using principal modes, a more complicated communication method. This situation is also discussed while considering the use of the spheres. Their applications include higher-dimensional quantum communications, quantum cryptography, and quantum networks. It is important to point out that in today’s commercial quantum communications systems, quantum states are single-photon states, but they are not entangled. This includes both discrete and continuous variable quantum key distribution systems. Thus, in this report, we also address single-photon systems that are not entangled, unless stated otherwise. Full article

(This article belongs to the Special Issue The Many Paths of Light: Unraveling Multimode Optical Fibers)

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17 pages, 3611 KB

Open AccessArticle

Autoencoder-Based End-to-End Underwater DCO-OFDM Communication System

by Hexi Liang, Wenzheng Ni, Kangle Wang, Jingwei Zhou, Jinlin Liu and Yong Ai

Photonics 2026, 13(9), 831; https://doi.org/10.3390/photonics13090831 - 30 Aug 2026

Abstract

To mitigate the delay spread and inter-symbol interference (ISI) induced by optical scattering in underwater wireless optical communication (UWOC), this paper introduces long short-term memory (LSTM), a convolutional block attention module (CBAM), and residual connections into a convolutional neural network autoencoder (CNN-AE), and [...] Read more.

To mitigate the delay spread and inter-symbol interference (ISI) induced by optical scattering in underwater wireless optical communication (UWOC), this paper introduces long short-term memory (LSTM), a convolutional block attention module (CBAM), and residual connections into a convolutional neural network autoencoder (CNN-AE), and proposes a CNN-LSTM-AE-based end-to-end DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) system. In the proposed system, convolutional layers in the encoder serve to extract local features; CBAM adaptively weights salient features along the channel and spatial dimensions; LSTM layers model the temporal dependencies of signal sequences; and residual connections are incorporated to improve the learning capability for subtle signal features, thereby enhancing the robustness of the system against multipath channels. A symmetric structure is adopted at the receiver, ultimately enabling end-to-end signal recovery. Simulation results show that, under typical clear ocean and coastal ocean channel conditions, the proposed system outperforms end-to-end systems based on a fully connected autoencoder (FC-AE) and a CNN-AE at different modulation orders, i.e., different numbers of bits per symbol. For example, under strong scattering conditions in the coastal ocean channel, when the number of bits per symbol is 2 and the bit error rate (BER) is

10^{- 3}

, the proposed system achieves signal-to-noise ratio (SNR) gains of approximately 3.33 dB and 1.82 dB over the two baselines. In terms of block error rate (BLER), SNR gains of approximately 4.52 dB and 2.19 dB are achieved over the two comparison systems, which substantiates the superior end-to-end transmission reliability of the proposed system. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications—2nd Edition)

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13 pages, 13724 KB

Open AccessArticle

Dark-Scene Neuromorphic Imaging for Human Pose Estimation

by Hezu Bai, Xulei Qin, Yayu Dai, Zhou Ge, Chengze Li, Chaofan Wu, Feng Shi, Hongchang Cheng, Haibo Fan and Shilong Li

Photonics 2026, 13(9), 830; https://doi.org/10.3390/photonics13090830 - 30 Aug 2026

Abstract

Neuromorphic imaging sensors (event cameras) offer a promising paradigm for computational imaging and human pose estimation (HPE) under extreme illumination conditions. Nevertheless, dark-scene background activity originating from photodiode dark current and circuit thermal noise, together with hot-pixel noise, severely corrupts event streams and [...] Read more.

Neuromorphic imaging sensors (event cameras) offer a promising paradigm for computational imaging and human pose estimation (HPE) under extreme illumination conditions. Nevertheless, dark-scene background activity originating from photodiode dark current and circuit thermal noise, together with hot-pixel noise, severely corrupts event streams and impedes reliable HPE in low-light scenarios. To address this issue, we propose an adaptive event denoising framework built upon a spatiotemporal Gaussian-weighted neighborhood model with a dynamic thresholding mechanism. It can effectively suppress background activity and hot-pixel noise while preserving edge and motion details critical for pose estimation. Leveraging this denoising front-end, we construct a complete dark-scene neuromorphic HPE pipeline by transferring the pre-trained MediaPipe model onto event-based time-surfaces. Quantitative and qualitative evaluations on public and self-collected datasets demonstrate that our approach outperforms state-of-the-art denoising methods with an improvement of over 20% on public benchmarks and over 30% on self-collected dark-scene data. We expect our work to pave the way toward reliable dark-scene human–robot interaction through robust neuromorphic pose estimation. Full article

(This article belongs to the Special Issue Computational Optical Imaging: Progress and Future Prospects)

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13 pages, 2053 KB

Open AccessArticle

Numerical Experiment Based on Monte Carlo Stochastic Algorithm: Control of Strong-Field Double Ionization Dynamics by Carrier-Envelope Phase

by Yuxing Bai and Xiaolei Hao

Photonics 2026, 13(9), 829; https://doi.org/10.3390/photonics13090829 - 29 Aug 2026

Abstract

Nonsequential double ionization is a fundamental process in ultrafast strong-field physics, containing rich information about electron correlation dynamics. In few-cycle laser fields, the carrier-envelope phase becomes a key parameter for controlling electron behavior. However, this process involves nonlinear mechanisms such as multi-electron stochastic [...] Read more.

Nonsequential double ionization is a fundamental process in ultrafast strong-field physics, containing rich information about electron correlation dynamics. In few-cycle laser fields, the carrier-envelope phase becomes a key parameter for controlling electron behavior. However, this process involves nonlinear mechanisms such as multi-electron stochastic dynamics and complex Coulomb interactions, posing significant challenges to traditional analytical theories. To address this, this work develops a numerical experimental approach based on a Monte Carlo stochastic algorithm, transforming the quantum problem into a computable stochastic sampling task. Through statistical sampling and final-state analysis of tens of millions of quantum trajectories, the central regulatory role of the carrier-envelope phase is systematically revealed. The computational results show that this phase can not only independently regulate the yields of double ionization and frustrated double ionization but also control the branching ratio between them, with a peak-to-peak modulation amplitude of approximately 20%. Additionally, it enables fine-tuning of the electron momentum correlation distribution. The physical mechanisms underlying these regulatory effects are clearly elucidated through analysis of the quantum trajectories. This work not only clarifies the critical role of the carrier-envelope phase in few-cycle intense-field double ionization but also demonstrates the powerful capability of the Monte Carlo stochastic algorithm in revealing the intrinsic stochasticity of strong-field physics and achieving precise physical control, providing an example for the deep integration of intense-field physics and computational science. Full article

(This article belongs to the Special Issue Laser-Driven Ultrafast Dynamics and Imaging in Atoms and Molecules)

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13 pages, 14751 KB

Open AccessArticle

Joint Optimization of Scanning Strategy and Initialization for Accelerated Lensless Coded Ptychography

by Pengcheng Yan, Lingzhi Jiang, Yufei Liu, Cong Zhang, Yangchen Cai, Tianjun Wang, Kai Zhu, Bindi Xu, Xian Zuo, Shaowei Jiang and Liming Yang

Photonics 2026, 13(9), 828; https://doi.org/10.3390/photonics13090828 - 28 Aug 2026

Abstract

Lensless coded ptychography (CP) acquires multiple intensity measurements by translating either the object or the coded sensor, and reconstructs high-resolution, large field-of-view images via iterative phase retrieval. However, conventional CP suffers from slow reconstruction due to two limitations. First, periodic uniform scanning requires [...] Read more.

Lensless coded ptychography (CP) acquires multiple intensity measurements by translating either the object or the coded sensor, and reconstructs high-resolution, large field-of-view images via iterative phase retrieval. However, conventional CP suffers from slow reconstruction due to two limitations. First, periodic uniform scanning requires dense measurements to maintain sufficient measurement diversity, resulting in a large number of raw measurements and a high computational burden for iterative phase retrieval. Second, random initialization leads to slow convergence and increases the risk of stagnation in local minima. To address these limitations, we propose a joint optimization of the sampling strategy and initialization for accelerated reconstruction. Specifically, a continuous non-uniform scanning strategy preserves measurement diversity while reducing the number of required measurements. In addition, a low-resolution regularized ptychographic iterative engine (rPIE)-based initialization provides a more accurate starting point and accelerates the convergence of iterative phase retrieval. Both simulations and experiments demonstrate that the proposed approach achieves approximately three-fold faster convergence while maintaining high reconstruction fidelity. The proposed approach offers an effective solution for high-throughput imaging applications, including digital pathology and label-free quantitative phase imaging. Full article

(This article belongs to the Special Issue Computational Methods for Advanced Optical Imaging)

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29 pages, 11451 KB

Open AccessReview

Optical Tweezers: From Light Scattering Fundamentals to Single-Molecule Manipulation and Cellular Mechanosensing

by Jie He, Yanan Zhao and Tianli Wu

Photonics 2026, 13(9), 827; https://doi.org/10.3390/photonics13090827 - 28 Aug 2026

Abstract

Optical force originates from momentum transfer in the interaction between light and matter, forming the physical foundation of optical tweezers. Over the past four decades, optical tweezers have undergone a paradigm shift from passive manipulation tools to active platforms for bioregulation. This review [...] Read more.

Optical force originates from momentum transfer in the interaction between light and matter, forming the physical foundation of optical tweezers. Over the past four decades, optical tweezers have undergone a paradigm shift from passive manipulation tools to active platforms for bioregulation. This review focuses on the latest advances in the field of optical mechanics, ranging from fundamental theory to biological applications. First, the fundamentals of light scattering in different regimes and the resultant optical forces, as well as the conditions for stable trapping, are described. Then, advanced optical tweezer platforms developed on various physical principles are reviewed. Applications from single-molecule force spectroscopy to optical probing of cellular mechanosensitive responses are discussed. At the end, future directions toward in vivo manipulation, high-throughput automation, and intelligent real-time feedback control are provided. Full article

(This article belongs to the Special Issue Optical Tweezers: From Light Scattering Theory to Micro/Nanoparticle Manipulation)

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19 pages, 9646 KB

Open AccessArticle

Dual-Channel Photonic Crystal Fiber Sensor Integrating Hyperbolic Mode Resonance and Lossy Mode Resonance for Simultaneous Measurement of RI and Temperature

by Qirui Shu, Miaomiao Yan, Ying He and Yanfang Yang

Photonics 2026, 13(9), 826; https://doi.org/10.3390/photonics13090826 - 28 Aug 2026

Abstract

Hyperbolic mode resonance (HMR) has emerged as a solution to conventional sensing limitations. HMR is introduced into a photonic crystal fiber (PCF) for sensing in this research. A dual-channel PCF sensor for both refractive index (RI) and temperature measurement is proposed. The device [...] Read more.

Hyperbolic mode resonance (HMR) has emerged as a solution to conventional sensing limitations. HMR is introduced into a photonic crystal fiber (PCF) for sensing in this research. A dual-channel PCF sensor for both refractive index (RI) and temperature measurement is proposed. The device has two independent sensing channels on the dual-side polishing surface of the PCF. An Au/SnO₂ bilayer in the RI channel induces HMR, while the SnO₂/PDMS bilayer in the temperature channel induces lossy mode resonance, with PDMS as the temperature-sensitive material. By introducing an Au layer to excite the HMR effect, the sensor achieves enhanced resonance depth and figure of merit (FOM) without sacrificing sensitivity. When the RI ranges from 1.34 to 1.426 and the temperature varies from 25 °C to 100 °C, the optimized sensor achieves a maximum RI sensitivity of 9100 nm/RIU and a maximum temperature sensitivity of −5 nm/°C. A

2 \times 2

sensitivity-matrix analysis confirms effective dual-parameter decoupling, with demodulation errors below

5 \times 10^{- 4}

RIU and 0.1 °C under simultaneous RI and temperature variations. This PCF sensor offers high sensitivity, quantifiably low crosstalk, wide detection range and good fabrication tolerance, making it promising for biomedical, environmental and industrial applications. Full article

(This article belongs to the Special Issue Optical Fiber Sensors: Refractivity and Interferometric Applications)

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13 pages, 3585 KB

Open AccessArticle

Enhancement and Manipulation of Near-Field Thermal Radiation Heat Transfer in a Two-Body System Based on the Lorentz Model

by Jincheng Wang, Ronghui Yang, Shiqi Yang, Qijun Ma and Wei Huang

Photonics 2026, 13(9), 825; https://doi.org/10.3390/photonics13090825 - 28 Aug 2026

Abstract

In this paper, we systematically investigate the near-field radiative heat transfer in a two-body system described by the Lorentz model. Under a finite temperature difference between the upper and lower plates, we explore the evolution of radiative heat transfer by tuning the longitudinal [...] Read more.

In this paper, we systematically investigate the near-field radiative heat transfer in a two-body system described by the Lorentz model. Under a finite temperature difference between the upper and lower plates, we explore the evolution of radiative heat transfer by tuning the longitudinal and transverse optical phonon frequencies. Our results show that the near-field radiative heat flux can exceed the blackbody radiation limit by several orders of magnitude. Both the longitudinal and transverse phonon frequencies serve as effective control knobs, enabling broadband modulation of the spectral heat transfer. Furthermore, mirror-symmetric modulation of these phonon frequencies in both plates significantly enhances the tunability of the near-field heat transfer. The findings obtained in this study provide valuable theoretical insights and a fundamental reference framework for designing actively tunable near-field thermal devices based on similar material platforms. Full article

(This article belongs to the Section Optical Interaction Science)

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19 pages, 16483 KB

Open AccessArticle

Broadband Photonic LFM Microwave Signal Generation via Segmented Parabolic Waveform and Recirculating Phase Accumulation

by Wen Xie, Xinyu Jin, Cheng Gu, Boxiong Cui, Chenyang Ma, Yilun Ma, Zhengyang Xie, Xin Zhao, Pengwei Gong and Zheng Zheng

Photonics 2026, 13(9), 824; https://doi.org/10.3390/photonics13090824 - 28 Aug 2026

Abstract

We propose and experimentally demonstrate a broadband photonic approach to linear frequency-modulated (LFM) microwave signal generation based on segmented parabolic phase modulation and synchronous recirculating phase accumulation. The expanded quadratic-phase trajectory is mapped onto an amplitude-limited segmented parabolic waveform, thereby increasing the effective [...] Read more.

We propose and experimentally demonstrate a broadband photonic approach to linear frequency-modulated (LFM) microwave signal generation based on segmented parabolic phase modulation and synchronous recirculating phase accumulation. The expanded quadratic-phase trajectory is mapped onto an amplitude-limited segmented parabolic waveform, thereby increasing the effective phase slope without requiring a proportional increase in the peak driving voltage. Meanwhile, a synchronized optical recirculating loop enables coherent phase accumulation over multiple modulation passes, providing further bandwidth enhancement. Numerical simulations verify the bandwidth scalability enabled by this two-stage phase-enhancement mechanism, achieving a maximum simulated bandwidth of 39.04 GHz. Experimentally, an LFM microwave signal with a duration of 512 ns and a bandwidth of 10.6 GHz is successfully generated, corresponding to a chirp rate of 20.70 GHz/μs, a time–bandwidth product (TBWP) of 5427.2, and a pulse-compression ratio (PCR) of 5429.48. The frequency-sweep linearity of the generated signal is measured to be 0.868%, demonstrating excellent linear frequency-sweep performance. Multiple independent measurements further confirm the stability and repeatability of the system, with the maximum measured bandwidth reaching 11.65 GHz. These results demonstrate that the proposed approach provides a scalable route to broadband LFM microwave signal generation, combining high chirp rates, large TBWPs, excellent frequency-sweep linearity, and good repeatability. Full article

(This article belongs to the Special Issue Microwave Photonics: Devices, Systems and Emerging Applications)

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14 pages, 17199 KB

Open AccessArticle

Broadened and Flattened Optical Frequency Comb Based on Directly Modulated DFB Semiconductor Laser

by Tong Xu, Huatao Zhu, Yongcheng Li, Xin Zhang, Xing Yang, Haijing Li, Yibo Liu and Feng Jiang

Photonics 2026, 13(9), 823; https://doi.org/10.3390/photonics13090823 - 28 Aug 2026

Abstract

A simple scheme for generating a broadened and flattened optical frequency comb (OFC) based on a directly modulated distributed feedback (DFB) semiconductor laser cascaded with electro-optic modulators is proposed. The initial OFC is generated by a gain-switched DFB semiconductor laser. The number of [...] Read more.

A simple scheme for generating a broadened and flattened optical frequency comb (OFC) based on a directly modulated distributed feedback (DFB) semiconductor laser cascaded with electro-optic modulators is proposed. The initial OFC is generated by a gain-switched DFB semiconductor laser. The number of comb lines is further expanded by cascading a phase modulator and the flatness performance is optimized by adjusting the power of the radio frequency (RF) and bias voltage of the cascaded Mach–Zehnder modulator. Moreover, the spacing of comb lines can be flexibly tuned by varying the frequency of the RF signal, and an OFC with seven spectral lines within a 4.72 dB power variation is demonstrated. The 17-line OFC with a frequency spacing of 12.5 GHz and a flatness of 3 dB is further applied to an optical transmission system, which enables error-free transmission of a non-return-to-zero data signal. The proposed scheme is expected to find applications in flexible-grid optical transmission systems. Full article

(This article belongs to the Section Optical Communication and Network)

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15 pages, 5087 KB

Open AccessArticle

Vortex Beam-Coupled Cassegrain Front-End Free-Space Optical Links with LSTM and Transformer-Based Signal Recovery

by Jiyeon Baek, Yuna Lee and Hyunchae Chun

Photonics 2026, 13(9), 822; https://doi.org/10.3390/photonics13090822 - 28 Aug 2026

Abstract

Free-space optical (FSO) communication provides high-capacity wireless transmission but suffers from reduced optical coupling efficiency when compact Cassegrain telescopes are employed because the secondary mirror blocks the central portion of the incident beam. This paper proposes a vortex beam-assisted FSO communication system that [...] Read more.

Free-space optical (FSO) communication provides high-capacity wireless transmission but suffers from reduced optical coupling efficiency when compact Cassegrain telescopes are employed because the secondary mirror blocks the central portion of the incident beam. This paper proposes a vortex beam-assisted FSO communication system that combines aperture-matched optical coupling with machine learning-based signal recovery. Unlike a conventional Gaussian beam, the annular intensity distribution of a Laguerre–Gaussian vortex beam is matched to the unobstructed annular aperture of a centrally obscured Cassegrain telescope, thereby reducing obstruction-induced optical loss. The coupling characteristics are analyzed using an annular aperture overlap model and experimentally validated in a 100 m free-space optical link employing Cassegrain transmitter and receiver front-ends. The mean measured telescope-output power is increased by more than 30% over the Gaussian reference. To overcome the system-induced signal aliasing, Transformer and long short-term memory (LSTM) equalizers are optimized and applied. Both models substantially outperform optimized threshold detection, while the LSTM achieves the lowest observed error rate with markedly fewer multiply–accumulate operations than the Transformer. These results show that aperture-matched optical coupling and computationally efficient sequence equalization, such as LSTM is a crucial component of compact, high-performance telescope-assisted FSO systems. Full article

(This article belongs to the Special Issue Machine Learning and Artificial Intelligence for Optical Networks)

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16 pages, 5905 KB

Open AccessArticle

Imaging Fungal Hyphae and Infection-Induced Stromal Changes in Ex Vivo Porcine Corneas Using Line-Field Spectral Domain Optical Coherence Microscopy

by Samuel Lawman, Keri McLean, Sharon Mason, Yao-Chun Shen, Stephen B. Kaye and Yalin Zheng

Photonics 2026, 13(9), 821; https://doi.org/10.3390/photonics13090821 - 28 Aug 2026

Abstract

Microbial keratitis requires rapid diagnosis to enable timely, pathogen-directed treatment. Corneal confocal microscopy (CCM) is used to quickly diagnose acanthamoeba and fungal keratitis, but it relies on undesirable contact objective lenses, and no supplier has maintained a stable market presence. Here, we present [...] Read more.

Microbial keratitis requires rapid diagnosis to enable timely, pathogen-directed treatment. Corneal confocal microscopy (CCM) is used to quickly diagnose acanthamoeba and fungal keratitis, but it relies on undesirable contact objective lenses, and no supplier has maintained a stable market presence. Here, we present a preliminary investigation of the potential abilities of non-contact optical coherence microscopy (OCM) as an emerging alternative technology. Using an ex vivo porcine corneal infection model, we demonstrate that OCM can produce en face images, similar to CCM, as well as 3D visualizations of fungi hyphae in stromal tissue. Like CCM, bacterial pathogens were too small to directly identify in OCM images. Nevertheless, OCM detected that ex vivo infection induced systematic changes to the stroma that are consistent with the breakdown of keratocyte structure. These proof-of-concept results demonstrate the potential of non-contact OCM for imaging microbial keratitis and support further in vivo evaluation. Full article

(This article belongs to the Section Biophotonics and Biomedical Optics)

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13 pages, 3252 KB

Open AccessArticle

Study on the Performance Degradation Mechanism of Low-Light-Level Image Intensifiers in Humid and Hot Environments

by Meng Zhu, Mengmeng Liu, Mingqian Xu, Ru Yao, Rongxuan Liang, De Song and Weijun Chen

Photonics 2026, 13(9), 820; https://doi.org/10.3390/photonics13090820 - 27 Aug 2026

Abstract

To analyze the performance stability of image intensifiers under humid and hot environments, this study systematically investigates the device performance degradation mechanism through a combination of natural exposure tests, humidity variation aging tests, disassembly analysis of failed samples, and a vacuum decay model. [...] Read more.

To analyze the performance stability of image intensifiers under humid and hot environments, this study systematically investigates the device performance degradation mechanism through a combination of natural exposure tests, humidity variation aging tests, disassembly analysis of failed samples, and a vacuum decay model. Experimental and simulated results reveal that moisture ingress in a humid and hot environment serves as the dominant factor governing the luminance gain stability of devices. Microscopic defects such as cracks, voids, and delamination phenomena existing at the sealing interface between ceramic rings and metal gaskets are the direct cause of moisture penetration. The attenuation of luminance gain accelerates significantly with increasing ambient humidity, demonstrating that water vapor is the primary driving factor for the environmental failure of devices. This study clarifies the attenuation mechanism of luminance gain for image intensifiers under humid and hot environments, providing a theoretical basis for the optimization of packaging processes and the improvement in environmental adaptability of image intensifiers. Full article

(This article belongs to the Special Issue Semiconductor Optoelectronic Devices: Characterizations, Design and Fabrication)

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23 pages, 28039 KB

Open AccessArticle

Vertically Resolved Aerosol Optical-State Identification by Multi-Wavelength Raman–Mie Polarization Lidar in Contrasting Inland and Coastal Environments

by Zhen Zhang, Zhigang Li, Zhichao Bu and Yaru Dai

Photonics 2026, 13(9), 819; https://doi.org/10.3390/photonics13090819 - 27 Aug 2026

Abstract

Multi-wavelength Raman–Mie polarization lidar provides vertically resolved measurements of aerosol scattering, particle shape, and wavelength-dependent response. We analyzed quality-controlled clear-sky observations from June 2025 to March 2026 to identify aerosol optical regimes at inland Beijing Nanjiao and coastal Beihai. The feature space comprised [...] Read more.

Multi-wavelength Raman–Mie polarization lidar provides vertically resolved measurements of aerosol scattering, particle shape, and wavelength-dependent response. We analyzed quality-controlled clear-sky observations from June 2025 to March 2026 to identify aerosol optical regimes at inland Beijing Nanjiao and coastal Beihai. The feature space comprised

\log_{10} (β_{532})

,

δ_{p, 532}

, and

{AE}_{β, 355 / 532}

, representing scattering intensity, particle nonsphericity, and size-sensitive spectral response. K-means was applied independently at each site. At Beijing Nanjiao, a high-depolarization, strong-scattering, and low-

{AE}_{β, 355 / 532}

regime was concentrated in the lowest observed layer, whereas lower-loading regimes occurred more frequently aloft. Its dust AOD and dust fraction were descriptively 37.5% and 19.2% above the site means, although the dust-related inter-regime differences were not significant after FDR correction. Beihai was dominated by low-depolarization regimes separated mainly by scattering intensity and wavelength response. Its strongest-scattering regime showed total, sea salt, OC, and sulfate AOD enhancements of 33.0%, 15.5%, 24.5%, and 41.9%, respectively; the inter-regime differences were significant for total and sulfate AODs but not for sea salt AOD. MERRA-2 aerosol diagnostics and trajectory analyses provided auxiliary regional context for interpreting the lidar-defined optical states and were not used as clustering inputs or direct chemical validation. These results demonstrate that combined polarization and multi-wavelength lidar sensing can distinguish vertically varying aerosol optical states that are obscured in surface or column-integrated observations. Full article

(This article belongs to the Section Lasers, Light Sources and Sensors)

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