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Photonics, Volume 10, Issue 7 (July 2023) – 156 articles

Cover Story (view full-size image): Sb-based materials have demonstrated the potential to create avalanche photodiodes with breakthrough performance. Over the past 50 years, there have been many efforts to develop avalanche photodiodes that can achieve multiplication noise as low as those fabricated from Si, but that operate at the longer wavelengths required for applications such as optical communications, night-vision imaging, and, more recently, quantum information processing and transmission and LIDAR. Avalanche photodiodes fabricated from AlInAsSb and AlGaAsSb now offer that capability for the SWIR and MWIR spectral regions. Since these are III–V compounds with high absorption coefficients and high carrier saturation velocities, they also demonstrate excellent performance in the near-infrared region and the MWIR. View this paper
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14 pages, 797 KiB  
Review
Atmospheric Pre-Compensation of Ground-to-Space Communications with Adaptive Optics: Past, Present and Future—A Field Review
by Noelia Martinez
Photonics 2023, 10(7), 858; https://doi.org/10.3390/photonics10070858 - 24 Jul 2023
Cited by 1 | Viewed by 1761
Abstract
Conventionally used in astronomy, adaptive optics (AO) systems measure and correct for turbulence and, therefore, have the capability to mitigate the impact of the atmosphere on the ground-to-space communication links. Historically, there have been two main streams, respectively, advocating to use or not [...] Read more.
Conventionally used in astronomy, adaptive optics (AO) systems measure and correct for turbulence and, therefore, have the capability to mitigate the impact of the atmosphere on the ground-to-space communication links. Historically, there have been two main streams, respectively, advocating to use or not use adaptive optics on optical communications. This paper aims to present a comprehensive review of the field of AO-based uplink pre-compensation. It will cover the technical challenges associated with uplink correction, provide an overview of the state-of-the-art research and demonstrations from the early days to the present, and discuss the future prospects of AO-based uplink pre-compensation and potential trade-offs between performance, cost, and operational considerations. Full article
(This article belongs to the Special Issue New Perspectives in Free-Space Optical Communications and Networks)
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13 pages, 2459 KiB  
Communication
Nonparaxial Focusing of Partially Coherent Gaussian Schell-Model and Bessel-Correlated Beams in Free Space
by Nikolai I. Petrov
Photonics 2023, 10(7), 857; https://doi.org/10.3390/photonics10070857 - 24 Jul 2023
Cited by 1 | Viewed by 799
Abstract
The nonparaxial focusing of partially coherent beams in free space has been studied using the coherent-state and coherent-mode decomposition methods. Analytical expressions for the width and angular divergence of partially coherent Gaussian Schell-model (GSM) beams have been obtained using the coherent-state method. It [...] Read more.
The nonparaxial focusing of partially coherent beams in free space has been studied using the coherent-state and coherent-mode decomposition methods. Analytical expressions for the width and angular divergence of partially coherent Gaussian Schell-model (GSM) beams have been obtained using the coherent-state method. It has been shown that the focusing plane is shifted in the opposite axial direction compared to the geometric focusing plane. The influence of the nonparaxiality and spatial coherence of Bessel-correlated vortex beams on the intensity distribution and displacement of the focus plane has been analyzed. It has been shown that the shift of the focus plane increases with a decrease in the coherence radius of the source. A smaller diffraction spread has been shown for partially coherent Bessel-correlated beams compared to GSM beams. Full article
(This article belongs to the Special Issue Light Focusing and Optical Vortices)
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7 pages, 2711 KiB  
Communication
Vortex Beam in a Turbulent Kerr Medium for Atmospheric Communication
by Andrey D. Bulygin, Yury E. Geints and Ilia Y. Geints
Photonics 2023, 10(7), 856; https://doi.org/10.3390/photonics10070856 - 24 Jul 2023
Cited by 3 | Viewed by 984
Abstract
The dynamics of the topological charge of a vortex optical beam propagating in turbulent air while accounting for the cubic nonlinearity is theoretically considered. In a number of examples, we show that the optical beam, self-focusing, manifests itself ambiguously depending on the optical [...] Read more.
The dynamics of the topological charge of a vortex optical beam propagating in turbulent air while accounting for the cubic nonlinearity is theoretically considered. In a number of examples, we show that the optical beam, self-focusing, manifests itself ambiguously depending on the optical wave power. At near-critical values of beam power, self-focusing leads to enhanced spatial localization of optical vortices and substantial suppression of vortex walk-off relative to the beam axis caused by air turbulence. However, with increasing optical intensity, the modulation instability imposed by cubic nonlinearity becomes significant and contributes jointly with medium turbulence and leads to faster divergence of vortex beams. Full article
(This article belongs to the Special Issue Nonlinear and Ultrafast Optics: Fundamentals and Applications)
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12 pages, 2917 KiB  
Article
The Light-Trapping Character of Pit Arrays on the Surface of Solar Cells
by Baohua Zhu, Le Chen, Song Ye and Wei Luo
Photonics 2023, 10(7), 855; https://doi.org/10.3390/photonics10070855 - 24 Jul 2023
Cited by 2 | Viewed by 1005
Abstract
Surfaces with light-trapping structures are widely used in solar cells to enhance light capturing and to transform efficiency. The study of light-trapping character is important for light-trapping structures in solar cells. In the present study, the light-trapping character for the regular hemisphere pit [...] Read more.
Surfaces with light-trapping structures are widely used in solar cells to enhance light capturing and to transform efficiency. The study of light-trapping character is important for light-trapping structures in solar cells. In the present study, the light-trapping character for the regular hemisphere pit arrays (RHPAs) in solar cells was intensively investigated in terms of reducing light reflection, suppressing light escape, and increasing the length of the optical path. Results show that the RHPAs can decrease surface reflectivity by ~54% compared with the plane structure, and can reflect ~33% of the light that has not been absorbed back into the absorption layer of the solar cell. The total optical path of the cell with the RHPAs structure remarkably increased from 2ω to 4ω. To verify the theoretical research conclusions, we produced the glass structure samples with different aspect ratios by using micro/nanometer-processing technology. The reflection ratios for silicon wafers covered by plane and RHPAs glass samples were tested. The test results were compared with the theoretical calculation results, which showed consistency. Full article
(This article belongs to the Topic Advances in Solar Technologies)
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15 pages, 2130 KiB  
Article
High-Order Harmonics Generation in Selenium-Containing Plasmas
by Rashid A. Ganeev
Photonics 2023, 10(7), 854; https://doi.org/10.3390/photonics10070854 - 24 Jul 2023
Cited by 2 | Viewed by 899
Abstract
The studies of the high-order harmonics generated in Se-containing plasmas are reported. The ablation of selenium in a vacuum allowed for the formation of a plasma demonstrating high-order harmonics generation and resonance enhancement of the harmonic at the shortest wavelength reported so far [...] Read more.
The studies of the high-order harmonics generated in Se-containing plasmas are reported. The ablation of selenium in a vacuum allowed for the formation of a plasma demonstrating high-order harmonics generation and resonance enhancement of the harmonic at the shortest wavelength reported so far (λ ≈ 22.9 nm, Eph ≈ 54.14 eV). This harmonic corresponds to the 35th order of the 800-nm-class lasers. The influence of the presence of selenium in the molecular state (ZnSe and HgSe) on the suppression of the resonance effect during harmonics generation in plasma is studied. The enhanced 35th harmonic was analyzed by different methods of plasma formation using nanosecond, picosecond, and femtosecond pulses. The enhancement factor of the resonance-enhanced harmonic was measured to be 32× compared with the neighboring lower-order harmonics in the case of the picosecond-pulses-induced Se plasma. The role of the strong ionic transition of Se in the region of 22.7 nm in the observation of the resonance-induced enhancement of a single harmonic is discussed. Full article
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15 pages, 6480 KiB  
Article
Research on Indoor Visible Light Location Based on Fusion Clustering Algorithm
by Chenghu Ke, Yuting Shu and Xizheng Ke
Photonics 2023, 10(7), 853; https://doi.org/10.3390/photonics10070853 - 23 Jul 2023
Viewed by 1011
Abstract
Aiming at the problem of large positioning errors in the boundary area, a new location fingerprint location method based on a fusion clustering algorithm is proposed. This clustering-based method embodies the idea of rough location first and then fine location. Firstly, the edge [...] Read more.
Aiming at the problem of large positioning errors in the boundary area, a new location fingerprint location method based on a fusion clustering algorithm is proposed. This clustering-based method embodies the idea of rough location first and then fine location. Firstly, the edge regions of the received signal strength (RSS) samples which are greatly affected by reflection are divided using the k-medoids algorithm, and then the center part is clustered via density-based spatial clustering of applications with noise (DBSCAN). In the actual location estimation stage, the points to be measured can only be located in one of the classified areas, and combined with the optimal k-nearest neighbor algorithm (WOKNN) to match the location. The results show that the average positioning error of the algorithm is 13 cm in an indoor environment of 5 m × 5 m × 3 m. Compared with the traditional method without clustering, the positioning accuracy of the edge area is increased by 21%, and the overall improvement is 33.8%, which proves that the proposed algorithm effectively improves the efficiency of real-time positioning and indoor positioning accuracy. Full article
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22 pages, 10357 KiB  
Review
Deep Learning and Adjoint Method Accelerated Inverse Design in Photonics: A Review
by Zongyong Pan and Xiaomin Pan
Photonics 2023, 10(7), 852; https://doi.org/10.3390/photonics10070852 - 23 Jul 2023
Cited by 5 | Viewed by 2679
Abstract
For photonic applications, the inverse design method plays a critical role in the optimized design of photonic devices. According to its two ingredients, inverse design in photonics can be improved from two aspects: to find solutions to Maxwell’s equations more efficiently and to [...] Read more.
For photonic applications, the inverse design method plays a critical role in the optimized design of photonic devices. According to its two ingredients, inverse design in photonics can be improved from two aspects: to find solutions to Maxwell’s equations more efficiently and to employ a more suitable optimization scheme. Various optimization algorithms have been employed to handle the optimization: the adjoint method (AM) has become the one of the most widely utilized ones because of its low computational cost. With the rapid development of deep learning (DL) in recent years, inverse design has also benefited from DL algorithms, leading to a new pattern of photon inverse design. Unlike the AM, DL can be an efficient solver of Maxwell’s equations, as well as a nice optimizer, or even both, in inverse design. In this review, we discuss the development of the AM and DL algorithms in inverse design, and the advancements, advantages, and disadvantages of the AM and DL algorithms in photon inverse design. Full article
(This article belongs to the Special Issue Recent Trends in Computational Photonics)
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10 pages, 3291 KiB  
Communication
Efficient 1054 nm Raman Random Fiber Laser
by Pan Wang, Shengtao Lin, Jiaojiao Zhang, Xingyu Bao, Longqun Ni, Yifei Qi and Zinan Wang
Photonics 2023, 10(7), 851; https://doi.org/10.3390/photonics10070851 - 22 Jul 2023
Cited by 2 | Viewed by 1177
Abstract
Low–coherence laser is regarded as the key to mitigating laser–plasma instability (LPI) in laser–driven inertial confinement fusion (ICF), where LPI can decrease the laser energy coupled to the target. With the merits of low coherence, high spectral stability, and flexible output characteristics, the [...] Read more.
Low–coherence laser is regarded as the key to mitigating laser–plasma instability (LPI) in laser–driven inertial confinement fusion (ICF), where LPI can decrease the laser energy coupled to the target. With the merits of low coherence, high spectral stability, and flexible output characteristics, the Raman random fiber laser (RRFL) is considered to be a candidate light source in ICF. In this paper, the 1054 nm RRFL with high slope efficiency is achieved for the first time. In the RRFL pump source design section, we have optimized the ytterbium–doped fiber (YDF) length by simulation and amplified the power by Master Oscillator Power Amplifier (MOPA) to realize a 1011 nm YDF laser with 47.3 dB optical signal–to–noise ratio (OSNR). In terms of RRFL cavity design, a fiber loop mirror and Rayleigh scattering in the HI 1060 Flex fiber provide wideband point feedback and random distributed feedback, respectively. Based on this system, we achieve an RRFL output with 0.4 nm half–maximum full width, 182% slope efficiency, and 41.3 dB OSNR. This work will provide guidance for the application of RRFL in high–energy–density physics research. Full article
(This article belongs to the Special Issue High Power Fiber Laser and Amplifiers)
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8 pages, 1117 KiB  
Article
Numerical Analysis of the Effect of Decentered Refractive Segmented Extended Depth of Focus (EDoF) Intraocular Lenses on Predicted Visual Outcomes
by Scott García, Luis Salvá, Salvador García-Delpech, Anabel Martínez-Espert and Vicente Ferrando
Photonics 2023, 10(7), 850; https://doi.org/10.3390/photonics10070850 - 22 Jul 2023
Viewed by 897
Abstract
This study aimed to evaluate the optical performance of a rotationally asymmetric intraocular lens (IOL) when it is decentered relative to the visual axis. The FEMTIS Comfort IOL (Teleon Surgical B.V., Spankeren, The Netherlands) was assessed using ray tracing software in the Atchison [...] Read more.
This study aimed to evaluate the optical performance of a rotationally asymmetric intraocular lens (IOL) when it is decentered relative to the visual axis. The FEMTIS Comfort IOL (Teleon Surgical B.V., Spankeren, The Netherlands) was assessed using ray tracing software in the Atchison model eye at apertures of 3.0 mm and 4.5 mm. The metric used for assessment was the through-the-focus area under the modulation transfer function (TF-MTFa). Decentrations of 0.2 mm and 0.4 mm were considered. Our results indicated that the MTFa defocus curves exhibited significant differences depending on the direction of vertical decentration. Downward decentrations shifted the MTFa curve towards virtual vergences, resulting in improved optical quality at far distances but decreased optical quality at intermediate and near vision. Conversely, upward decentrations produced the opposite effect. Since, on one hand, this lens is fixed within the capsulorhexis during surgery, demonstrating excellent stability, and on the other hand, the precise centration of the capsulorhexis can be made accurately off the visual axis, these results provide surgeons with the opportunity to plan various clinical scenarios to optimize surgical outcomes with this IOL by selecting the optimal location for capsulorhexis centration in each patient. Full article
(This article belongs to the Special Issue Visual Optics)
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13 pages, 3161 KiB  
Article
Enhanced Yb:YAG Active Mirrors for High Power Laser Amplifiers
by Vladimir A. Petrov, Gleb V. Kuptsov, Alyona O. Kuptsova, Victor V. Atuchin, Elena V. Stroganova and Victor V. Petrov
Photonics 2023, 10(7), 849; https://doi.org/10.3390/photonics10070849 - 21 Jul 2023
Cited by 6 | Viewed by 1087
Abstract
The work is aimed at the investigation of the influence of nonlinear active ions concentration profiles in Yb:YAG laser elements on temperature distribution and wavefront distortions during amplification using sub-kilowatt level diode pumping. A mathematical model is presented for the theoretical study of [...] Read more.
The work is aimed at the investigation of the influence of nonlinear active ions concentration profiles in Yb:YAG laser elements on temperature distribution and wavefront distortions during amplification using sub-kilowatt level diode pumping. A mathematical model is presented for the theoretical study of the amplification process in crystals with cubic crystal system. A detailed comparison of Yb:YAG active elements with the same thickness and absorbed pumping power, but with various concentration profiles of Yb3+, ions is carried out. It is shown that the use of active elements with an increasing dopant concentration in the pump beam direction allows one to optimize the temperature profile inside the active element and, thus, reduce the thermal-induced wavefront distortions of the amplified radiation. Modeling is carried out for the experimentally grown crystal with linear concentration gradient profile. It is shown that the linear doping profile with a gradient of 0.65 at.%/mm allows increasing the small-signal gain up to 10% and decreasing the thermal-induced wavefront distortions by ~15%. Full article
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12 pages, 3177 KiB  
Communication
Focusing of Radially Polarized Electromagnetic Waves by a Parabolic Mirror
by Zerihun Tadele Godana, János Hebling and László Pálfalvi
Photonics 2023, 10(7), 848; https://doi.org/10.3390/photonics10070848 - 21 Jul 2023
Viewed by 1219
Abstract
It is well-known that a strong longitudinal electric field and a small spot size are observed when radially polarized beams are tightly focused using a high numerical aperture parabolic mirror. The longitudinal electric field component can accelerate electrons along the propagation axis at [...] Read more.
It is well-known that a strong longitudinal electric field and a small spot size are observed when radially polarized beams are tightly focused using a high numerical aperture parabolic mirror. The longitudinal electric field component can accelerate electrons along the propagation axis at high intensities in the focal region, which opens an application in particle acceleration. In this paper, we present a rigorous derivation of the electric field obtained when a radially polarized, monochromatic, flat-top beam is focused by a parabolic mirror. The formulae were deduced from the Stratton–Chu integral known from vector diffraction theory. We examined the influence of the focusing parameters on the distribution of both the longitudinal and radial electric field components. In the small numerical aperture and short wavelength regimes, excellent agreement was found with the results obtained from the Rayleigh–Sommerfeld formula. The calculation method can be adapted for various beam types and for electromagnetic pulses as well. Full article
(This article belongs to the Special Issue Photonics: Theory, Technologies and Applications)
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8 pages, 4421 KiB  
Communication
Photonic-Assisted Microwave Frequency Measurement Using High Q-Factor Microdisk with High Accuracy
by Mengyao Zhao, Wenyu Wang, Lei Shi, Chicheng Che and Jianji Dong
Photonics 2023, 10(7), 847; https://doi.org/10.3390/photonics10070847 - 21 Jul 2023
Cited by 3 | Viewed by 1288
Abstract
Frequency measurement plays a crucial role in radar, communication, and various applications. The photonic-assisted frequency measurement method offers several advantages, including resistance to electromagnetic interference, broad bandwidth, and low power consumption. Notably, frequency-to-time mapping enables the measurement of various microwave signal types, such [...] Read more.
Frequency measurement plays a crucial role in radar, communication, and various applications. The photonic-assisted frequency measurement method offers several advantages, including resistance to electromagnetic interference, broad bandwidth, and low power consumption. Notably, frequency-to-time mapping enables the measurement of various microwave signal types, such as single-frequency, multiple-frequency, frequency hopping, and chirped signals. However, the accuracy of this method is currently limited due to the absence of resonant devices with high-quality factors, which are essential for achieving higher-precision measurements. In this work, a frequency measurement method based on an ultrahigh-quality-factor microdisk is proposed. By establishing a correlation between the time difference and the frequency to be measured, a reduction in measurement error to below 10 MHz within a frequency measurement range of 3 GHz is realized. Our work introduces a new approach to frequency measurement using optical devices, opening new possibilities in this field. Full article
(This article belongs to the Special Issue Microwave Photonic Techniques)
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17 pages, 7679 KiB  
Article
Twisted Silica Few-Mode Hollow GeO2-Doped Ring-Core Microstructured Optical Fiber
by Anton V. Bourdine, Vladimir V. Demidov, Egishe V. Ter-Nersesyants, Grigori A. Pchelkin, Dmitriy N. Shurupov, Alexander V. Khokhlov, Alexandra S. Matrosova, Andrey I. Kashin, Sergei V. Bureev, Michael V. Dashkov, Alexander S. Evtushenko, Elena S. Zaitseva, Azat R. Gizatulin, Ivan K. Meshkov, Amogh A. Dyavangoudar, Ankur Saharia, Manish Tiwari, Alexander A. Vasilets, Vasiliy S. Elagin, Ghanshyam Singh and Konstantin V. Dukelskiiadd Show full author list remove Hide full author list
Photonics 2023, 10(7), 846; https://doi.org/10.3390/photonics10070846 - 21 Jul 2023
Cited by 1 | Viewed by 1075
Abstract
This work presents the first instance of a silica few-mode microstructured optical fiber (MOF) being successfully fabricated with a hollow GeO2-doped ring core and by strongly inducing twisting up to 790 revolutions per meter. Some technological issues that occurred during the [...] Read more.
This work presents the first instance of a silica few-mode microstructured optical fiber (MOF) being successfully fabricated with a hollow GeO2-doped ring core and by strongly inducing twisting up to 790 revolutions per meter. Some technological issues that occurred during the manufacturing of the GeO2-doped supporting elements for the large hollow cores are also described, which complicated the spinning of the MOFs discussed above. We also provide the results of the tests performed for the pilot samples—designed and manufactured using the untwisted and twisted MOFs described above—which were characterized by an outer diameter of 65 µm, a hollow ring core with an inner diameter of 30.5 µm, under a wall thickness of 1.7 µm, and a refractive index difference of Δn = 0.030. Moreover, their geometrical parameters, basic transmission characteristics, and the measurements of the far-field laser beam profile patterns are also provided. Full article
(This article belongs to the Special Issue Technologies and Applications of Large Core Optical Fibers)
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10 pages, 4268 KiB  
Communication
Photoactive Properties of Transport Sol-Gel Layers Based on Strontium Titanate for Perovskite Solar Cells
by Alina V. Semchenko, Gagik Y. Ayvazyan, Viktoriya V. Malyutina-Bronskaya, Sergei A. Khakhomov, Dmitry L. Kovalenko, Andrei A. Boiko, Vitali V. Sidski, Anton V. Nestsiaronak, Alexander A. Mayevsky, Konstantin D. Danilchenko, Dmitry V. Zhigulin, Vladimir A. Pilipenko, R. Subasri and Nikolai V. Gaponenko
Photonics 2023, 10(7), 845; https://doi.org/10.3390/photonics10070845 - 21 Jul 2023
Cited by 3 | Viewed by 1128
Abstract
In this work, we have investigated the photocurrent and spectral sensitivity of the silicon/SrTiO3:xNb/perovskite structures. The sol–gel method carried out the deposition of undoped SrTiO3 layers as well as niobium-doped (SrTiO3:Nb) layers at atomic concentrations of 3 and [...] Read more.
In this work, we have investigated the photocurrent and spectral sensitivity of the silicon/SrTiO3:xNb/perovskite structures. The sol–gel method carried out the deposition of undoped SrTiO3 layers as well as niobium-doped (SrTiO3:Nb) layers at atomic concentrations of 3 and 6% Nb. The perovskite layer, CH3NH3PbI3−xClx, has been deposited by the vacuum co-evaporation technique. The layers have been characterized by scanning electron microscopy and X-ray diffraction measurements. The volt–ampere characteristics and spectral sensitivity of the fabricated samples have been measured under illumination with selective wavelengths of 405, 450, 520, 660, 780, 808, 905, 980, and 1064 nm of laser diodes. We have shown that for different configurations of applied voltage between silicon, SrTiO3:xNb, and CH3NH3PbI3−xClx, the structures are photosensitive ones with a variation of photocurrent from microamperes to milliamperes depending on Nb concentration in SrTiO3, and the highest photocurrent and spectral sensitivity values are observed when a SrTiO3:Nb layer with 3 at.% of Nb is used. A possible application of the proposed structure with a SrTiO3:Nb layer for perovskite solar cells and photodetectors is being discussed. Full article
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17 pages, 11338 KiB  
Article
Analysis and Experiment of Laser Energy Distribution of Laser Wireless Power Transmission Based on a Powersphere Receiver
by Tiefeng He, Guoliang Zheng, Qingyang Wu, Haixuan Huang, Lili Wan, Keyan Xu, Tianyu Shi and Zhijian Lv
Photonics 2023, 10(7), 844; https://doi.org/10.3390/photonics10070844 - 21 Jul 2023
Cited by 3 | Viewed by 1860
Abstract
Laser wireless power transmission (WPT) is one of the most important technologies in the field of long-range power transfer. This technique uses a laser as a transmission medium instead of conventional physical or electrical connections to perform WPT. It has the characteristics of [...] Read more.
Laser wireless power transmission (WPT) is one of the most important technologies in the field of long-range power transfer. This technique uses a laser as a transmission medium instead of conventional physical or electrical connections to perform WPT. It has the characteristics of long transmission distance and flexible operation. The existing laser wireless power transmission system uses photovoltaic cells as a receiver, which convert light into electricity. Due to the contradiction between the Gaussian distribution of laser and the uniform illumination requirements of photovoltaic cells, the laser wireless power transmission technology has problems such as low transmission efficiency and small output power. Therefore, understanding the energy distribution changes in the laser during transmission, especially the energy change after the laser is transmitted to each key device, and analyzing the influencing factors of the energy distribution state, are of great significance in improving the transmission efficiency and reducing the energy loss in the system. This article utilizes the optical software Lighttools as a tool to establish a laser wireless power transmission model based on a powersphere. This model is used to study the energy distribution changes in the laser as it passes through various components, and to analyze the corresponding influencing factors. To further validate the simulation results, an experimental platform was constructed using a semiconductor laser, beam expander, Fresnel lens, and powersphere as components. A beam quality analyzer was used to measure and analyze the laser energy distribution of each component except for the powersphere. The output voltage and current values of various regions of the powersphere were measured using a multimeter. The energy distribution of the powersphere was reflected based on the linear relationship between photo-generated current, voltage, and light intensity. The experimental results obtained were in good agreement with the simulation results. Simulations and experiments have shown that using a beam expander can reduce divergence angle and energy loss, while employing large-aperture focusing lens can enhance energy collection and output power, providing a basis for improving the efficiency of laser wireless power transfer. Full article
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13 pages, 7289 KiB  
Article
Single-Frequency Continuous-Wave Self-Sweeping Fiber Laser Based on Separated Gain and Absorption Dynamics Gratings
by Ekaterina K. Kashirina, Ivan A. Lobach and Sergey I. Kablukov
Photonics 2023, 10(7), 843; https://doi.org/10.3390/photonics10070843 - 21 Jul 2023
Viewed by 1037
Abstract
A new cavity scheme for a self-sweeping fiber laser with separated gain and absorption dynamics gratings is presented. The scheme is experimentally realized in an Er-doped ring fiber laser generating in the continuous-wave (CW) regime near the wavelength of 1604 nm. Switching between [...] Read more.
A new cavity scheme for a self-sweeping fiber laser with separated gain and absorption dynamics gratings is presented. The scheme is experimentally realized in an Er-doped ring fiber laser generating in the continuous-wave (CW) regime near the wavelength of 1604 nm. Switching between single longitudinal mode stabilization and wavelength self-sweeping regimes is demonstrated by controlling the intracavity losses in the laser. The pump power and optimization of the intracavity losses made it possible to demonstrate record sweeping range of 2.6 nm in a single-frequency self-sweeping regime in the telecommunication L-band. Full article
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10 pages, 1770 KiB  
Article
All-Fiber Narrow-Bandwidth Mode-Locked Laser Based on Polarization-Dependent Helical Long-Period Grating
by Ying Wan, Chen Jiang, Zuxing Zhang, Yaya Mao, Jianxin Ren, Jianxiang Wen and Yunqi Liu
Photonics 2023, 10(7), 842; https://doi.org/10.3390/photonics10070842 - 21 Jul 2023
Cited by 2 | Viewed by 1296
Abstract
As a crucial component of nonlinear polarization rotation (NPR) mode locking, optical fiber gratings offer advantages such as polarization modulation capability, stability, fiber compatibility, and preparation maturity, making them a vital technological foundation for achieving NPR mode locking. Here, a polarization-maintaining fiber helical [...] Read more.
As a crucial component of nonlinear polarization rotation (NPR) mode locking, optical fiber gratings offer advantages such as polarization modulation capability, stability, fiber compatibility, and preparation maturity, making them a vital technological foundation for achieving NPR mode locking. Here, a polarization-maintaining fiber helical long-period grating (PMF-HLPG) was designed and fabricated as a polarizer using the CO2-laser direct-write technique to realize the NPR effect. A homemade fiber Bragg grating (FBG) was also introduced into the laser system to enable a narrow-bandwidth lasing output and wavelength tunability. Based on the PMF-HLPG and FBG mentioned above, an all-fiber mode-locked laser with a spectra bandwidth of 0.15 nm was constructed to generate stable short pulses with a fundamental repetition rate of 12.7122 MHz and a pulse duration of 30.08 ps. In particular, its signal-to-noise ratio is up to 84.5 dB, showing the high stability of the laser. Further, the operating wavelength of the laser can be tuned from 1559.65 nm to 1560.29 nm via heating the FBG while maintaining its mode-locked state with stability. The results indicate that the PMF-HLPG could be used as a polarizer to meet the NPR mechanism for ultrashort pulse laser applications in optical communication, optical sensing, and biomedical imaging. Full article
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9 pages, 2483 KiB  
Communication
Non-Contact Real-Time Measurement of Refractive Index of Flowing Liquid Based on Optical Coherence Tomography
by Hongbo Fu, Weijian Gao, Zixin Lin, Zhemin Zeng, Wen Shi and Jian Zhang
Photonics 2023, 10(7), 841; https://doi.org/10.3390/photonics10070841 - 20 Jul 2023
Viewed by 1130
Abstract
The refractive index (RI) is one of the liquid’s most important physical parameters of liquid. The change of RI usually indicates the change of liquid properties, and this relation leads to broad application. This paper proposes a method based on optical coherence tomography [...] Read more.
The refractive index (RI) is one of the liquid’s most important physical parameters of liquid. The change of RI usually indicates the change of liquid properties, and this relation leads to broad application. This paper proposes a method based on optical coherence tomography (OCT) to measure the RI of liquid in real time. A series of targeted image-processing techniques play a vital role in this method. Benefiting from the advantages of OCT, this method does not require the operators to come into direct contact with liquids. Moreover, the principles and techniques adopted in this method imply that the measuring range of RI of unknown liquids is unlimited. The effectiveness of this method is validated by the experiment measuring the RI of sucrose liquids with gradient concentration. Another experiment thoroughly verifies this method’s feasibility in predicting sucrose concentration based on real-time measured RI. This simple and easy proposed method indicates an immense potential application prospect in the industry and scientific research. Full article
(This article belongs to the Special Issue Advanced Techniques in Biomedical Optical Imaging)
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10 pages, 4213 KiB  
Communication
Study on Cortisol Sensing Principle Based on Fluorophore and Aptamer Competitive Assay on Polymer Optical Fiber
by Vivek Semwal, Jonas Højgaard, Emil Møller, Ole Bang and Jakob Janting
Photonics 2023, 10(7), 840; https://doi.org/10.3390/photonics10070840 - 20 Jul 2023
Viewed by 1232
Abstract
In this study, we present a polymer optical fiber fluorophore/aptamer competitive assay-based cortisol sensing principle. We developed a low-cost, two-fiber perpendicular design for fluorophore-based sensing with less input light interference and high output signal intensity. The design is suitable for narrow stokes shift [...] Read more.
In this study, we present a polymer optical fiber fluorophore/aptamer competitive assay-based cortisol sensing principle. We developed a low-cost, two-fiber perpendicular design for fluorophore-based sensing with less input light interference and high output signal intensity. The design is suitable for narrow stokes shift fluorophores. We have demonstrated the cortisol sensing principle based on the competition between tagged and normal cortisol. To date, the sensing design has exhibited a slow response, and we identified possible modifications for improvement. Our estimation shows that with miniaturization and a modified sensor assay compartment design, a less than one-hour response time can be achieved. The reported sensing principle and low-cost new design will be helpful for the future development of fluorophore-based fiber optic aptasensors that can potentially be used in a wet environment for online sensing. Full article
(This article belongs to the Special Issue Editorial Board Members’ Collection Series: Photonics Sensors)
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12 pages, 4342 KiB  
Communication
Self-Mixing Interferometry Cooperating with Frequency Division Multiplexing for Multiple-Dimensional Displacement Measurement
by Dongmei Guo, Zhanwu Xie, Qin Yang, Wei Xia, Yanguang Yu and Ming Wang
Photonics 2023, 10(7), 839; https://doi.org/10.3390/photonics10070839 - 20 Jul 2023
Cited by 1 | Viewed by 1079
Abstract
In this study, a multiple-dimensional displacement measurement technology is demonstrated by using self-mixing interferometry (SMI) cooperating with a frequency division multiplexing (FDM) technique. The proposed SMI configuration with a single laser generates three modulated light beams with different carrier frequencies. Each beam is [...] Read more.
In this study, a multiple-dimensional displacement measurement technology is demonstrated by using self-mixing interferometry (SMI) cooperating with a frequency division multiplexing (FDM) technique. The proposed SMI configuration with a single laser generates three modulated light beams with different carrier frequencies. Each beam is incident on a planar grating with its own auto-collimation diffraction angle. The diffracted beams return to the laser cavity and then self-mixing interference occurs. An algorithm based on FDM is developed for multiple-dimensional displacement reconstruction from a single SMI signal. Experiments are conducted to verify the proposed approach. This paper shows an attractive sensing system for multiple-dimensional displacement featuring compact configuration, high resolution and better immunity to environmental disturbances. Full article
(This article belongs to the Special Issue Optical Measurement Systems)
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13 pages, 2360 KiB  
Article
Tuning the Structural, Electronic, and Optical Properties of Monolayer Graphene through Heteroatom Doping: A First-Principles Study with Future Light Sensing Applications
by Ahmed Adel A. Abdelazeez, Amira Ben Gouider Trabelsi, Fatemah H. Alkallas, Salem AlFaify, Mohd. Shkir, Tahani A. Alrebdi, Kholoud S. Almugren, Feodor V. Kusmatsev and Mohamed Rabia
Photonics 2023, 10(7), 838; https://doi.org/10.3390/photonics10070838 - 20 Jul 2023
Cited by 1 | Viewed by 1038
Abstract
This study explores the effects of Si and Si-P heteroatoms doping and co-doping on a monolayer graphene surface through density functional analysis. The results suggest that doping with Si and co-doping with Si-P significantly alters the bonding arrangement of the atoms surrounding the [...] Read more.
This study explores the effects of Si and Si-P heteroatoms doping and co-doping on a monolayer graphene surface through density functional analysis. The results suggest that doping with Si and co-doping with Si-P significantly alters the bonding arrangement of the atoms surrounding the graphene sheet. Additionally, the surface of the graphene material had a high concentration of electrons in both Si doping and Si-P co-doping, based on electron population analysis. The HOMO–LUMO gap of graphene sheets was found to decrease in the following order: pristine graphene sheet > Si-doped graphene sheet > Si-P co-doped graphene sheet. Furthermore, a TD-DFT study revealed that the absorption wavelength of Si and Si-P co-doped graphene systems had a greater shift to a lower range compared to pristine graphene. The order of decreasing absorption wavelength is Si-P co-doped graphene, Si doped graphene, and pristine graphene. These materials are suggested to have a high potential for photodetector applications due to their broad absorption range. Full article
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10 pages, 2854 KiB  
Communication
Second-Order Statistics of Partially Coherent Beams with Laguerre Non-Uniform Coherence Properties under Turbulence
by Yang Zhao, Zhiwen Yan, Yibo Wang, Liming Liu, Xinlei Zhu, Bohan Guo and Jiayi Yu
Photonics 2023, 10(7), 837; https://doi.org/10.3390/photonics10070837 - 20 Jul 2023
Cited by 1 | Viewed by 884
Abstract
We use the extended Huygens–Fresnel integral to analyze the propagation properties of a class of partially coherent beams with Laguerre non-uniform coherence properties (called Laguerre non-uniformly correlated beams) in free space and in a turbulent atmosphere. We focus on how different initial beam [...] Read more.
We use the extended Huygens–Fresnel integral to analyze the propagation properties of a class of partially coherent beams with Laguerre non-uniform coherence properties (called Laguerre non-uniformly correlated beams) in free space and in a turbulent atmosphere. We focus on how different initial beam orders and coherence lengths affect the propagation behavior of the beams, such as the evolution of intensity, degree of coherence, propagation factor, and beam wander. Our results show that non-uniform coherence properties play a role in resisting the degrading effects of turbulence. Furthermore, adjusting the initial beam parameter of the non-uniform coherence structure, i.e., increasing the beam order and decreasing the coherence, can further improve the turbulence resistance of the beams. Our results have potential applications in free-space optical communications. Full article
(This article belongs to the Special Issue Free-Space Optical Communication: Physics and Applications)
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12 pages, 9818 KiB  
Communication
Regenerative Shaper of Ultrashort Light Pulses
by Kęstutis Regelskis, Gustas Liaugminas and Julijanas Želudevičius
Photonics 2023, 10(7), 836; https://doi.org/10.3390/photonics10070836 - 20 Jul 2023
Viewed by 907
Abstract
In this article, we propose and demonstrate an ultrashort light pulse regenerative shaper based on a closed-loop double-stage Mamyshev regenerator with an electrically controllable acousto-optic switch. This setup allowed us to apply an initial pulse from an external pulse source and to reshape [...] Read more.
In this article, we propose and demonstrate an ultrashort light pulse regenerative shaper based on a closed-loop double-stage Mamyshev regenerator with an electrically controllable acousto-optic switch. This setup allowed us to apply an initial pulse from an external pulse source and to reshape the pulse to an ultrashort, high-quality, compressible one, the parameters of which were determined by the parameters of the regenerative circuit (RC), due to repeated pulse round-trips in the closed-loop RC. It was found that after 3–7 round-trips the energy of the regenerated pulse was almost independent of the energy of the initial pulse. However, at least 20 round-trips in the RC were required to shape the steady repetitive pulse. In addition, we demonstrated, both experientially and numerically, that the generation of periodic pulse sequences with limited pump power can lead to a rearrangement of the pulse sequences, which results in variations in the length of the pulse sequence from period to period. Full article
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19 pages, 3433 KiB  
Review
Quartz-Enhanced Photoacoustic Spectroscopy in the Terahertz Spectral Range
by Alexey P. Votintsev, Alexey V. Borisov, Didar R. Makashev, Mariya Y. Stoyanova and Yury V. Kistenev
Photonics 2023, 10(7), 835; https://doi.org/10.3390/photonics10070835 - 19 Jul 2023
Cited by 2 | Viewed by 1510
Abstract
Infrared laser photo-acoustic spectroscopy provides very high sensitivity of a gas sample analysis when high-power tunable laser radiation sources and resonant photo-acoustic detectors (PADs) are used. In the resonant PAD, the acoustic signal generated by absorbed laser radiation in a measurement chamber is [...] Read more.
Infrared laser photo-acoustic spectroscopy provides very high sensitivity of a gas sample analysis when high-power tunable laser radiation sources and resonant photo-acoustic detectors (PADs) are used. In the resonant PAD, the acoustic signal generated by absorbed laser radiation in a measurement chamber is amplified proportionally to a Q-factor of the acoustic resonator. But, compact tunable high-power lasers (with power above 100 mW) still are not widely spread in the terahertz spectral range. One of the ways to achieve an acceptable sensitivity of terahertz photo-acoustic spectroscopy is using PADs with a very high Q-factor. The latter can be achieved using PAD with a quartz tuning fork. The current state in this field is presented in the review. Full article
(This article belongs to the Special Issue Biomedical Spectroscopy: Techniques and Applications)
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14 pages, 869 KiB  
Article
Success Rate of Direct Pulp Capping with Conventional Procedures Using Ca (OH)2 and Bioactive Tricalcium Silicate Paste vs. Laser-Assisted Procedures (Diode 980 nm, CO2, and Er: YAG)
by Samir Nammour, Marwan El Mobadder, Melanie Namour, Saad Houeis, Daniel Heysselaer, Paul Nahas, Carmen D. Todea, Jacek Matys, Kinga Grzech-Leśniak, Aneta Olszewska, Wassim El Mobadder, Ilaria Giovannacci, Markus Laky, Amaury Namour and Sabine Geerts
Photonics 2023, 10(7), 834; https://doi.org/10.3390/photonics10070834 - 18 Jul 2023
Cited by 1 | Viewed by 2027
Abstract
Direct pulp capping (DPC) is reliable in pulp exposure management. Objective: This study aimed to assess the success rate of DPC materials and different laser protocols. The included procedures were CO2 laser (n = 1147), Er: YAG laser (n = [...] Read more.
Direct pulp capping (DPC) is reliable in pulp exposure management. Objective: This study aimed to assess the success rate of DPC materials and different laser protocols. The included procedures were CO2 laser (n = 1147), Er: YAG laser (n = 69), and 980 nm diode laser (n = 124), on the one hand, and Ca (OH)2 (n = 376) and bioactive tricalcium silicate paste, on the other (n = 279). Materials and methods: Data from 1995 DPC cases were included. For laser groups, irradiation was used to coagulate the pulp exposure followed by Ca (OH)2 placement. Data with follow-up at 12, 24, and 36 months post-treatment were included. The irradiation parameters for the CO2 laser were as follows: energy density per pulse of 141 J/cm², 1 W power, 0.3 mm beam diameter, 100 ms pulse duration, and 1 Hz, and a series of five pulses maximum were delivered during 5 s. For the 980 diode lasers: 1.5 W power, continuous wave (CW), 400 μm fiber diameter, contact mode, 190.98 W/cm2 power density, and total delivered energy density of 2387 J/cm2. For the Er: YAG laser: 0.5 W output power, 9.95 J/cm2 energy density, a beam diameter of 0.8 mm, 300 µsec pulse duration, 10 Hz, non-contact mode, irradiation with air without water spray, and an average irradiation time of 8–10 s. Results: At the 3-year follow-up, the success percentages were as follows: CO2 (88.01%) > Ca (OH)2 (75.72%) > diode (70.01%) > Er: YAG (54.55%) > bioactive tricalcium silicate paste (51.1%). The timing of permanent filling (immediate or delayed), patient age, size of pulp exposure, tooth type, and exposure etiology significantly affected the success rate. Patients aged ≤ 35 years presented higher success (70.91%) compared to those ≥ 36 years (61.2%). Immediate permanent fillings increase the success rate (71.41%) compared to delayed permanent fillings (65.93%). Exposure in molars and premolars significantly lowers the success rate (60.3%) compared to canines and incisors (72.1%). Idiopathic pulp exposure presented higher success (72.58%) compared to caries-related causes (63.7%). Conclusion: The highest success rate was in the CO2 laser group followed by the diode and Ca (OH)2, Er: YAG, and bioactive tricalcium silicate material (biodentine) groups. The age factor, filling timing, size of exposure, tooth type, and exposure etiology can significantly affect the success rate of DPC. Full article
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22 pages, 4678 KiB  
Review
A Brief Review on Nonlinear Photonic Crystals Induced by Direct Femtosecond Laser Writing
by Yaolan Tian, Qingbo Li, Lili Yan, Xiangdong Cao and Xian Zhao
Photonics 2023, 10(7), 833; https://doi.org/10.3390/photonics10070833 - 18 Jul 2023
Viewed by 1321
Abstract
Direct femtosecond laser writing or inscription is a useful technique, and it has been employed to engineer various materials in many applications including nonlinear photonic crystals, which are of periodically patterned second-order nonlinearity to get and control the coherent light at new frequencies. [...] Read more.
Direct femtosecond laser writing or inscription is a useful technique, and it has been employed to engineer various materials in many applications including nonlinear photonic crystals, which are of periodically patterned second-order nonlinearity to get and control the coherent light at new frequencies. By manipulation of second-order nonlinearity, either erased or poled, quasi-phase matching has been achieved in several crystals, especially three-dimensional nonlinear photonic crystals have been originally proposed and proved to be truly three-dimensional. Here we shortly review on the recent advances in the research field of nonlinear photonic crystals inscribed by femtosecond laser, as well as look into the future in this field. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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15 pages, 4838 KiB  
Article
Investigation of Oceanic Turbulence Random Phase Screen Generation Methods for UWOC
by Ruixian Jiang, Kexin Wang, Xinke Tang and Xu Wang
Photonics 2023, 10(7), 832; https://doi.org/10.3390/photonics10070832 - 18 Jul 2023
Cited by 2 | Viewed by 1444
Abstract
Underwater wireless optical communication (UWOC) has recently gained great research interest due to its capability of transmitting data underwater with high data rate and low latency. However, oceanic turbulence seriously degrades the optical signal quality and hence the performance of practical UWOC systems. [...] Read more.
Underwater wireless optical communication (UWOC) has recently gained great research interest due to its capability of transmitting data underwater with high data rate and low latency. However, oceanic turbulence seriously degrades the optical signal quality and hence the performance of practical UWOC systems. Establishing more accurate and efficient phase screen models is in demand for studying the oceanic turbulence effect. In this paper, techniques for generating underwater random phase screens are studied and supplemented. A promising hybrid method combining sparse spectrum and Zernike polynomials methods is proposed and investigated, which generates phase screens with improved accuracy and efficiency. Full article
(This article belongs to the Special Issue Underwater Wireless Optical Communication, Sensor and Network)
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18 pages, 1248 KiB  
Article
Partially Coherent Cylindrical Vector Sources
by Massimo Santarsiero, Juan Carlos González de Sande, Olga Korotkova, Rosario Martínez-Herrero, Gemma Piquero and Franco Gori
Photonics 2023, 10(7), 831; https://doi.org/10.3390/photonics10070831 - 17 Jul 2023
Viewed by 815
Abstract
A new class of stationary electromagnetic sources radiating outward from the surface of an infinitely long cylinder is introduced via vectorial coherent mode representation. First, two particular types of such sources are discussed: with either an electric or magnetic field aligned with the [...] Read more.
A new class of stationary electromagnetic sources radiating outward from the surface of an infinitely long cylinder is introduced via vectorial coherent mode representation. First, two particular types of such sources are discussed: with either an electric or magnetic field aligned with the cylinder’s axis. The former case represents a scalar scenario, while the latter leads to the two-component electric field. The combination of these two types of sources is then considered by forming the three-component electric field vector. An extension to the stationary case is then made in which the electric field correlations are shown to be described by the intrinsically 3×3 cross-spectral density matrix. Several known theories of electromagnetic coherence and polarization are then invoked for the analysis of radiation, on and off the source surface. The results for the spectral density, degree of coherence, and degree of polarization are then discussed in detail. The effects of mutual correlation of modes are also outlined. The new family of sources is of importance for any application involving cylindrical sources with controllable radiation. Full article
(This article belongs to the Special Issue Advances and Application of Structured Light)
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15 pages, 4701 KiB  
Article
High-Sensitivity Sensor Based on Diametrical Graphene Strip Plasma-Induced Transparency
by Aijun Zhu, Pengcheng Bu, Lei Cheng, Cong Hu and Rabi Mahapatra
Photonics 2023, 10(7), 830; https://doi.org/10.3390/photonics10070830 - 17 Jul 2023
Cited by 7 | Viewed by 1147
Abstract
In this paper, two parallel graphene strip structures are adopted to achieve tunable plasma-induced transparency (PIT) sensors in the terahertz band. Both graphene bands act as bright modes, and a PIT window appears due to the weak hybridization between them. A Lorentzian oscillation [...] Read more.
In this paper, two parallel graphene strip structures are adopted to achieve tunable plasma-induced transparency (PIT) sensors in the terahertz band. Both graphene bands act as bright modes, and a PIT window appears due to the weak hybridization between them. A Lorentzian oscillation coupling model is fitted to the simulation results of the proposed structure by the finite-difference time-domain (FDTD) method and is in good agreement with the simulation results. The performance of the PIT system can be controlled by tuning the geometrical parameters of the structure. In addition, the resonant frequency of the PIT window can be dynamically adjusted by changing the chemical potential and carrier mobility of the graphene strips. When the chemical potential of graphene increases from 0.2 eV to 1 eV, the amplitude modulation depth of the PIT window (2.832 THz, 3.684 THz, and 4.386 THz) can reach 92.39%, 96.14%, and 90.4%, respectively. Furthermore, due to its dispersion characteristics, the realized PIT window has a sensitive response to the surrounding medium, and the sensitivity can be as high as 1.25 THz/RIU. This PIT effect-based graphene microstructure has important implications for the future design of terahertz modulators, optical switches, and ultrasensitive sensors. Full article
(This article belongs to the Special Issue Terahertz Metamaterials and Device Applications)
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10 pages, 4068 KiB  
Communication
Effect of Contact Angle on Friction Properties of Superhydrophobic Nickel Surface
by Junyuan Huang, Zhiwei Zhu, Ling Zhang, Dongdong Guo, Zhen Niu and Wei Zhang
Photonics 2023, 10(7), 829; https://doi.org/10.3390/photonics10070829 - 17 Jul 2023
Cited by 2 | Viewed by 1269
Abstract
Due to their excellent performance, superhydrophobic materials have received a lot of attention and research in friction reduction and wear resistance. However, the effect of different contact angles of superhydrophobicity on friction and wear properties has not been thoroughly studied. In this paper, [...] Read more.
Due to their excellent performance, superhydrophobic materials have received a lot of attention and research in friction reduction and wear resistance. However, the effect of different contact angles of superhydrophobicity on friction and wear properties has not been thoroughly studied. In this paper, a nanosecond pulsed laser was used to realize the preparation of a superhydrophobic nickel surface, which indeed reduced the coefficient of friction but also increased the wear volume when compared to the unprocessed surface. As the contact angle of the superhydrophobic nickel surface increased, the coefficient of friction gradually increased, and the wear volume decreased gradually in superhydrophobic nickel surfaces. When the laser energy density was 1 J/cm2, the contact angle was 150.3° and the minimum friction coefficient was 0.4. However, when the laser energy density was 50 J/cm2, the maximum contact angle was 156.4° and the minimum wear volume was 4.23 × 107 μm3. The friction direction also influenced the tribological properties of the superhydrophobic-textured surface. This method makes it possible to process superhydrophobic surfaces with more suitable friction and wear properties. Full article
(This article belongs to the Special Issue Fiber Laser and Their Applications)
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