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Photonics, Volume 11, Issue 2 (February 2024) – 91 articles

Cover Story (view full-size image): The head-on scattering of electrons, with energies from a few MeV to 5 GeV, off ultra-short and ultra-intense laser pulses at petawatt intensities is investigated. Radiation reaction effects are included, through the correction terms given by the Landau–Lifshitz equation. Full paraxial fields for the laser are used, including their longitudinal electric and magnetic components, and both the fundamental Gaussian TEM00 mode as well as the orbital angular momentum mode with (l,p) = (1,0) are studied. We compare the expected behavior, as regards the influence of radiation reaction, at near-infrared (NIR) and at vacuum ultraviolet (VUV) or X-ray wavelengths. View this paper
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16 pages, 18828 KiB  
Article
Dark Light Image-Enhancement Method Based on Multiple Self-Encoding Prior Collaborative Constraints
by Lei Guan, Jiawei Dong, Qianxi Li, Jijiang Huang, Weining Chen and Hao Wang
Photonics 2024, 11(2), 190; https://doi.org/10.3390/photonics11020190 - 19 Feb 2024
Viewed by 820
Abstract
The purpose of dark image enhancement is to restore dark images to visual images under normal lighting conditions. Due to the ill-posedness of the enhancement process, previous enhancement algorithms often have overexposure, underexposure, noise increases and artifacts when dealing with complex and changeable [...] Read more.
The purpose of dark image enhancement is to restore dark images to visual images under normal lighting conditions. Due to the ill-posedness of the enhancement process, previous enhancement algorithms often have overexposure, underexposure, noise increases and artifacts when dealing with complex and changeable images, and the robustness is poor. This article proposes a new enhancement approach consisting in constructing a dim light enhancement network with more robustness and rich detail features through the collaborative constraint of multiple self-coding priors (CCMP). Specifically, our model consists of two prior modules and an enhancement module. The former learns the feature distribution of the dark light image under normal exposure as an a priori term of the enhancement process through multiple specific autoencoders, implicitly measures the enhancement quality and drives the network to approach the truth value. The latter fits the curve mapping of the enhancement process as a fidelity term to restore global illumination and local details. Through experiments, we concluded that the new method proposed in this article can achieve more excellent quantitative and qualitative results, improve detail contrast, reduce artifacts and noise, and is suitable for dark light enhancement in multiple scenes. Full article
(This article belongs to the Special Issue Optical Imaging and Measurements)
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12 pages, 5141 KiB  
Article
Numerical Simulation and Experimental Investigation of ps Pulsed Laser Modification inside 4H-SiC Material
by Yiying Song, Shusen Zhao, Hongzhi He, Han Liang, Zhanfeng Dai, Xuechun Lin and Guling Zhang
Photonics 2024, 11(2), 189; https://doi.org/10.3390/photonics11020189 - 19 Feb 2024
Viewed by 767
Abstract
Silicon Carbide (SiC) is the predominant substrate material for optoelectronic-integrated devices. However, it challenges the wafer-slicing process because of its high hardness, brittleness, and other material characteristics. Laser processing has gained prominence as the primary method, leveraging its merits of high efficiency, precision, [...] Read more.
Silicon Carbide (SiC) is the predominant substrate material for optoelectronic-integrated devices. However, it challenges the wafer-slicing process because of its high hardness, brittleness, and other material characteristics. Laser processing has gained prominence as the primary method, leveraging its merits of high efficiency, precision, and micro-destructiveness. In this study, a finite element method is applied to calculate the temperature field distribution resulting from the electric field of a Gaussian beam. The simulation considers laser propagation inside 4H-SiC, non-linear absorption, and spherical aberration induced by the refractive index of the material. The influence of laser pulse energy and focusing depth are considered. The results indicate that the modification depths decrease with the increasing focusing depth. With the increase of laser pulse energy, the depth of the modification layer increases continuously. Moreover, an experimental setup has been devised to furnish valuable references in validating the proposed model. Full article
(This article belongs to the Special Issue Advanced Lasers and Their Applications)
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23 pages, 5841 KiB  
Review
Solid-State Color Centers for Single-Photon Generation
by Greta Andrini, Francesco Amanti, Fabrizio Armani, Vittorio Bellani, Vincenzo Bonaiuto, Simone Cammarata, Matteo Campostrini, Thu Ha Dao, Fabio De Matteis, Valeria Demontis, Giovanni Di Giuseppe, Sviatoslav Ditalia Tchernij, Simone Donati, Andrea Fontana, Jacopo Forneris, Roberto Francini, Luca Frontini, Roberto Gunnella, Simone Iadanza, Ali Emre Kaplan, Cosimo Lacava, Valentino Liberali, Francesco Marzioni, Elena Nieto Hernández, Elena Pedreschi, Paolo Piergentili, Domenic Prete, Paolo Prosposito, Valentino Rigato, Carlo Roncolato, Francesco Rossella, Andrea Salamon, Matteo Salvato, Fausto Sargeni, Jafar Shojaii, Franco Spinella, Alberto Stabile, Alessandra Toncelli, Gabriella Trucco and Valerio Vitaliadd Show full author list remove Hide full author list
Photonics 2024, 11(2), 188; https://doi.org/10.3390/photonics11020188 - 19 Feb 2024
Viewed by 2625
Abstract
Single-photon sources are important for integrated photonics and quantum technologies, and can be used in quantum key distribution, quantum computing, and sensing. Color centers in the solid state are a promising candidate for the development of the next generation of single-photon sources integrated [...] Read more.
Single-photon sources are important for integrated photonics and quantum technologies, and can be used in quantum key distribution, quantum computing, and sensing. Color centers in the solid state are a promising candidate for the development of the next generation of single-photon sources integrated in quantum photonics devices. They are point defects in a crystal lattice that absorb and emit light at given wavelengths and can emit single photons with high efficiency. The landscape of color centers has changed abruptly in recent years, with the identification of a wider set of color centers and the emergence of new solid-state platforms for room-temperature single-photon generation. This review discusses the emerging material platforms hosting single-photon-emitting color centers, with an emphasis on their potential for the development of integrated optical circuits for quantum photonics. Full article
(This article belongs to the Special Issue Photonic Integrated Circuits for Information, Computing and Sensing)
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14 pages, 3686 KiB  
Article
Fabrication of Micro/Nano Dual Needle Structures with Morphological Gradient Based on Two-Photon Polymerization Laser Direct Writing with Proactive Focus Compensation
by Chenxi Xu, Chen Zhang, Wei Zhao, Yining Liu, Ziyu Li, Zeyu Wang, Baole Lu, Kaige Wang and Jintao Bai
Photonics 2024, 11(2), 187; https://doi.org/10.3390/photonics11020187 - 18 Feb 2024
Viewed by 763
Abstract
Micro/nano structures with morphological gradients possess unique physical properties and significant applications in various research domains. This study proposes a straightforward and precise method for fabricating micro/nano structures with morphological gradients utilizing single-voxel synchronous control and a nano-piezoelectric translation stage in a two-photon [...] Read more.
Micro/nano structures with morphological gradients possess unique physical properties and significant applications in various research domains. This study proposes a straightforward and precise method for fabricating micro/nano structures with morphological gradients utilizing single-voxel synchronous control and a nano-piezoelectric translation stage in a two-photon laser direct writing technique. To address the defocusing issue in large-scale fabrication, a methodology for laser focus dynamic proactive compensation was developed based on fluorescence image analysis, which can achieve high-precision compensation of laser focus within the entire range of the nano-piezoelectric translation stage. Subsequently, the fabrication of micro/nano dual needle structures with morphological gradients were implemented by employing different writing speeds and voxel positions. The minimum height of the tip in the dual needle structure is 80 nm, with a linewidth of 171 nm, and a dual needle total length reaching 200 μm. Based on SEM (scanning electron microscope) and AFM (atomic force microscope) characterization, the dual needle structures fabricated by the method proposed in this study exhibit high symmetry and nanoscale gradient accuracy. Additionally, the fabrication of hexagonal lattice periodic structures assembled from morphological gradient needle structures and the size gradient Archimedean spiral structures validate the capability of the single voxel-based fabrication and proactive focus compensation method for complex gradient structure fabrication. Full article
(This article belongs to the Special Issue New Advances in Ultrashort Pulse Fiber Lasers and Their Applications)
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18 pages, 6450 KiB  
Article
Design and Analysis of Optomechanical Micro-Gyroscope for Angular-Vibration Detection
by Jamal N. A. Hassan, Wenyi Huang, Xing Yan, Senyu Zhang, Dingwei Chen, Guangjun Wen and Yongjun Huang
Photonics 2024, 11(2), 186; https://doi.org/10.3390/photonics11020186 - 18 Feb 2024
Viewed by 874
Abstract
Micro-gyroscopes based on the Coriolis principle are widely employed in inertial navigation, motion control, and vibration analysis applications. Conventional micro-gyroscopes often exhibit limitations, including elevated noise levels and suboptimal performance metrics. Conversely, the advent of cavity optomechanical system technology heralds an innovative approach [...] Read more.
Micro-gyroscopes based on the Coriolis principle are widely employed in inertial navigation, motion control, and vibration analysis applications. Conventional micro-gyroscopes often exhibit limitations, including elevated noise levels and suboptimal performance metrics. Conversely, the advent of cavity optomechanical system technology heralds an innovative approach to micro-gyroscope development. This method enhances the device’s capabilities, offering elevated sensitivity, augmented precision, and superior resolution. This paper presents our main contributions which include a novel dual-frame optomechanical gyroscope, a unique photonic crystal cavity design, and advanced numerical simulation and optimization methods. The proposed design utilizes an optical cavity formed between dual oscillating frames, whereby input rotation induces a measurable phase shift via optomechanical coupling. Actuation of the frames is achieved electrostatically via an interdigitated comb-drive design. Through theoretical modeling based on cavity optomechanics and finite element simulation, the operating principle and performance parameters are evaluated in detail. The results indicate an expected angular rate sensitivity of 22.8 mV/°/s and an angle random walk of 7.1 × 10−5 °/h1/2, representing superior precision to existing micro-electromechanical systems gyroscopes of comparable scale. Detailed analysis of the optomechanical transduction mechanism suggests this dual-frame approach could enable angular vibration detection with resolution exceeding state-of-the-art solutions. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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15 pages, 4426 KiB  
Article
Polarization-Based Digital Histology of Human Skin Biopsies Assisted by Deep Learning
by Deyan Ivanov, Lidia Zaharieva, Victoria Mircheva, Petranka Troyanova, Ivan Terziev, Razvigor Ossikovski, Tatiana Novikova and Tsanislava Genova
Photonics 2024, 11(2), 185; https://doi.org/10.3390/photonics11020185 - 18 Feb 2024
Viewed by 781
Abstract
Mueller polarimetry has proven to be a powerful optical technique to complement medical doctors in their conventional histology analysis. In this work, various degenerative and malignant human skin lesions were evaluated ex vivo using imaging Mueller polarimetry. The Mueller matrix images of thin [...] Read more.
Mueller polarimetry has proven to be a powerful optical technique to complement medical doctors in their conventional histology analysis. In this work, various degenerative and malignant human skin lesions were evaluated ex vivo using imaging Mueller polarimetry. The Mueller matrix images of thin sections of biopsies were recorded and the differential decomposition of Mueller matrices was applied pixel-wise to extract the polarization fingerprint of the specimens under study. To improve the classification accuracy, a deep learning model was created. The results indicate the sensitivity of polarimetry to different skin lesions and healthy skin zones and their differentiation, while using standard histological analysis as a ground truth. In particular, the deep learning model was found sufficiently accurate to detect and differentiate between all eight classes in the data set. Special attention was paid to the overfitting problem and the reduction of the loss function of the model. Our approach is an effort in establishing digital histology for clinical applications by complementing medical doctors in their diagnostic decisions. Full article
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26 pages, 5282 KiB  
Article
Nanoscale Dots, Grids, Ripples, and Hierarchical Structures on PET by UV Laser Processing
by Gerda Buchberger, Martin Kührer, Günter Hesser and Johannes Heitz
Photonics 2024, 11(2), 184; https://doi.org/10.3390/photonics11020184 - 18 Feb 2024
Viewed by 750
Abstract
Nanostructures can be produced on poly(ethylene terephthalate) (PET) foils by using a krypton fluoride (KrF) excimer laser with a wavelength of 248 nm and a pulse duration of about 20 ns. We show that surface nanoripples, nanodots, nanogrids, and hybrid patterns of ripples [...] Read more.
Nanostructures can be produced on poly(ethylene terephthalate) (PET) foils by using a krypton fluoride (KrF) excimer laser with a wavelength of 248 nm and a pulse duration of about 20 ns. We show that surface nanoripples, nanodots, nanogrids, and hybrid patterns of ripples with dots or finer ripples on top can be fabricated. The effects of a water layer in front of the PET foil and of cooling during laser processing were investigated. For pattern formation, several irradiation parameters (pulse number, pulse energy, and polarization) were varied systematically. The spatial periods of the ripples were changed by adjusting the angle of incidence of the laser beam. All nanostructures were characterized by scanning electron microscopy, and relevant morphological parameters, such as peak-to-peak distances and spatial periods, were assessed. Shapes and heights of some structures were characterized by using focused ion beam cuts to avoid the tip-sample convolution effects typical of atomic force microscopy images. We further demonstrate nanoripple formation on PET foils as thin as 12 µm, 6 µm, and 1.4 µm. The remarkable variety of nanostructures on PET we present here enables customized fabrication for a wide range of applications. Full article
(This article belongs to the Special Issue Emerging Trends in Laser Processing Techniques)
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8 pages, 2763 KiB  
Article
High Electric Field Enhancement Induced by Modal Coupling for a Plasmonic Dimer Array on a Metallic Film
by Jiawei Liu, Ziming Meng and Jinyun Zhou
Photonics 2024, 11(2), 183; https://doi.org/10.3390/photonics11020183 - 17 Feb 2024
Viewed by 637
Abstract
A giant electric field on a subwavelength scale is highly beneficial for boosting the light–matter interaction. In this paper, we investigated a hybrid structure consisting of a hemispheric dimer array and a gold film and realized resonant mode coupling of the surface lattice [...] Read more.
A giant electric field on a subwavelength scale is highly beneficial for boosting the light–matter interaction. In this paper, we investigated a hybrid structure consisting of a hemispheric dimer array and a gold film and realized resonant mode coupling of the surface lattice resonance (SLR) and surface plasmon polariton (SPP). Mode coupling is demonstrated by observing anti-crossing in reflection spectra, which corresponds to Rabi splitting. Although the resonance coupling does not enter the strong coupling regime, an improved quality factor (Q~350) and stronger electric field enhancement in the gap region of the dimer (i.e., hot spot) in our hybrid structure are obtained compared to those of the single dimer or dimer array only. Remarkably, the magnitude of electric field enhancement over 500 can be accessible. Such high field enhancement makes our hybridized structure a versatile platform for the realization of ultra-sensitive biosensing, low-threshold nanolasing, low-power nonlinear optical devices, etc. Full article
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13 pages, 1203 KiB  
Article
Enhancing Dynamic Performance in K-Rb-21Ne Co-Magnetometers through Atomic Density Optimization
by Lv Yang, Haoying Pang and Wei Quan
Photonics 2024, 11(2), 182; https://doi.org/10.3390/photonics11020182 - 16 Feb 2024
Viewed by 707
Abstract
The K-Rb-21Ne co-magnetometer exhibits poorer dynamic performance due to the larger equivalent magnetic field generated by alkali metal atoms. In this study, the impact of the atomic number density of alkali metal atoms and noble gas atoms in the cell on [...] Read more.
The K-Rb-21Ne co-magnetometer exhibits poorer dynamic performance due to the larger equivalent magnetic field generated by alkali metal atoms. In this study, the impact of the atomic number density of alkali metal atoms and noble gas atoms in the cell on the dynamic performance of the atomic ensemble is investigated quantitatively. Relationships between the slow-decay term in the transient response attenuation of the Spin-Exchange Relaxation-Free (SERF) co-magnetometer to interference magnetic fields and the number densities of noble gas atoms as well as alkali metal atoms are established. Based on the established model, the relationship between the number density of 21Ne atoms and dynamic performance is investigated using cells with five different noble gas pressures. Then, we investigate the impact of the number density of alkali metal atoms using a cell with a pressure of 2.1 atm at different temperatures. The results indicate that, as the number density of alkali metal atoms or noble gas atoms in the cell increases, the dynamic performance of the system improves, which provides a theoretical basis for the design of cell parameters for SERF co-magnetometers. Full article
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15 pages, 3959 KiB  
Article
Sub-Bin Delayed High-Range Accuracy Photon-Counting 3D Imaging
by Hao-Meng Yin, Hui Zhao, Ming-Yang Yang, Yong-An Liu, Li-Zhi Sheng and Xue-Wu Fan
Photonics 2024, 11(2), 181; https://doi.org/10.3390/photonics11020181 - 16 Feb 2024
Viewed by 620
Abstract
The range accuracy of single-photon-array three-dimensional (3D) imaging systems is limited by the time resolution of the array detectors. We introduce a method for achieving super-resolution in 3D imaging through sub-bin delayed scanning acquisition and fusion. Its central concept involves the generation of [...] Read more.
The range accuracy of single-photon-array three-dimensional (3D) imaging systems is limited by the time resolution of the array detectors. We introduce a method for achieving super-resolution in 3D imaging through sub-bin delayed scanning acquisition and fusion. Its central concept involves the generation of multiple sub-bin difference histograms through sub-bin shifting. Then, these coarse time-resolution histograms are fused with multiplied averages to produce finely time-resolved detailed histograms. Finally, the arrival times of the reflected photons with sub-bin resolution are extracted from the resulting fused high-time-resolution count distribution. Compared with the sub-delayed with the fusion method added, the proposed method performs better in reducing the broadening error caused by coarsened discrete sampling and background noise error. The effectiveness of the proposed method is examined at different target distances, pulse widths, and sub-bin scales. The simulation analytical results indicate that small-scale sub-bin delays contribute to superior reconstruction outcomes for the proposed method. Specifically, implementing a sub-bin temporal resolution delay of a factor of 0.1 for a 100 ps echo pulse width substantially reduces the system ranging error by three orders of magnitude. Furthermore, Monte Carlo simulations allow to describe a low signal-to-background noise ratio (0.05) characterised by sparsely reflected photons. The proposed method demonstrates a commendable capability to simultaneously achieve wide-ranging super-resolution and denoising. This is evidenced by the detailed depth distribution information and substantial reduction of 95.60% in the mean absolute error of the reconstruction results, confirming the effectiveness of the proposed method in noisy scenarios. Full article
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16 pages, 4382 KiB  
Article
Multispectral Narrowband Frustrated Total Internal Reflection Filter with Inclusions of Plasmonic Nanoparticles
by Nikolai I. Petrov
Photonics 2024, 11(2), 180; https://doi.org/10.3390/photonics11020180 - 16 Feb 2024
Viewed by 688
Abstract
A spatial-frequency thin-film filter with inclusions of nanoparticles operating in the visible range of the spectrum is investigated. The effect of nanoparticles embedded in the central and lateral layers of the frustrated total internal reflection filter on the spectral characteristics, considering the frequency [...] Read more.
A spatial-frequency thin-film filter with inclusions of nanoparticles operating in the visible range of the spectrum is investigated. The effect of nanoparticles embedded in the central and lateral layers of the frustrated total internal reflection filter on the spectral characteristics, considering the frequency dispersion, is investigated. It is shown that plasmonic effects cause the splitting of the filter bandwidth into a set of narrow-band spectral lines and the angular splitting of the incident beam into a set of output beams. It is demonstrated that due to the difference in the resonance conditions for s- and p-polarization waves, the spectral lines of transparency do not coincide, which indicates the possibility of using the filter as a polarizer. Full article
(This article belongs to the Special Issue Recent Advances in Diffractive Optics)
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17 pages, 5844 KiB  
Article
Optical–Mechanical Integration Analysis and Validation of LiDAR Integrated Systems with a Small Field of View and High Repetition Frequency
by Lu Li, Kunming Xing, Ming Zhao, Bangxin Wang, Jianfeng Chen and Peng Zhuang
Photonics 2024, 11(2), 179; https://doi.org/10.3390/photonics11020179 - 16 Feb 2024
Viewed by 774
Abstract
Integrated systems are facing complex and changing environments with the wide application of atmospheric LiDAR in civil, aerospace, and military fields. Traditional analysis methods employ optical software to evaluate the optical performance of integrated systems, and cannot comprehensively consider the influence of optical [...] Read more.
Integrated systems are facing complex and changing environments with the wide application of atmospheric LiDAR in civil, aerospace, and military fields. Traditional analysis methods employ optical software to evaluate the optical performance of integrated systems, and cannot comprehensively consider the influence of optical and mechanical coupling on the optical performance of the integrated system, resulting in the unsatisfactory accuracy of the analysis results. Optical–mechanical integration technology provides a promising solution to this problem. A small-field-of-view LiDAR system with high repetition frequency, low energy, and single-photon detection technology was taken as an example in this study, and the Zernike polynomial fitting algorithm was programmed to enable transmission between optical and mechanical data. Optical–mechanical integration technology was employed to obtain the optical parameters of the integrated system under a gravity load in the process of designing the optical–mechanical structure of the integrated system. The experimental validation results revealed that the optical–mechanical integration analysis of the divergence angle of the transmission unit resulted in an error of 2.586%. The focal length of the telescope increased by 89 μm, its field of view was 244 μrad, and the error of the detector target surface spot was 4.196%. The continuous day/night detection results showed that the system could accurately detect the temporal and spatial variations in clouds and aerosols. The inverted optical depths were experimentally compared with those obtained using a solar photometer. The average optical depth was 0.314, as detected using LiDAR, and 0.329, as detected by the sun photometer, with an average detection error of 4.559%. Therefore, optical–mechanical integration analysis can effectively improve the stability of the structure of highly integrated and complex optical systems. Full article
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14 pages, 5703 KiB  
Article
The Design of a Photonic Crystal Fiber for Hydrogen Cyanide Gas Detection
by Abdolreza Pourfathi Fard, Somayeh Makouei, Morad Danishvar and Sebelan Danishvar
Photonics 2024, 11(2), 178; https://doi.org/10.3390/photonics11020178 - 16 Feb 2024
Viewed by 725
Abstract
Hydrogen cyanide gas is a dangerous and fatal gas that is one of the causes of air pollution in the environment. A small percentage of this gas causes poisoning and eventually death. In this paper, a new PCF is designed that offers high [...] Read more.
Hydrogen cyanide gas is a dangerous and fatal gas that is one of the causes of air pollution in the environment. A small percentage of this gas causes poisoning and eventually death. In this paper, a new PCF is designed that offers high sensitivity and low confinement loss in the absorption wavelength of hydrogen cyanide gas. The proposed structure consists of circular layers that are located around the core, which is also composed of circular microstructures. The finite element method (FEM) is used to simulate the results. According to the results, the PCF gives a high relative sensitivity of 65.13% and a low confinement loss of 1.5 × 10−3 dB/m at a wavelength of 1.533 µm. The impact of increasing the concentration of hydrogen cyanide gas on the relative sensitivity and confinement loss is investigated. The high sensitivity and low confinement losses of the designed PCF show that this optical structure could be a good candidate for the detection of this gas in industrial and medical environments. Full article
(This article belongs to the Special Issue Advanced Photonic Sensing and Measurement II)
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12 pages, 3654 KiB  
Article
Upconversion Photonic Doppler Velocimetry Based on Stimulated Brillouin Scattering
by Long Chen, Cangli Liu, Heli Ma, Longhuang Tang, Xing Jia, Tianjiong Tao, Shenggang Liu, Yongchao Chen, Xiang Wang, Jian Wu, Chengjun Li and Jidong Weng
Photonics 2024, 11(2), 177; https://doi.org/10.3390/photonics11020177 - 16 Feb 2024
Viewed by 662
Abstract
Optical up-conversion photonic Doppler velocimetry (PDV) based on stimulated Brillouin Scattering (SBS) with an all-fiber link structure is proposed in this article. Because SBS limits the laser power transmitted by a fiber over long distances, the probe does not have enough outgoing light [...] Read more.
Optical up-conversion photonic Doppler velocimetry (PDV) based on stimulated Brillouin Scattering (SBS) with an all-fiber link structure is proposed in this article. Because SBS limits the laser power transmitted by a fiber over long distances, the probe does not have enough outgoing light to reach the measured surface and cannot receive the signal light. Traditionally, SBS is avoided, but it is a phase-conjugated light and shifts down relative to the source light, so it can be used as a reference light in the laser interference structure to achieve up-conversion heterodyne velocimetry. Compared with general homodyne velocimetry (DPS), SBS-PDV naturally upconverts and has more interference fringes and higher resolution at low-speed measurement. In the gas multiple reflection impact compression experiment, the velocity measurement results of SBS-PDV and dual-laser heterodyne Velocimetry (DLHV) are basically consistent, and the accuracy is better than 0.8%. Due to its coaxial heterodyne optical path, this kind of photonic Doppler velocimetry is suitable for low-velocity and long-distance practical applications in the field of shock wave physics. Full article
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14 pages, 12997 KiB  
Article
Harnessing Multistability: A Novel Approach to Optical Logic Gate Construction Using Erbium-Doped Fiber Lasers
by Safara Bibi, Guillermo Huerta-Cuellar, José Luís Echenausía-Monroy, Rider Jaimes-Reátegui, Juan Hugo García-López and Alexander N. Pisarchik
Photonics 2024, 11(2), 176; https://doi.org/10.3390/photonics11020176 - 15 Feb 2024
Viewed by 653
Abstract
We present an innovative method harnessing multistability within a diode-pumped erbium-doped fiber laser to construct logic gates. Our approach involves manipulating the intensity of external noise to regulate the probability of transitioning among four concurrent attractors. In this manner, we facilitate the realization [...] Read more.
We present an innovative method harnessing multistability within a diode-pumped erbium-doped fiber laser to construct logic gates. Our approach involves manipulating the intensity of external noise to regulate the probability of transitioning among four concurrent attractors. In this manner, we facilitate the realization of OR, AND, NOR, and NAND logic operations, aligning with the coexisting period-1, period-3, period-4, and period-5 orbits. Employing detrended fluctuation analysis, we establish equilibrium in the probability distributions of these states. The obtained results denote a substantial advancement in the field of optical logic gate development, representing a pivotal stride toward the seamless integration of an all-optical logic gate within laser oscillator-based systems. Full article
(This article belongs to the Special Issue Advanced Photonic Sensing and Measurement II)
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16 pages, 1648 KiB  
Article
Spatial Relation Awareness Module for Phase Unwrapping
by Chiori Azuma, Tomoyoshi Ito and Tomoyoshi Shimobaba
Photonics 2024, 11(2), 175; https://doi.org/10.3390/photonics11020175 - 14 Feb 2024
Viewed by 679
Abstract
Phase unwrapping is a technique used to recover the original phase from the wrapped phase in the range (π,π]. Various methods have been proposed for phase unwrapping. In particular, methods using convolutional neural networks (CNNs) have been [...] Read more.
Phase unwrapping is a technique used to recover the original phase from the wrapped phase in the range (π,π]. Various methods have been proposed for phase unwrapping. In particular, methods using convolutional neural networks (CNNs) have been extensively researched because of their high robustness against noise and fast inference speed. However, conventional CNN-based methods discard the local position information and relationships between pixels in the convolution process, resulting in poor phase-unwrapping performance. To obtain better phase unwrapping results, we propose a module that combines a global convolution network, which applies convolutional layers with a kernel size equivalent to that of the feature maps, and CoordConv, which acquires the positional relationships between pixels. We validated the performance of the proposed method by comparing it with a quality-guided path algorithm and deep learning-based phase unwrapping methods and found that the proposed method is highly robust against noise. Full article
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15 pages, 4962 KiB  
Article
Single-Pixel Infrared Hyperspectral Imaging via Physics-Guided Generative Adversarial Networks
by Dong-Yin Wang, Shu-Hang Bie, Xi-Hao Chen and Wen-Kai Yu
Photonics 2024, 11(2), 174; https://doi.org/10.3390/photonics11020174 - 12 Feb 2024
Viewed by 920
Abstract
A physics-driven generative adversarial network (GAN) was utilized to demonstrate a single-pixel hyperspectral imaging (HSI) experiment in the infrared spectrum, eliminating the need for extensive dataset training in most data-driven deep neural networks. Within the GAN framework, the physical process of single-pixel imaging [...] Read more.
A physics-driven generative adversarial network (GAN) was utilized to demonstrate a single-pixel hyperspectral imaging (HSI) experiment in the infrared spectrum, eliminating the need for extensive dataset training in most data-driven deep neural networks. Within the GAN framework, the physical process of single-pixel imaging (SPI) was integrated into the generator, and its estimated one-dimensional (1D) bucket signals and the actual 1D bucket signals were employed as constraints in the objective function to update the network’s parameters and optimize the generator with the assistance of the discriminator. In comparison to single-pixel infrared HSI methods based on compressive sensing and physics-driven convolution neural networks, our physics-driven GAN-based single-pixel infrared HSI exhibits superior imaging performance. It requires fewer samples and achieves higher image quality. We believe that our physics-driven network will drive practical applications in computational imaging, including various SPI-based techniques. Full article
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18 pages, 7756 KiB  
Article
Stray Light Analysis and Suppression for an Infrared Fourier Imaging Spectrometer
by Chenzhao Ben, Honghai Shen, Xiao Yu, Lingtong Meng, Huishi Cheng and Ping Jia
Photonics 2024, 11(2), 173; https://doi.org/10.3390/photonics11020173 - 12 Feb 2024
Viewed by 964
Abstract
To improve the accuracy of infrared radiation characteristics measurement in the aviation field, an infrared Fourier transform imaging spectrometer based on a double-swing solid angle reflector was designed. This imaging spectrometer operates in the 3–5 μm wavelength range and has a field of [...] Read more.
To improve the accuracy of infrared radiation characteristics measurement in the aviation field, an infrared Fourier transform imaging spectrometer based on a double-swing solid angle reflector was designed. This imaging spectrometer operates in the 3–5 μm wavelength range and has a field of view of 1.7° × 1.7°. This article presents a comprehensive analysis of the system’s stray light and also studies the impact of external stray light on the imaging quality, along with the influence of internal stray light on the interference effects and the spectral resolution. It also present the design of a hood that suppresses the point source transmittance of the external stray light down to the order of 10−4. Based on this, we propose a method that incorporates the introduction of wedge and inclination angles. Additionally, a numerical range is provided for the addition of these angles on the beam splitter mirror and compensation plate. This ensures the effective suppression of any internal stray light. This study fills the gap in the knowledge about Fourier transform imaging spectrometers operating in the mid-infrared band for aviation applications, and proposes a suppression method suitable for interference systems, which is also suitable for Fourier transform imaging spectrometers based on other types of interferometers. This study broadens the application field of Fourier transform imaging spectrometers in stray light, and has great significance to promote the development of Fourier transform imaging spectrometer. Full article
(This article belongs to the Special Issue Advances in Photoelectric Tracking Systems)
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12 pages, 15814 KiB  
Article
Optical Signal Attenuation through Smog in Controlled Laboratory Conditions
by Hira Khalid, Sheikh Muhammad Sajid, Muhammad Imran Cheema and Erich Leitgeb
Photonics 2024, 11(2), 172; https://doi.org/10.3390/photonics11020172 - 12 Feb 2024
Viewed by 837
Abstract
Free-space optical (FSO) communication is a line-of-sight (LOS) communication technology that uses light, typically lasers, to transmit data through the atmosphere. FSO can provide high data transfer rates, but factors like weather conditions can affect its performance. Like fog, smog also degrades the [...] Read more.
Free-space optical (FSO) communication is a line-of-sight (LOS) communication technology that uses light, typically lasers, to transmit data through the atmosphere. FSO can provide high data transfer rates, but factors like weather conditions can affect its performance. Like fog, smog also degrades the availability and reliability of FSO links, as the particulate matter (PM) present in smog scatters the light beam, causing perceptible attenuation. In this paper, we have investigated the attenuation of an optical signal under laboratory-controlled smog conditions, using both theoretical and experimental approaches. A 6 m long acrylic chamber is used to contain artificial smog and measure the optical attenuation through it. The experimental result shows that smog attenuation is approximately 1.705 times more than fog attenuation. The findings of this study offer valuable insights into the effects of smog on optical links and can contribute to the development and optimization of these systems in regions with high levels of smog. Full article
(This article belongs to the Special Issue New Advances in Optical Wireless Communication)
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7 pages, 1873 KiB  
Communication
Compact Diode-Pumped 946 nm Nd:YAG Laser with Good Beam Quality
by Rubel C. Talukder and Arkady Major
Photonics 2024, 11(2), 171; https://doi.org/10.3390/photonics11020171 - 12 Feb 2024
Viewed by 843
Abstract
A continuous-wave (CW), high-power, quasi-three-level Nd:YAG laser operating at 946 nm is reported. The beam quality of the laser is greatly improved. The laser consists of a composite Nd:YAG rod end pumped by a fiber-coupled diode laser inside a simple concave-plane cavity. At [...] Read more.
A continuous-wave (CW), high-power, quasi-three-level Nd:YAG laser operating at 946 nm is reported. The beam quality of the laser is greatly improved. The laser consists of a composite Nd:YAG rod end pumped by a fiber-coupled diode laser inside a simple concave-plane cavity. At an incident pump power of 31 W, a maximum CW output of 9.98 W was obtained at 946 nm, with a beam quality factor of M2~5. The corresponding optical-to-optical efficiency was 32.2% with respect to the incident pump power. To the best of our knowledge, this is the highest output power at 946 nm with such a beam quality ever generated by diode-pumped Nd:YAG laser with bulk crystals. Full article
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17 pages, 4567 KiB  
Article
Asymmetrical Cross-Polarization Coupling in a Whispering-Gallery Microresonator
by Karleyda Sandoval and A. T. Rosenberger
Photonics 2024, 11(2), 170; https://doi.org/10.3390/photonics11020170 - 11 Feb 2024
Viewed by 756
Abstract
Cross-polarization coupling between transverse electric (TE) and transverse magnetic (TM) whispering-gallery modes in an optical microresonator produces effects such as coupled-mode induced transparency (CMIT). The detailed analytical theory of this coupling indicates that the TE-to-TM and TM-to-TE couplings may have different strengths. Using [...] Read more.
Cross-polarization coupling between transverse electric (TE) and transverse magnetic (TM) whispering-gallery modes in an optical microresonator produces effects such as coupled-mode induced transparency (CMIT). The detailed analytical theory of this coupling indicates that the TE-to-TM and TM-to-TE couplings may have different strengths. Using an experimental setup centered around a hollow bottle resonator and polarization-sensitive throughput detection, that had been used in previous CMIT experiments, this asymmetry was confirmed and studied. By fitting the throughput spectra of both polarizations to the numerical output of a basic model, the asymmetry parameter defined as the ratio of the coupling amplitudes was determined from the output power in the polarization orthogonal to that of the input. The results of many experiments give a range for this ratio, roughly from 0.2 to 4, that agrees with the range predicted by the detailed theory. An analytical approximation of this ratio shows that the main reason for the asymmetry is a difference in the axial orders of the coupled modes. In some experimental cases, the orthogonal output is not well fitted by the model that assumes a single mode of each polarization, and we demonstrate that this fitting discrepancy can be the result of additional mode interactions. Full article
(This article belongs to the Special Issue Advances in Optical Microresonators)
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16 pages, 3586 KiB  
Article
Planar Bilayer PT-Symmetric Systems and Resonance Energy Transfer
by Aliaksandr Arlouski and Andrey Novitsky
Photonics 2024, 11(2), 169; https://doi.org/10.3390/photonics11020169 - 10 Feb 2024
Viewed by 792
Abstract
Parity-time (PT) symmetry provides an outstanding improvement of photonic devices’ performance due to the remarkable physics behind it. Resonance energy transfer (RET) as an important characteristic mediating the molecules that can be tailored in the PT-symmetric environment, too. We study how planar bilayer [...] Read more.
Parity-time (PT) symmetry provides an outstanding improvement of photonic devices’ performance due to the remarkable physics behind it. Resonance energy transfer (RET) as an important characteristic mediating the molecules that can be tailored in the PT-symmetric environment, too. We study how planar bilayer PT-symmetric systems affect the process of resonance energy transfer occurring in the vicinity thereof. First, we investigate the reflectance and transmittance spectra of such systems by calculating reflection and transmission coefficients as well as total radiation amplification as functions of medium parameters. We obtain that reflectance and total amplification are greatest near the exceptional points of the PT-symmetric system. Then, we perform numerical calculations of the RET rate and investigate its dependence on the complex permittivity of the PT-symmetric medium, dipole orientation, frequency of radiation and layer thickness. Optically thick PT-symmetric systems may operate at lower gain at the expense of the appearance of chaotic-like behaviors. These appear owing to the dense oscillations in the reflectance and transmittance spectra and vividly manifest themselves as stochastic-like positions of the exceptional points for PT-symmetric bilayers. The RET rate, being a result of the field interference, can be significantly amplified and suppressed near exceptional points exhibiting a Fano-like lineshape. Full article
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11 pages, 6020 KiB  
Article
The Origin of Threshold Reduction in Random Lasers Based on MoS2/Au NPs: Charge Transfer
by Yanyan Huo, Ke Sun, Yuqian Zhang, Weihao Liu, Junkun Wang, Yuan Wan, Lina Zhao, Tingyin Ning, Zhen Li and Yingying Ren
Photonics 2024, 11(2), 168; https://doi.org/10.3390/photonics11020168 - 09 Feb 2024
Viewed by 829
Abstract
Random lasers have attracted much attention in recent years owing to their advantages of a simple fabrication process, low processing cost, and material flexibility for any lasing wavelengths. They provide a roadmap for the design of ultra-bright lighting, displays, etc. However, the threshold [...] Read more.
Random lasers have attracted much attention in recent years owing to their advantages of a simple fabrication process, low processing cost, and material flexibility for any lasing wavelengths. They provide a roadmap for the design of ultra-bright lighting, displays, etc. However, the threshold reduction in random nanolasers remains a challenge in practical applications. In this work, lower-threshold random laser action from monolayer molybdenum disulfide film-encapsulated Au nanoparticles (MoS2/Au NPs) is demonstrated. The observed laser action of the MoS2/Au NPs shows a lower threshold of about 0.564 µJ/mm2, which is about 46.2% lower than the threshold of random lasers based on Au NPs. We proposed that the charge transfer between MoS2 and the gain material is the main reason for the reduction in the random laser threshold. The finite-difference time-domain (FDTD) method was used to calculate the lasing action of these two nanostructures. When charge transfer is taken into account, the theoretically calculated threshold of the MoS2/Au NPs is reduced by 46.8% compared to Au NP samples, which is consistent with the experimental results. This study provides a new mechanism to achieve low-threshold and high-quality random lasers, which has the potential to facilitate the application of random lasers and the development of high-performance optoelectronic devices. Full article
(This article belongs to the Special Issue Coherence Properties of Light: From Theory to Applications)
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10 pages, 1947 KiB  
Article
Active Optical Tuning of Azopolymeric Whispering Gallery Mode Microresonators for Filter Applications
by Gabriel H. A. Jorge, Filipe A. Couto, Juliana M. P. Almeida, Victor A. S. Marques, Marcelo B. Andrade and Cleber R. Mendonça
Photonics 2024, 11(2), 167; https://doi.org/10.3390/photonics11020167 - 09 Feb 2024
Viewed by 891
Abstract
Light confinement provided by whispering gallery mode (WGM) microresonators is especially useful for integrated photonic circuits. In particular, the tunability of such devices has gained increased attention for active filtering and lasering applications. Traditional lithographic approaches for fabricating such devices, especially Si-based ones, [...] Read more.
Light confinement provided by whispering gallery mode (WGM) microresonators is especially useful for integrated photonic circuits. In particular, the tunability of such devices has gained increased attention for active filtering and lasering applications. Traditional lithographic approaches for fabricating such devices, especially Si-based ones, often restrict the device’s tuning due to the material’s inherent properties. Two-photon polymerization (2PP) has emerged as an alternative fabrication technique of sub-diffraction resolution 3D structures, in which compounds can be incorporated to further expand their applications, such as enabling active devices. Here, we exploited the advantageous characteristics of polymer-based devices and produced, via 2PP, acrylic-based WGM hollow microcylinders incorporated with the azoaromatic chromophore Disperse Red 13 (DR13). Within telecommunication range, we demonstrated the tuning of the microresonator’s modes by external irradiation within the dye’s absorption peak (at 514 nm), actively inducing a blueshift at a rate of 1.2 nm/(Wcm−2). Its thermo-optical properties were also investigated through direct heating, and the compatibility of both natural phenomena was also confirmed by finite element simulations. Such results further expand the applicability of polymeric microresonators in optical and photonic devices since optically active filtering was exhibited. Full article
(This article belongs to the Special Issue Advances in Optical Microresonators)
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19 pages, 31766 KiB  
Article
Measurement and Evaluation Method of Distributed Optical Fiber Acoustic Sensing Performance
by Zhiyuan Xie, Yuwei Sun, Anqiang Lv and Qian Xu
Photonics 2024, 11(2), 166; https://doi.org/10.3390/photonics11020166 - 08 Feb 2024
Viewed by 717
Abstract
Distributed acoustic sensing incorporates multiple indicators, and there exists a mutually constraining relationship among these indicators. Different application fields have varying requirements for indicators. Therefore, indicator testing and comprehensive evaluations are crucial for engineering applications. In this paper, we conducted a theoretical analysis [...] Read more.
Distributed acoustic sensing incorporates multiple indicators, and there exists a mutually constraining relationship among these indicators. Different application fields have varying requirements for indicators. Therefore, indicator testing and comprehensive evaluations are crucial for engineering applications. In this paper, we conducted a theoretical analysis of key indicators, including frequency response, sensitivity, spatial resolution, sensing distance, multi-point perturbation, and temperature influence. The indicator test scheme was developed, and a test system was constructed. The test data were analyzed and compared in the time-frequency domain. A performance evaluation method for distributed acoustic sensing, based on the analytic hierarchy process, is proposed, and a comprehensive evaluation example focused on high-frequency applications is presented. The results show that the test scheme and method presented in this paper can accurately measure the upper limits of each indicator of distributed acoustic sensing. The proposed comprehensive evaluation method enables the assessment of sensor performance and applicability based on engineering practices. It addresses the challenge of evaluating distributed acoustic sensing with multiple indicators and offers an efficient approach for equipment development and engineering applications. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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10 pages, 1054 KiB  
Article
Associations between Ocular Biometry, Refractive Error, and Body Characteristics
by Veronica Noya-Padin, Noelia Nores-Palmas, Jacobo Garcia-Queiruga, Maria J. Giraldez, Hugo Pena-Verdeal and Eva Yebra-Pimentel
Photonics 2024, 11(2), 165; https://doi.org/10.3390/photonics11020165 - 08 Feb 2024
Viewed by 853
Abstract
Myopia is a refractive error widely spread throughout the world, usually related to excessive axial length (AL) of the eye. This elongation could have severe consequences, even leading to blindness. However, AL varies among subjects, and it may be correlated with other anthropometric [...] Read more.
Myopia is a refractive error widely spread throughout the world, usually related to excessive axial length (AL) of the eye. This elongation could have severe consequences, even leading to blindness. However, AL varies among subjects, and it may be correlated with other anthropometric parameters. The aim of this study was to evaluate the relationships between AL, body height, refractive error, and sex. A total of 72 eyes of 36 myopic participants with a mean age of 11.1 ± 1.42 years (ranging from 8 to 14 years) were included in the study. Participants underwent objective refraction by NVision-K5001, AL measurement by Topcon MYAH biometer, and body height measurement. Significant correlations were observed between AL, body height, and spherical equivalent (SE) (Spearman’s correlation, all p ≤ 0.016). When participants were grouped by AL, significant differences were observed for body height and SE, and when grouped by height percentile, significant differences were observed for AL and SE (Kruskal–Wallis test, all p ≤ 0.006). There was a significant difference in SE, AL, and body height between genders (Mann–Whitney U test, all p ≤ 0.038). AL relates to the refractive state of the eye and is also influenced by individual anatomical characteristics. Full article
(This article belongs to the Special Issue Latest Developments in Ocular Biometry)
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15 pages, 12173 KiB  
Article
Deep Learning for Polarization Optical System Automated Design
by Haodong Shi, Ruihan Fan, Chunfeng He, Jiayu Wang, Shuai Yang, Miao Xu, Hongyu Sun, Yingchao Li and Qiang Fu
Photonics 2024, 11(2), 164; https://doi.org/10.3390/photonics11020164 - 08 Feb 2024
Viewed by 788
Abstract
Aiming at the problem that traditional design methods make it difficult to control the polarization aberration distribution of optical systems quickly and accurately, this study proposes an automatic optimization design method for polarization optical systems based on deep learning. The unsupervised training model [...] Read more.
Aiming at the problem that traditional design methods make it difficult to control the polarization aberration distribution of optical systems quickly and accurately, this study proposes an automatic optimization design method for polarization optical systems based on deep learning. The unsupervised training model based on ray tracing and polarized ray tracing was constructed by learning the reference lens structural feature data from the optical lens library, and the generalization ability of the deep neural network model was improved to achieve the automatic optimization design of the polarized optical system. The design results show that the optical system structure optimized by the network model is close to the reference lens in the full field of view and the full spectrum and that the optical system structure can be designed for different focal length requirements. The success rate of 1 million groups of initial structures designed is better than 96.403%, and the polarization effect of the optical system is effectively controlled. The proposed deep learning approach to optical design provides a new solution for future complex optical systems and also provides an effective way to improve the design accuracy of special optical systems such as polarization optical systems. Full article
(This article belongs to the Special Issue Design and Applications of Polarized Optical System)
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13 pages, 2473 KiB  
Article
Terahertz Polarization-Resolved Spectra of the Metamaterial Formed by Optimally Shaped Omega Elements on a Silicon Substrate at Oblique Incidence of Waves
by Andrew V. Lyakhnovich, Igor V. Semchenko, Andrey L. Samofalov, Maksim A. Podalov, George V. Sinitsyn, Alexandr Y. Kravchenko and Sergei A. Khakhomov
Photonics 2024, 11(2), 163; https://doi.org/10.3390/photonics11020163 - 07 Feb 2024
Viewed by 854
Abstract
The reflection and transmission spectra of a metamaterial formed by omega-shaped elements with pre-calculated optimal parameters on a silicon substrate have been recorded in the terahertz range at oblique incidence for the s- and p-polarizations of the incident wave. The spectra were interpreted [...] Read more.
The reflection and transmission spectra of a metamaterial formed by omega-shaped elements with pre-calculated optimal parameters on a silicon substrate have been recorded in the terahertz range at oblique incidence for the s- and p-polarizations of the incident wave. The spectra were interpreted within the dipole radiation theory of electromagnetic waves. Both measurement results and analysis provide evidence supporting the presence of a pronounced polarization anisotropy impact in the reflection and transmission of the metamaterial. The potential of these materials to be utilized in the development of devices that control the polarization properties of THz radiation across a wide spectral range is examined. Full article
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13 pages, 3218 KiB  
Article
Enhanced THz Circular-Polarization Detection in Miniaturized Chips with Chiral Antennas
by Fangzhe Li, Jing Zhou, Jie Deng, Jinyong Shen, Tianyun Zhu, Wenji Jing, Xu Dai, Jiexian Ye, Yujie Zhang, Junwei Huang and Xiaoshuang Chen
Photonics 2024, 11(2), 162; https://doi.org/10.3390/photonics11020162 - 07 Feb 2024
Viewed by 857
Abstract
Recent advancements in terahertz (THz) wave technology have highlighted the criticality of circular-polarization detection, fostering the development of more compact, efficient on-chip THz circular-polarization detectors. In response to this technological imperative, we presented a chiral-antenna-integrated GaAs/AlGaAs quantum well (QW) THz detector. The chiral [...] Read more.
Recent advancements in terahertz (THz) wave technology have highlighted the criticality of circular-polarization detection, fostering the development of more compact, efficient on-chip THz circular-polarization detectors. In response to this technological imperative, we presented a chiral-antenna-integrated GaAs/AlGaAs quantum well (QW) THz detector. The chiral antenna selectively couples the incident light of a specific circular-polarization state into a surface-plasmon polariton wave that enhances the absorptance of the QWs by a factor of 12 relative to a standard 45° faceted device, and reflects a significant amount of the incident light of the orthogonal circular-polarization state. The circular-polarization selectivity is further enhanced by the QWs with a strong intrinsic anisotropy, resulting in a circular-polarization extinction ratio (CPER) as high as 26 at 6.52 THz. In addition, the operation band of the device can be adjusted by tuning the structural parameters of the chiral structure. Moreover, the device preserves a high performance for oblique incidence within a range of ±5°, and the device architecture is compatible with a focal plane array. This report communicates a promising approach for the development of miniaturized on-chip THz circular-polarization detectors. Full article
(This article belongs to the Special Issue Advanced Photonic Sensing and Measurement II)
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11 pages, 4124 KiB  
Article
The SMILE Effect in the Beam Propagation Direction Affects the Beam Shaping of a Semiconductor Laser Bar Array
by Hongyou Zhang, Yu Hu, Shuihai Peng and Yong Liu
Photonics 2024, 11(2), 161; https://doi.org/10.3390/photonics11020161 - 07 Feb 2024
Viewed by 720
Abstract
Near-field bending of a laser diode bar (i.e., the SMILE effect) degrades the laser beam brightness, adversely affecting optical coupling and beam shaping. Previous reports mainly focused on the two-dimensional near-field bending of a laser diode bar. However, the near-field bending of a [...] Read more.
Near-field bending of a laser diode bar (i.e., the SMILE effect) degrades the laser beam brightness, adversely affecting optical coupling and beam shaping. Previous reports mainly focused on the two-dimensional near-field bending of a laser diode bar. However, the near-field bending of a laser diode bar not only occurs in the laser bar growth direction, but also in the beam propagation direction. The present article proposes the three-dimensional near-field bending of a laser diode array, which is commonly known as the three-dimensional spatial SMILE effect. Through theoretical and simulated investigations, it has been found that a laser bar array not only deforms in the fast axis direction to cause the traditional two-dimensional SMILE effect but also experiences an additional deformation of approximately 2 μm in the laser emission direction simultaneously. Due to the SMILE effect in the beam propagation direction, not all emitters are aligned in a straight line, and some emitters experience defocusing during collimation. Consequently, there is an increase in the residual divergence angle and beam width, resulting in a degradation of the laser bar array’s beam quality. According to the theoretical calculations, ZEMAX simulations, and experimental results, for a FAC (fast axis collimation) with a focal length of 300 μm, the divergence angle of single emitter after collimating in the fast axis increases from 4.95 mrad to 6.46 mrad when the offsetting of the working distance between the incident beam waist and FAC lens increases from 0 μm to 2 μm. Full article
(This article belongs to the Special Issue Laser Beam Propagation and Control)
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