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Editor’s Choice Articles

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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29 pages, 3831 KiB  
Review
AI-Enabled Intelligent Visible Light Communications: Challenges, Progress, and Future
by Jianyang Shi, Wenqing Niu, Yinaer Ha, Zengyi Xu, Ziwei Li, Shaohua Yu and Nan Chi
Photonics 2022, 9(8), 529; https://doi.org/10.3390/photonics9080529 - 29 Jul 2022
Cited by 25 | Viewed by 4906
Abstract
Visible light communication (VLC) is a highly promising complement to conventional wireless communication for local-area networking in future 6G. However, the extra electro-optical and photoelectric conversions in VLC systems usually introduce exceeding complexity to communication channels, in particular severe nonlinearities. Artificial intelligence (AI) [...] Read more.
Visible light communication (VLC) is a highly promising complement to conventional wireless communication for local-area networking in future 6G. However, the extra electro-optical and photoelectric conversions in VLC systems usually introduce exceeding complexity to communication channels, in particular severe nonlinearities. Artificial intelligence (AI) techniques are investigated to overcome the unique challenges in VLC, whereas considerable obstacles are found in practical VLC systems applied with intelligent learning approaches. In this paper, we present a comprehensive study of the intelligent physical and network layer technologies for AI-empowered intelligent VLC (IVLC). We first depict a full model of the visible light channel and discuss its main challenges. The advantages and disadvantages of machine learning in VLC are discussed and analyzed by simulation. We then present a detailed overview of advances in intelligent physical layers, including optimal coding, channel emulator, MIMO, channel equalization, and optimal decision. Finally, we envision the prospects of IVLC in both the intelligent physical and network layers. This article lays out a roadmap for developing machine learning-based intelligent visible light communication in 6G. Full article
(This article belongs to the Special Issue Advances in Visible Light Communication)
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10 pages, 3205 KiB  
Article
A Theoretical Design of Chiral Molecules through Conformational Lock towards Circularly Polarized Luminescence
by Lewen Wang, Tengfei He, Hailiang Liao, Yige Luo, Wen Ou, Yinye Yu, Wan Yue, Guankui Long, Xingzhan Wei and Yecheng Zhou
Photonics 2022, 9(8), 532; https://doi.org/10.3390/photonics9080532 - 29 Jul 2022
Viewed by 3070
Abstract
Circularly polarized (CP) light has shown great potential in quantum computing, optical communications, and three-dimensional displays. It is still a challenge to produce high-efficiency and high-purity CP light. Herein, we proposed a strategy to design chiral organic small molecules for CP light generation. [...] Read more.
Circularly polarized (CP) light has shown great potential in quantum computing, optical communications, and three-dimensional displays. It is still a challenge to produce high-efficiency and high-purity CP light. Herein, we proposed a strategy to design chiral organic small molecules for CP light generation. These kinds of chiral molecules are formed by achiral light-emitting groups and achiral alkyl chains through conformational lock, which indicates that chirality can also be introduced into achiral light-emitting groups through rational molecular design. The chirality of these molecules can be further tuned by changing the length of the alkyl chains connecting the diketopyrrolopyrrole unit. The chiroptical properties of these molecules are confirmed by calculated electronic circular dichroism and chiral emission spectra, and further confirmed in experiments. The strategy developed in this work will greatly enlarge the candidate library of chiral luminescent materials. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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11 pages, 1959 KiB  
Article
Photoacoustic Detection of Pollutants Emitted by Transportation System for Use in Automotive Industry
by Reza Hadjiaghaie Vafaie, Roya Shafiei Pour, Ardashir Mohammadzadeh, Jihad H. Asad and Amir Mosavi
Photonics 2022, 9(8), 526; https://doi.org/10.3390/photonics9080526 - 28 Jul 2022
Cited by 5 | Viewed by 1820
Abstract
In photoacoustic spectroscopy, the signal is inversely proportional to the resonant cell volume. Photoacoustic spectroscopy (PAS) is an absorption spectroscopy technique that is suitable for detecting gases at low concentrations. This desirable feature has created a growing interest in miniaturizing PA cells in [...] Read more.
In photoacoustic spectroscopy, the signal is inversely proportional to the resonant cell volume. Photoacoustic spectroscopy (PAS) is an absorption spectroscopy technique that is suitable for detecting gases at low concentrations. This desirable feature has created a growing interest in miniaturizing PA cells in recent years. In this paper, a simulation of a miniaturized H-type photoacoustic cell consisting of two buffer holes and a resonator was performed in order to detect CO, NH3, NO, and CH4 pollutants. These gases are the main components of the air pollutants that are produced by the automotive industry. The linear forms of the continuity, Navier–Stokes equations, and the energy equation were solved using the finite element method in a gaseous medium. The generated pressure could be measured by a MEMS sensor. Photoacoustic spectroscopy has proven to be a sensitive method for detecting pollutant gases. The objectives of the measurements were: determining the proper position of the pressure gauge sensor; measuring the frequency response; measuring the frequency response changes at different temperatures; studying the local velocity at the resonant frequency; and calculating the quality factor. The acoustic quality coefficient, acoustic response (pressure), local velocity, frequency response, and the effect of different temperatures on the frequency response were investigated. A frequency response measurement represents the fact that different gases have different resonance frequencies, for which CO and NO gases had values of 23.131 kHz and 23.329 kHz, respectively. The difference between these gases was 200 Hz. NH3 and CH4 gases with values of 21.206 kHz and 21.106 kHz were separable with a difference of 100 Hz. In addition, CO and NO gases had a difference of 2000 Hz compared to NH3 and CH4, which indicates the characteristic fingerprint of the designed cell in the detection of different gases. Better access to high-frequency acoustic signals was the goal of the presented model in this paper. Full article
(This article belongs to the Special Issue Photonic Application in the Automotive Industry)
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10 pages, 3268 KiB  
Communication
Detection of a Spinning Object at Different Beam Sizes Based on the Optical Rotational Doppler Effect
by Song Qiu, Ruoyu Tang, Xiangyang Zhu, Tong Liu and Yuan Ren
Photonics 2022, 9(8), 517; https://doi.org/10.3390/photonics9080517 - 25 Jul 2022
Cited by 3 | Viewed by 1916
Abstract
The rotational Doppler effect (RDE), as a counterpart of the conventional well-known linear Doppler effect in the rotating frame, has attracted increasing attention in recent years for rotating object detection. However, the effect of the beam size on the RDE is still an [...] Read more.
The rotational Doppler effect (RDE), as a counterpart of the conventional well-known linear Doppler effect in the rotating frame, has attracted increasing attention in recent years for rotating object detection. However, the effect of the beam size on the RDE is still an open question. In this article, we investigated the influence of the size of the probe light; i.e., the size of the ring-shaped orbital angular momentum (OAM)-carrying optical vortex (OV), on the RDE. Both the light coaxial and noncoaxial incident conditions were considered in our work. We analyzed the mechanism of the influence on the RDE under the light coaxial, lateral misalignment, and oblique incidence conditions based on the small-scatterer model. A proof-of-concept experiment was performed to verify the theoretical predictions. It was shown that both the signal-to-noise ratio and the frequency spectrum width were related to the OV size. The larger the beam size, the stronger the RDE signal observed in the practical detection. Especially in the lateral misalignment condition, the large OV size effectively reduced the signal spreading and enhanced the signal strength. These findings may be useful for practical application of the optical RDE in remote sensing and metrology. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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12 pages, 1105 KiB  
Review
Review of Quality Optimization of Electron Beam Based on Laser Wakefield Acceleration
by Kangnan Jiang, Wentao Wang, Ke Feng and Ruxin Li
Photonics 2022, 9(8), 511; https://doi.org/10.3390/photonics9080511 - 23 Jul 2022
Cited by 3 | Viewed by 2756
Abstract
Compared with state-of-the-art radio frequency accelerators, the gradient of laser wakefield accelerators is 3–4 orders of magnitude higher. This is of great significance in the development of miniaturized particle accelerators and radiation sources. Higher requirements have been proposed for the quality of electron [...] Read more.
Compared with state-of-the-art radio frequency accelerators, the gradient of laser wakefield accelerators is 3–4 orders of magnitude higher. This is of great significance in the development of miniaturized particle accelerators and radiation sources. Higher requirements have been proposed for the quality of electron beams, owing to the increasing application requirements of tabletop radiation sources, specifically with the rapid development of free-electron laser devices. This review briefly examines the electron beam quality optimization scheme based on laser wakefield acceleration and presents some representative studies. In addition, manipulation of the electron beam phase space by means of injection, plasma profile distribution, and laser evolution is described. This review of studies is beneficial for further promoting the application of laser wakefield accelerators. Full article
(This article belongs to the Special Issue Progress in Laser Accelerator and Future Prospects)
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15 pages, 13392 KiB  
Article
Flexible Multiplane Structured Illumination Microscope with a Four-Camera Detector
by Karl A. Johnson, Daniel Noble, Rosa Machado, Tristan C. Paul and Guy M. Hagen
Photonics 2022, 9(7), 501; https://doi.org/10.3390/photonics9070501 - 20 Jul 2022
Cited by 5 | Viewed by 2716
Abstract
Fluorescence microscopy provides an unparalleled tool for imaging biological samples. However, producing high-quality volumetric images quickly and without excessive complexity remains a challenge. Here, we demonstrate a four-camera structured illumination microscope (SIM) capable of simultaneously imaging multiple focal planes, allowing for the capture [...] Read more.
Fluorescence microscopy provides an unparalleled tool for imaging biological samples. However, producing high-quality volumetric images quickly and without excessive complexity remains a challenge. Here, we demonstrate a four-camera structured illumination microscope (SIM) capable of simultaneously imaging multiple focal planes, allowing for the capture of 3D fluorescent images without any axial movement of the sample. This setup allows for the acquisition of many different 3D imaging modes, including 3D time lapses, high-axial-resolution 3D images, and large 3D mosaics. We imaged mitochondrial motions in live cells, neuronal structure in Drosophila larvae, and imaged up to 130 µm deep into mouse brain tissue. After SIM processing, the resolution measured using one of the four cameras improved from 357 nm to 253 nm when using a 30×/1.05 NA objective. Full article
(This article belongs to the Special Issue Advances and Application of Super-resolution Optical Microscopy)
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10 pages, 21938 KiB  
Article
Exploration of Deformation of F-Actin during Macropinocytosis by Confocal Microscopy and 3D-Structured Illumination Microscopy
by Linyu Xu, Yanwei Zhang, Song Lang and Yan Gong
Photonics 2022, 9(7), 461; https://doi.org/10.3390/photonics9070461 - 30 Jun 2022
Cited by 2 | Viewed by 2048
Abstract
Since their invention, confocal microscopy and super-resolution microscopy have become important choices in cell biology research. Macropinocytosis is a critical form of endocytosis. Deformation of the cell membrane is thought to be closely related to the movement of F-actin during macropinocytosis. However, it [...] Read more.
Since their invention, confocal microscopy and super-resolution microscopy have become important choices in cell biology research. Macropinocytosis is a critical form of endocytosis. Deformation of the cell membrane is thought to be closely related to the movement of F-actin during macropinocytosis. However, it is still unclear how the morphology of F-actin and the membrane change during this process. In this study, confocal microscopy was utilized for macroscopic time-series imaging of the cell membranes and F-actin in cells induced by phorbol 12-myristate 13-acetate (PMA). Super-resolution structured illumination microscopy (SIM), which can overcome the diffraction limit, was used to demonstrate the morphological characteristics of F-actin filaments. Benefiting from the advantages of SIM in terms of resolution and 3D imaging, we speculated on the regular pattern of the deformation of F-actin during macropinocytosis. The detailed visualization of structures also helped to validate the speculation regarding the role of F-actin filaments in macropinocytosis in previous studies. The results obtained in this study will provide a better understanding of the mechanisms underlying macropinocytosis and endocytosis. Full article
(This article belongs to the Special Issue Advances and Application of Super-resolution Optical Microscopy)
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11 pages, 4314 KiB  
Communication
High-Efficiency and High-Power Multijunction InGaAs/InP Photovoltaic Laser Power Converters for 1470 nm
by Simon Fafard and Denis P. Masson
Photonics 2022, 9(7), 438; https://doi.org/10.3390/photonics9070438 - 21 Jun 2022
Cited by 32 | Viewed by 6069
Abstract
The high-efficiency capabilities of multijunction laser power converters are demonstrated for high-power applications with an optical input of around 1470 nm. The InP-based photovoltaic power converting III-V semiconductor devices are designed here, with 10 lattice-matched subcells (PT10-InGaAs/InP), using thin InGaAs absorbing layers connected [...] Read more.
The high-efficiency capabilities of multijunction laser power converters are demonstrated for high-power applications with an optical input of around 1470 nm. The InP-based photovoltaic power converting III-V semiconductor devices are designed here, with 10 lattice-matched subcells (PT10-InGaAs/InP), using thin InGaAs absorbing layers connected by transparent tunnel junctions. The results confirm that such long-wavelength power converter devices are capable of producing electrical output voltages greater than 4–5 V. The characteristics are compatible with common electronics requirements, and the optical input is well suited for propagation over long distances through fiber-based optical links. Conversion efficiencies of ~49% are measured at electrical outputs exceeding 7 W for an input wavelength of 1466 nm at 21 °C. The Power Converter Performance Chart has been updated with these PT10-InGaAs/InP results. Full article
(This article belongs to the Special Issue Latest Papers Related to OWPT 2019-22 on the Topics of Photovoltaic Components, Devices and Systems)
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13 pages, 2684 KiB  
Article
Electric Field Sensor Based on High Q Fano Resonance of Nano-Patterned Electro-Optic Materials
by Xiaowei Yin, Fengli Liu, Wentao Qiu, Can Liu, Heyuan Guan and Huihui Lu
Photonics 2022, 9(6), 431; https://doi.org/10.3390/photonics9060431 - 17 Jun 2022
Cited by 3 | Viewed by 3440
Abstract
This paper presents theoretical studies of Fano resonance based electric-field (E-field) sensors. E-field sensor based on two electro-optical (EO) materials i.e., barium titanate (BaTiO3, BTO) nanoparticles and relaxor ferroelectric material Pb(Mg1/3Nb2/3)O3-PbTiO [...] Read more.
This paper presents theoretical studies of Fano resonance based electric-field (E-field) sensors. E-field sensor based on two electro-optical (EO) materials i.e., barium titanate (BaTiO3, BTO) nanoparticles and relaxor ferroelectric material Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) combined with nanostructure are studied. As for the BTO based E-field sensor, a configuration of filling the BTO nanoparticles into a nano-patterned thin film silicon is proposed. The achieved resonance quality factor (Q) is 11,855 and a resonance induced electric field enhancement factor is of around 105. As for the design of PMN-PT based E-field sensor, a configuration by combining two square lattice air holes in PMN-PT thin film but with one offsetting hole left is chosen. The achieved resonance Q is of 9,273 and an electric field enhancement factor is of around 96. The resonance wavelength shift sensitivity of PMN-PT nanostructured can reach up to 4.768 pm/(V/m), while the BTO based nanostructure has a sensitivity of 0.1213 pm/(V/m). If a spectrum analyzer with 0.1 pm resolution is considered, then the minimum detection of the electric field Emin is 20 mV/m and 0.82 V/m for PMN-PT and BTO based nanostructures, respectively. The nano-patterned E-field sensor studied here are all dielectric, it has therefore the advantage of large measurement bandwidth, high measurement fidelity, high spatial resolution and high sensitivity. Full article
(This article belongs to the Special Issue Advances in Photonic Integrated Devices and Circuits)
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14 pages, 1924 KiB  
Review
Ultrafast Fiber Technologies for Compact Laser Wake Field in Medical Application
by Weijian Sha, Jean-Christophe Chanteloup and Gérard Mourou
Photonics 2022, 9(6), 423; https://doi.org/10.3390/photonics9060423 - 16 Jun 2022
Cited by 9 | Viewed by 3806
Abstract
Technologies, performances and maturity of ultrafast fiber lasers and fiber delivery of ultrafast pulses are discussed for the medical deployment of laser-wake-field acceleration (LWFA). The compact ultrafast fiber lasers produce intense laser pulses with flexible hollow-core fiber delivery to facilitate electron acceleration in [...] Read more.
Technologies, performances and maturity of ultrafast fiber lasers and fiber delivery of ultrafast pulses are discussed for the medical deployment of laser-wake-field acceleration (LWFA). The compact ultrafast fiber lasers produce intense laser pulses with flexible hollow-core fiber delivery to facilitate electron acceleration in the laser-stimulated wake field near treatment site, empowering endoscopic LWFA brachytherapy. With coherent beam combination of multiple fiber amplifiers, the advantages of ultrafast fiber lasers are further extended to bring in more capabilities in compact LWFA applications. Full article
(This article belongs to the Special Issue Progress in Laser Accelerator and Future Prospects)
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10 pages, 908 KiB  
Communication
Deterministic Entanglement Swapping with Hybrid Discrete- and Continuous-Variable Systems
by Shou-Bang Yang, Wen Ning, Ri-Hua Zheng, Zhen-Biao Yang and Shi-Biao Zheng
Photonics 2022, 9(6), 368; https://doi.org/10.3390/photonics9060368 - 25 May 2022
Cited by 1 | Viewed by 2381
Abstract
The study of entanglement between discrete and continuous variables is an important theoretical and experimental topic in quantum information processing, for which entanglement swapping is one of the interesting elements. Entanglement swapping allows two particles without interacting with each other in any way, [...] Read more.
The study of entanglement between discrete and continuous variables is an important theoretical and experimental topic in quantum information processing, for which entanglement swapping is one of the interesting elements. Entanglement swapping allows two particles without interacting with each other in any way, to form an entangled state by the action of another pair of entangled particles. In this paper, we propose an experimentally feasible scheme to realize deterministic entanglement swapping in the hybrid system with discrete and continuous variables. The process is achieved by preparing two pairs of entangled states, each is formed by a qubit and two quasi-orthogonal coherent state elements of a cavity, performing a Bell-state analysis through nonlocal operations on the continuous variable states of the two cavities, and projecting the two qubits into a maximally entangled state. The present scheme may be applied to other physical systems sustaining such hybrid discrete and continuous forms, providing a typical paradigm for entanglement manipulation through deterministic swapping operations. Full article
(This article belongs to the Special Issue Nonlinear and Quantum Optics in Coupled Structures: Fundamentals and Applications)
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9 pages, 3732 KiB  
Article
An Ultra-Broadband Polarization Beam Splitter Based on the Digital Meta-Structure at the 2 µm Waveband
by Jiefeng Xu, Yingjie Liu, Xiaoyuan Guo, Jiangbing Du and Ke Xu
Photonics 2022, 9(5), 361; https://doi.org/10.3390/photonics9050361 - 22 May 2022
Cited by 4 | Viewed by 3440
Abstract
The 2 μm waveband is considered to have great potential in optical communications. Driven by the demands on high-performance functional devices in this spectral band, various integrated photonic components have been demonstrated. In this work, an analog and digital topology optimization method is [...] Read more.
The 2 μm waveband is considered to have great potential in optical communications. Driven by the demands on high-performance functional devices in this spectral band, various integrated photonic components have been demonstrated. In this work, an analog and digital topology optimization method is proposed to design an ultra-broadband polarization beam splitter at the 2 μm waveband. Within an optical bandwidth of 213 nm, the excess losses of TE and TM modes are <0.53 dB and 0.3 dB, respectively. The corresponding polarization extinction ratios are >16.5 dB and 18.1 dB. The device has a very compact footprint of only 2.52 µm × 5.4 µm. According to our best knowledge, this is a benchmark demonstration of an ultra-broadband and ultra-compact polarization beam splitter enabled by the proposed optimization method. Full article
(This article belongs to the Special Issue Advances in Photonic Integrated Devices and Circuits)
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13 pages, 6405 KiB  
Article
Retinal and Choroidal Thickness in Myopic Young Adults
by Enrique J. Fernández, José A. Villa-Carpes, Rosa M. Martínez-Ojeda, Francisco J. Ávila and Juan M. Bueno
Photonics 2022, 9(5), 328; https://doi.org/10.3390/photonics9050328 - 10 May 2022
Cited by 3 | Viewed by 3284
Abstract
The retinal and the choroidal thickness were measured at four locations along the horizontal direction (foveola, one nasal to the fovea and two temporal) in a group of 43 young adults (mean age: 27.1 ± 3.9 years), with ocular refraction ranging from emmetropia [...] Read more.
The retinal and the choroidal thickness were measured at four locations along the horizontal direction (foveola, one nasal to the fovea and two temporal) in a group of 43 young adults (mean age: 27.1 ± 3.9 years), with ocular refraction ranging from emmetropia to high myopia (0 to −10D). Thickness values were obtained from OCT images centered at the foveal depression. The retinal thickness exhibited a correlation with refraction at all eccentricities but not at the fovea. When different subgroups of refraction were considered, the analysis of such correlations indicated that only the retinal thickness in the group of high myopia (refraction ≤ −6D) was statistically different from the other two groups (emmetropes: [−0.5, 0] D, and myopes: (−6, −0.5) D). No significant differences were found between emmetropic and myopic groups. In contrast to the retina, the choroidal thickness exhibited a significant correlation with refraction at the fovea, although such dependency only stood for high myopes (the choroid of myopes and emmetropes exhibited similar thickness). Correlation with refraction was also found at the nasal location, arising between emmetropic and high myopia groups. Other choroidal locations among groups did not exhibit relationship with the refraction. It is concluded that the differences in the choroid and retina thickness along the horizontal meridian as a function of refraction do not characterize the onset and progression of myopia at early stages, since they only manifest in the group of high myopia. Full article
(This article belongs to the Special Issue Ocular Imaging for Eye Care)
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16 pages, 1882 KiB  
Article
CASM: A Cost-Aware Switch Migration Strategy for Elastic Optical Inter-Datacenter Networks
by Yong Liu, Qian Meng, Zhonghua Shen and Fulong Yan
Photonics 2022, 9(5), 315; https://doi.org/10.3390/photonics9050315 - 6 May 2022
Cited by 2 | Viewed by 1741
Abstract
In inter-datacenter elastic optical networks, multi-controller deployment is adopted to improve the stability and scalability of the control plane. As the network scale increases, the traditional multi-controller deployment scheme ignores the dynamic characteristics of traffic, resulting in unbalanced load among multiple controllers. In [...] Read more.
In inter-datacenter elastic optical networks, multi-controller deployment is adopted to improve the stability and scalability of the control plane. As the network scale increases, the traditional multi-controller deployment scheme ignores the dynamic characteristics of traffic, resulting in unbalanced load among multiple controllers. In response to this problem, the existing switch migration mechanism is proposed to achieve balanced distribution of control loads. However, most of the existing research work does not consider the additional cost of switch migration, and the load balancing performance of the controller is not significantly improved after switch migration. In this paper, we propose a cost-aware switch migration (CASM) strategy for controller load balancing. The proposed CASM strategy first measures the controller load through multiple performance indicators that affect the controller load, and then judges whether the controller is overloaded or underloaded based on the controller’s response time to the request message, thereby improving the load balancing performance of the controller. Additionally, when selecting the switch to be migrated, the CASM selects the optimal switch for migration based on minimizing the migration cost, thereby reducing the cost of switch migration. The performance evaluation shows that CASM significantly improves load balancing performance of controllers and reduces the migration cost compared to existing solutions. Full article
(This article belongs to the Special Issue Optical Data Center Networks)
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10 pages, 3666 KiB  
Article
A Multi-Layer Erbium-Doped Air-Hole-Assisted Few-Mode Fiber with Ultra-Low Differential Modal Gain
by Zhiqi Li, Li Pei, Jingjing Zheng, Jianshuai Wang, Wenxuan Xu, Tigang Ning and Jing Li
Photonics 2022, 9(5), 305; https://doi.org/10.3390/photonics9050305 - 29 Apr 2022
Cited by 5 | Viewed by 2269
Abstract
The air-hole assisted few-mode fiber (AH-FMF) enables modal intensity balance and offers a profound prospect in gain equalization with the combination of adaptive ion doping. In this paper, we proposed an AH-FM-EDF with a multi-layered erbium doping profile. In AH-FM-EDF, due to the [...] Read more.
The air-hole assisted few-mode fiber (AH-FMF) enables modal intensity balance and offers a profound prospect in gain equalization with the combination of adaptive ion doping. In this paper, we proposed an AH-FM-EDF with a multi-layered erbium doping profile. In AH-FM-EDF, due to the central air hole, only the first radial order modes (LP01, LP11, LP21, and LP31) are supported, and all the modes are confined in the same high refractive index core region. The differential modal gain (DMG) is highly reduced by optimizing the erbium doping proportion in each layer. Compared with uniform doping, the DMG is reduced from 4 dB to 0.14 dB as triple-layer doping is deployed. Additionally, the proposed erbium-doped fiber performs well in gain flattening and fabrication tolerance over the whole C-band. Full article
(This article belongs to the Special Issue Optical Amplifiers: Progress, Challenges, and Future Prospects)
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17 pages, 7273 KiB  
Article
Evaluation of Structural and Optical Properties of Graphene Oxide-Polyvinyl Alcohol Thin Film and Its Potential for Pesticide Detection Using an Optical Method
by Nurul Illya Muhamad Fauzi, Yap Wing Fen, Jaafar Abdullah, Mazliana Ahmad Kamarudin, Nur Alia Sheh Omar, Faten Bashar Kamal Eddin, Nur Syahira Md Ramdzan and Wan Mohd Ebtisyam Mustaqim Mohd Daniyal
Photonics 2022, 9(5), 300; https://doi.org/10.3390/photonics9050300 - 28 Apr 2022
Cited by 18 | Viewed by 3635
Abstract
In the present work, graphene oxide (GO)–polyvinyl alcohol (PVA) composites thin film has been successfully synthesized and prepared by spin coating techniques. Then, the properties and morphology of the samples were characterized using Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), and atomic [...] Read more.
In the present work, graphene oxide (GO)–polyvinyl alcohol (PVA) composites thin film has been successfully synthesized and prepared by spin coating techniques. Then, the properties and morphology of the samples were characterized using Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), and atomic force microscopy (AFM). Experimental FTIR results for GO–PVA thin film demonstrated the existence of important functional groups such as -CH2 stretching, C=O stretching, and O–H stretching. Furthermore, UV-Vis analysis indicated that the GO–PVA thin film had the highest absorbance that can be observed at wavelengths ranging from 200 to 500 nm with a band gap of 4.082 eV. The surface morphology of the GO–PVA thin film indicated the thickness increased when in contact with carbaryl. The incorporation of the GO–PVA thin film with an optical method based on the surface plasmon resonance (SPR) phenomenon demonstrated a positive response for the detection of carbaryl pesticide as low as 0.02 ppb. This study has successfully proposed that the GO–PVA thin film has high potential as a polymer nanomaterial-based SPR sensor for pesticide detection. Full article
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11 pages, 1311 KiB  
Article
Angle-Dependent Magic Optical Trap for the 6S1/2↔nP3/2 Rydberg Transition of Cesium Atoms
by Jiandong Bai, Xin Wang, Xiaokai Hou, Wenyuan Liu and Junmin Wang
Photonics 2022, 9(5), 303; https://doi.org/10.3390/photonics9050303 - 28 Apr 2022
Cited by 4 | Viewed by 2879
Abstract
The existence of an anisotropic tensor part of atomic states with an angular momentum greater than 1/2 causes their dynamic polarizabilities to be very sensitive to the polarization direction of the laser field. Therefore, the magic wavelength of the transition between two atomic [...] Read more.
The existence of an anisotropic tensor part of atomic states with an angular momentum greater than 1/2 causes their dynamic polarizabilities to be very sensitive to the polarization direction of the laser field. Therefore, the magic wavelength of the transition between two atomic states also depends on the polarization angle between the quantized axis and the polarization vector. We perform a calculation of the magic conditions of the 6S1/2↔nP3/2 (n = 50–90) Rydberg transition of cesium atoms by introducing an auxiliary electric diople transition connected to the target Rydberg state and a low-excited state. The magic condition is determined by the intersection of dynamic polarizabilities of the 6S1/2 ground state and the nP3/2 Rydberg state. The dynamic polarizability is calculated by using the sum-over-states method. Furthermore, we analyze the dependence of magic detuning on the polarization angle for a linearly polarized trapping laser and establish the relationship between magic detuning and a principal quantum number of the Rydberg state at the magic angle. The magic optical dipole trap can confine the ground-state and Rydberg-state atoms simultaneously, and the differential light shift in the 6S1/2↔nP3/2 transition can be canceled under the magic condition. It is of great significance for the application of long-lifetime high-repetition-rate accurate manipulation of Rydberg atoms on high-fidelity entanglement and quantum logic gate operation. Full article
(This article belongs to the Special Issue Optical Quantum Manipulation of Rydberg Atoms)
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12 pages, 2940 KiB  
Communication
Flexible Silicon Dimer Nanocavity with Electric and Magnetic Enhancement
by Chengda Pan, Yajie Bian, Yuchan Zhang, Shiyu Zhang, Xiaolei Zhang, Botao Wu, Qingyuan Jin and E Wu
Photonics 2022, 9(4), 267; https://doi.org/10.3390/photonics9040267 - 18 Apr 2022
Cited by 1 | Viewed by 2394
Abstract
High-index dielectrics have recently been regarded as promising building blocks in nanophotonics owing to optical electric and magnetic Mie resonances. In particular, silicon is gaining great interest as the backbone of modern technology. Here, silicon dimer nanocavities with different sizes of silicon nanospheres [...] Read more.
High-index dielectrics have recently been regarded as promising building blocks in nanophotonics owing to optical electric and magnetic Mie resonances. In particular, silicon is gaining great interest as the backbone of modern technology. Here, silicon dimer nanocavities with different sizes of silicon nanospheres were constructed using a probe nanomanipulation method and interacted with a few-layered R6G membrane to investigate the enhancement of electric and magnetic mode coupling. The evidence of the enhancement of fluorescence and slightly prolonged lifetime of R6G indicated the existence of nanocavities. In addition, the simulated electric and magnetic field distributions and decomposed mode of nanocavity were used to analyze the contribution of electric and magnetic modes to the R6G enhanced fluorescence. Such silicon dimer is a flexible nanocavity with electric and magnetic mode enhancement and has promising applications in sensing and all-dielectric metamaterials or nanophotonic devices. Full article
(This article belongs to the Topic Optical and Optoelectronic Materials and Applications)
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45 pages, 17409 KiB  
Review
Phase-Inserted Fiber Gratings and Their Applications to Optical Filtering, Optical Signal Processing, and Optical Sensing: Review
by Chengliang Zhu, Lei Wang and Hongpu Li
Photonics 2022, 9(4), 271; https://doi.org/10.3390/photonics9040271 - 18 Apr 2022
Cited by 8 | Viewed by 4159
Abstract
Phase-inserted fiber gratings (PI-FGs) refer to those gratings where there exist a number of the phase-shifts (spatial spacing) among different sections (or local periods) of the gratings themselves. All the PI-FGs developed to date can mainly be divided into three categories: phase-shifted gratings, [...] Read more.
Phase-inserted fiber gratings (PI-FGs) refer to those gratings where there exist a number of the phase-shifts (spatial spacing) among different sections (or local periods) of the gratings themselves. All the PI-FGs developed to date can mainly be divided into three categories: phase-shifted gratings, phase-only sampled gratings, and phase-modulated gratings, of which the utilized gratings could be either the Bragg ones (FBGs) or the long-period ones (LPGs). As results of the proposed the PI-FGs where the numbers, quantities, and positions of the inserted phases along the fiber direction are optimally selected, PI-FGs have already been designed and used as various complex filters such as the ultra-narrow filters, the triangular (edge) filters, the high channel-count filters, and the flat-top band-pass/band-stop filters, which, however, are extremely difficult or even impossible to be realized by using the ordinary fiber gratings. In this paper, we have briefly but fully reviewed the past and recent advances on PI-FGs, in which the principles and design methods, the corresponding fabrication techniques, and applications of the different PI-FGs to the fields of optical filtering, optical signal processing, and optical sensing, etc., have been highlighted. Full article
(This article belongs to the Special Issue Advancements in Fiber Bragg Grating Research II)
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14 pages, 50636 KiB  
Article
A Light Field Display Realization with a Nematic Liquid Crystal Microlens Array and a Polymer Dispersed Liquid Crystal Film
by Hui Li, Yancheng He, Yi Yu, Yuntao Wu, Shuiping Zhang and Yanduo Zhang
Photonics 2022, 9(4), 244; https://doi.org/10.3390/photonics9040244 - 7 Apr 2022
Cited by 4 | Viewed by 2712
Abstract
This study demonstrates a light field display system using a nematic liquid crystal (LC) microlens array (MLA) and a polymer dispersed liquid crystal (PDLC) film. LC-MLA without polarization effects presented high-resolution intermediate 3D images by adopting a depolarization algorithm. The adopted PDLC film [...] Read more.
This study demonstrates a light field display system using a nematic liquid crystal (LC) microlens array (MLA) and a polymer dispersed liquid crystal (PDLC) film. LC-MLA without polarization effects presented high-resolution intermediate 3D images by adopting a depolarization algorithm. The adopted PDLC film modulated the reconstructed 3D images to deliver full-parallax images efficiently with a wide FOV. The experimental result shows that the peak signal to noise ratio (PSNR) value of photograph accurate display results improves compared to the pure LC-MLA method. The proposed method is an essential step toward high-quality light field display. Full article
(This article belongs to the Special Issue Optical MEMS for 3D Imaging Applications)
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10 pages, 1924 KiB  
Communication
Processing-Speed Enhancement in a Delay-Laser-Based Reservoir Computer by Optical Injection
by Ziyue Li, Song-Sui Li, Xihua Zou, Wei Pan and Lianshan Yan
Photonics 2022, 9(4), 240; https://doi.org/10.3390/photonics9040240 - 4 Apr 2022
Cited by 4 | Viewed by 2105
Abstract
A delay-laser-based reservoir computer (RC) usually has its processing speed limited by the transient response of laser dynamics. Here, we study a simple all-optical approach to enhancing the processing speed by introducing optical injection to the reservoir layer of conventional RC that consists [...] Read more.
A delay-laser-based reservoir computer (RC) usually has its processing speed limited by the transient response of laser dynamics. Here, we study a simple all-optical approach to enhancing the processing speed by introducing optical injection to the reservoir layer of conventional RC that consists of a semiconductor laser with a delay loop. Using optical injection, the laser’s transient response effectively accelerates due to the speeded carrier-photon resonance. In the chaotic time-series prediction task, the proposed RC achieves good performance in a flexible range of injection detuning frequency under sufficient injection rate. Using proper injection parameters, the prediction error is significantly reduced and stabilized when using high processing speed. For achieving a prediction error below 0.006, the optical injection enhances the processing speed by an order of magnitude of about 5 GSample/s. Moreover, the proposed RC extends the advantage to the handwritten digit recognition task by achieving better word error rate. Full article
(This article belongs to the Special Issue Semiconductor Lasers)
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19 pages, 2857 KiB  
Article
Parabola-Like Gold Nanobowtie on Sapphire Substrate as Nano-Cavity
by Wenbing Li, Zhuo Yang, Jiali Zhang, Xin Tong, Yuheng Zhang, Bo Liu and Chao Ping Chen
Photonics 2022, 9(3), 193; https://doi.org/10.3390/photonics9030193 - 17 Mar 2022
Cited by 2 | Viewed by 3774
Abstract
Plasmonic metallic nanostructures have attracted much interest for their ability to manipulate light on a subwavelength scale and for their related applications in various fields. In this work, a parabola-like gold nanobowtie (PGNB) on a sapphire substrate was designed as a nano-cavity for [...] Read more.
Plasmonic metallic nanostructures have attracted much interest for their ability to manipulate light on a subwavelength scale and for their related applications in various fields. In this work, a parabola-like gold nanobowtie (PGNB) on a sapphire substrate was designed as a nano-cavity for confining light waves in a nanoscale gap region. The near-field optical properties of the innovative PGNB structure were studied comprehensively, taking advantage of the time-resolved field calculation based on a finite-difference time-domain algorithm (FDTD). The calculation result showed that the resonance wavelength of the nano-cavity was quite sensitive to the geometry of the PGNB. The values that related to the scattering and absorption properties of the PGNB, such as the scattering cross section, absorption cross section, extinction cross section, scattering ratio, and also the absorption ratio, were strongly dependent on the geometrical parameters which affected the surface area of the nanobowtie. Increased sharpness of the gold tips on the parabola-like nano-wings benefited the concentration of high-density charges with opposite electric properties in the narrow gold tips with limited volume, thus, resulting in a highly enhanced electric field in the nano-cavity under illumination of the light wave. Reduction of the gap size between the two gold nano-tips, namely, the size of the nano-cavity, decreased the distance that the electric potential produced by the highly concentrated charges on the surface of each gold nano-tip had to jump across, therefore, causing a significantly enhanced field in the nano-cavity. Further, alignment of the linearly polarized electric field of the incident light wave with the symmetric axis of the PGNB efficiently enabled the free electrons in the PGNB to concentrate on the surface of the sharp gold tips with a high density, thus, strongly improving the field across the nano-cavity. The research provides a new insight for future design, nanofabrication, and characterization of PGNBs for applications in devices that relate to enhancing photons emission, improving efficiency for energy harvesting, and improving sensitivity for infrared detection. Full article
(This article belongs to the Special Issue Semiconductor Lasers)
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14 pages, 11858 KiB  
Article
The Performance Improvement of VLC-OFDM System Based on Reservoir Computing
by Bingyao Cao, Kechen Yuan, Hu Li, Shuaihang Duan, Yuwen Li and Yuanjiang Ouyang
Photonics 2022, 9(3), 185; https://doi.org/10.3390/photonics9030185 - 14 Mar 2022
Cited by 8 | Viewed by 3038
Abstract
Nonlinear effects have been restricting the development of high-speed visible light communication (VLC) systems. Neural network (NN) has become an effective means to alleviate the nonlinearity of a VLC system due to its powerful ability to fit complicated functions. However, the complex training [...] Read more.
Nonlinear effects have been restricting the development of high-speed visible light communication (VLC) systems. Neural network (NN) has become an effective means to alleviate the nonlinearity of a VLC system due to its powerful ability to fit complicated functions. However, the complex training process of traditional NN limits its application in high-speed VLC. Without performance penalty, reservoir computing (RC) simplifies the training process of NN by training only part of the network connection weights, and has become an alternative scheme to NN. For the indoor visible light orthogonal frequency division multiplexing (VLC-OFDM) system, this paper studies the signal recovery effect of the pilot-assisted reservoir computing (PA-RC) frequency domain equalization algorithm. The pilot information is added to the feature engineering of RC to improve the accuracy of channel estimation by traditional least squares (LS) algorithm. The performance of 64 quadrature amplitude modulation (QAM) signal under different transmission rates and peak to peak voltage (Vpp) conditions is demonstrated in the experiments. Compared with the traditional frequency domain equalization algorithms, PA-RC can further expand the Vpp range that meets the 7% hard-decision forward error correction (FEC) limit of 3.8 × 10−3. At the rate of 240 Mbps, the BER of the system is reduced by about 90%, and the utilization rate of the available frequency band of the system reaches 100%. The results show that PA-RC can effectively improve the transmission performance of VLC system well, and has strong generalization ability. Full article
(This article belongs to the Special Issue Visible Light Communication (VLC))
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12 pages, 4375 KiB  
Article
Nonlinear Dynamics of Interband Cascade Laser Subjected to Optical Feedback
by Hong Han, Xumin Cheng, Zhiwei Jia and K. Alan Shore
Photonics 2021, 8(9), 366; https://doi.org/10.3390/photonics8090366 - 31 Aug 2021
Cited by 9 | Viewed by 2850
Abstract
We present a theoretical study of the nonlinear dynamics of a long external cavity delayed optical feedback-induced interband cascade laser (ICL). Using the modified Lang–Kobayashi equations, we numerically investigate the effects of some key parameters on the first Hopf bifurcation point of ICL [...] Read more.
We present a theoretical study of the nonlinear dynamics of a long external cavity delayed optical feedback-induced interband cascade laser (ICL). Using the modified Lang–Kobayashi equations, we numerically investigate the effects of some key parameters on the first Hopf bifurcation point of ICL with optical feedback, such as the delay time (τf), pump current (I), linewidth enhancement factor (LEF), stage number (m) and feedback strength (fext). It is found that compared with τf, I, LEF and m have a significant effect on the stability of the ICL. Additionally, our results show that an ICL with few stage numbers subjected to external cavity optical feedback is more susceptible to exhibiting chaos. The chaos bandwidth dependences on m, I and fext are investigated, and 8 GHz bandwidth mid-infrared chaos is observed. Full article
(This article belongs to the Special Issue Nonlinear Dynamics of Semiconductor Lasers and Their Applications)
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39 pages, 1986 KiB  
Review
Nondegenerate Bright Solitons in Coupled Nonlinear Schrödinger Systems: Recent Developments on Optical Vector Solitons
by S. Stalin, R. Ramakrishnan and M. Lakshmanan
Photonics 2021, 8(7), 258; https://doi.org/10.3390/photonics8070258 - 5 Jul 2021
Cited by 24 | Viewed by 3997
Abstract
Nonlinear dynamics of an optical pulse or a beam continue to be one of the active areas of research in the field of optical solitons. Especially, in multi-mode fibers or fiber arrays and photorefractive materials, the vector solitons display rich nonlinear phenomena. Due [...] Read more.
Nonlinear dynamics of an optical pulse or a beam continue to be one of the active areas of research in the field of optical solitons. Especially, in multi-mode fibers or fiber arrays and photorefractive materials, the vector solitons display rich nonlinear phenomena. Due to their fascinating and intriguing novel properties, the theory of optical vector solitons has been developed considerably both from theoretical and experimental points of view leading to soliton-based promising potential applications. Mathematically, the dynamics of vector solitons can be understood from the framework of the coupled nonlinear Schrödinger (CNLS) family of equations. In the recent past, many types of vector solitons have been identified both in the integrable and non-integrable CNLS framework. In this article, we review some of the recent progress in understanding the dynamics of the so called nondegenerate vector bright solitons in nonlinear optics, where the fundamental soliton can have more than one propagation constant. We address this theme by considering the integrable two coupled nonlinear Schrödinger family of equations, namely the Manakov system, mixed 2-CNLS system (or focusing-defocusing CNLS system), coherently coupled nonlinear Schrödinger (CCNLS) system, generalized coupled nonlinear Schrödinger (GCNLS) system and two-component long-wave short-wave resonance interaction (LSRI) system. In these models, we discuss the existence of nondegenerate vector solitons and their associated novel multi-hump geometrical profile nature by deriving their analytical forms through the Hirota bilinear method. Then we reveal the novel collision properties of the nondegenerate solitons in the Manakov system as an example. The asymptotic analysis shows that the nondegenerate solitons, in general, undergo three types of elastic collisions without any energy redistribution among the modes. Furthermore, we show that the energy sharing collision exhibiting vector solitons arises as a special case of the newly reported nondegenerate vector solitons. Finally, we point out the possible further developments in this subject and potential applications. Full article
(This article belongs to the Special Issue Optical Solitons: Current Status)
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13 pages, 1806 KiB  
Article
Controlling Microresonator Solitons with the Counter-Propagating Pump
by Zhiwei Fan and Dmitry V. Skryabin
Photonics 2021, 8(7), 239; https://doi.org/10.3390/photonics8070239 - 26 Jun 2021
Cited by 1 | Viewed by 2743
Abstract
Considering a bidirectionally pumped ring microresonator, we provide a concise derivation of the model equations allowing us to eliminate the repetition rate terms and reduce the nonlinear interaction between the counter-propagating waves to the power-dependent shifts of the resonance frequencies. We present the [...] Read more.
Considering a bidirectionally pumped ring microresonator, we provide a concise derivation of the model equations allowing us to eliminate the repetition rate terms and reduce the nonlinear interaction between the counter-propagating waves to the power-dependent shifts of the resonance frequencies. We present the simulation results of the soliton control by swiping the frequency of the counter-propagating wave in the forward and backward directions and with the soliton-blockade effect either present or not. We highlight the non-reciprocity of the forward and backward scans. Furthermore, we report the soliton crystals and breathers existing in the vicinity of the blockade interval. Full article
(This article belongs to the Special Issue Optical Solitons: Current Status)
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9 pages, 1471 KiB  
Article
Electron Spill-Out Effect in Singular Metasurfaces
by Fan Yang and Kun Ding
Photonics 2021, 8(5), 154; https://doi.org/10.3390/photonics8050154 - 5 May 2021
Cited by 1 | Viewed by 3221
Abstract
The electron spill-out effect is considered in a singular metasurface. Using the hydrodynamic model, we found that electron spill-out effectively smears the sharp singularity. The introduction of the electron spill-out effect also significantly changes the reflection spectrum, charge distribution, field profile for a [...] Read more.
The electron spill-out effect is considered in a singular metasurface. Using the hydrodynamic model, we found that electron spill-out effectively smears the sharp singularity. The introduction of the electron spill-out effect also significantly changes the reflection spectrum, charge distribution, field profile for a singular metasurface. Therefore, this spill-out contribution is crucial and cannot be ignored for a realistic description of optical response in a singular system. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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9 pages, 3247 KiB  
Article
Low Noise Short Wavelength Infrared Avalanche Photodetector Using SB-Based Strained Layer Superlattice
by Arash Dehzangi, Jiakai Li and Manijeh Razeghi
Photonics 2021, 8(5), 148; https://doi.org/10.3390/photonics8050148 - 30 Apr 2021
Cited by 5 | Viewed by 5125
Abstract
We demonstrate low noise short wavelength infrared (SWIR) Sb-based type II superlattice (T2SL) avalanche photodiodes (APDs). The SWIR GaSb/(AlAsSb/GaSb) APD structure was designed based on impact ionization engineering and grown by molecular beam epitaxy on a GaSb substrate. At room temperature, the device [...] Read more.
We demonstrate low noise short wavelength infrared (SWIR) Sb-based type II superlattice (T2SL) avalanche photodiodes (APDs). The SWIR GaSb/(AlAsSb/GaSb) APD structure was designed based on impact ionization engineering and grown by molecular beam epitaxy on a GaSb substrate. At room temperature, the device exhibits a 50% cut-off wavelength of 1.74 µm. The device was revealed to have an electron-dominated avalanching mechanism with a gain value of 48 at room temperature. The electron and hole impact ionization coefficients were calculated and compared to give a better prospect of the performance of the device. Low excess noise, as characterized by a carrier ionization ratio of ~0.07, has been achieved. Full article
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15 pages, 2696 KiB  
Article
Akinetic Swept-Source Master–Slave-Enhanced Optical Coherence Tomography
by Manuel J. Marques, Ramona Cernat, Jason Ensher, Adrian Bradu and Adrian Podoleanu
Photonics 2021, 8(5), 141; https://doi.org/10.3390/photonics8050141 - 24 Apr 2021
Cited by 5 | Viewed by 3605
Abstract
This paper presents a different approach for processing the signal from interferometers driven by swept sources that exhibit non-linear tuning during stable time intervals. Such sources are, for example, those commercialised by Insight, which are electrically tunable and akinetic. These Insight sources use [...] Read more.
This paper presents a different approach for processing the signal from interferometers driven by swept sources that exhibit non-linear tuning during stable time intervals. Such sources are, for example, those commercialised by Insight, which are electrically tunable and akinetic. These Insight sources use a calibration procedure to skip frequencies already included in a spectral sweep, i.e., a process of “clearing the spectrum”. For the first time, the suitability of the Master–Slave (MS) procedure is evaluated as an alternative to the conventional calibration procedure for such sources. Here, the MS process is applied to the intact, raw interferogram spectrum delivered by an optical coherence tomography (OCT) system. Two modalities are investigated to implement the MS processing, based on (i) digital generation of the Master signals using the OCT interferometer and (ii) down-conversion using a second interferometer driven by the same swept source. The latter allows near-coherence-limited operation at a large axial range (>80 mm), without the need for a high sampling rate digitiser card to cope with the large frequency spectrum generated, which can exceed several GHz. In both cases, the depth information is recovered with some limitations as described in the text. Full article
(This article belongs to the Special Issue Optical Fiber Interferometric Sensors: New Production Methodologies and Novel Applications)
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18 pages, 53862 KiB  
Review
Metasurface Holography in the Microwave Regime
by Guanyu Shang, Zhuochao Wang, Haoyu Li, Kuang Zhang, Qun Wu, Shah Nawaz Burokur and Xumin Ding
Photonics 2021, 8(5), 135; https://doi.org/10.3390/photonics8050135 - 22 Apr 2021
Cited by 30 | Viewed by 9926
Abstract
Hologram technology has attracted a great deal of interest in a wide range of optical fields owing to its potential use in future optical applications, such as holographic imaging and optical data storage. Although there have been considerable efforts to develop holographic technologies [...] Read more.
Hologram technology has attracted a great deal of interest in a wide range of optical fields owing to its potential use in future optical applications, such as holographic imaging and optical data storage. Although there have been considerable efforts to develop holographic technologies using conventional optics, critical issues still hinder their future development. A metasurface, as an emerging multifunctional device, can manipulate the phase, magnitude, polarization and resonance properties of electromagnetic fields within a sub-wavelength scale, opening up an alternative for a compact holographic structure and high imaging quality. In this review paper, we first introduce the development history of holographic imaging and metasurfaces, and demonstrate some applications of metasurface holography in the field of optics. We then summarize the latest developments in holographic imaging in the microwave regime. These functionalities include phase- and amplitude-based design, polarization multiplexing, wavelength multiplexing, spatial asymmetric propagation, and a reconfigurable mechanism. Finally, we conclude briefly on this rapidly developing research field and present some outlooks for the near future. Full article
(This article belongs to the Special Issue Holography)
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21 pages, 7371 KiB  
Review
Nonreciprocal and Topological Plasmonics
by Kunal Shastri, Mohamed Ismail Abdelrahman and Francesco Monticone
Photonics 2021, 8(4), 133; https://doi.org/10.3390/photonics8040133 - 20 Apr 2021
Cited by 21 | Viewed by 5020
Abstract
Metals, semiconductors, metamaterials, and various two-dimensional materials with plasmonic dispersion exhibit numerous exotic physical effects in the presence of an external bias, for example an external static magnetic field or electric current. These physical phenomena range from Faraday rotation of light propagating in [...] Read more.
Metals, semiconductors, metamaterials, and various two-dimensional materials with plasmonic dispersion exhibit numerous exotic physical effects in the presence of an external bias, for example an external static magnetic field or electric current. These physical phenomena range from Faraday rotation of light propagating in the bulk to strong confinement and directionality of guided modes on the surface and are a consequence of the breaking of Lorentz reciprocity in these systems. The recent introduction of relevant concepts of topological physics, translated from condensed-matter systems to photonics, has not only given a new perspective on some of these topics by relating certain bulk properties of plasmonic media to the surface phenomena, but has also led to the discovery of new regimes of truly unidirectional, backscattering-immune, surface-wave propagation. In this article, we briefly review the concepts of nonreciprocity and topology and describe their manifestation in plasmonic materials. Furthermore, we use these concepts to classify and discuss the different classes of guided surface modes existing on the interfaces of various plasmonic systems. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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15 pages, 2236 KiB  
Article
A New Type of Shape-Invariant Beams with Structured Coherence: Laguerre-Christoffel-Darboux Beams
by Rosario Martínez-Herrero, Massimo Santarsiero, Gemma Piquero and Juan Carlos González de Sande
Photonics 2021, 8(4), 134; https://doi.org/10.3390/photonics8040134 - 20 Apr 2021
Cited by 11 | Viewed by 2819
Abstract
A new class of sources presenting structured coherence properties is introduced and analyzed. They are obtained as the incoherent superposition of coherent Laguerre-Gaussian modes with suitable coefficients. This ensures that the shape of the intensity profile and the spatial coherence features of the [...] Read more.
A new class of sources presenting structured coherence properties is introduced and analyzed. They are obtained as the incoherent superposition of coherent Laguerre-Gaussian modes with suitable coefficients. This ensures that the shape of the intensity profile and the spatial coherence features of the propagated beams are invariant during paraxial approximation. A simple analytical expression is obtained for the cross-spectral density of the sources of this class, regardless of the number of superposed modes. Properties of these sources are analyzed and described by several examples. Full article
(This article belongs to the Special Issue Structured Light Coherence)
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13 pages, 9494 KiB  
Article
High-Sensitivity Optical-Resolution Photoacoustic Microscopy with an Optical-Acoustic Combiner Based on an Off-Axis Parabolic Acoustic Mirror
by Xiang Zhang, Yang Liu, Chao Tao, Jie Yin, Zizhong Hu, Songtao Yuan, Qinghuai Liu and Xiaojun Liu
Photonics 2021, 8(4), 127; https://doi.org/10.3390/photonics8040127 - 18 Apr 2021
Cited by 10 | Viewed by 3364
Abstract
Optical-resolution photoacoustic microscopy (OR-PAM) is a promising noninvasive biomedical imaging technology with label-free optical absorption contrasts. Performance of OR-PAM is usually closely related to the optical-acoustic combiner. In this study, we propose an optical-acoustic combiner based on a flat acoustic reflector and an [...] Read more.
Optical-resolution photoacoustic microscopy (OR-PAM) is a promising noninvasive biomedical imaging technology with label-free optical absorption contrasts. Performance of OR-PAM is usually closely related to the optical-acoustic combiner. In this study, we propose an optical-acoustic combiner based on a flat acoustic reflector and an off-axis parabolic acoustic mirror with a conical bore. Quantitative simulation and experiments demonstrated that this combiner can provide better acoustic focusing performance and detection sensitivity. Moreover, OR-PAM is based on the combiner suffer low optical disorders, which guarantees the good resolution. In vivo experiments of the mouse brain and the iris were also conducted to show the practicability of the combiner in biomedicine. This proposed optical-acoustic combiner realizes a high-quality optical-acoustic confocal alignment with minimal optical disorders and acoustic insertion loss, strong acoustic focusing, and easy implementation. These characteristics might be useful for improving the performance of OR-PAM. Full article
(This article belongs to the Special Issue Photoacoustic Imaging for Biomedical Applications)
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13 pages, 3697 KiB  
Article
Microshape Measurement Method Using Speckle Interferometry Based on Phase Analysis
by Yasuhiko Arai
Photonics 2021, 8(4), 112; https://doi.org/10.3390/photonics8040112 - 8 Apr 2021
Cited by 7 | Viewed by 3556
Abstract
A method for the measurement of the shape of a fine structure beyond the diffraction limit based on speckle interferometry has been reported. In this paper, the mechanism for measuring the shape of the fine structure in speckle interferometry using scattered light as [...] Read more.
A method for the measurement of the shape of a fine structure beyond the diffraction limit based on speckle interferometry has been reported. In this paper, the mechanism for measuring the shape of the fine structure in speckle interferometry using scattered light as the illumination light is discussed. Furthermore, by analyzing the phase distribution of the scattered light from the surface of the measured object, this method can be used to measure the shapes of periodic structures and single silica microspheres beyond the diffraction limit. Full article
(This article belongs to the Special Issue Advances in Optical Metrology)
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10 pages, 6981 KiB  
Article
Joule-Level Twelve-Pass LD End-Pumped Bonded Neodymium Glass Laser Amplifier
by Long Pan, Shengzhe Ji, Wenfa Huang, Jiangtao Guo, Xinghua Lu, Jiangfeng Wang, Wei Fan, Xuechun Li and Jianqiang Zhu
Photonics 2021, 8(4), 96; https://doi.org/10.3390/photonics8040096 - 30 Mar 2021
Cited by 1 | Viewed by 3160
Abstract
This paper reports on a Joule-level multi-pass laser amplification device with diode end-pumped square-rod neodymium glass (Nd:glass) bonded to K9 glass. The device generated 1.17 J pulse energy at 1 Hz and 1053 nm. The optical-to-optical efficiency was 13.01%, and the effective energy [...] Read more.
This paper reports on a Joule-level multi-pass laser amplification device with diode end-pumped square-rod neodymium glass (Nd:glass) bonded to K9 glass. The device generated 1.17 J pulse energy at 1 Hz and 1053 nm. The optical-to-optical efficiency was 13.01%, and the effective energy extraction efficiency was 44.23%. Comparing Nd:glass of the same specification without K9 glass under the same conditions, the thermal wave aberration of the former was 85.71% of that of the latter, which is 0.78 um. The near-field modulation degree at the highest energy output was 1.42 within 90% of the spot, and the far-field energy concentration was 81.88% within the 2.5-fold diffraction limit. The Nd:glass bonding method of the square rod is relatively novel in laser amplification systems pumped by the diode end face and can be further studied in future works. Full article
(This article belongs to the Special Issue Laser Amplifiers)
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11 pages, 435 KiB  
Communication
Quest for New Data: Ionizing Radiation Metrology in the Presence of Laser-Assisted Scattering Processes
by Davide Bianco, Filomena Loffredo, Maria Quarto and Luigi Santamaria Amato
Photonics 2021, 8(4), 94; https://doi.org/10.3390/photonics8040094 - 27 Mar 2021
Viewed by 2321
Abstract
Radiation metrology is crucial in space, for instance in monitoring the conditions on-board space vehicles. The energy released in matter by ionizing radiation is due to the atomic and molecular ionization processes, which have been investigated for several decades from both a theoretical [...] Read more.
Radiation metrology is crucial in space, for instance in monitoring the conditions on-board space vehicles. The energy released in matter by ionizing radiation is due to the atomic and molecular ionization processes, which have been investigated for several decades from both a theoretical and an experimental point of view. Electronic excitation and ionization cross-section are of particular interest in radiation physics, because of their role in the radiation–matter interaction process. Recently, experimental findings have shown that the interplay with a laser field can strongly modify the electronic interaction probabilities and emission spectra. These phenomena are still not completely understood from a theoretical point of view, and the available empirical data concern a few, simple atomic species. We represent a possible dosimetric effect of the interaction with laser light, inferring from experiments the characteristics of laser-assisted cross-sections. Using a Monte-Carlo calculation for simulating the micro-dosimetric aspects of the irradiation of a simple geometry, we show the need of new experimental data and more detailed theoretical approaches to these phenomena in complex molecular systems. Full article
(This article belongs to the Special Issue Spectroscopy, Metrology and Quantum Technology for Space Science and Astrophysics)
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11 pages, 1754 KiB  
Article
Wide Field-of-View, High-Resolution Endoscopic Lens Design with Low F-Number for Disposable Endoscopy
by Dongmok Kim, Sehui Chang and Hyuk-Sang Kwon
Photonics 2021, 8(4), 89; https://doi.org/10.3390/photonics8040089 - 24 Mar 2021
Cited by 8 | Viewed by 6512
Abstract
In the past few decades, video endoscopy has become one of the primary medical devices in diverse clinical fields for examination, treatment, and early disease diagnosis of the gastrointestinal tract. For an accurate diagnosis, an endoscopic camera offering bright and wide field-of-view images [...] Read more.
In the past few decades, video endoscopy has become one of the primary medical devices in diverse clinical fields for examination, treatment, and early disease diagnosis of the gastrointestinal tract. For an accurate diagnosis, an endoscopic camera offering bright and wide field-of-view images is required while maintaining its compact dimensions to enter the long, narrow, and dark tract inside of the body. Recent endoscopic lenses successfully provide wide fields-of-view and have compact sizes for the system; however, their f-numbers still remain at 2.8 or higher. Therefore, further improvement in f-numbers is required to compensate for the restricted illumination system of the endoscopic probe. Here, we present a low f-number endoscopic lens design while providing wide field-of-view and high-resolution imaging. The proposed lens system achieved a low f-number of 2.2 and a field-of-view of 140 deg. The modulation transfer function (MTF) is over 20% at 180 lp/mm, and relative illumination is more than 60% in the full field. Additionally, the proposed lens is designed for a 1/4” 5-megapixel complementary metal-oxide-semiconductor (CMOS) image sensor with a pixel size of 1.4 µm. This all-plastic lens design could help develop a high-performance disposable endoscope that prevents the risk of infection or cross-contamination with mass manufacture and low cost. Full article
(This article belongs to the Section Biophotonics and Biomedical Optics)
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6 pages, 432 KiB  
Communication
Coherence Stokes Parameters in the Description of Electromagnetic Coherence
by Tero Setälä, Kimmo Saastamoinen and Ari T. Friberg
Photonics 2021, 8(3), 85; https://doi.org/10.3390/photonics8030085 - 22 Mar 2021
Cited by 7 | Viewed by 3348
Abstract
The two-point counterparts of the traditional Stokes parameters, which are called the coherence Stokes parameters, have recently been extensively used for assessing the coherence properties of random electromagnetic light beams. In this work, we highlight their importance by emphasizing two features associated with [...] Read more.
The two-point counterparts of the traditional Stokes parameters, which are called the coherence Stokes parameters, have recently been extensively used for assessing the coherence properties of random electromagnetic light beams. In this work, we highlight their importance by emphasizing two features associated with them. First, the role of polarization in electromagnetic coherence is significantly elucidated when the coherence Stokes parameters are used. Second, the normalized coherence Stokes parameters should be regarded as the true electromagnetic counterparts of the normalized scalar-field correlation coefficient. Full article
(This article belongs to the Special Issue Structured Light Coherence)
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29 pages, 1073 KiB  
Article
The Overlap Factor Model of Spin-Polarised Coupled Lasers
by Martin Vaughan, Hadi Susanto, Ian Henning and Mike Adams
Photonics 2021, 8(3), 83; https://doi.org/10.3390/photonics8030083 - 20 Mar 2021
Cited by 1 | Viewed by 2795
Abstract
A general model for the dynamics of arrays of coupled spin-polarised lasers is derived. The general model is able to deal with waveguides of any geometry with any number of supported normal modes. A unique feature of the model is that it allows [...] Read more.
A general model for the dynamics of arrays of coupled spin-polarised lasers is derived. The general model is able to deal with waveguides of any geometry with any number of supported normal modes. A unique feature of the model is that it allows for independent polarisation of the pumping in each laser. The particular geometry is shown to be introduced via ’overlap factors’, which are a generalisation of the optical confinement factor. These factors play an important role in determining the laser dynamics. The model is specialised to the case of a general double-guided structure, which is shown to reduce to both the spin flip model in a single cavity and the coupled mode model for a pair of guides in the appropriate limit. This is applied to the particular case of a circular-guide laser pair, which is analysed and simulated numerically. It is found that increasing the ellipticity of the pumping tends to reduce the region of instability in the plane of pumping strength versus guide separation. Full article
(This article belongs to the Special Issue Nonlinear Dynamics of Semiconductor Lasers and Their Applications)
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13 pages, 18109 KiB  
Article
Propagation of a Modified Complex Lorentz–Gaussian-Correlated Beam in a Marine Atmosphere
by Baoyin Sun, Han Lü, Dan Wu, Fei Wang and Yangjian Cai
Photonics 2021, 8(3), 82; https://doi.org/10.3390/photonics8030082 - 19 Mar 2021
Cited by 9 | Viewed by 2706
Abstract
In this paper, we study the second-order statistics of a modified complex Lorentz–Gaussian-correlated (MCLGC) beam, which is a new type of partially coherent beam capable of producing an Airy-like intensity pattern in the far field, propagation through marine atmospheric turbulence. The propagation formula [...] Read more.
In this paper, we study the second-order statistics of a modified complex Lorentz–Gaussian-correlated (MCLGC) beam, which is a new type of partially coherent beam capable of producing an Airy-like intensity pattern in the far field, propagation through marine atmospheric turbulence. The propagation formula of spectral density is derived by the extended Huygens–Fresnel integral, which could explicitly indicate the interaction of turbulence on the beams’ spectral density under propagation. The influences of the structure constant of the turbulence, initial coherence width and wavelength on the spectral density are investigated in detail through numerical examples. In addition, analytical expressions for the r.m.s beam width, divergence angle and M2 factor of the MCLGC beam in the marine turbulence are also derived with the help of the Wigner distribution function. The results reveal that the beam spreads much faster, and the M2 factor deteriorates severely with the increase of the structure constant and the decrease of the inner scale size, whereas the outer scale size has little effect on these two quantities. Full article
(This article belongs to the Special Issue Structured Light Coherence)
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14 pages, 1744 KiB  
Article
Optical Machine Learning Using Time-Lens Deep Neural NetWorks
by Luhe Zhang, Caiyun Li, Jiangyong He, Yange Liu, Jian Zhao, Huiyi Guo, Longfei Zhu, Mengjie Zhou, Kaiyan Zhu, Congcong Liu and Zhi Wang
Photonics 2021, 8(3), 78; https://doi.org/10.3390/photonics8030078 - 15 Mar 2021
Cited by 8 | Viewed by 5667
Abstract
As a high-throughput data analysis technique, photon time stretching (PTS) is widely used in the monitoring of rare events such as cancer cells, rough waves, and the study of electronic and optical transient dynamics. The PTS technology relies on high-speed data collection, and [...] Read more.
As a high-throughput data analysis technique, photon time stretching (PTS) is widely used in the monitoring of rare events such as cancer cells, rough waves, and the study of electronic and optical transient dynamics. The PTS technology relies on high-speed data collection, and the large amount of data generated poses a challenge to data storage and real-time processing. Therefore, how to use compatible optical methods to filter and process data in advance is particularly important. The time-lens proposed, based on the duality of time and space as an important data processing method derived from PTS, achieves imaging of time signals by controlling the phase information of the timing signals. In this paper, an optical neural network based on the time-lens (TL-ONN) is proposed, which applies the time-lens to the layer algorithm of the neural network to realize the forward transmission of one-dimensional data. The recognition function of this optical neural network for speech information is verified by simulation, and the test recognition accuracy reaches 95.35%. This architecture can be applied to feature extraction and classification, and is expected to be a breakthrough in detecting rare events such as cancer cell identification and screening. Full article
(This article belongs to the Special Issue Advanced Technique and Future Perspective for Next Generation Optical Fiber Communications)
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16 pages, 2849 KiB  
Article
Engineering of TiO2 or ZnO—Graphene Oxide Nanoheterojunctions for Hybrid Solar Cells Devices
by Duarte Carreira, Paulo A. Ribeiro, Maria Raposo and Susana Sério
Photonics 2021, 8(3), 75; https://doi.org/10.3390/photonics8030075 - 12 Mar 2021
Cited by 8 | Viewed by 3730
Abstract
It is currently of huge importance to find alternatives to fossil fuels to produce clean energy and to ensure the energy demands of modern society. In the present work, two types of hybrid solar cell devices were developed and characterized. The photoactive layers [...] Read more.
It is currently of huge importance to find alternatives to fossil fuels to produce clean energy and to ensure the energy demands of modern society. In the present work, two types of hybrid solar cell devices were developed and characterized. The photoactive layers of the hybrid heterojunctions comprise poly (allylamine chloride) (PAH) and graphene oxide (GO) and TiO2 or ZnO films, which were deposited using the layer-by-layer technique and DC-reactive magnetron sputtering, respectively, onto fluorine-doped tin oxide (FTO)-coated glass substrates. Scanning electron microscopy evidenced a homogeneous inorganic layer, the surface morphology of which was dependent on the number of organic bilayers. The electrical characterization pointed out that FTO/(PAH/GO)50/TiO2/Al, FTO/(PAH/GO)30/ZnO/Al, and FTO/(PAH/GO)50/ZnO/Al architectures were the only ones to exhibit a diode behavior, and the last one experienced a decrease in current in a low-humidity environment. The (PAH/GO)20 impedance spectroscopy study further revealed the typical impedance of a parallel RC circuit for a dry environment, whereas in a humid environment, it approached the impedance of a series of three parallel RC circuits, indicating that water and oxygen contribute to other conduction processes. Finally, the achieved devices should be encapsulated to work successfully as solar cells. Full article
(This article belongs to the Special Issue Photonics, Optics and Laser Technology)
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6 pages, 1061 KiB  
Article
Computational Modeling and Simulation to Increase Laser Shooting Accuracy of Autonomous LEO Trackers
by Jose M. Gambi, Maria L. Garcia del Pino, Jonathan Mosser and Ewa B. Weinmüller
Photonics 2021, 8(2), 55; https://doi.org/10.3390/photonics8020055 - 18 Feb 2021
Cited by 4 | Viewed by 2483
Abstract
In this paper, we introduce a computational procedure that enables autonomous LEO laser trackers endowed with INSs to increase the current accuracy when shooting at middle distant medium-size LEO debris targets. The code is designed for the trackers to throw the targets into [...] Read more.
In this paper, we introduce a computational procedure that enables autonomous LEO laser trackers endowed with INSs to increase the current accuracy when shooting at middle distant medium-size LEO debris targets. The code is designed for the trackers to throw the targets into the atmosphere by means of ablations. In case that the targets are eclipsed to the trackers by the Earth, the motions of the trackers and targets are modeled by equations that contain post-Newtonian terms accounting for the curvature of space. Otherwise, when the approaching targets become visible for the trackers, we additionally use more accurate equations, which allow to account for the local bending of the laser beams aimed at the targets. We observe that under certain circumstances the correct shooting configurations that allow to safely and efficiently shoot down the targets, differ from the current estimations by distances that may be larger than the size of many targets. In short, this procedure enables to estimate the optimal shooting instants for any middle distant medium-size LEO debris target. Full article
(This article belongs to the Special Issue Laser Interaction with Materials)
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14 pages, 4887 KiB  
Article
Mie Resonance Engineering in Two Disks
by Evgeny Bulgakov, Konstantin Pichugin and Almas Sadreev
Photonics 2021, 8(2), 49; https://doi.org/10.3390/photonics8020049 - 13 Feb 2021
Cited by 13 | Viewed by 3888
Abstract
Recently the recipes to achieve the high-Q subwavelength resonances in an isolated dielectric disk have been reported based on avoided crossing (anticrossing) of the TE resonances under variation of the aspect ratio of the disk. In a silicon disk that recipe gives an [...] Read more.
Recently the recipes to achieve the high-Q subwavelength resonances in an isolated dielectric disk have been reported based on avoided crossing (anticrossing) of the TE resonances under variation of the aspect ratio of the disk. In a silicon disk that recipe gives an enhancement of the Q factor by one order of magnitude. In the present paper we present the approach based on engineering of the spherical Mie resonances with high orbital index in two coaxial disks by two-fold avoided crossing of the resonant modes of the disks. At the first step we select the resonant modes of single disk which are degenerate because of the opposite symmetry. Approaching of the second disk removes this degeneracy because of interaction between the disks. As a result at certain distances we realize the hybridized anti-bonding resonant modes whose morphology becomes close to the spherical Mie resonant mode with high orbital index. Respectively the Q factor of the anti-bonding resonant mode can be enhanced by three orders of magnitude compared to the case of single disk. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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48 pages, 3859 KiB  
Article
Accurate Power-Efficient Format-Scalable Multi-Parallel Optical Digital-to-Analogue Conversion
by Moshe Nazarathy and Ioannis Tomkos
Photonics 2021, 8(2), 38; https://doi.org/10.3390/photonics8020038 - 4 Feb 2021
Cited by 16 | Viewed by 4166
Abstract
In optical transmitters generating multi-level constellations, optical modulators are preceded by Electronic Digital-to-Analog-Converters (eDAC). It is advantageous to use eDAC-free Optical Analog to Digital Converters (oDAC) to directly convert digital bitstreams into multilevel PAM/QAM optical signals. State-of-the-art oDACs are based on Segmented Mach-Zehnder-Modulators [...] Read more.
In optical transmitters generating multi-level constellations, optical modulators are preceded by Electronic Digital-to-Analog-Converters (eDAC). It is advantageous to use eDAC-free Optical Analog to Digital Converters (oDAC) to directly convert digital bitstreams into multilevel PAM/QAM optical signals. State-of-the-art oDACs are based on Segmented Mach-Zehnder-Modulators (SEMZM) using multiple modulation segments strung along the MZM waveguides to serially accumulate binary-modulated optical phases. Here we aim to assess performance limits of the Serial oDACs (SEMZM) and introduce an alternative improved Multi-Parallel oDAC (MPoDAC) architecture, in particular based on arraying multiple binary-driven MZMs in parallel: Multi-parallel MZM (MPMZM) oDAC. We develop generic methodologies of oDAC specification and optimization encompassing both SEMZM and MPMZM options in Direct-Detection (DD) and Coherent-Detection (COH) implementations. We quantify and compare intrinsic performance limits of the various serial/parallel DD/COH subclasses for general constellation orders, comparing with the scant prior-work on the multi-parallel option. A key finding: COH-MPMZM is the only class synthesizing ‘perfect’ (equi-spaced max-full-scale) constellations while maximizing energy-efficiency-SEMZM/MPMZM for DD are less accurate when maximal energy-efficiency is required. In particular, we introduce multiple variants of PAM4|8 DD and QAM16|64 COH MPMZMs, working out their accuracy vs. energy-efficiency-and-complexity tradeoffs, establishing their format-reconfigurability (format-flexible switching of constellation order and/or DD/COH). Full article
(This article belongs to the Special Issue Reconfigurable Photonic Interconnects)
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9 pages, 3087 KiB  
Review
Inverted p-down Design for High-Speed Photodetectors
by Masahiro Nada, Fumito Nakajima, Toshihide Yoshimatsu, Yasuhiko Nakanishi, Atsushi Kanda, Takahiko Shindo, Shoko Tatsumi, Hideaki Matsuzaki and Kimikazu Sano
Photonics 2021, 8(2), 39; https://doi.org/10.3390/photonics8020039 - 4 Feb 2021
Cited by 12 | Viewed by 3836
Abstract
We discuss the structural consideration of high-speed photodetectors used for optical communications, focusing on vertical illumination photodetectors suitable for device fabrication and optical coupling. We fabricate an avalanche photodiode that can handle 100-Gbit/s four-level pulse-amplitude modulation (50 Gbaud) signals, and pin photodiodes for [...] Read more.
We discuss the structural consideration of high-speed photodetectors used for optical communications, focusing on vertical illumination photodetectors suitable for device fabrication and optical coupling. We fabricate an avalanche photodiode that can handle 100-Gbit/s four-level pulse-amplitude modulation (50 Gbaud) signals, and pin photodiodes for 100-Gbaud operation; both are fabricated with our unique inverted p-side down (p-down) design. Full article
(This article belongs to the Special Issue Advanced Ultra High Speed Optoelectronic Devices)
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14 pages, 24671 KiB  
Article
Topological-Insulator-Based Gap-Surface Plasmon Metasurfaces
by Andreas Aigner, Stefan A. Maier and Haoran Ren
Photonics 2021, 8(2), 40; https://doi.org/10.3390/photonics8020040 - 4 Feb 2021
Cited by 3 | Viewed by 5797
Abstract
Topological insulators (TIs) have unique highly conducting symmetry-protected surface states while the bulk is insulating, making them attractive for various applications in condensed matter physics. Recently, topological insulator materials have been tentatively applied for both near- and far-field wavefront manipulation of electromagnetic waves, [...] Read more.
Topological insulators (TIs) have unique highly conducting symmetry-protected surface states while the bulk is insulating, making them attractive for various applications in condensed matter physics. Recently, topological insulator materials have been tentatively applied for both near- and far-field wavefront manipulation of electromagnetic waves, yielding superior plasmonic properties in the ultraviolet (UV)-to-visible wavelength range. However, previous reports have only demonstrated inefficient wavefront control based on binary metasurfaces that were digitalized on a TI thin film or non-directional surface plasmon polariton (SPP) excitation. Here, we numerically demonstrated the plasmonic capabilities of the TI Bi2Te3 as a material for gap–surface plasmon (GSP) metasurfaces. By employing the principle of the geometric phase, a far-field beam-steering metasurface was designed for the visible spectrum, yielding a cross-polarization efficiency of 34% at 500 nm while suppressing the co-polarization to 0.08%. Furthermore, a birefringent GSP metasurface design was studied and found to be capable of directionally exciting SPPs depending on the incident polarization. Our work forms the basis for accurately controlling the far- and near-field responses of TI-based GSP metasurfaces in the visible spectral range. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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14 pages, 5322 KiB  
Article
Proposal of Highly Efficient Quantum Well Microring Resonator-Loaded Optical Phase Modulator Integrated with Antenna-Coupled Electrodes for Radio-over-Fiber
by Hiro Kamada and Taro Arakawa
Photonics 2021, 8(2), 37; https://doi.org/10.3390/photonics8020037 - 3 Feb 2021
Cited by 4 | Viewed by 4400
Abstract
Radio-over-fiber (RoF) technology for low-loss, high-speed millimeter-wave transmission using optical fibers has been attracting attention. We propose a highly efficient microring resonator (MRR)-loaded InGaAs/InAlAs multiple-quantum-well (MQW) phase modulator with an antenna coupled electrode (ACE) for 60 GHz RoF systems, and its modulation characteristics [...] Read more.
Radio-over-fiber (RoF) technology for low-loss, high-speed millimeter-wave transmission using optical fibers has been attracting attention. We propose a highly efficient microring resonator (MRR)-loaded InGaAs/InAlAs multiple-quantum-well (MQW) phase modulator with an antenna coupled electrode (ACE) for 60 GHz RoF systems, and its modulation characteristics are theoretically discussed. This modulator is able to directly convert wireless millimeter-wave (MMW) signals into optical signals without an external power supply. The MRR used in the waveguide structure increases the optical phase change obtained by the unique quantum confinement Stark effect in the MQW through phase enhancement effects, while the ACE based on a coupled microstrip resonant electrode applies a strong standing wave electric field to the waveguide. The proposed modulator is expected to provide tens of times higher phase modulation efficiency than a conventional gap-embedded planar antenna-integrated modulator. Full article
(This article belongs to the Special Issue Radio over Fiber)
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6 pages, 3875 KiB  
Communication
Effect of Si Doping on the Performance of GaN Schottky Barrier Ultraviolet Photodetector Grown on Si Substrate
by Fangzhou Liang, Wen Chen, Meixin Feng, Yingnan Huang, Jianxun Liu, Xiujian Sun, Xiaoning Zhan, Qian Sun, Qibao Wu and Hui Yang
Photonics 2021, 8(2), 28; https://doi.org/10.3390/photonics8020028 - 23 Jan 2021
Cited by 8 | Viewed by 3175
Abstract
GaN Schottky barrier ultraviolet photodetectors with unintentionally doped GaN and lightly Si-doped n−-GaN absorption layers were successfully fabricated, respectively. The high-quality GaN films on the Si substrate both have a fairly low dislocation density and point defect concentration. More importantly, the [...] Read more.
GaN Schottky barrier ultraviolet photodetectors with unintentionally doped GaN and lightly Si-doped n−-GaN absorption layers were successfully fabricated, respectively. The high-quality GaN films on the Si substrate both have a fairly low dislocation density and point defect concentration. More importantly, the effect of Si doping on the performance of the GaN-on-Si Schottky barrier ultraviolet photodetector was studied. It was found that light Si doping in the absorption layer can significantly increase the responsivity under reverse bias, which might be attributed to the persistent photoconductivity that originates from the lowering of the Schottky barrier height. In addition, the devices with unintentionally doped GaN demonstrated a relatively high-speed photo response. We briefly studied the mechanism of changes in Schottky barrier, dark current and the characteristic of response time. Full article
(This article belongs to the Special Issue Wide Bandgap Semiconductor Photonic Devices)
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21 pages, 6504 KiB  
Article
Image Deconvolution with Hybrid Reweighted Adaptive Total Variation (HRATV) for Optoacoustic Tomography
by Chen Yang, Yang Jiao, Xiaohua Jian and Yaoyao Cui
Photonics 2021, 8(2), 25; https://doi.org/10.3390/photonics8020025 - 20 Jan 2021
Cited by 4 | Viewed by 3090
Abstract
Optoacoustic tomography (OAT) is a hybrid biomedical imaging modality that usually employs a transducer array to detect laser-generated ultrasonic signals. The reconstructed image suffers low contrast and degraded resolution due to the limited bandwidth and the spatial directivity of the transducer element. Here, [...] Read more.
Optoacoustic tomography (OAT) is a hybrid biomedical imaging modality that usually employs a transducer array to detect laser-generated ultrasonic signals. The reconstructed image suffers low contrast and degraded resolution due to the limited bandwidth and the spatial directivity of the transducer element. Here, we introduce a modified image deconvolution method with a hybrid reweighted adaptive total variation tailored to improve the image quality of OAT. The effectiveness and the parameter dependency of the proposed method are verified on standard test images. The performance of the proposed method in OAT is then characterized on both simulated phantoms and in vivo mice experiments, which demonstrates that the modified deconvolution algorithm is able to restore the sharp edges and fine details in OAT simultaneously. The signal-to-noise ratios (SNRs) of the target structures in mouse liver and brain were improved by 4.90 and 12.69 dB, respectively. We also investigated the feasibility of using Fourier ring correlation (FRC) as an indicator of the image quality to monitor the deconvolution progress in OAT. Based on the experimental results, a practical guide for image deconvolution in OAT was summarized. We anticipate that the proposed method will be a promising post-processing tool to enhance the visualization of micro-structures in OAT. Full article
(This article belongs to the Section Biophotonics and Biomedical Optics)
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