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Photonics
Volume 12
Issue 8
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Photonics, Volume 12, Issue 8 (August 2025) – 69 articles

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16 pages, 2415 KiB  
Review
Recycling Technologies for Extracting Gallium from Light-Emitting Diodes
by Laraib Mustafa, Muhammad Usman, Shazma Ali, Ahmed Ali and Anis Naveed
Photonics 2025, 12(8), 808; https://doi.org/10.3390/photonics12080808 - 12 Aug 2025
Abstract
Light-emitting diodes (LEDs) are made up of precious metals, e.g., gallium. These elements can be recovered and reused, reducing the need for new raw materials. Proper recycling prevents harmful substances in LEDs, such as lead and arsenic, from contaminating the environment. Recycling LEDs [...] Read more.
Light-emitting diodes (LEDs) are made up of precious metals, e.g., gallium. These elements can be recovered and reused, reducing the need for new raw materials. Proper recycling prevents harmful substances in LEDs, such as lead and arsenic, from contaminating the environment. Recycling LEDs uses less energy compared to producing new ones, leading to lower carbon emissions. The valuable metal gallium faces the challenge of supply and demand due to the surge in its demand, the difficulty of separating it from minerals, and processing issues during extraction. In this review, we describe the methods for recycling gallium from LEDs by using different techniques such as pyrolysis (95% recovery), oxalic acid leaching (83.2% recovery), HCL acid leaching of coal fly ash (90–95% recovery), subcritical water treatment (80.5% recovery), supercritical ethanol (93.10% recovery), oxidation and subsequent leaching (91.4% recovery), and vacuum metallurgy separation (90% recovery). Based on our analysis, hydrometallurgy is the best approach for recovering gallium. It is reported that approximately 5% of the waste from LEDs is adequately recycled, whereas the total gallium potential wasted throughout production is over 93%. By recycling LEDs, we can minimize waste, conserve resources, and promote sustainable practices. Thus, recycling LEDs is essential for strengthening a circular economy. Full article
(This article belongs to the Special Issue Light-Emitting Diodes: Technology, Advances, Challenges and Applications)
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13 pages, 2876 KiB  
Article
Comparative Study of Raman Laser Generation Techniques in Cold Atomic Gravimeters
by Rui Xu, Fangjun Qin, Zhichao Ding, Hao Chen, An Li, Dongyi Li, Jiaqing Sun, Haibo Zhang and Chenxi Ge
Photonics 2025, 12(8), 807; https://doi.org/10.3390/photonics12080807 - 11 Aug 2025
Abstract
In the measurement process of cold atomic gravimeters, Raman laser plays an important role both in the state preparation stage and in the atomic interference stage. This paper discusses Raman laser generation techniques. The optical phase-locked loop (OPLL) method and the electro-optical modulation [...] Read more.
In the measurement process of cold atomic gravimeters, Raman laser plays an important role both in the state preparation stage and in the atomic interference stage. This paper discusses Raman laser generation techniques. The optical phase-locked loop (OPLL) method and the electro-optical modulation (EOM) method are compared from a theoretical point of view. An OPLL system and an EOM system were constructed separately. The two schemes were tested in terms of linewidth, phase noise and long-term stability. The experimental results were analyzed and discussed. Based on the results, recommendations are given for the selection of Raman laser schemes under different scenarios. Full article
(This article belongs to the Section New Applications Enabled by Photonics Technologies and Systems)
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13 pages, 3051 KiB  
Article
Impact of LD Spectra on Efficiency of Yb-Doped Fiber Laser
by Fengyun Li, Yi Shi, Chun Zhang, Qiuhui Chu, Lingli Huang, Haoyu Zhang, Qiang Shu, Yu Wen, Xingchen Jiang, Zixiang Gao, Honghuan Lin and Rumao Tao
Photonics 2025, 12(8), 806; https://doi.org/10.3390/photonics12080806 - 11 Aug 2025
Abstract
The spectral characteristics of pump laser diodes (LDs) introduce significant ambiguity into the performance evaluation of high-power ytterbium-doped fiber lasers (YDFLs), obscuring their intrinsic efficiency and hindering reliable system design. Here, we introduce a rigorous quantitative framework that decouples these pump-induced effects by [...] Read more.
The spectral characteristics of pump laser diodes (LDs) introduce significant ambiguity into the performance evaluation of high-power ytterbium-doped fiber lasers (YDFLs), obscuring their intrinsic efficiency and hindering reliable system design. Here, we introduce a rigorous quantitative framework that decouples these pump-induced effects by referencing laser performance to the absorbed, rather than the launched, pump power. Our analysis demonstrates that the widely reported discrepancies in conventional optical-to-optical (OO) and slope efficiencies are governed almost entirely by variations in pump absorption, while the influence of the quantum defect is negligible. This approach provides a robust metric for intrinsic laser performance that is independent of the LD’s spectral properties, proving particularly valuable for systems pumped by non-wavelength-stabilized LDs (nWS-LDs). We uncover a non-monotonic evolution of the unabsorbed residual pump power, revealing that the peak thermal load on system components occurs at an intermediate operational state, not at maximum pump power. This finding challenges conventional thermal management strategies and is critical for ensuring the long-term operational reliability of high-power YDFLs. Full article
(This article belongs to the Special Issue Advancements in High-Power Optical Fibers and Fiber Lasers)
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14 pages, 2038 KiB  
Article
Electro-Optic Toffoli Logic Based on Hybrid Surface Plasmons
by Zhixun Liang, Yunying Shi, Yunfei Yi and Yuanyuan Fan
Photonics 2025, 12(8), 805; https://doi.org/10.3390/photonics12080805 - 11 Aug 2025
Abstract
Reversible gates theoretically do not result in energy loss during the calculation process. The Toffoli gate is a universal reversible logic gate, and any reversible circuit can be constructed from the Toffoli gate. This paper presents a hybrid electro-optic Toffoli logic that uses [...] Read more.
Reversible gates theoretically do not result in energy loss during the calculation process. The Toffoli gate is a universal reversible logic gate, and any reversible circuit can be constructed from the Toffoli gate. This paper presents a hybrid electro-optic Toffoli logic that uses an HSPP Switch (hybrid surface plasmon polariton switch), waveguide coupler, and Y-shaped splitter. The hybrid electro-optic Toffoli logic operation is applied via voltage regulation, and the FDTD simulation is used for this research. The modeling and simulation results show that the device’s operating speed is up to 61.62 GHz; the power consumption for transmitting 1 bit is only 13.44 fJ; the average insertion loss is 6.4 dB, and the average extinction ratio of each output port is 19.7 dB. Full article
(This article belongs to the Special Issue Thermal Radiation and Micro-/Nanophotonics)
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12 pages, 2763 KiB  
Article
Damage Characteristics of Silicon Solar Cells Induced by Nanosecond Pulsed Laser
by Hao Chang, Weijing Zhou, Yingjie Ma, Zhilong Jian, Xiaoyuan Quan and Chenyu Xiao
Photonics 2025, 12(8), 804; https://doi.org/10.3390/photonics12080804 - 11 Aug 2025
Abstract
The damage characteristics of monocrystalline silicon solar cells irradiated by a nanosecond pulsed laser were investigated in a vacuum environment. An 8 ns pulsed laser was used with a 1064 nm wavelength, a 2.0 J maximum pulse energy, and a millimeter-scale ablation spot [...] Read more.
The damage characteristics of monocrystalline silicon solar cells irradiated by a nanosecond pulsed laser were investigated in a vacuum environment. An 8 ns pulsed laser was used with a 1064 nm wavelength, a 2.0 J maximum pulse energy, and a millimeter-scale ablation spot diameter. The cells were irradiated by a laser with varying fluences, irradiation positions, and pulse numbers. The damage mechanism was discussed in combination with the degradation of electrical properties, the morphology of surface damage, and electroluminescence images. A single pulse mainly caused surface heating and deformation, while multi-pulse irradiation led to the formation of melting ablation craters. More severe performance degradation was caused by irradiation at the grid line site due to fracture of the grid line electrodes. Moreover, monocrystalline silicon cells showed excellent damage resistance to fixed-position irradiations at non-gridded line areas. This work reveals, for the first time in vacuum, that grid-line fracture dominates performance degradation—enabling targeted hardening for space solar cells. Full article
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17 pages, 2652 KiB  
Article
First-Principles and Device-Level Investigation of β-AgGaO2 Ferroelectric Semiconductors for Photovoltaic Applications
by Wen-Jie Hu, Xin-Yu Zhang, Xiao-Tong Zhu, Yan-Li Hu, Hua-Kai Xu, Xiang-Fu Xu, You-Da Che, Xing-Yuan Chen, Li-Ting Niu and Bing Dai
Photonics 2025, 12(8), 803; https://doi.org/10.3390/photonics12080803 - 11 Aug 2025
Abstract
Ferroelectric semiconductors, with their inherent spontaneous polarization, present a promising approach for efficient charge separation, making them attractive for photovoltaic applications. The potential of β-AgGaO2, a polar ternary oxide with an orthorhombic Pna21 structure, as a light-absorbing material is evaluated. [...] Read more.
Ferroelectric semiconductors, with their inherent spontaneous polarization, present a promising approach for efficient charge separation, making them attractive for photovoltaic applications. The potential of β-AgGaO2, a polar ternary oxide with an orthorhombic Pna21 structure, as a light-absorbing material is evaluated. First-principles computational analysis reveals that β-AgGaO2 possesses an indirect bandgap of 2.1 eV and exhibits pronounced absorption within the visible spectral range. Optical simulations suggest that a 300 nm thick absorber layer could theoretically achieve a power conversion efficiency (PCE) of 20%. Device-level simulations using SCAPS-1D evaluate the influence of hole and electron transport layers on solar cell performance. Among the tested hole transport materials, Cu2FeSnS4 (CFTS) achieves the highest PCE of 14%, attributed to its optimized valence band alignment and reduced recombination losses. In contrast, no significant improvements were observed with the electron transport layers tested. These findings indicate the potential of β-AgGaO2 as a ferroelectric photovoltaic absorber and emphasize the importance of band alignment and interface engineering for optimizing device performance. Full article
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9 pages, 1588 KiB  
Communication
Sub-60 fs, 1300 nm Laser Pulses Generation from Soliton Self-Frequency Shift Pumped by Femtosecond Yb-Doped Fiber Laser
by Hongyuan Xuan, Kong Gao, Xingyang Zou, Ze Zhang, Wenchao Qiao and Yizhou Liu
Photonics 2025, 12(8), 802; https://doi.org/10.3390/photonics12080802 - 10 Aug 2025
Viewed by 65
Abstract
We report on the generation of 1300 nm ultrashort laser pulses via the soliton self-frequency shift in a high-nonlinearity fiber, pumped by the 41.9 MHz, 67.9 fs, 1073 nm femtosecond laser emitted from an Yb-doped fiber laser system. A numerical simulation was applied [...] Read more.
We report on the generation of 1300 nm ultrashort laser pulses via the soliton self-frequency shift in a high-nonlinearity fiber, pumped by the 41.9 MHz, 67.9 fs, 1073 nm femtosecond laser emitted from an Yb-doped fiber laser system. A numerical simulation was applied to investigate the spectral broadening process driven by the soliton self-frequency shift with increased pump power. The experimental results are in good agreement with the numerical results, delivering a 33 mW, 57.8 fs 1300 nm Raman soliton filtered by a longpass filter. The impact of the polarization direction of the injected pump laser on the soliton self-frequency shift process was also further investigated. The root means squares of the Yb-doped fiber laser and the nonlinearly spectral broadened laser were 0.19%@1h and 0.23%@1h, respectively. Full article
(This article belongs to the Special Issue Ultrafast Optics: From Fundamental Dynamics to Transformative Technologies)
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12 pages, 4710 KiB  
Article
Generation of Higher-Order Hermite–Gaussian Modes Based on Physical Model and Deep Learning
by Tai Chen, Chengcai Jiang, Jia Tao, Long Ma and Longzhou Cao
Photonics 2025, 12(8), 801; https://doi.org/10.3390/photonics12080801 - 10 Aug 2025
Viewed by 75
Abstract
The higher-order Hermite–Gaussian (HG) modes exhibit complex spatial distributions and find a wide range of applications in fields such as quantum information processing, optical communications, and precision measurements. In recent years, the advancement of deep learning has emerged as an effective approach for [...] Read more.
The higher-order Hermite–Gaussian (HG) modes exhibit complex spatial distributions and find a wide range of applications in fields such as quantum information processing, optical communications, and precision measurements. In recent years, the advancement of deep learning has emerged as an effective approach for generating higher-order HG modes. However, the traditional data-driven deep learning method necessitates a substantial amount of labeled data for training, entails a lengthy data acquisition process, and imposes stringent requirements on system stability. In practical applications, these methods are confronted with challenges such as the high cost of data labeling. This paper proposes a method that integrates a physical model with deep learning. By utilizing only a single intensity distribution of the target optical field and incorporating the physical model, the training of the neural network can be accomplished, thereby eliminating the dependency of traditional data-driven deep learning methods on large datasets. Experimental results demonstrate that, compared with the traditional data-driven deep learning method, the method proposed in this paper yields a smaller root mean squared error between the generated higher-order HG modes. The quality of the generated modes is higher, while the training time of the neural network is shorter, indicating greater efficiency. By incorporating the physical model into deep learning, this approach overcomes the limitations of traditional deep learning methods, offering a novel solution for applying deep learning in light field manipulation, quantum physics, and other related fields. Full article
(This article belongs to the Section Data-Science Based Techniques in Photonics)
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16 pages, 2069 KiB  
Article
High-Efficiency Mid-Infrared Transmission Modulator Based on Graphene Plasmon Resonance and Photonic Crystal Defect States
by Jiduo Dong, Qing Zang, Linlong Tang, Binbin Wei, Xiangxing Bai, Hao Zhang, Chunheng Liu, Haofei Shi, Hongyan Shi, Yang Liu and Yueguang Lu
Photonics 2025, 12(8), 800; https://doi.org/10.3390/photonics12080800 - 9 Aug 2025
Viewed by 129
Abstract
With the continuous exploration and advancement of communication frequency bands, terahertz and mid-to-far-infrared communication systems have attracted significant attention in recent years. Modulators are essential components in these systems, making the enhancement of modulator performance in the infrared and terahertz bands a prominent [...] Read more.
With the continuous exploration and advancement of communication frequency bands, terahertz and mid-to-far-infrared communication systems have attracted significant attention in recent years. Modulators are essential components in these systems, making the enhancement of modulator performance in the infrared and terahertz bands a prominent research focus. In this study, we propose a high-performance infrared transmission-type modulator based on the plasmon resonance effect of graphene nanoribbons. This design synergistically exploits near-field enhancement from metal slits and defect states in one-dimensional photonic crystals to strengthen light–graphene interactions. The modulator achieves a modulation depth exceeding 80% and an operating bandwidth greater than 4 THz in the mid-infrared range, enabling efficient signal modulation for free-space optical communication. Importantly, the proposed design alleviates experimental challenges typically associated with the need for high graphene mobility and a wide Fermi energy tuning range in conventional approaches, thereby improving its practical feasibility. Moreover, the approach is scalable to far-infrared and terahertz bands, offering valuable insights for advancing signal modulation technologies across these spectral regions. Full article
(This article belongs to the Special Issue Metamaterials and Nanophotonics: Fundamentals and Applications)
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14 pages, 2685 KiB  
Article
In Vivo Optical Coherence Tomography for Diagnostic Characterization of Enamel Defects in Molar Incisor Hypomineralization: A Case-Control Study
by Fortunato Buttacavoli, Clara Buttacavoli, Giovanna Giuliana, Giuseppina Campisi and Vera Panzarella
Photonics 2025, 12(8), 799; https://doi.org/10.3390/photonics12080799 - 9 Aug 2025
Viewed by 124
Abstract
Molar incisor hypomineralization (MIH) is characterized by systemic hypomineralization affecting one to four first permanent molars (FPMs), often accompanied by lesions in incisors and potentially involving other primary or permanent teeth. MIH poses clinical challenges, including hypersensitivity, susceptibility to pulp involvement, and aesthetic [...] Read more.
Molar incisor hypomineralization (MIH) is characterized by systemic hypomineralization affecting one to four first permanent molars (FPMs), often accompanied by lesions in incisors and potentially involving other primary or permanent teeth. MIH poses clinical challenges, including hypersensitivity, susceptibility to pulp involvement, and aesthetic concerns. Optical Coherence Tomography (OCT), an advanced, non-invasive imaging modality, has gained interest as a potential diagnostic tool in dentistry. This exploratory observational case-control study aims to compare the structural characteristics of MIH-affected and healthy teeth using in vivo OCT, focusing on identifying qualitative imaging patterns associated with enamel hypomineralization. This study included 50 mild MIH-affected permanent teeth from pediatric patients and 50 healthy permanent teeth as controls. Representative OCT scans were acquired, analyzed, and compared for both groups. In OCT imaging, healthy enamel and dentin appeared as two distinct superimposed layers defined by the dentin-enamel junction. Conversely, MIH-affected teeth exhibited characteristic subsurface hyper-reflective zones, indicative of hypomineralized enamel, with deeper hypo-reflective shadowing. This first in vivo study applying OCT to MIH-affected teeth demonstrates its potential as a non-invasive technique for the real-time assessment of enamel structural anomalies, supporting its future role in monitoring remineralization therapies and improving early detection strategies in pediatric dental care. Full article
(This article belongs to the Special Issue New Perspectives in Biomedical Optics and Optical Imaging)
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17 pages, 5039 KiB  
Article
Enhancement of Self-Collimation via Nonlinear Symmetry Breaking in Hexagonal Photonic Crystals
by Ozgur Onder Karakilinc
Photonics 2025, 12(8), 798; https://doi.org/10.3390/photonics12080798 - 8 Aug 2025
Viewed by 108
Abstract
This study proposes the use of a low-symmetry hexagonal photonic crystal (LSHPC) incorporating Kerr-type nonlinearity to enhance self-collimation. The equifrequency contours (EFCs) of a C2-symmetric LSHPC composed of nonlinear LiNbO3 rods are analyzed as a function of the nonlinear refractive [...] Read more.
This study proposes the use of a low-symmetry hexagonal photonic crystal (LSHPC) incorporating Kerr-type nonlinearity to enhance self-collimation. The equifrequency contours (EFCs) of a C2-symmetric LSHPC composed of nonlinear LiNbO3 rods are analyzed as a function of the nonlinear refractive index. The self-collimation characteristics, transmission spectrum, group velocity dispersion (GVD), and third-order dispersion (TOD) are investigated using the Plane Wave Expansion (PWE) and Finite Difference Time Domain (FDTD) methods. The results demonstrate that increasing the nonlinear index leads to a significant flattening of the EFCs, which enhances self-collimation performance. Furthermore, symmetry-lowering perturbations improve beam confinement and enable all-angle self-collimation. These findings highlight the potential of Kerr-type nonlinear photonic crystals for integrated photonic circuits requiring precise control over light propagation. Full article
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13 pages, 1888 KiB  
Article
Femtosecond-Laser Direct Writing of Double-Line and Tubular Depressed-Cladding Waveguides in Ultra-Low-Expansion Glass
by Yuhao Wu, Sixuan Guo, Guanghua Cheng, Feiran Wang, Xu Wang and Yunjie Zhang
Photonics 2025, 12(8), 797; https://doi.org/10.3390/photonics12080797 - 8 Aug 2025
Viewed by 186
Abstract
Addressing the stability requirements of photonic integrated devices operating over wide temperature ranges, this work achieves controlled fabrication of femtosecond-laser direct-written Type II double-line waveguides and Type III depressed-cladding tubular waveguides within ultra-low-expansion LAS glass-ceramics. The light-guiding mechanisms were elucidated through finite element [...] Read more.
Addressing the stability requirements of photonic integrated devices operating over wide temperature ranges, this work achieves controlled fabrication of femtosecond-laser direct-written Type II double-line waveguides and Type III depressed-cladding tubular waveguides within ultra-low-expansion LAS glass-ceramics. The light-guiding mechanisms were elucidated through finite element modeling. The influences of laser writing parameters and waveguide geometric structures on guiding performance were systematically investigated. Experimental results demonstrate that the double-line waveguides exhibit optimal single-mode guiding performance at 30 μm spacing and 120 mW writing power. For the tubular depressed-cladding waveguides, both single-mode and multi-mode fields are attainable across a broad processing parameter window. Large-mode-area characteristics manifested in the 50 μm core waveguide, exhibiting an edge-shifted intensity profile for higher-order modes that generated a hollow beam, enabling applications in atom guidance and particle trapping. Full article
(This article belongs to the Special Issue Direct Ultrafast Laser Writing in Photonics and Optoelectronics)
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18 pages, 5585 KiB  
Article
A CNN-GS Hybrid Algorithm for Generating Pump Light Fields in Atomic Magnetometers
by Miaohui Song, Ying Liu, Feijie Lu, Qian Cao and Yueyang Zhai
Photonics 2025, 12(8), 796; https://doi.org/10.3390/photonics12080796 - 7 Aug 2025
Viewed by 137
Abstract
Atomic magnetometers (AMs), recognized for their ultra-high magnetic sensitivity, demand highly uniform pump light fields to maximize measurement accuracy. In this paper, a phase modulation-based method using convolutional neural networks (CNN) and the Gerchberg–Saxton (GS) algorithm is proposed to generate the pumping light [...] Read more.
Atomic magnetometers (AMs), recognized for their ultra-high magnetic sensitivity, demand highly uniform pump light fields to maximize measurement accuracy. In this paper, a phase modulation-based method using convolutional neural networks (CNN) and the Gerchberg–Saxton (GS) algorithm is proposed to generate the pumping light field, and the model was trained using a supervised learning approach with a custom dataset. The specific training settings are as follows: the backpropagation algorithm was adopted as the training algorithm, and the Adam optimization method was used for network training, with a learning rate of 0.001 and a total of 100 training epochs, utilizing a liquid crystal spatial light modulator (LCSLM) to regulate the light field phase distribution dynamically. By transforming Gaussian beams into flat-top beams, the method significantly enhances polarization uniformity within vapor cells, leading to improved magnetometric sensitivity. The proposed hybrid algorithm reduces the mean square error from 35% to 19% and peak non-uniformity from 21% to 7.6%. A reflective LCSLM-based optical setup is implemented to produce circular and square flat-top beams with a measured non-uniformity of 5.1%, resulting in an enhancement of magnetic sensitivity from 14.04 fT/Hz1/2 to 7.80 fT/Hz1/2. Full article
(This article belongs to the Special Issue Advancements in Optical Techniques for Enhancing the Sensitivity and Precision of Atomic Magnetometers)
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13 pages, 1269 KiB  
Article
Contrast-Enhancing Spatial–Frequency Deconvolution-Aided Interferometric Scattering Microscopy (iSCAT)
by Xiang Zhang and Hao He
Photonics 2025, 12(8), 795; https://doi.org/10.3390/photonics12080795 - 7 Aug 2025
Viewed by 172
Abstract
Interferometric scattering microscopy (iSCAT) is widely used for label-free tracking of nanoparticles and single molecules. However, its ability to identify small molecules is limited by low imaging contrast blurred with noise. Frame-averaging methods are widely used for reducing background noise but require hundreds [...] Read more.
Interferometric scattering microscopy (iSCAT) is widely used for label-free tracking of nanoparticles and single molecules. However, its ability to identify small molecules is limited by low imaging contrast blurred with noise. Frame-averaging methods are widely used for reducing background noise but require hundreds of frames to produce a single frame as a trade-off. To address this, we applied a spatial–frequency domain deconvolution algorithm to suppress background noise and amplify the signal for each frame, achieving an improvement of ∼ 3-fold without hardware modification. This enhancement is achieved by compensating for missing information within the optical transfer function (OTF) boundary, while high-frequency components (noise) beyond this boundary are filtered. The resulting deconvolution process provides linear signal amplification, making it ideal for quantitative analysis in mass photometry. Additionally, the localization error is reduced by 20%. Comparisons with traditional denoising algorithms revealed that these methods often extract the side lobes. In contrast, our deconvolution approach preserves signal integrity while enhancing sensitivity. This work highlights the potential of image processing techniques to significantly improve the detection sensitivity of iSCAT for small molecule analysis. Full article
(This article belongs to the Special Issue Research, Development and Application of Raman Scattering Technology)
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15 pages, 4886 KiB  
Article
Fabrication of Diffractive Optical Elements to Generate Square Focal Spots via Direct Laser Lithography and Machine Learning
by Hieu Tran Doan Trung, Young-Sik Ghim and Hyug-Gyo Rhee
Photonics 2025, 12(8), 794; https://doi.org/10.3390/photonics12080794 - 6 Aug 2025
Viewed by 210
Abstract
Recently, diffractive optics systems have garnered increasing attention due to their myriad benefits in various applications, such as creating vortex beams, Bessel beams, or optical traps, while refractive optics systems still exhibit some disadvantages related to materials, substrates, and intensity shapes. The manufacturing [...] Read more.
Recently, diffractive optics systems have garnered increasing attention due to their myriad benefits in various applications, such as creating vortex beams, Bessel beams, or optical traps, while refractive optics systems still exhibit some disadvantages related to materials, substrates, and intensity shapes. The manufacturing of diffractive optical elements has become easier due to the development of lithography techniques such as direct laser writing, photo lithography, and electron beam lithography. In this paper, we improve the results from previous research and propose a new methodology to design and fabricate advanced binary diffractive optical elements that achieve a square focal spot independently, reducing reliance on additional components. By integrating a binary square zone plate with an axicon zone plate of the same scale, we employ machine learning for laser path optimization and direct laser lithography for manufacturing. This streamlined approach enhances simplicity, accuracy, efficiency, and cost effectiveness. Our upgraded binary diffractive optical elements are ready for real-world applications, marking a significant improvement in optical capabilities. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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12 pages, 1583 KiB  
Article
Photothermal Performance Testing of Lithium Niobate After Ion Beam Radiation
by Junyi Liu, Daiyong Lin, Xing Peng, Yao Wu, Jian Li, Ziqiang Hu, Zhixuan He, Jiaqi Wang, Yuxia Tan, Xiaoshu Xu and Shuo Qiao
Photonics 2025, 12(8), 793; https://doi.org/10.3390/photonics12080793 - 6 Aug 2025
Viewed by 193
Abstract
To investigate the evolution of the optothermal properties of lithium niobate with ion beam irradiation parameters, the thermal effect theory was analyzed, and ion beam irradiation technology was used to modify lithium niobate samples. The transmittance of lithium niobate crystals after ion beam [...] Read more.
To investigate the evolution of the optothermal properties of lithium niobate with ion beam irradiation parameters, the thermal effect theory was analyzed, and ion beam irradiation technology was used to modify lithium niobate samples. The transmittance of lithium niobate crystals after ion beam irradiation and the relationship between their optothermal properties and transmittance were studied. The results show that the average surface optothermal signal of lithium niobate exhibits a significant dependence on ion beam parameters. When the ion beam voltage is 800 V, the ion beam current is 30 mA, and the irradiation time is 60 s, a distinct absorption peak is observed on the surface of lithium niobate, with an average surface optothermal signal of 5377.34 ppm, demonstrating potential for all-optical modulation. Full article
(This article belongs to the Section Optical Interaction Science)
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16 pages, 2772 KiB  
Article
Double Demodulation Incorporates Reciprocal Modulation and Residual Amplitude Modulation Feedback to Enhance the Bias Performance of RFOG
by Zhijie Yang, Xiaolong Yan, Guoguang Chen and Xiaoli Tian
Photonics 2025, 12(8), 792; https://doi.org/10.3390/photonics12080792 - 5 Aug 2025
Viewed by 160
Abstract
The suppression of Rayleigh backscattering noise in a resonant fiber optic gyro (RFOG) is accompanied by the emergence of residual amplitude modulation (RAM) effects, which impact the bias performance of the RFOG output. In this paper, we propose a double demodulation technique that [...] Read more.
The suppression of Rayleigh backscattering noise in a resonant fiber optic gyro (RFOG) is accompanied by the emergence of residual amplitude modulation (RAM) effects, which impact the bias performance of the RFOG output. In this paper, we propose a double demodulation technique that integrates reciprocal modulation and RAM feedback. By utilizing reciprocal modulation–demodulation along with a RAM feedback control method, we effectively suppress both RAM and laser frequency noise. Furthermore, the inherent suppression characteristics of the double modulation–demodulation scheme facilitate effective backscatter noise reduction. As a result, the gyro angular random walk of the RFOG has improved to 3°/h, and the long-term bias instability has been enhanced to 0.1°/h over a test duration of 10 h. Full article
(This article belongs to the Special Issue Emerging Trends in Optical Fiber Sensors and Sensing Techniques)
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13 pages, 3292 KiB  
Article
Topological Large-Area Waveguide States Based on THz Photonic Crystals
by Yulin Zhao, Feng Liang, Jingsen Li, Jianfei Han, Jiangyu Chen, Haihua Hu, Ke Zhang and Yuanjie Yang
Photonics 2025, 12(8), 791; https://doi.org/10.3390/photonics12080791 - 5 Aug 2025
Viewed by 231
Abstract
Terahertz (THz) has attracted substantial attention owing to its unique advantages in high-speed communications. However, conventional THz waveguide systems are inherently constrained by high transmission losses, stringent fabrication precision requirements, and extreme sensitivity to structural defects. Topological edge states with topological protection have [...] Read more.
Terahertz (THz) has attracted substantial attention owing to its unique advantages in high-speed communications. However, conventional THz waveguide systems are inherently constrained by high transmission losses, stringent fabrication precision requirements, and extreme sensitivity to structural defects. Topological edge states with topological protection have driven significant advancements in THz wave manipulation. Nevertheless, the width of the topological waveguide based on edge states remains restricted. In this work, we put forward a type of spin photonic crystal with three-layer heterostructures, where large-area topological waveguide states are demonstrated. The results show that these topological waveguide states are localized within the region of Dirac photonic crystals. They also display spin-momentum-locking characteristics and maintain strong robustness against defects and sharp bends. Furthermore, a THz beam splitter and a topological beam modulator are implemented. The designed heterostructures expand the applications of multi-functional topological devices and provide a prospective pathway for overcoming the waveguide bottleneck in THz applications. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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13 pages, 3882 KiB  
Article
Thermal Damage Characterization of Detector Induced by Nanosecond Pulsed Laser Irradiation
by Zhilong Jian, Weijing Zhou, Hao Chang, Yingjie Ma, Xiaoyuan Quan and Zikang Wang
Photonics 2025, 12(8), 790; https://doi.org/10.3390/photonics12080790 - 5 Aug 2025
Viewed by 235
Abstract
Experimental and simulation analysis was conducted on the effects of 532 nm nanosecond laser-induced thermal damage on the front-side illuminated CMOS detector. The study examined CMOS detector output images at different stages of damage, including point damage, line damage, and complete failure, and [...] Read more.
Experimental and simulation analysis was conducted on the effects of 532 nm nanosecond laser-induced thermal damage on the front-side illuminated CMOS detector. The study examined CMOS detector output images at different stages of damage, including point damage, line damage, and complete failure, and correlated these with microscopic structural changes observed through optical and scanning electron microscopy. A finite element model was used to study the thermal–mechanical coupling effect during laser irradiation. The results indicated that at a laser energy density of 78.9 mJ/cm2, localized melting occurs within photosensitive units in the epitaxial layer, manifesting as an irreversible white bright spot appearing in the detector output image (point damage). When the energy density is further increased to 241.9 mJ/cm2, metal routings across multiple pixel units melt, resulting in horizontal and vertical black lines in the output image (line damage). Upon reaching 2005.4 mJ/cm2, the entire sensor area failed to output any valid image due to thermal stress-induced delamination of the silicon dioxide insulation layer, with cracks propagating to the metal routing and epitaxial layers, ultimately causing structural deformation and device failure (complete failure). Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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25 pages, 3310 KiB  
Article
Real-Time Signal Quality Assessment and Power Adaptation of FSO Links Operating Under All-Weather Conditions Using Deep Learning Exploiting Eye Diagrams
by Somia A. Abd El-Mottaleb and Ahmad Atieh
Photonics 2025, 12(8), 789; https://doi.org/10.3390/photonics12080789 - 4 Aug 2025
Viewed by 206
Abstract
This paper proposes an intelligent power adaptation framework for Free-Space Optics (FSO) communication systems operating under different weather conditions exploiting a deep learning (DL) analysis of received eye diagram images. The system incorporates two Convolutional Neural Network (CNN) architectures, LeNet and Wide Residual [...] Read more.
This paper proposes an intelligent power adaptation framework for Free-Space Optics (FSO) communication systems operating under different weather conditions exploiting a deep learning (DL) analysis of received eye diagram images. The system incorporates two Convolutional Neural Network (CNN) architectures, LeNet and Wide Residual Network (Wide ResNet) algorithms to perform regression tasks that predict received signal quality metrics such as the Quality Factor (Q-factor) and Bit Error Rate (BER) from the received eye diagram. These models are evaluated using Mean Squared Error (MSE) and the coefficient of determination (R2 score) to assess prediction accuracy. Additionally, a custom CNN-based classifier is trained to determine whether the BER reading from the eye diagram exceeds a critical threshold of 10−4; this classifier achieves an overall accuracy of 99%, correctly detecting 194/195 “acceptable” and 4/5 “unacceptable” instances. Based on the predicted signal quality, the framework activates a dual-amplifier configuration comprising a pre-channel amplifier with a maximum gain of 25 dB and a post-channel amplifier with a maximum gain of 10 dB. The total gain of the amplifiers is adjusted to support the operation of the FSO system under all-weather conditions. The FSO system uses a 15 dBm laser source at 1550 nm. The DL models are tested on both internal and external datasets to validate their generalization capability. The results show that the regression models achieve strong predictive performance, and the classifier reliably detects degraded signal conditions, enabling the real-time gain control of the amplifiers to achieve the quality of transmission. The proposed solution supports robust FSO communication under challenging atmospheric conditions including dry snow, making it suitable for deployment in regions like Northern Europe, Canada, and Northern Japan. Full article
(This article belongs to the Special Issue Editorial Board Members’ Collection Series: Optical Wireless Communication)
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17 pages, 4522 KiB  
Article
A Blue LED Spectral Simulation Method Using Exponentially Modified Gaussian Functions with Superimposed Asymmetric Pseudo-Voigt Corrections
by Hongru Zhuang, Yanfei Wang, Caihong Dai, Ling Li, Zhifeng Wu and Jiang Pan
Photonics 2025, 12(8), 788; https://doi.org/10.3390/photonics12080788 - 4 Aug 2025
Viewed by 181
Abstract
Accurately simulating the asymmetric spectral profiles of blue LEDs is crucial for photobiological research, yet it remains a challenge for traditional symmetric models. This study proposes a novel spectral simulation model that effectively captures these asymmetries. The proposed model structure is partly motivated [...] Read more.
Accurately simulating the asymmetric spectral profiles of blue LEDs is crucial for photobiological research, yet it remains a challenge for traditional symmetric models. This study proposes a novel spectral simulation model that effectively captures these asymmetries. The proposed model structure is partly motivated by known broadening and dispersion mechanisms observed in real LED spectra; it employs a ‘base model + correction’ framework, where an Exponentially Modified Gaussian (EMG) function captures the primary spectral shape and falling edge and an Asymmetric Pseudo-Voigt (APV) function corrects the deviations on the rising edge. Requiring only the central wavelength and bandwidth as user inputs, the simulation results exhibit a high degree of agreement with the experimental data spectra. The model provides a rapid and robust tool for pre-evaluating light sources against regulatory criteria (e.g., >99% of the spectral intensity is in the 400–500 nm band), thereby enhancing the efficiency of experimental design in blue light protection studies. Full article
(This article belongs to the Special Issue Light-Emitting Diodes: Technology, Advances, Challenges and Applications)
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14 pages, 2905 KiB  
Article
Optimal Design of a Lightweight Terahertz Absorber Featuring Ultra-Wideband Polarization-Insensitive Characteristics
by Yafeng Hao, Tengteng Li, Pu Zhu, Fupeng Ma, Huijia Wu, Cheng Lei, Meihong Liu, Ting Liang and Jianquan Yao
Photonics 2025, 12(8), 787; https://doi.org/10.3390/photonics12080787 - 4 Aug 2025
Viewed by 381
Abstract
Metamaterial absorbers in terahertz (THz) based bands have garnered significant attention for their potential applications in military stealth, terahertz imaging, and other fields. Nevertheless, the limited bandwidth, low absorption rate, and heavy weight greatly reduce the further development and wide application of terahertz [...] Read more.
Metamaterial absorbers in terahertz (THz) based bands have garnered significant attention for their potential applications in military stealth, terahertz imaging, and other fields. Nevertheless, the limited bandwidth, low absorption rate, and heavy weight greatly reduce the further development and wide application of terahertz absorbers. To solve these problems, we propose a polystyrene (PS)-based ultra-broadband metamaterial absorber integrated with a polyethylene terephthalate (PET) double-sided adhesive layer and a patterned indium tin oxide (ITO) film through the simulation method, which operates in the THz band. The electromagnetic wave absorption properties and underlying physical absorption mechanisms of the proposed metamaterial absorbers are comprehensively modeled and rigorously numerically simulated. The research demonstrates the metamaterial absorber can achieve absorption performance of over 90% for fully polarized incident waves in the ultra-wideband range of 1.2–10 THz, especially achieving perfect absorption characteristics of over 99.9% near 1.8–1.9 THz and 5.8–6.2 THz. The proposed absorber has a lightweight physical property of 0.7 kg/m2 and polarization-insensitive characteristic, and it achieves a broad-angle that allows a range of incidence angles up to 60°. The simulation research results of this article provide theoretical support for the design of terahertz absorbers with ultra-wideband absorption characteristics. Full article
(This article belongs to the Special Issue Metamaterials and Nanophotonics: Fundamentals and Applications)
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13 pages, 2630 KiB  
Article
Photodynamic Therapy in the Management of MDR Candida spp. Infection Associated with Palatal Expander: In Vitro Evaluation
by Cinzia Casu, Andrea Butera, Alessandra Scano, Andrea Scribante, Sara Fais, Luisa Ladu, Alessandra Siotto-Pintor and Germano Orrù
Photonics 2025, 12(8), 786; https://doi.org/10.3390/photonics12080786 - 4 Aug 2025
Viewed by 259
Abstract
The aim of this work is to evaluate the effectiveness of antimicrobial photodynamic therapy (aPDT) against oral MDR (multi-drug-resistant) Candida spp. infections related to orthodontic treatment with palatal expanders through in vitro study. Methods: PDT protocol: Curcumin + H2O2 was [...] Read more.
The aim of this work is to evaluate the effectiveness of antimicrobial photodynamic therapy (aPDT) against oral MDR (multi-drug-resistant) Candida spp. infections related to orthodontic treatment with palatal expanders through in vitro study. Methods: PDT protocol: Curcumin + H2O2 was used as a photosensitizer activated by a 460 nm diode LED lamp, with an 8 mm blunt tip for 2 min in each spot of interest. In vitro simulation: A palatal expander sterile device was inserted into a custom-designed orthodontic bioreactor, realized with 10 mL of Sabouraud dextrose broth plus 10% human saliva and infected with an MDR C. albicans clinical isolate CA95 strain to reproduce an oral palatal expander infection. After 48 h of incubation at 37 °C, the device was treated with the PDT protocol. Two samples before and 5 min after the PDT process were taken and used to contaminate a Petri dish with a Sabouraud field to evaluate Candida spp. CFUs (colony-forming units). Results: A nearly 99% reduction in C. albicans colonies in the palatal expander biofilm was found after PDT. Conclusion: The data showed the effectiveness of using aPDT to treat palatal infection; however, specific patient oral micro-environment reproduction (Ph values, salivary flow, mucosal adhesion of photosensitizer) must be further analyzed. Full article
(This article belongs to the Section Biophotonics and Biomedical Optics)
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11 pages, 2306 KiB  
Article
Optical Path Design of an Integrated Cavity Optomechanical Accelerometer with Strip Waveguides
by Chengwei Xian, Pengju Kuang, Zhe Li, Yi Zhang, Changsong Wang, Rudi Zhou, Guangjun Wen, Yongjun Huang and Boyu Fan
Photonics 2025, 12(8), 785; https://doi.org/10.3390/photonics12080785 - 4 Aug 2025
Viewed by 238
Abstract
To improve the efficiency and stability of the system, this paper proposes a monolithic integrated optical path design for a cavity optomechanical accelerometer based on a 250 nm top silicon thickness silicon-on-insulator (SOI) wafer instead of readout through U-shape fiber coupling. Finite Element [...] Read more.
To improve the efficiency and stability of the system, this paper proposes a monolithic integrated optical path design for a cavity optomechanical accelerometer based on a 250 nm top silicon thickness silicon-on-insulator (SOI) wafer instead of readout through U-shape fiber coupling. Finite Element Analysis (FEA) and Finite-Difference Time-Domain (FDTD) methods are employed to systematically investigate the performance of key optical structures, including the resonant modes and bandgap characteristics of photonic crystal (PhC) microcavities, transmission loss of strip waveguides, coupling efficiency of tapered-lensed fiber-to-waveguide end-faces, coupling characteristics between strip waveguides and PhC waveguides, and the coupling mechanism between PhC waveguides and microcavities. Simulation results demonstrate that the designed PhC microcavity achieves a quality factor (Q-factor) of 2.26 × 105 at a 1550 nm wavelength while the optimized strip waveguide exhibits a low loss of merely 0.2 dB over a 5000 μm transmission length. The strip waveguide to PhC waveguide coupling achieves 92% transmittance at the resonant frequency, corresponding to a loss below 0.4 dB. The optimized edge coupling structure exhibits a transmittance of 75.8% (loss < 1.2 dB), with a 30 μm coupling length scheme (60% transmittance, ~2.2 dB loss) ultimately selected based on process feasibility trade-offs. The total optical path system loss (input to output) is 5.4 dB. The paper confirms that the PhC waveguide–microcavity evanescent coupling method can effectively excite the target cavity mode, ensuring optomechanical coupling efficiency for the accelerometer. This research provides theoretical foundations and design guidelines for the fabrication of high-precision monolithic integrated cavity optomechanical accelerometers. Full article
(This article belongs to the Special Issue Emerging Trends in Spectral Analysis with Optical Sensors: Modern Approaches and Applications)
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11 pages, 1709 KiB  
Article
Beam Profile Prediction of High-Repetition-Rate SBS Pulse Compression Using Convolutional Neural Networks
by Hongli Wang, Chaoshuai Liu, Panpan Yan and Qinglin Niu
Photonics 2025, 12(8), 784; https://doi.org/10.3390/photonics12080784 - 4 Aug 2025
Viewed by 169
Abstract
Fast prediction of beam quality in SBS pulse compression for high-repetition-rate operation is urgently important for SBS experimental parameter acquisition. In this study, a fast computational prediction model for SBS beam profiles is developed using a convolutional neural network (CNN) method, which is [...] Read more.
Fast prediction of beam quality in SBS pulse compression for high-repetition-rate operation is urgently important for SBS experimental parameter acquisition. In this study, a fast computational prediction model for SBS beam profiles is developed using a convolutional neural network (CNN) method, which is trained and validated using experimental data from SBS pulse compression experiments. The CNN method can predict beam spot images for experimental conditions in the range of 100–500 Hz repetition rates and 5–40 mJ injection energy. The proposed CNN-based SBS beam profile prediction model has a fast convergence of the loss function and an average error of 15% with respect to the experimental results, indicating a high accuracy of the model. The CNN-based prediction model achieves an average error of 11.8% for beam profile prediction across various experimental conditions, demonstrating its potential for SBS beam profile characterization. The CNN method could provide a fast means for predicting the characteristic law of the beam intensity distribution in high-repetition-rate SBS pulse compression systems. 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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7 pages, 183 KiB  
Editorial
Editorial Board Members’ Collection Series: Nonlinear Photonics
by Luigi Sirleto
Photonics 2025, 12(8), 783; https://doi.org/10.3390/photonics12080783 - 4 Aug 2025
Viewed by 196
Abstract
Photonics has often been defined as the key technology of the 21st century [...] Full article
(This article belongs to the Special Issue Editorial Board Members' Collection Series: Nonlinear Photonics)
17 pages, 7323 KiB  
Article
Line Laser 3D Measurement Method and Experiments of Gears
by Yanqiang Sun, Zhaoyao Shi, Bo Yu and Meichuan Li
Photonics 2025, 12(8), 782; https://doi.org/10.3390/photonics12080782 - 4 Aug 2025
Viewed by 221
Abstract
Line laser measurement, as a typical method of laser triangulation, makes the acquisition of 3D tooth-surface data more accurate, efficient, and informative. Thus, a line laser 3D measurement model of gears is established, and a specialized polyhedral artifact with specific geometric features is [...] Read more.
Line laser measurement, as a typical method of laser triangulation, makes the acquisition of 3D tooth-surface data more accurate, efficient, and informative. Thus, a line laser 3D measurement model of gears is established, and a specialized polyhedral artifact with specific geometric features is invented to determine the pose parameters of the line laser sensor in measuring space. Based on this, a single-spindle gear-measuring instrument is developed and a series of experimental studies are conducted for gears with different module and flank directions in this instrument, including profile deviation, helix deviation, pitch deviation, topological deviation, etc. A comparative experiment with traditional contact measurement methods validates the correctness of the methods mentioned in this paper for the accurate evaluation of tested gears. In further research, the mining and utilization of big data obtained from the line laser 3D measurement of gears will be an important topic. Full article
(This article belongs to the Special Issue Advancements in Optical Metrology and Imaging)
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11 pages, 4858 KiB  
Communication
Customized Chirality of an Optical Vortex Pair: Helical Dichroism and Enantioselective Force
by Xingxing Han, Haibo Niu, Jing Shi, Weili Dong and Jiajie Wang
Photonics 2025, 12(8), 781; https://doi.org/10.3390/photonics12080781 - 4 Aug 2025
Viewed by 159
Abstract
Tailoring the chirality of an optical vortex is crucial for advancing helical chiroptical spectroscopy techniques in various scenarios and attracts great attention. In contrast to the single vortex, the optical vortex pair exhibits richer, fantastic chirality properties due to its additional adjustment parameters. [...] Read more.
Tailoring the chirality of an optical vortex is crucial for advancing helical chiroptical spectroscopy techniques in various scenarios and attracts great attention. In contrast to the single vortex, the optical vortex pair exhibits richer, fantastic chirality properties due to its additional adjustment parameters. Here, a comprehensive investigation of the chirality for linearly polarized optical vortex pairs based on the vector angular spectrum decomposition method is conducted. The numerical results show that the magnitudes and distributions of local chirality density, helical dichroism, and enantioselective force of the optical vortex pair can be flexibly customized by the position as well as sign combination of vortices, and can vary during free space propagation. The underlying physical mechanism behind these phenomena is ascribed to the interplay of two vortices. Our work can deepen the understanding of the chirality for multiple vortices and open-up the prospect for relevant applications in chiral recognition and manipulation. Full article
(This article belongs to the Special Issue Realization and Application of Vortex Laser)
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14 pages, 10145 KiB  
Article
Wavefront-Corrected Algorithm for Vortex Optical Transmedia Wavefront-Sensorless Sensing Based on U-Net Network
by Shangjun Yang, Yanmin Zhao, Binkun Liu, Shuguang Zou and Chenghu Ke
Photonics 2025, 12(8), 780; https://doi.org/10.3390/photonics12080780 - 1 Aug 2025
Viewed by 174
Abstract
Atmospheric and oceanic turbulence can severely degrade the orbital angular momentum (OAM) mode purity of vortex beams in cross-media optical links. Here, we propose a hybrid correction framework that fuses multiscale phase-screen modeling with a lightweight U-Net predictor for phase-distortion—driven solely by measured [...] Read more.
Atmospheric and oceanic turbulence can severely degrade the orbital angular momentum (OAM) mode purity of vortex beams in cross-media optical links. Here, we propose a hybrid correction framework that fuses multiscale phase-screen modeling with a lightweight U-Net predictor for phase-distortion—driven solely by measured optical intensity—and augments it with a feed-forward, Gaussian-reference subtraction scheme for iterative compensation. In our experiments, this approach boosts the l = 3 mode purity from 38.4% to 98.1%. Compared to the Gerchberg–Saxton algorithm, the Gaussian-reference feed-forward method achieves far lower computational complexity and greater robustness, making real-time phase recovery feasible for OAM-based communications over heterogeneous channels. Full article
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11 pages, 1758 KiB  
Article
Nonlinear Absorption Properties of Phthalocyanine-like Squaraine Dyes
by Fan Zhang, Wuyang Shi, Xixiao Li, Yigang Wang, Leilei Si, Wentao Gao, Meng Qi, Minjie Zhou, Jiajun Ma, Ao Li, Zhiqiang Li, Hongming Wang and Bing Jin
Photonics 2025, 12(8), 779; https://doi.org/10.3390/photonics12080779 - 1 Aug 2025
Viewed by 214
Abstract
This study synthesizes and comparatively investigates two squaric acid-based phthalocyanine-like dyes, SNF and its long-chain alkylated derivative LNF, to systematically elucidate the influence of peripheral hydrophobic groups on their third-order nonlinear optical (NLO) properties. The NLO characteristics were comprehensively characterized using femtosecond Z-scan [...] Read more.
This study synthesizes and comparatively investigates two squaric acid-based phthalocyanine-like dyes, SNF and its long-chain alkylated derivative LNF, to systematically elucidate the influence of peripheral hydrophobic groups on their third-order nonlinear optical (NLO) properties. The NLO characteristics were comprehensively characterized using femtosecond Z-scan and I-scan techniques at both 800 nm and 900 nm. Both dyes exhibited strong saturable absorption (SA), confirming their potential as saturable absorbers. Critically, the comparative analysis revealed that SNF exhibits a significantly greater nonlinear absorption coefficient (β) compared to LNF under identical conditions. For instance, at 800 nm, the β of SNF was approximately 3–5 times larger than that of LNF. This result conclusively demonstrates that the introduction of long hydrophobic alkyl chains attenuates the NLO response. Furthermore, I-scan measurements revealed excellent SA performance, with high modulation depths (e.g., LNF: 43.0% at 900 nm) and low saturation intensities. This work not only clarifies the structure–property relationship in these D-A-D dyes but also presents a clear strategy for modulating the NLO properties of organic chromophores for applications in near-infrared pulsed lasers. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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