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Closed Form Analytic Expressions for the Evanescent and Traveling Components of the Electromagnetic Green Function and for Defocused Hemispherical Focusing of Electromagnetic Waves
Space–Time Duality in Optics: Its Origin and Applications
Entanglement Dynamics of Two Giant Atoms Embedded in a One-Dimensional Photonic Lattice with a Synthetic Gauge Field
Non-Hermitian Control of Tri-Photon and Quad-Photon Using Parallel Multi-Dressing Quantization

Journal Description

Photonics

Photonics is an international, scientific, peer-reviewed, open access journal on the science and technology of optics and photonics, published monthly online by MDPI.

Open Access— free for readers, with article processing charges (APC) paid by authors or their institutions.
High Visibility: indexed within Scopus, SCIE (Web of Science), Inspec, Ei Compendex, CAPlus / SciFinder, and other databases.
Journal Rank: CiteScore - Q2 (Instrumentation)
Rapid Publication: manuscripts are peer-reviewed and a first decision is provided to authors approximately 14.8 days after submission; acceptance to publication is undertaken in 1.9 days (median values for papers published in this journal in the first half of 2025).
Recognition of Reviewers: reviewers who provide timely, thorough peer-review reports receive vouchers entitling them to a discount on the APC of their next publication in any MDPI journal, in appreciation of the work done.
Companion journal: Lights.

Impact Factor: 1.9 (2024); 5-Year Impact Factor: 2.0 (2024)

Imprint Information Journal Flyer Open Access ISSN: 2304-6732

Latest Articles

15 pages, 1458 KiB

Open AccessArticle

Standing Wave Photon Structures in Constraint Spaces

by Donglin Zu

Photonics 2025, 12(9), 841; https://doi.org/10.3390/photonics12090841 - 22 Aug 2025

Based on the single-photon structure model, the standing wave electric 4-photon (SWE4-P) composite, the standing wave magnetic 4-photon (SWM4-P) composite in one-dimensional longitudinal constraint space, and the standing wave 8-photon (SW8-P) composite structure in a laser microcavity are derived. The electromagnetic field of [...] Read more.

Based on the single-photon structure model, the standing wave electric 4-photon (SWE4-P) composite, the standing wave magnetic 4-photon (SWM4-P) composite in one-dimensional longitudinal constraint space, and the standing wave 8-photon (SW8-P) composite structure in a laser microcavity are derived. The electromagnetic field of the TM₀₁₀ mode in a microwave cylindrical resonant cavity is studied and analyzed, and the photon structure basic unit of this mode is identified as the standing wave cylindrical 8-photon composite structure. The cylindrical photon is of the same size as the cavity volume, the photon volume being V = πR²L. The standing wave 8-photon composite structure contains an SWE4-P composite and an SWM4-P composite, with a phase difference of 90°. Therefor, the energy unit of the TM₀₁₀ mode in the cavity is 8ℏω. Full article

15 pages, 7090 KiB

Open AccessArticle

Design of a Transmitting Optical System for Large-Angle MEMS Lidar with High Spatial Resolution

by Jiajie Wu, Jianjie Yu, Yang Qi, Shuo Wang, Chunzhu Yu, Yonglun Liu and Qingyan Li

Photonics 2025, 12(9), 840; https://doi.org/10.3390/photonics12090840 - 22 Aug 2025

Lidar has been extensively used in various applications, such as autonomous driving, robot navigation, and drone obstacle avoidance, due to its advantages of a high resolution, high-ranging accuracy, and strong anti-interference ability. The micro-electro-mechanical systems (MEMS) lidar technology approach has gained popularity due [...] Read more.

Lidar has been extensively used in various applications, such as autonomous driving, robot navigation, and drone obstacle avoidance, due to its advantages of a high resolution, high-ranging accuracy, and strong anti-interference ability. The micro-electro-mechanical systems (MEMS) lidar technology approach has gained popularity due to its miniaturization and semi-solid state. However, the small scanning angle of the MEMS scanning micromirror and the associated radar system cause issues, such as a limited scanning range and low spatial resolution, which hinder the wider use of MEMS lidar. To address the problems caused by the small scanning angle of the MEMS micromirror and the limitations of the current optical system, this study suggests a new MEMS lidar transmitting optical system that offers a wide scanning angle and high spatial resolution. It is based on an array reflector group and a Fresnel lens, which enables the large-angle scanning of the target area while maintaining high spatial resolution. The scanning range is 120° × 60°, the spatial resolution is 0.05° × 0.25°, and the beam-filling ratio reaches 90.63%. Full article

(This article belongs to the Special Issue Advanced Methods in Exploring Light–Matter Interactions and Nonlinear Effects Optics Applications)

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19 pages, 5861 KiB

Open AccessArticle

Research on the Initial Orientation Technology of the View Axis for Underwater Laser Communication Dynamic Platforms Based on Coordinate Transformation Matrix Positioning Model

by Jun Ma, Yunjie Teng, Yang Liu, Mingyang Zhang, Cheng Qiu, Hao Qin and Yanpu Li

Photonics 2025, 12(9), 839; https://doi.org/10.3390/photonics12090839 - 22 Aug 2025

To address the challenge of directly applying space laser communication systems to dynamic underwater environments, this paper integrates coordinate transformation matrices with underwater positioning systems, proposing an ultra-short baseline (USBL) system combined with a coordinate transformation-based underwater positioning model. The model is designed [...] Read more.

To address the challenge of directly applying space laser communication systems to dynamic underwater environments, this paper integrates coordinate transformation matrices with underwater positioning systems, proposing an ultra-short baseline (USBL) system combined with a coordinate transformation-based underwater positioning model. The model is designed to effectively compensate for underwater dynamic disturbances, enhance the pointing accuracy of the system, and achieve stable tracking between underwater platforms. Simulation results demonstrate that the proposed model can enhance system tracking accuracy to 130.31 μrad (an improvement of 32.24%). Through underwater experiments, the results demonstrate that the underwater positioning model enables the system to achieve a pointing accuracy of 2.82 mrad (an improvement of 39.87%) and a tracking accuracy of 181.70 μrad (an improvement of 31.46%). Additionally, it can achieve underwater communication at 50 m with a data rate of 10 Mbps, providing a reference for future research on dynamic underwater laser communication. Full article

(This article belongs to the Special Issue Stable Control Technology and Image Perception Technology in Optoelectronic Servo Systems)

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17 pages, 1794 KiB

Open AccessArticle

Improved Ultra-Dense Connection Provision Capability of Concurrent Upstream and Direct Inter-ONU Communication IMDD PONs by P2MP Flexible Optical Transceivers

by Lin Chen, Han Yang, Shenming Jiang, Wei Jin, Jiaxiang He, Roger Philip Giddings, Yi Huang, Md. Saifuddin Faruk, Xingwen Yi and Jianming Tang

Photonics 2025, 12(9), 838; https://doi.org/10.3390/photonics12090838 - 22 Aug 2025

To cost-effectively meet 6G latency requirements, concurrent upstream and direct inter-optical network unit (ONU) communication passive optical networks (PONs) based on flexible point-to-multipoint (P2MP) optical transceivers and intensity modulation and direct detection (IMDD) have been reported to enable direct communications among different ONUs [...] Read more.

To cost-effectively meet 6G latency requirements, concurrent upstream and direct inter-optical network unit (ONU) communication passive optical networks (PONs) based on flexible point-to-multipoint (P2MP) optical transceivers and intensity modulation and direct detection (IMDD) have been reported to enable direct communications among different ONUs within the same PON without passing data to the optical line terminal (OLT). However, the previously reported P2MP transceivers suffer from high DSP complexity for establishing ultra-dense connections. For such application scenarios, the PON’s remote nodes also have high inter-ONU signal power losses. To effectively solve these technical challenges, this paper experimentally showcases (a) new P2MP transceivers by utilizing parallel multi-channel aggregation/de-aggregation and advanced extended Gaussian function (EGF)-based orthogonal digital filter banks, along with (b) low inter-ONU signal power loss-remote nodes. By introducing these two techniques into a 27 km, >54.31 Gbit/s concurrent upstream and direct inter-ONU communication IMDD PON, comprehensive experimental explorations of the PON’s performances were undertaken for the first time. The remote node is capable of supporting 128 ONUs. The results show that the new P2MP transceivers lead to >75% (>40%) reductions in overall transmitter (receiver multi-channel de-aggregation) DSP complexity, and they can also equip the PONs with an enhanced capability of providing ultra-dense connections. The experimental results also show that the PON allows each ONU to flexibly change its upstream and inter-ONU communication channel count without considerably compromising its performance. Therefore, the PON outperforms those of previously reported works in terms of ensuring low DSP complexity, highly robust transmission performance, and enhanced capabilities of flexibly accommodating numerous applications with diverse requirements regarding traffic characteristics, thus making it suitable for ultra-dense connection application scenarios. Full article

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

13 pages, 6253 KiB

Open AccessArticle

Temperature Control Based on Fuzzy Neural Networks for High-Power Laser Diodes

by Nan Li, Kaixuan Wang, Huadong Lu, Yaohui He and Xiaoli Jin

Photonics 2025, 12(9), 837; https://doi.org/10.3390/photonics12090837 - 22 Aug 2025

High-power laser diodes (LDs) inherently generate considerable heat during current loading, which presents substantial challenges to the stable operation of laser systems. This study reports a machine learning-based approach that is to be applied to LD temperature control systems, in which a fuzzy [...] Read more.

High-power laser diodes (LDs) inherently generate considerable heat during current loading, which presents substantial challenges to the stable operation of laser systems. This study reports a machine learning-based approach that is to be applied to LD temperature control systems, in which a fuzzy neural network (FNN) algorithm is integrated with a proportional-integral-derivative (PID) controller to create an FNN-PID control architecture. The proposed algorithms synergistically integrate fuzzy rule-based systems with neural network learning frameworks, and, furthermore, facilitate adaptive parameter optimization while preserving the interpretability of the decision-making process. Applying the optimized algorithm temperature controller to the LD with output optical power of 110 W @ 888 nm, compared with the conventional PID, the FNN-PID algorithm has shortened the temperature settling time by 77% during 100 W heat generation in LD, the long-term temperature fluctuation is decreased from ±0.126% to ±0.06%, the corresponding optical power steady-state precision is decreased from ±0.09% to ±0.04%, and the step response time of temperature and corresponding power are reduced by 73.4% and 70% from 25 °C to 27 °C, respectively. The FNN-PID outperforms conventional methods (the PID algorithm and the Fuzzy-PID algorithm) in managing thermal fluctuations, and it offers potential for precise laser control applications to enhance beam quality and stability. Full article

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11 pages, 1849 KiB

Open AccessArticle

Miniaturized Multicolor Femtosecond Laser Based on Quartz-Encapsulated Nonlinear Frequency Conversion

by Bosong Yu, Siying Wang, Aimin Wang, Yizhou Liu and Lishuang Feng

Photonics 2025, 12(9), 836; https://doi.org/10.3390/photonics12090836 - 22 Aug 2025

Ultrafast lasers operating at 740 nm and 820 nm have attracted widespread attention as two-photon light sources for the detection of biological metabolism. Here, we report on a solid-like quartz-encapsulated femtosecond laser with a repetition rate of 80 MHz, delivering 740 nm and [...] Read more.

Ultrafast lasers operating at 740 nm and 820 nm have attracted widespread attention as two-photon light sources for the detection of biological metabolism. Here, we report on a solid-like quartz-encapsulated femtosecond laser with a repetition rate of 80 MHz, delivering 740 nm and 820 nm femtosecond laser pulses. This home-built laser system was realized by employing an erbium-doped 1560 nm fiber laser as the fundamental laser source. A quartz-encapsulated nonlinear frequency conversion stage, consisting of a second-harmonic generation (SHG) stage and self-phase modulation (SPM)-based nonlinear spectral broadening stage, was utilized to deliver 30 mW, 53.7 fs, 740 nm laser pulses and the 15 mW, 60.8 fs, 820 nm laser pulses. Further imaging capabilities of both wavelengths were validated using a custom-built inverted two-photon microscope. Clear imaging results were obtained from mouse kidney sections and pollen samples by collecting the corresponding fluorescence signals. The achieved results demonstrate the great potential of this laser source for advanced two-photon microscopy in metabolic detection. Full article

(This article belongs to the Special Issue Advances in Solid-State Laser Technology and Applications)

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13 pages, 9516 KiB

Open AccessArticle

Rapid Full-Field Surface Topography Measurement of Large-Scale Wafers Using Interferometric Imaging

by Ruifang Ye, Jiarui Zeng, Heyan Zhang, Yujie Su and Huihui Li

Photonics 2025, 12(9), 835; https://doi.org/10.3390/photonics12090835 - 22 Aug 2025

Rapid full-field surface topography measurement for large-scale wafers remains challenging due to limitations in speed, system complexity, and scalability. This work presents a interferometric system based on thin-film interference for high-precision wafer profiling. An optical flat serves as the reference surface, forming a [...] Read more.

Rapid full-field surface topography measurement for large-scale wafers remains challenging due to limitations in speed, system complexity, and scalability. This work presents a interferometric system based on thin-film interference for high-precision wafer profiling. An optical flat serves as the reference surface, forming a parallel air-gap structure with the wafer under test. A large-aperture collimated beam is introduced via an off-axis parabolic mirror to generate high-contrast interference fringes across the entire field of view. Once the wafer is fully illuminated, topographic information is directly extracted from the fringe pattern. Comparative measurements with a commercial interferometer show relative deviations below

3 %

in bow and warp, confirming the system’s accuracy and stability. With its simple optical layout, low cost, and robust performance, the proposed method shows strong potential for industrial applications in wafer inspection and online surface monitoring. Full article

(This article belongs to the Special Issue Advances in Interferometric Optics and Applications)

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11 pages, 1618 KiB

Open AccessArticle

Measurement of Enhanced Inversion Factor of InGaAs-Based Well-Island Composite Structure by Photoluminescence Spectra from Dual Facets

by Xing Ge, Qingnan Yu, Zixuan Chen, Zeng Jin, Xinyang Qi, Ru Wang, Kang Meng, Wei Wang, Hongxu Li, Gang Liu and Junjie Wu

Photonics 2025, 12(9), 834; https://doi.org/10.3390/photonics12090834 - 22 Aug 2025

The inversion factor is an important physical parameter for assessing and revealing the performance of semiconductor lasers, providing insights into the carrier-injected band-filling effect and radiation characteristics. In this paper, the carrier inversion factor (P_f) is measured to elucidate the luminescence [...] Read more.

The inversion factor is an important physical parameter for assessing and revealing the performance of semiconductor lasers, providing insights into the carrier-injected band-filling effect and radiation characteristics. In this paper, the carrier inversion factor (P_f) is measured to elucidate the luminescence mechanism of an InGaAs-based well-island composite (WIC) structure, formed by the self-assembly migration of indium atoms and exhibiting excellent spectral properties. P_f is obtained by collecting the amplified photoluminescence (PL) spectra from dual facets of the device, with carrier concentrations ranging from 9.0 × 10¹⁷ to 9.4 × 10¹⁷ cm⁻³. Compared with classical InGaAs/GaAs quantum well structures under the same operating conditions, the inversion level in the WIC structure can be as high as 2.2. Simulation results reveal enhanced quasi-Fermi-level separation and broadened spectral bandwidth. The research is of great significance in the development of new types of quantum-confined lasers with wide spectral output. Full article

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10 pages, 6301 KiB

Open AccessArticle

Study on Diamond NV Centers Excited by Green Light Emission from OLEDs

by Yangyang Guo, Xin Li, Fuwen Shi, Wenjun Wang and Bo Li

Photonics 2025, 12(9), 833; https://doi.org/10.3390/photonics12090833 - 22 Aug 2025

This study demonstrates the feasibility of exciting NV centers using ITO-anode OLED devices, followed by the fabrication of GO/PEDOT:PSS hybrid anodes via spin-coating. Through interfacial modification, the OLED devices exhibit significantly enhanced luminescence intensity, leading to improved NV center excitation efficiency. Experimental results [...] Read more.

This study demonstrates the feasibility of exciting NV centers using ITO-anode OLED devices, followed by the fabrication of GO/PEDOT:PSS hybrid anodes via spin-coating. Through interfacial modification, the OLED devices exhibit significantly enhanced luminescence intensity, leading to improved NV center excitation efficiency. Experimental results show that the optimized GO/PEDOT:PSS (40%) hybrid anode device achieves a lower turn-on voltage, with the NV center fluorescence peak intensity reaching 3.7 times that of the ITO-anode device, confirming the enhanced excitation effect through interfacial engineering of the light source. By integrating NV centers with OLED technology, this work establishes a new approach for efficient excitation. This integration approach provides a new pathway for applications such as quantum sensing. Full article

(This article belongs to the Special Issue Recent Progress in Single-Photon Generation and Detection)

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20 pages, 6699 KiB

Open AccessArticle

Low-Light Image Enhancement with Residual Diffusion Model in Wavelet Domain

by Bing Ding, Desen Bu, Bei Sun, Yinglong Wang, Wei Jiang, Xiaoyong Sun and Hanxiang Qian

Photonics 2025, 12(9), 832; https://doi.org/10.3390/photonics12090832 - 22 Aug 2025

In low-light optical imaging, the scarcity of incident photons and the inherent limitations of imaging sensors lead to challenges such as low signal-to-noise ratio, limited dynamic range, and degraded contrast, severely compromising image quality and optical information integrity. To address these challenges, we [...] Read more.

In low-light optical imaging, the scarcity of incident photons and the inherent limitations of imaging sensors lead to challenges such as low signal-to-noise ratio, limited dynamic range, and degraded contrast, severely compromising image quality and optical information integrity. To address these challenges, we propose a novel low-light image enhancement technique, LightenResDiff, which combines a residual diffusion model with the discrete wavelet transform. The core innovation of LightenResDiff lies in it accurately restoring the low-frequency components of an image through the residual diffusion model, effectively capturing and reconstructing its fundamental structure, contours, and global features. Additionally, the dual cross-coefficients recovery module (DCRM) is designed to process high-frequency components, enhancing fine details and local contrast. Moreover, the perturbation compensation module (PCM) mitigates noise sources specific to low-light optical environments, such as dark current noise and readout noise, significantly improving overall image fidelity. Experimental results across four widely-used benchmark datasets demonstrate that LightenResDiff outperforms existing methods both qualitatively and quantitatively, surpassing the current state-of-the-art techniques. Full article

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12 pages, 3310 KiB

Open AccessArticle

Resolution Enhancement in Extreme Ultraviolet Ptychography Using a Refined Illumination Probe and Small-Etendue Source

by Seungchan Moon, Junho Hong, Taeho Lee and Jinho Ahn

Photonics 2025, 12(8), 831; https://doi.org/10.3390/photonics12080831 - 21 Aug 2025

Extreme ultraviolet (EUV) ptychography is a promising actinic mask metrology technique capable of providing aberration-free images with subwavelength resolution. However, its performance is fundamentally constrained by the strong absorption of EUV light and the limited detection of high-frequency diffraction signals, which are critical [...] Read more.

Extreme ultraviolet (EUV) ptychography is a promising actinic mask metrology technique capable of providing aberration-free images with subwavelength resolution. However, its performance is fundamentally constrained by the strong absorption of EUV light and the limited detection of high-frequency diffraction signals, which are critical for resolving fine structural details. In this study, we demonstrate significant improvements in EUV ptychographic imaging by implementing an upgraded EUV source system with reduced source etendue and applying an illumination aperture to spatially refine the probe. This approach effectively enhances the photon flux and spatial coherence, resulting in an increased signal-to-noise ratio of the high-frequency diffraction components and an extended maximum detected spatial frequency. Simulations and experimental measurements using a Siemens star pattern confirmed that the refined probe enabled more robust phase retrieval and higher-resolution image reconstruction. Consequently, we achieved a half-pitch resolution of 46 nm, corresponding to a critical dimension of 11.5 nm at the wafer plane. These findings demonstrate the enhanced capability of EUV ptychography as a high-fidelity actinic metrology tool for next-generation EUV mask characterization. Full article

(This article belongs to the Special Issue Computational Imaging for Semiconductor Devices Metrology Applications)

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9 pages, 1887 KiB

Open AccessArticle

Tunable High-Power 420 nm Laser with External Cavity Frequency Doubling: Toward Efficient Rubidium Rydberg Excitation

by Zhongxiao Xu, Xin Jia, Keyu Qin, Weisen Wang, Yaoting Zhou and Donghao Li

Photonics 2025, 12(8), 830; https://doi.org/10.3390/photonics12080830 - 21 Aug 2025

The external cavity frequency doubling technique serves as a potent method for generating short-wavelength lasers, yet achieving high-power outputs remains challenging due to the thermal lens effect. This study systematically investigates the generation mechanism of the thermal lens effect and its impact on [...] Read more.

The external cavity frequency doubling technique serves as a potent method for generating short-wavelength lasers, yet achieving high-power outputs remains challenging due to the thermal lens effect. This study systematically investigates the generation mechanism of the thermal lens effect and its impact on laser performance. By optimizing the bow-tie cavity design and leveraging a large beam waist of 106 µm to suppress thermal-induced distortions, we demonstrate a tunable 420 nm laser with up to 800 mW of output power and a peak conversion efficiency of 77%. The fundamental light source, a Ti:Sa laser locked to an ultra-stable cavity, ensures a narrow linewidth, flexible tunability, and long-term frequency stability. This high-performance blue laser enables the efficient Rydberg excitation of rubidium atoms, presenting critical applications in quantum computing, quantum simulation, and quantum precision measurement. Full article

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26 pages, 3282 KiB

Open AccessReview

Linear-Mode Gain HgCdTe Avalanche Photodiodes for Weak-Target Spaceborne Photonic System

by Hui Yu, Zhichao Zhang, Ming Liu, Weirong Xing, Qing Wu, Yi Zhang, Weiting Zhang, Jialin Xu and Qiguang Tan

Photonics 2025, 12(8), 829; https://doi.org/10.3390/photonics12080829 - 20 Aug 2025

Spectroscopic observations of Earth-like exoplanets and ultra-faint galaxies–top scientific priorities for the coming decades–involve measuring broadband signals at rates of only a few photons per square meter per hour. This imposes exceptional requirements on the detector performance, necessitating dark currents below 1 e [...] Read more.

Spectroscopic observations of Earth-like exoplanets and ultra-faint galaxies–top scientific priorities for the coming decades–involve measuring broadband signals at rates of only a few photons per square meter per hour. This imposes exceptional requirements on the detector performance, necessitating dark currents below 1 e⁻/pixel/kilo second, read noise under 1 e⁻/pixel/frame, and the ability to handle large-format arrays–capabilities that are not yet met by most existing infrared detectors. In addition, spaceborne LiDAR systems require photodetectors with exceptional sensitivity, compact size, low power consumption, and multi-channel capability to facilitate long-range range finding, topographic mapping, and active spectroscopy without increasing the instrument burden. MCT Avalanche photodiodes arrays offer high internal gain, pixelation, and photon-counting performance across SW to MW wavelengths needed for multi-beam and multi-wavelength measurements, marking them as a critical enabling technology for next-generation planetary and Earth science LiDAR missions. This work reports the latest progress in developing Hg_1−xCd_xTe linear-mode e-APDs at premier industrial research institutions, including relevant experimental data, simulations and major project planning. Related studies are summarized to demonstrate the practical and iterative approach for device fabrication, which have a transformative impact on the evolution of this discipline. Full article

(This article belongs to the Special Issue Emerging Trends in Photodetector Technologies)

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18 pages, 3063 KiB

Open AccessArticle

Diffuse Correlation Blood Flow Tomography Based on Conv-TransNet Model

by Xiaojuan Zhang, Wen Yan, Peng Zhang, Xiaogang Tong, Haifeng Zhou and Yu Shang

Photonics 2025, 12(8), 828; https://doi.org/10.3390/photonics12080828 - 20 Aug 2025

Diffuse correlation tomography (DCT) is an emerging technique for detecting diseases associated with localized abnormal perfusion from near-infrared light intensity temporal autocorrelation functions (g₂(τ)). However, a critical drawback of traditional reconstruction methods is the imbalance between optical measurements [...] Read more.

Diffuse correlation tomography (DCT) is an emerging technique for detecting diseases associated with localized abnormal perfusion from near-infrared light intensity temporal autocorrelation functions (g₂(τ)). However, a critical drawback of traditional reconstruction methods is the imbalance between optical measurements and the voxels to be reconstructed. To address this issue, this paper proposes Conv-TransNet, a convolutional neural network (CNN)–Transformer hybrid model that directly maps g₂(τ) data to blood flow index (BFI) images. For model training and testing, we constructed a dataset of 18,000 pairs of noise-free and noisy g₂(τ) data with their corresponding BFI images. In simulation validation, the root mean squared error (RMSE) for the five types of anomalies with noisy data are 2.13%, 4.43%, 2.15%, 4.05%, and 4.39%, respectively. The MJR (misjudgment ratio)of them are close to zero. In the phantom experiments, the CONTRAST of the quasi-solid cross-shaped anomaly reached 0.59, with an MJR of 2.21%. Compared with the traditional Nth-order linearization (NL) algorithm, the average CONTRAST of the speed-varied liquid tubular anomaly increased by 0.55. These metrics also demonstrate the superior performance of our method over traditional CNN-based approaches. The experimental results indicate that the Conv-TransNet model would achieve more accurate and robust reconstruction, suggesting its potential as an alternative for blood flow imaging. Full article

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

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19 pages, 1299 KiB

Open AccessArticle

Structured Emission and Entanglement Dynamics of a Giant Atom in a Photonic Creutz Ladder

by Vassilios Yannopapas

Photonics 2025, 12(8), 827; https://doi.org/10.3390/photonics12080827 - 20 Aug 2025

We explore the spontaneous emission dynamics of a giant atom coupled to a photonic Creutz ladder, focusing on how flat-band frustration and synthetic gauge fields shape atom–photon interactions. The Creutz ladder exhibits perfectly flat bands, Aharonov–Bohm caging, and topological features arising from its [...] Read more.

We explore the spontaneous emission dynamics of a giant atom coupled to a photonic Creutz ladder, focusing on how flat-band frustration and synthetic gauge fields shape atom–photon interactions. The Creutz ladder exhibits perfectly flat bands, Aharonov–Bohm caging, and topological features arising from its nontrivial hopping structure. By embedding the giant atom at multiple spatially separated sites, we reveal interference-driven emission control and the formation of nonradiative bound states. Using both spectral and time-domain analyses, we uncover strong non-Markovian dynamics characterized by persistent oscillations, long-lived entanglement, and recoherence cycles. The emergence of bound-state poles in the spectral function is accompanied by spatially localized photonic profiles and directionally asymmetric emission, even in the absence of band dispersion. Calculations of von Neumann entropy and atomic purity confirm the formation of coherence-preserving dressed states in the flat-band regime. Furthermore, the spacetime structure of the emitted field displays robust zig-zag interference patterns and synthetic chirality, underscoring the role of geometry and topology in photon transport. Our results demonstrate how flat-band photonic lattices can be leveraged to engineer tunable atom–photon entanglement, suppress radiative losses, and create structured decoherence-free subspaces for quantum information applications. Full article

(This article belongs to the Special Issue Recent Progress in Optical Quantum Information and Communication)

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13 pages, 690 KiB

Open AccessArticle

Design and Optimization of Polarization-Maintaining Low-Loss Hollow-Core Anti-Resonant Fibers Based on a Multi-Objective Genetic Algorithm

by Zhiling Li, Yingwei Qin, Jingjing Ren, Xiaodong Huang and Yanan Bao

Photonics 2025, 12(8), 826; https://doi.org/10.3390/photonics12080826 - 20 Aug 2025

In this work, a novel polarization-maintaining hollow-core fiber structure featuring a semi-circular nested dual-ring geometry is proposed. To simultaneously optimize two inherently conflicting performance metrics, namely, birefringence and confinement loss, a multi objective genetic algorithm is employed for geometric parameter tuning, resulting in [...] Read more.

In this work, a novel polarization-maintaining hollow-core fiber structure featuring a semi-circular nested dual-ring geometry is proposed. To simultaneously optimize two inherently conflicting performance metrics, namely, birefringence and confinement loss, a multi objective genetic algorithm is employed for geometric parameter tuning, resulting in a set of Pareto-optimal solutions. At the target wavelength of 1550 nm, the first optimal design achieves birefringence exceeding

1 \times 10^{- 4}

over a 1275 nm bandwidth while maintaining confinement loss around

10^{0}

dB/m; the second design maintains birefringence above

1 \times 10^{- 4}

across a 1000 nm spectral range, with confinement loss on the order of

10^{- 1}

dB/m. These optimized designs offer a promising approach for improving the performance of polarization-sensitive applications such as interferometric sensing and high coherence laser systems. The results confirm the suitability of multi-objective genetic algorithms for integrated multi-objective fiber optimization and provide a new strategy for designing low-loss and high-birefringence fiber devices. Full article

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14 pages, 4281 KiB

Open AccessArticle

Joint Rx IQ Imbalance Compensation and Timing Recovery for Faster-than-Nyquist WDM Systems

by Jialin You

Photonics 2025, 12(8), 825; https://doi.org/10.3390/photonics12080825 - 19 Aug 2025

Faster-than-Nyquist (FTN) tight filtering introduces serious inter-symbol interference (ISI) impairment, leading to an insufficient compensation range for conventional IQ imbalance compensation algorithms. Furthermore, receiver (Rx) IQ imbalance and ISI impairments significantly increase the convergence cost required by the squared Gardner phase detector (SGPD) [...] Read more.

Faster-than-Nyquist (FTN) tight filtering introduces serious inter-symbol interference (ISI) impairment, leading to an insufficient compensation range for conventional IQ imbalance compensation algorithms. Furthermore, receiver (Rx) IQ imbalance and ISI impairments significantly increase the convergence cost required by the squared Gardner phase detector (SGPD) timing recovery algorithm to establish a timing synchronization loop. This paper proposes a joint Rx IQ compensation and timing recovery scheme. By embedding a two-stage IQ imbalance compensation algorithm into the timing recovery feedback loop, the proposed scheme could effectively estimate and compensate for Rx IQ imbalance. Meanwhile, thanks to the innovative scheme, which equalizes Rx IQ imbalance and ISI during the timing feedback loop, the convergence cost of timing recovery could be reduced compared with the conventional blind frequency domain (BFD) scheme. The simulation results of 128 GBaud polarization multiplexing (PM) 16-quadrature amplitude modulation (QAM) FTN wavelength division multiplexing (WDM) transmission systems demonstrate that the proposed scheme could bring about 14%, 12.5%, and 16.6% improvements in the compensation range for Rx IQ amplitude imbalance, phase imbalance, and skew, respectively, compared with the conventional one. Meanwhile, the convergence cost is reduced by at least 31% with a 0.9 acceleration factor. In addition, 40 GBaud PM-16QAM FTN experiment results show that the proposed scheme could bring about a 0.8 dB improvement in the optical signal noise ratio (OSNR) compared with the conventional BFD scheme. Full article

(This article belongs to the Special Issue Optical Communication Networks: Challenges and Opportunities)

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10 pages, 1397 KiB

Open AccessArticle

Encoding and Verification of Composite Vortex Beams with Spaced Orbital Angular Momentum

by Tianpeng Xu, Xinping Han, Xiaodie Wang, Sichen Lei, Pengfei Wu and Huiqin Wang

Photonics 2025, 12(8), 824; https://doi.org/10.3390/photonics12080824 - 19 Aug 2025

A novel encoding method based on the orbital angular momentum (OAM) mode and radial mode of composite vortex beams is proposed. The superposition of two vortex beams generates 32 different types of composite vortex beams: one of them is a Laguerre–Gaussian (LG) beam [...] Read more.

A novel encoding method based on the orbital angular momentum (OAM) mode and radial mode of composite vortex beams is proposed. The superposition of two vortex beams generates 32 different types of composite vortex beams: one of them is a Laguerre–Gaussian (LG) beam with a fixed OAM mode and radial mode, and the other is a LG beam containing four radial modes (p = 0, 1, 2, 3) and eight OAM modes with the same interval (l = ±3, ±5, ±7, ±9). A specially designed composite fork-shaped grating (CFG) is utilized to generate the intensity array pattern, and the received composite vortex beam is diffracted into a Gaussian beam with the relevant coordinates. Based on the coordinates and the number of bright rings in the intensity pattern, the OAM modes and radial modes of the two vortex beams composing the superposition state are determined, and finally the received composite vortex beam is decoded into the initially propagated information sequence. The correctness and effectiveness of the proposed encoding are confirmed through the comparative analysis of the correlation of the optical fields at both the transmitter and receiver in the two scenarios of interval and non-interval encoding. The proposed encoding method can significantly improve the efficiency of information transmission and its resistance to interference, holding great potential for future applications in free-space optical communication. Full article

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22 pages, 9617 KiB

Open AccessArticle

An Improved PCA and Jacobian-Enhanced Whale Optimization Collaborative Method for Point Cloud Registration

by Haiman Chu, Jingjing Fan, Zai Luo, Yinbao Cheng, Yingqi Tang and Yaru Li

Photonics 2025, 12(8), 823; https://doi.org/10.3390/photonics12080823 - 19 Aug 2025

Scanned data often contain substantial outliers due to environmental interference, which drastically decreases the performance of traditional registration algorithms. To address this issue, this article proposes an improved principal component analysis (PCA) and Jacobian-enhanced whale optimization collaborative method for point cloud registration. First, [...] Read more.

Scanned data often contain substantial outliers due to environmental interference, which drastically decreases the performance of traditional registration algorithms. To address this issue, this article proposes an improved principal component analysis (PCA) and Jacobian-enhanced whale optimization collaborative method for point cloud registration. First, an improved PCA point cloud initial registration algorithm is proposed by introducing the normal vector local information to set the screening conditions. This algorithm can streamline the original set of 48 candidate rotation matrices down to 4, achieving rapid point cloud registration at the data level between the scanned and model point clouds. Second, a Jacobian whale optimization algorithm for fine registration (JWOA-FR) is proposed by incorporating local gradient information. The algorithm employs gradient descent on optimal whale individuals to dynamically guide global search updates, thereby enhancing both registration accuracy and efficiency. Finally, a threshold is set to remove the outliers contained in the workpieces based on the information of the matched point pairs. The iterative closest point (ICP) algorithm is further used to improve registration accuracy for data without outliers. The experimental results showed that registration errors of large workpieces 1, 2, and 3 were 2.0755 mm, 2.3955 mm, and 2.5823 mm, respectively, after outlier removal, which indicates that the proposed method is applicable to data with outliers, and the registration accuracy meets the requirements. Full article

(This article belongs to the Special Issue Advancements in Optics and Laser Measurement)

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9 pages, 807 KiB

Open AccessCommunication

Optimization of the Saturable Absorption of 2D Bi₂Te₃ Layers

by Nayla Jimenez de la Vega, Arjun Karimbana Kandy, Fabien Lemarchand, Andrea Campos, Martiane Cabié, Carine Perrin-Pellegrino, Julien Lumeau, Jean-Yves Natoli and Konstantinos Iliopoulos

Photonics 2025, 12(8), 822; https://doi.org/10.3390/photonics12080822 - 19 Aug 2025

The saturable absorption of 2D Bi₂Te₃ layers is studied by using the Z-scan technique employing infrared 400 fs laser pulses. Optimization of the nonlinearities has been carried out by measuring the third-order nonlinear susceptibilities as a function of the film [...] Read more.

The saturable absorption of 2D Bi₂Te₃ layers is studied by using the Z-scan technique employing infrared 400 fs laser pulses. Optimization of the nonlinearities has been carried out by measuring the third-order nonlinear susceptibilities as a function of the film thickness. A thorough optimization of the thin film annealing conditions has been performed and is presented. For each thickness, the annealing parameters have been separately investigated. Scanning electron microscopy, X-ray diffraction, and UV-Vis spectrophotometry studies have also been performed on the as-deposited and crystallized 2D layers. Full article

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