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Photonics
Volume 8
Issue 7
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Photonics, Volume 8, Issue 7 (July 2021) – 58 articles

Cover Story (view full-size image): Two recent scientific breakthroughs announced a new phase in the exploration of organic light emission—firstly, the indication of lasing in an organic diode and, secondly, demonstration of the high-speed response of μ-OLEDs. For electrically pumped organic host–guest diode lasers, a new rate-equation model is developed that treats the Stoke-shifted re-absorption in a self-consistent manner. The class-B nature of these lasers is demonstrated in simulations by the manifestation of damped GHz relaxation oscillations following the ultra-fast switch-on of the laser. It is shown that laser operation is characterized by: (1) The presence of a pump–current threshold; (2) Linewidth narrowing by several orders of magnitude on passing the threshold; (3) Gain clamping. Finally, the prospect for CW steady-state lasing is discussed. View this paper
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11 pages, 842 KiB  
Communication
Remote Quantum-Safe Authentication of Entities with Physical Unclonable Functions
by Georgios M. Nikolopoulos
Photonics 2021, 8(7), 289; https://doi.org/10.3390/photonics8070289 - 20 Jul 2021
Cited by 5 | Viewed by 2921
Abstract
Physical unclonable functions have been shown to be a useful resource of randomness for implementing various cryptographic tasks including entity authentication. All the related entity authentication protocols that have been discussed in the literature so far, either they are vulnerable to an emulation [...] Read more.
Physical unclonable functions have been shown to be a useful resource of randomness for implementing various cryptographic tasks including entity authentication. All the related entity authentication protocols that have been discussed in the literature so far, either they are vulnerable to an emulation attack, or they are limited to short distances. Hence, quantum-safe remote entity authentication over large distances remains an open question. In the first part of this work, we discuss the requirements that an entity authentication protocol has to offer, to be useful for remote entity authentication in practice. Subsequently, we propose a protocol, which can operate over large distances, and offers security against both classical and quantum adversaries. The proposed protocol relies on standard techniques, it is fully compatible with the infrastructure of existing and future photonic networks, and it can operate in parallel with other quantum protocols, including QKD protocols. Full article
(This article belongs to the Special Issue Photonics Technologies for Smart Quantum Networks)
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11 pages, 25676 KiB  
Communication
Ag-Yb Alloy-Novel Tunable Plasmonic Material
by Suetying Ching, Chakming Chan, Jack Ng and Kokwai Cheah
Photonics 2021, 8(7), 288; https://doi.org/10.3390/photonics8070288 - 20 Jul 2021
Cited by 2 | Viewed by 2231
Abstract
Metals are commonly used in plasmonic devices because of their strong plasmonic property. However, such properties are not easily tuned. For applications such as spatial light modulators and beam steering, tunable plasmonic properties are essential, and neither metals nor other plasmonic materials possess [...] Read more.
Metals are commonly used in plasmonic devices because of their strong plasmonic property. However, such properties are not easily tuned. For applications such as spatial light modulators and beam steering, tunable plasmonic properties are essential, and neither metals nor other plasmonic materials possess truly tunable plasmonic properties. In this work, we show that the silver alloy silver–ytterbium (Ag-Yb) possesses tunable plasmonic properties; its plasmonic response strength can be adjusted as a function of Yb concentration. Such tunability can be explained in terms of the influence of Yb on bound charge and interaction of its dielectric with the dielectric of Ag. The change in transition characteristics progressively weakens Ag’s plasmonic properties. With a spectral ellipsometric measurement, it was shown that the Ag-Yb alloy thin film retains the properties of Ag with high transmission efficiency. The weakened surface plasmon coupling strength without dramatic change in the coupling wavelengths implies that the tunability of the Ag-Yb alloy is related to its volume ratio. The principle mechanism of the plasmonic change is theoretically explained using a model. This work points to a potential new type of tunable plasmonic material. Full article
(This article belongs to the Section Optoelectronics and Optical Materials)
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22 pages, 7365 KiB  
Review
Types/Applications of Photoacoustic Contrast Agents: A Review
by Jaehun Jung, Yongho Jang, Mingyun Kim and Hyuncheol Kim
Photonics 2021, 8(7), 287; https://doi.org/10.3390/photonics8070287 - 19 Jul 2021
Cited by 6 | Viewed by 3179
Abstract
Ultrasound imaging, one of the common diagnosis techniques, is frequently used since it is safe, cost-efficient technique and real-time imaging can be conducted. However, various organs and tissues reflect ultrasonic waves, which leads to difficulty in imaging small biomolecules and to a low [...] Read more.
Ultrasound imaging, one of the common diagnosis techniques, is frequently used since it is safe, cost-efficient technique and real-time imaging can be conducted. However, various organs and tissues reflect ultrasonic waves, which leads to difficulty in imaging small biomolecules and to a low spatial resolution for deep-tissue images. As such, there have been significant advances in photonics and optical molecular probes in recent years, and photoacoustic (PA) tomography (PAT) has emerged as a promising modality that can overcome the limitations of ultrasound. PAT relies on the photoacoustic effect, which is the conversion of absorbed optical energy into acoustic energy. Since fewer biomolecules exhibit the photoacoustic effect compared to the scattering or reflection of ultrasound, PAT can be employed to generate high-resolution images. PAT also has a number of other advantages when compared to conventional biomedical imaging modalities such as optical tomography, ultrasound imaging, computed tomography, positron emission tomography and magnetic resonance imaging. This review provides a general overview of the contrast agents used for PAT, including organic, inorganic and hybrid contrast agents, and describes their application. This review also identifies limitations of current PAT contrast agents and suggests future research directions for their development. Full article
(This article belongs to the Special Issue Photoacoustic Imaging and Systems)
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12 pages, 3427 KiB  
Article
Three-Dimensional (3D) Printing Implemented by Computer-Generated Holograms for Generation of 3D Layered Images in Optical Near Field
by Chung-Fei Lee, Wei-Feng Hsu, Tzu-Hsuan Yang and Ren-Jei Chung
Photonics 2021, 8(7), 286; https://doi.org/10.3390/photonics8070286 - 19 Jul 2021
Cited by 3 | Viewed by 3176
Abstract
Photocurable three-dimensional (3D) printing is a stepwise layer-by-layer fabrication process widely used in the manufacture of highly specialized objects. Current 3D printing techniques are easily implemented; however, the build rate is slow and the surface quality is less than ideal. Holographic 3D display [...] Read more.
Photocurable three-dimensional (3D) printing is a stepwise layer-by-layer fabrication process widely used in the manufacture of highly specialized objects. Current 3D printing techniques are easily implemented; however, the build rate is slow and the surface quality is less than ideal. Holographic 3D display (3DHD) technology makes it possible to reform planar wavefronts into a 3D intensity distribution, which appears as a 3D image in space. This paper examined the application of holographic imaging technology to 3D printing based on photocurable polymers. The proposed system uses a 3DHD diffractive optics system based on a liquid-crystal-on-silicon spatial light modulator (LCoS-SLM), wherein a 3D layered image is created in the optical near field, based on a computer-generated hologram (CGH) optimized using the iterative angular spectrum algorithm (IASA) and a circular IASA. From a single CGH, multiple 2D sliced images are created in space to form a 3D optical image used to initiate the photopolymerization of photocurable resin to form 3D objects. In experiments, the proposed 3D printing system was used to create five polymer objects with a maximum axial length of 25 mm and minimum feature width of 149 μm. The phase-only CGH reformed the incident light into a distribution of optical intensity with high diffraction efficiency suitable for photocuring. Despite limitations pertaining to fabrication area and axial complexity in this initial study, the proposed method demonstrated high light efficiency, high resolution in the lateral direction, rapid fabrication, and good object continuity. Full article
(This article belongs to the Special Issue Holography)
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14 pages, 712 KiB  
Article
Effective Control of the Optical Bistability of a Three-Level Quantum Emitter near a Nanostructured Plasmonic Metasurface
by Hamid R. Hamedi, Emmanuel Paspalakis and Vassilios Yannopapas
Photonics 2021, 8(7), 285; https://doi.org/10.3390/photonics8070285 - 17 Jul 2021
Cited by 3 | Viewed by 4779
Abstract
We study, theoretically, the phenomena optical bistability and multistability of a hybrid quantum-plasmonic system immersed within an optical ring cavity. The hybrid quantum-plasmonic system consists of a three-level V-type quantum emitter and a two-dimensional plasmonic metasurface of gold nanoshells. The quantum emitter [...] Read more.
We study, theoretically, the phenomena optical bistability and multistability of a hybrid quantum-plasmonic system immersed within an optical ring cavity. The hybrid quantum-plasmonic system consists of a three-level V-type quantum emitter and a two-dimensional plasmonic metasurface of gold nanoshells. The quantum emitter and the plasmonic metasurface are placed in close proximity to each other so that a strong quantum interference of spontaneous emission occurs, which enables the strong modification of optical-bistability/ multistability hysteresis curves. Along with this, the strong interaction between the emitter and the plasmonic metasurface allows for active control of the corresponding bistable threshold intensity. Furthermore, we show that by varying the metasurface-emitter separation, a transition from bistability to multistability of the hybrid system is observed. Lastly, by introducing an additional incoherent pumping in the system, we have the emergence of phenomena, such as probe absorption and gain, with or without population inversion. The results may find technological application in on-chip nanoscale photonic devices, optoelectronics and solid-state quantum information science. Full article
(This article belongs to the Special Issue Plasmonic Metasurfaces)
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9 pages, 1874 KiB  
Article
Scaling the Retinal Image of the Wide-Angle Eye Using the Nodal Point
by Michael J. Simpson
Photonics 2021, 8(7), 284; https://doi.org/10.3390/photonics8070284 - 17 Jul 2021
Cited by 9 | Viewed by 5866
Abstract
Angles subtended at the second nodal point of the eye (NP2) are approximately the same as input visual angles over a very large angular range, despite the nodal point being a paraxial lens property. Raytracing using an average pseudophakic eye showed that the [...] Read more.
Angles subtended at the second nodal point of the eye (NP2) are approximately the same as input visual angles over a very large angular range, despite the nodal point being a paraxial lens property. Raytracing using an average pseudophakic eye showed that the angular nodal point criterion was only valid up to about 10°, and that the linear relationship was due instead to the cornea and lens initially creating chief ray angles at the exit pupil that are about 0.83 times input values for this particular eye, and then by the retina curving around to meet the rays in a manner that compensates for increasing angle. This linear relationship is then also maintained when retinal intersections are calculated relative to other axial points, with angles rescaled approximately using the equation R/(R + delta), where delta is the axial distance from the center of a spherical retina of radius R. Angles at NP2 approximately match the input angles, but the terminology is misleading because this is not a paraxial property of the eye. Chief rays are used with finite raytracing to determine the actual behavior. Full article
(This article belongs to the Special Issue Ocular Imaging for Eye Care)
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17 pages, 8859 KiB  
Article
Formation of Inverse Energy Flux in the Case of Diffraction of Linearly Polarized Radiation by Conventional and Generalized Spiral Phase Plates
by Andrey V. Ustinov, Svetlana N. Khonina and Alexey P. Porfirev
Photonics 2021, 8(7), 283; https://doi.org/10.3390/photonics8070283 - 16 Jul 2021
Cited by 5 | Viewed by 2235
Abstract
Recently, there has been increased interest in the shaping of light fields with an inverse energy flux to guide optically trapped nano- and microparticles towards a radiation source. To generate inverse energy flux, non-uniformly polarized laser beams, especially higher-order cylindrical vector beams, are [...] Read more.
Recently, there has been increased interest in the shaping of light fields with an inverse energy flux to guide optically trapped nano- and microparticles towards a radiation source. To generate inverse energy flux, non-uniformly polarized laser beams, especially higher-order cylindrical vector beams, are widely used. Here, we demonstrate the use of conventional and so-called generalized spiral phase plates for the formation of light fields with an inverse energy flux when they are illuminated with linearly polarized radiation. We present an analytical and numerical study of the longitudinal and transverse components of the Poynting vector. The conditions for maximizing the negative value of the real part of the longitudinal component of the Poynting vector are obtained. Full article
(This article belongs to the Special Issue Polarized Light and Optical Systems)
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9 pages, 3729 KiB  
Article
Characterization of Ex Vivo Nonmelanoma Skin Tissue Using Raman Spectroscopy
by Hieu T. M. Nguyen, Yao Zhang, Austin J. Moy, Xu Feng, Katherine R. Sebastian, Jason S. Reichenberg, Matthew C. Fox, Mia K. Markey and James W. Tunnell
Photonics 2021, 8(7), 282; https://doi.org/10.3390/photonics8070282 - 16 Jul 2021
Cited by 7 | Viewed by 2481
Abstract
Raman spectroscopy has shown great potential in detecting nonmelanoma skin cancer accurately and quickly; however, little direct evidence exists on the sensitivity of measurements to the underlying anatomy. Here, we aimed to correlate Raman measurements directly to the underlying tissue anatomy. We acquired [...] Read more.
Raman spectroscopy has shown great potential in detecting nonmelanoma skin cancer accurately and quickly; however, little direct evidence exists on the sensitivity of measurements to the underlying anatomy. Here, we aimed to correlate Raman measurements directly to the underlying tissue anatomy. We acquired Raman spectra of ex vivo skin tissue from 25 patients undergoing Mohs surgery with a fiber probe. We utilized a previously developed biophysical model to extract key biomarkers in the skin from the Raman spectra. We then examined the correlations between the biomarkers and the major skin structures (including the dermis, sebaceous glands, hair follicles, fat, and two types of nonmelanoma skin cancer—basal cell carcinoma (BCC) and squamous cell carcinoma (SCC)). SCC had a significantly different concentration of keratin, collagen, and nucleic acid than normal structures, while ceramide differentiated BCC from normal structures. Our findings identified the key proteins, lipids, and nucleic acids that discriminate nonmelanoma tumors and healthy skin using Raman spectroscopy. These markers may be promising surgical guidance tools for detecting tumors in resection margins. Full article
(This article belongs to the Special Issue Tissue Optics)
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21 pages, 14803 KiB  
Review
Advances in Endoscopic Photoacoustic Imaging
by Yan Li, Gengxi Lu, Qifa Zhou and Zhongping Chen
Photonics 2021, 8(7), 281; https://doi.org/10.3390/photonics8070281 - 16 Jul 2021
Cited by 25 | Viewed by 5559
Abstract
Photoacoustic (PA) imaging is able to provide extremely high molecular contrast while maintaining the superior imaging depth of ultrasound (US) imaging. Conventional microscopic PA imaging has limited access to deeper tissue due to strong light scattering and attenuation. Endoscopic PA technology enables direct [...] Read more.
Photoacoustic (PA) imaging is able to provide extremely high molecular contrast while maintaining the superior imaging depth of ultrasound (US) imaging. Conventional microscopic PA imaging has limited access to deeper tissue due to strong light scattering and attenuation. Endoscopic PA technology enables direct delivery of excitation light into the interior of a hollow organ or cavity of the body for functional and molecular PA imaging of target tissue. Various endoscopic PA probes have been developed for different applications, including the intravascular imaging of lipids in atherosclerotic plaque and endoscopic imaging of colon cancer. In this paper, the authors review representative probe configurations and corresponding preclinical applications. In addition, the potential challenges and future directions of endoscopic PA imaging are discussed. Full article
(This article belongs to the Special Issue Photoacoustic Imaging and Systems)
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19 pages, 7082 KiB  
Article
Computing 3D Phase-Type Holograms Based on Deep Learning Method
by Huadong Zheng, Jianbin Hu, Chaojun Zhou and Xiaoxi Wang
Photonics 2021, 8(7), 280; https://doi.org/10.3390/photonics8070280 - 15 Jul 2021
Cited by 15 | Viewed by 3678
Abstract
Computer holography is a technology that use a mathematical model of optical holography to generate digital holograms. It has wide and promising applications in various areas, especially holographic display. However, traditional computational algorithms for generation of phase-type holograms based on iterative optimization have [...] Read more.
Computer holography is a technology that use a mathematical model of optical holography to generate digital holograms. It has wide and promising applications in various areas, especially holographic display. However, traditional computational algorithms for generation of phase-type holograms based on iterative optimization have a built-in tradeoff between the calculating speed and accuracy, which severely limits the performance of computational holograms in advanced applications. Recently, several deep learning based computational methods for generating holograms have gained more and more attention. In this paper, a convolutional neural network for generation of multi-plane holograms and its training strategy is proposed using a multi-plane iterative angular spectrum algorithm (ASM). The well-trained network indicates an excellent ability to generate phase-only holograms for multi-plane input images and to reconstruct correct images in the corresponding depth plane. Numerical simulations and optical reconstructions show that the accuracy of this method is almost the same with traditional iterative methods but the computational time decreases dramatically. The result images show a high quality through analysis of the image performance indicators, e.g., peak signal-to-noise ratio (PSNR), structural similarity (SSIM) and contrast ratio. Finally, the effectiveness of the proposed method is verified through experimental investigations. Full article
(This article belongs to the Special Issue Holography)
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17 pages, 3269 KiB  
Article
Organic Diode Laser Dynamics: Rate-Equation Model, Reabsorption, Validation and Threshold Predictions
by Daan Lenstra, Alexis P.A. Fischer, Amani Ouirimi, Alex Chamberlain Chime, Nixson Loganathan and Mahmoud Chakaroun
Photonics 2021, 8(7), 279; https://doi.org/10.3390/photonics8070279 - 15 Jul 2021
Cited by 5 | Viewed by 3440
Abstract
We present and analyze a simple model based on six rate equations for an electrically pumped organic diode laser. The model applies to organic host-guest systems and includes Stoke-shifted reabsorption in a self-consistent manner. With the validated model for the Alq3:DCM host-guest system, [...] Read more.
We present and analyze a simple model based on six rate equations for an electrically pumped organic diode laser. The model applies to organic host-guest systems and includes Stoke-shifted reabsorption in a self-consistent manner. With the validated model for the Alq3:DCM host-guest system, we predict the threshold for short-pulse laser operation. We predict laser operation characterized by damped relaxation oscillations in the GHz regime and several orders of magnitude linewidth narrowing. Prospect for CW steady-state laser operation is discussed. Full article
(This article belongs to the Special Issue Nonlinear Dynamics of Semiconductor Lasers and Their Applications)
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17 pages, 3656 KiB  
Article
Target Tracking and Ranging Based on Single Photon Detection
by Zhikang Li, Bo Liu, Huachuang Wang, Zhen Chen, Qun Zhang, Kangjian Hua and Jing Yang
Photonics 2021, 8(7), 278; https://doi.org/10.3390/photonics8070278 - 15 Jul 2021
Cited by 7 | Viewed by 2782
Abstract
In order to achieve non-cooperative target tracking and ranging in conditions of a weak echo signal, this paper presents a real-time acquisition, pointing, tracking (APT), and ranging (APTR) lidar system based on single photon detection. With this system, an active target APT mechanism [...] Read more.
In order to achieve non-cooperative target tracking and ranging in conditions of a weak echo signal, this paper presents a real-time acquisition, pointing, tracking (APT), and ranging (APTR) lidar system based on single photon detection. With this system, an active target APT mechanism based on a single photon detector is proposed. The target tracking and ranging strategy and the simulation of target APT are presented. Experiments in the laboratory show that the system has good performance to achieve the acquisition, pointing and ranging of a static target, and track a dynamic target (angular velocity around 3 mrad/s) under the condition of extremely weak echo signals (a dozen photons). Meanwhile, through further theoretical analysis, it can be proven that the mechanism has stronger tracking and detection ability in long distance. It can achieve the active tracking of the target with a lateral velocity of hundreds of meters per second at about one hundred kilometers distance. This means that it has the ability of fast long-distance non-cooperative target tracking and ranging, only by using a single-point single photon detector. Full article
(This article belongs to the Special Issue Smart Pixels and Imaging)
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12 pages, 4027 KiB  
Article
An Inexpensive Portable Self-Reference Module for Digital Holographic Microscopy
by Xin Fan, Zhengyuan Tang, Kevin O’Dwyer and Bryan M. Hennelly
Photonics 2021, 8(7), 277; https://doi.org/10.3390/photonics8070277 - 14 Jul 2021
Cited by 6 | Viewed by 2755
Abstract
This paper describes a novel optical system that can be integrated to the image port of an existing brightfield microscope in order to enhance the microscope with the features of digital holographic microscopy. The proposed system is modular and portable. It is relatively [...] Read more.
This paper describes a novel optical system that can be integrated to the image port of an existing brightfield microscope in order to enhance the microscope with the features of digital holographic microscopy. The proposed system is modular and portable. It is relatively inexpensive and robust to vibrations due to its compact design. An additional benefit is that the system does not need to undergo path-length realignment if the sample is changed, unlike several other architectures. The module is based on a square in-line Mach–Zender architecture but achieves the off-axis condition using two sets of wedge prism pairs. This design offers a significant advantage over competing Mach–Zender nearly common-path modules in terms of path length matching of object and reference wavefields for the case of low-temporal coherence sources, which are preferable for low noise phase imaging. An additional advantage that the proposed system has when compared with similar modules is the facility to continuously vary the tilt angles of the object and reference wavefields that are incident on the sensor, which enables the module to be readily adapted to any given microscope and camera. We provide a detailed overview of the module design and construction. Experimental results are demonstrated on a micro-lens array as well as buccal epithelial cells. We also provide a detailed discussion on the relationship between the proposed self-reference module and related common-path and nearly common-path holographic modules that have previously been proposed in the literature. Full article
(This article belongs to the Special Issue Holography)
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12 pages, 4045 KiB  
Article
Dynamic Speckle Illumination Digital Holographic Microscopy by Doubly Scattered System
by Yun Liu, Peihua Bu, Mingxing Jiao, Junhong Xing, Ke Kou, Tianhong Lian, Xian Wang and Yumeng Liu
Photonics 2021, 8(7), 276; https://doi.org/10.3390/photonics8070276 - 14 Jul 2021
Cited by 7 | Viewed by 2220
Abstract
The coherent noise always exists in digital holographic microscopy due to the laser source, degrading the image quality. A method of speckle suppression using the dynamic speckle illumination, produced by double-moving diffusers, is presented in digital holographic microscopy. The space–time correlation functions are [...] Read more.
The coherent noise always exists in digital holographic microscopy due to the laser source, degrading the image quality. A method of speckle suppression using the dynamic speckle illumination, produced by double-moving diffusers, is presented in digital holographic microscopy. The space–time correlation functions are theoretically analyzed from the statistics distribution in the doubly and singly scattered system, respectively. The configuration of double-moving diffusers is demonstrated to have better performance in speckle suppression compared with the single diffuser and moving-static double diffusers cases. The experiment results verify the feasibility of the approach. The presented approach only requires a single shot interferogram to realize the speckle reduction, accordingly it has the potential application in real-time measurement. Full article
(This article belongs to the Special Issue Holography)
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12 pages, 2743 KiB  
Review
Challenges in 3D Live Cell Imaging
by Herbert Schneckenburger and Verena Richter
Photonics 2021, 8(7), 275; https://doi.org/10.3390/photonics8070275 - 13 Jul 2021
Cited by 11 | Viewed by 4223
Abstract
A short overview on 3D live cell imaging is given. Relevant samples are described and various problems and challenges—including 3D imaging by optical sectioning, light scattering and phototoxicity—are addressed. Furthermore, enhanced methods of wide-field or laser scanning microscopy together with some relevant examples [...] Read more.
A short overview on 3D live cell imaging is given. Relevant samples are described and various problems and challenges—including 3D imaging by optical sectioning, light scattering and phototoxicity—are addressed. Furthermore, enhanced methods of wide-field or laser scanning microscopy together with some relevant examples and applications are summarized. In the future one may profit from a continuous increase in microscopic resolution, but also from molecular sensing techniques in the nanometer range using e.g., non-radiative energy transfer (FRET). Full article
(This article belongs to the Special Issue Tissue Optics)
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11 pages, 3285 KiB  
Article
Visual Adaptation to Scattering in Myopes
by José A. Villa-Carpes, Juan M. Bueno and Enrique J. Fernández
Photonics 2021, 8(7), 274; https://doi.org/10.3390/photonics8070274 - 13 Jul 2021
Cited by 4 | Viewed by 2347
Abstract
Myopes exhibit a larger capability of adaptation to defocus. Adaptation produces a boost in visual performance that can be characterized through different metrics. The ability of myopes to adapt to other sources of blur, such as diffusion, has not been studied so far. [...] Read more.
Myopes exhibit a larger capability of adaptation to defocus. Adaptation produces a boost in visual performance that can be characterized through different metrics. The ability of myopes to adapt to other sources of blur, such as diffusion, has not been studied so far. In this work, a group of 20 myopes with normal vision underwent high-contrast visual acuity (VA) measurements under different viewing conditions, wearing their refractive correction with or without a diffuser (Bangerter filter, BF). VA decreased immediately after wearing the BF of density 0.6, showing a significant relationship with the ocular refraction. After 40 minutes of binocular vision through the BF, a statistically significant increase (p = 0.02) in VA from 0.54 to 0.62 in decimal scale (from 0.3 to 0.2 logMAR) was obtained. No correlation with the refraction was observed. After removing the diffuser, VA returned to baseline. A control group (17 subjects) underwent the same experimental protocol but without diffuser filters. No significant changes in VA were found in this group. We describe a new type of contrast adaptation to blur in myopes caused by scattering, rather than by defocus. The effects of low scattering levels in vision might be relevant in the analysis of early stage of cataract, amblyopia treatments, and myopia understanding. Full article
(This article belongs to the Special Issue Visual Optics and Ophthalmology)
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9 pages, 2804 KiB  
Article
Super-Resolution Image Reconstruction Based on Single-Molecule Localization Algorithm
by Lixin Liu, Meijie Qi, Yujie Liu, Xinzhu Xue, Danni Chen and Junle Qu
Photonics 2021, 8(7), 273; https://doi.org/10.3390/photonics8070273 - 12 Jul 2021
Cited by 6 | Viewed by 2697
Abstract
Fluorescence imaging is an important and efficient tool in cell biology and biomedical research. In order to observe the dynamics of biological macromolecules such as DNA, RNA and proteins in live cells, it is extremely necessary to surpass the Abbe diffraction limit in [...] Read more.
Fluorescence imaging is an important and efficient tool in cell biology and biomedical research. In order to observe the dynamics of biological macromolecules such as DNA, RNA and proteins in live cells, it is extremely necessary to surpass the Abbe diffraction limit in microscopic imaging. Single-molecule localization microscopy (SMLM) is a sort of super-resolution imaging technique that can obtain a large number of images of sparse fluorescent molecules by the use of photoswitchable fluorescent probes and single-molecule localization technology. The center positions of fluorescent molecules in the images are precisely located, and then the entire sample pattern is reconstructed with super resolution. In this paper, we present a single-molecule localization algorithm (SMLA) that is based on blind deconvolution and centroid localization (BDCL) method. Single-molecule localization and image reconstruction of 15,000/9990 frames of original images of tubulins are accomplished. In addition, this fluorophore localization algorithm is used to localize high particle-density images. The results show that our BDCL-SMLA method is a reasonable attempt and useful method for SMLM imaging when the imaging system is unknown. Full article
(This article belongs to the Section Biophotonics and Biomedical Optics)
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11 pages, 6451 KiB  
Article
Enhanced Performance of Nanotextured Silicon Solar Cells with Excellent Light-Trapping Properties
by Bingfei Dou, Rui Jia, Zhao Xing, Xiaojiang Yao, Dongping Xiao, Zhi Jin and Xinyu Liu
Photonics 2021, 8(7), 272; https://doi.org/10.3390/photonics8070272 - 9 Jul 2021
Cited by 4 | Viewed by 2167
Abstract
Light-trapping nanostructures have been widely used for improving solar cells’ performance, but the higher surface recombination and poor electrode contact introduced need to be addressed. In this work, silicon nanostructures were synthesized via silver-catalyzed etching to texturize solar cells. Atomic-layer-deposited Al2O [...] Read more.
Light-trapping nanostructures have been widely used for improving solar cells’ performance, but the higher surface recombination and poor electrode contact introduced need to be addressed. In this work, silicon nanostructures were synthesized via silver-catalyzed etching to texturize solar cells. Atomic-layer-deposited Al2O3 passivated the nanotextured cells. A surface recombination velocity of 126 cm/s was obtained, much lower than the 228 cm/s of the SiNX-passivated one. Additionally, the open-circuit voltage (VOC) of the nanotextured cells improved significantly from 582 to 610 mV, as did the short-circuit current (JSC) from 25.5 to 31 mA/cm2. Furthermore, the electrode contact property was enhanced by light-induced plating. A best efficiency of 13.3% for nano-textured cells was obtained, which is higher than the planar cell’s 12%. Full article
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14 pages, 14106 KiB  
Article
An Improved Large-Field Microscopic Speckle Interferometry System for Dynamic Displacement Measurement of MEMS
by Chenjia Gao, Zhan Gao, Yuhao Niu, Xu Wang, Jieming Zhao and Lin Deng
Photonics 2021, 8(7), 271; https://doi.org/10.3390/photonics8070271 - 9 Jul 2021
Cited by 8 | Viewed by 2975
Abstract
The traditional microscopic speckle interferometer has limited applications in engineering due to its small field of view. In this paper, we propose a large-field microscopic speckle interferometer which embeds two doublet lens groups in the improved Mach–Zehnder optical path structure to expand its [...] Read more.
The traditional microscopic speckle interferometer has limited applications in engineering due to its small field of view. In this paper, we propose a large-field microscopic speckle interferometer which embeds two doublet lens groups in the improved Mach–Zehnder optical path structure to expand its field of view. At the same time, the new system can reduce the coherent noise of reflected light in the optical path. We use this new system to measure the dynamic displacement process of the entire surface of the microchips. The experimental results show that our improved measurement system can achieve large-field, real-time and high-precision dynamic measurement of micro-electromechanical systems (MEMS). Full article
(This article belongs to the Special Issue Optical Fiber Interferometric Sensors: New Production Methodologies and Novel Applications)
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12 pages, 4099 KiB  
Communication
A High-Sensitivity SPR Sensor with Bimetal/Silicon/Two-Dimensional Material Structure: A Theoretical Analysis
by Pengyu Zhang, Junxian Wang, Guoquan Chen, Jian Shen, Chaoyang Li and Tingting Tang
Photonics 2021, 8(7), 270; https://doi.org/10.3390/photonics8070270 - 9 Jul 2021
Cited by 21 | Viewed by 3353
Abstract
In this paper, we reported a theoretical study of a novel Surface plasmon resonance (SPR) biosensor composed of BK7 prism, gold (Au)/silver (Ag) bimetallic layer, silicon and two-dimensional (2D) materials. The bimetallic layer combines the advantages of Au and Ag and the high [...] Read more.
In this paper, we reported a theoretical study of a novel Surface plasmon resonance (SPR) biosensor composed of BK7 prism, gold (Au)/silver (Ag) bimetallic layer, silicon and two-dimensional (2D) materials. The bimetallic layer combines the advantages of Au and Ag and the high refractive index silicon layer enhances the electric field on the surface of the sensor, so that the sensor has a better overall performance in terms of sensitivity and figure of merit (FOM). Compared with ordinary dielectrics, 2D materials have excellent photoelectric properties, such as larger specific surface area, higher carrier density and stronger adsorption capacity, which improve the detection ability of the sensor. The sensitivity of the optimized sensor achieves 297.2°/RIU, 274°/RIU and 246°/RIU when the silicon layer is covered with graphene, MXene (Ti3T2Cx) and MoS2, respectively. Compared with the traditional SPR sensor, the sensitivity of the structure has been significantly improved, and its excellent performance has broad application prospects in biosensing and other fields. Full article
(This article belongs to the Special Issue Bio-Integrated Photonic Materials and Devices)
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Graphical abstract

25 pages, 1097 KiB  
Article
Quantum Optical Aspects of High-Harmonic Generation
by Sándor Varró
Photonics 2021, 8(7), 269; https://doi.org/10.3390/photonics8070269 - 9 Jul 2021
Cited by 14 | Viewed by 3554
Abstract
The interaction of electrons with strong laser fields is usually treated with semiclassical theory, where the laser is represented by an external field. There are analytic solutions for the free electron wave functions, which incorporate the interaction with the laser field exactly, but [...] Read more.
The interaction of electrons with strong laser fields is usually treated with semiclassical theory, where the laser is represented by an external field. There are analytic solutions for the free electron wave functions, which incorporate the interaction with the laser field exactly, but the joint effect of the atomic binding potential presents an obstacle for the analysis. Moreover, the radiation is a dynamical system, the number of photons changes during the interactions. Thus, it is legitimate to ask how can one treat the high order processes nonperturbatively, in such a way that the electron-atom interaction and the quantized nature of radiation be simultaneously taken into account? An analytic method is proposed to answer this question in the framework of nonrelativistic quantum electrodynamics. As an application, a quantum optical generalization of the strong-field Kramers-Heisenberg formula is derived for describing high-harmonic generation. Our formalism is suitable to analyse, among various quantal effects, the possible role of arbitrary photon statistics of the incoming field. The present paper is dedicated to the memory of Prof. Dr. Fritz Ehlotzky, who had significantly contributed to the theory of strong-field phenomena over many decades. Full article
(This article belongs to the Special Issue Quantum Optics in Strong Laser Fields)
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14 pages, 3090 KiB  
Article
Controlled Coherence Plasmonic Light Sources
by Greg Gbur and Matt Smith
Photonics 2021, 8(7), 268; https://doi.org/10.3390/photonics8070268 - 8 Jul 2021
Cited by 2 | Viewed by 2312
Abstract
Through a computational model, we study the coherence converting capabilities of an array of holes in a surface plasmon-supporting metal plate, with an eye towards the creation of controlled coherence plasmonic light sources. We evaluate how the average coherence and transmission of the [...] Read more.
Through a computational model, we study the coherence converting capabilities of an array of holes in a surface plasmon-supporting metal plate, with an eye towards the creation of controlled coherence plasmonic light sources. We evaluate how the average coherence and transmission of the hole array depends on the parameters of the array, such as the array geometry, lattice constant, and hole size. We show that the location of coherence bandgaps and resonances can be estimated through a simple formula and that increases in coherence are strongly correlated with increases in transmission. Full article
(This article belongs to the Special Issue Structured Light Coherence)
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25 pages, 5580 KiB  
Review
Recent Progress of Electrically Pumped AlGaN Diode Lasers in the UV-B and -C Bands
by Syed M. N. Hasan, Weicheng You, Md Saiful Islam Sumon and Shamsul Arafin
Photonics 2021, 8(7), 267; https://doi.org/10.3390/photonics8070267 - 8 Jul 2021
Cited by 18 | Viewed by 6676
Abstract
The development of electrically pumped semiconductor diode lasers emitting at the ultraviolet (UV)-B and -C spectral bands has been an active area of research over the past several years, motivated by a wide range of emerging applications. III-Nitride materials and their alloys, in [...] Read more.
The development of electrically pumped semiconductor diode lasers emitting at the ultraviolet (UV)-B and -C spectral bands has been an active area of research over the past several years, motivated by a wide range of emerging applications. III-Nitride materials and their alloys, in particular AlGaN, are the material of choice for the development of this ultrashort-wavelength laser technology. Despite significant progress in AlGaN-based light-emitting diodes (LEDs), the technological advancement and innovation in diode lasers at these spectral bands is lagging due to several technical challenges. Here, the authors review the progress of AlGaN electrically-pumped lasers with respect to very recent achievements made by the scientific community. The devices based on both thin films and nanowires demonstrated to date will be discussed in this review. The state-of-the-art growth technologies, such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD); and various foreign substrates/templates used for the laser demonstrations will be highlighted. We will also outline technical challenges associated with the laser development, which must be overcome in order to achieve a critical technological breakthrough and fully realize the potential of these lasers. Full article
(This article belongs to the Special Issue Wide Bandgap Semiconductor Photonic Devices)
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23 pages, 3092 KiB  
Review
A Comprehensive Survey of Readout Strategies for SiPMs Used in Nuclear Imaging Systems
by Shahram Hatefi Hesari, Mohammad Aminul Haque and Nicole McFarlane
Photonics 2021, 8(7), 266; https://doi.org/10.3390/photonics8070266 - 7 Jul 2021
Cited by 8 | Viewed by 5805
Abstract
Silicon photomultipliers (SiPMs) offer advantages such as lower relative cost, smaller size, and lower operating voltages compared to photomultiplier tubes. A SiPM’s readout circuit topology can significantly affect the characteristics of an imaging array. In nuclear imaging and detection, energy, timing, and position [...] Read more.
Silicon photomultipliers (SiPMs) offer advantages such as lower relative cost, smaller size, and lower operating voltages compared to photomultiplier tubes. A SiPM’s readout circuit topology can significantly affect the characteristics of an imaging array. In nuclear imaging and detection, energy, timing, and position are the primary characteristics of interest. Nuclear imaging has applications in the medical, astronomy, and high energy physics fields, making SiPMs an active research area. This work is focused on the circuit topologies required for nuclear imaging. We surveyed the readout strategies including the front end preamplification topology choices of transimpedance amplifier, charge amplifier, and voltage amplifier. In addition, a review of circuit topologies suitable for energy, timing, and position information extraction was performed along with a summary of performance limitations and current challenges. Full article
(This article belongs to the Special Issue Emerging Photonic Devices, Circuits and Systems)
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23 pages, 3631 KiB  
Review
In-Fiber Interferometric-Based Sensors: Overview and Recent Advances
by Amalia Miliou
Photonics 2021, 8(7), 265; https://doi.org/10.3390/photonics8070265 - 7 Jul 2021
Cited by 70 | Viewed by 6571
Abstract
In-fiber interferometric-based sensors are a rapidly growing field, as these sensors exhibit many desirable characteristics compared to their regular fiber-optic counterparts and are being implemented in many promising devices. These sensors have the capability to make extremely accurate measurements on a variety of [...] Read more.
In-fiber interferometric-based sensors are a rapidly growing field, as these sensors exhibit many desirable characteristics compared to their regular fiber-optic counterparts and are being implemented in many promising devices. These sensors have the capability to make extremely accurate measurements on a variety of physical or chemical quantities such as refractive index, temperature, pressure, curvature, concentration, etc. This article is a comprehensive overview of the different types of in-fiber interferometric sensors that presents and discusses recent developments in the field. Basic configurations, a brief approach of the operating principle and recent applications are introduced for each interferometric architecture, making it easy to compare them and select the most appropriate one for the application at hand. Full article
(This article belongs to the Special Issue Advancements in Fiber Bragg Grating Research)
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16 pages, 3290 KiB  
Article
Adaptation of the Standard Off-Axis Digital Holographic Microscope to Achieve Variable Magnification
by Xin Fan, John J. Healy, Kevin O’Dwyer, Julianna Winnik and Bryan M. Hennelly
Photonics 2021, 8(7), 264; https://doi.org/10.3390/photonics8070264 - 7 Jul 2021
Cited by 5 | Viewed by 3393
Abstract
Traditional microscopy provides only for a small set of magnifications using a finite set of microscope objectives. Here, a novel architecture is proposed for quantitative phase microscopy that requires only a simple adaptation of the traditional off-axis digital holographic microscope. The architecture has [...] Read more.
Traditional microscopy provides only for a small set of magnifications using a finite set of microscope objectives. Here, a novel architecture is proposed for quantitative phase microscopy that requires only a simple adaptation of the traditional off-axis digital holographic microscope. The architecture has the key advantage of continuously variable magnification, resolution, and Field-of-View, by simply moving the sample. The method is based on combining the principles of traditional off-axis digital holographic microscopy and Gabor microscopy, which uses a diverging spherical wavefield for magnification. We present a proof-of-concept implementation and ray-tracing is used to model the magnification, Numerical Aperture, and Field-of-View as a function of sample position. Experimental results are presented using a micro-lens array and shortcomings of the method are highlighted for future work; in particular, the problem of aberration is highlighted, which results from imaging far from the focal plane of the infinity corrected microscope objective. Full article
(This article belongs to the Special Issue Holography)
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10 pages, 5694 KiB  
Communication
Describing High-Order Harmonic Generation Using Quantum Optical Models
by Péter Földi, István Magashegyi, Ákos Gombköto and Sándor Varró
Photonics 2021, 8(7), 263; https://doi.org/10.3390/photonics8070263 - 6 Jul 2021
Cited by 12 | Viewed by 2788
Abstract
Optical generation of high-order harmonics is a prototypical example of nonlinear light–matter interactions in the high-field regime. Quantum optical effects have recently been demonstrated to have a significant influence on this phenomenon. These findings underline the importance of understanding the dynamics of the [...] Read more.
Optical generation of high-order harmonics is a prototypical example of nonlinear light–matter interactions in the high-field regime. Quantum optical effects have recently been demonstrated to have a significant influence on this phenomenon. These findings underline the importance of understanding the dynamics of the quantized electromagnetic field during high-order harmonic generation. In the following, we discuss the challenges that are related to the theoretical description of this process and summarize the results that were obtained using the high-field, multimode generalization of well-known quantum optical models that are based on the concept of the two-level atom. Full article
(This article belongs to the Special Issue Quantum Optics in Strong Laser Fields)
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13 pages, 4306 KiB  
Article
Comparison of Refractive and Visual Outcomes after Transepithelial Photorefractive Keratectomy (TransPRK) in Low versus Moderate Myopia
by Diego de Ortueta, Dennis von Rüden and Samuel Arba-Mosquera
Photonics 2021, 8(7), 262; https://doi.org/10.3390/photonics8070262 - 6 Jul 2021
Cited by 5 | Viewed by 3040
Abstract
Is it possible to obtain good results in myopia of 2 or fewer diopters (D) with transepithelial photorefractive keratectomy (TransPRK) changing the optical zone and epithelium thickness? We retrospectively analyzed two groups of 296 eyes with a minimum follow-up of 4 months. Group [...] Read more.
Is it possible to obtain good results in myopia of 2 or fewer diopters (D) with transepithelial photorefractive keratectomy (TransPRK) changing the optical zone and epithelium thickness? We retrospectively analyzed two groups of 296 eyes with a minimum follow-up of 4 months. Group A had 2 or less D, treated with an optical zone (OZ) 0.2 mm bigger than recommended, and a central epithelium thickness of 60 microns, and group B had 2 D to 5 D, with the recommended optical zone, and a 55-micron epithelium ablation at the center. The outcomes were not different between the two myopic ranges; the postop uncorrected distance visual acuity was 20/20 ± 4 in both groups (p = 0.2), which was −0.3 ± 0.8 lines worse than the preoperative corrected distance visual acuity in both groups (p = 0.5). The safety of the treatments resulted in a change of 0.0 ± 0.7 lines in the low myopia group, versus a gain of +0.1 ± 0.8 lines in the moderate myopia group (p = 0.1). The deviation from the intended target was −0.04 ± 0.33 D in the low myopia group and +0.07 ± 0.32 D in the moderate myopia group (p < 0.0001); the postoperative spherical equivalent was 0.00 ± 0.33 D in the low myopia group and +0.10 ± 0.31 D in the moderate myopia group (p < 0.0001). The postop refractive astigmatism was 0.32 ± 0.16 D in both groups (p = 0.5). In conclusion, the refractive and visual outcomes after TransPRK are comparable in low myopia changing the optical zone and epithelium thickness versus moderate myopia with standard optical zone and epithelium thickness. Full article
(This article belongs to the Special Issue Visual Optics and Ophthalmology)
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11 pages, 5252 KiB  
Communication
Thermal Noise in Cubic Optical Cavities
by Guanjun Xu, Dongdong Jiao, Long Chen, Linbo Zhang, Ruifang Dong, Tao Liu and Junbiao Wang
Photonics 2021, 8(7), 261; https://doi.org/10.3390/photonics8070261 - 6 Jul 2021
Cited by 9 | Viewed by 2516
Abstract
Thermal noise in optical cavities sets a fundamental limit to the frequency instability of ultra-stable lasers. Numata et al. derived three equations based on strain energy and the fluctuation–dissipation theorem to estimate the thermal noise contributions of the spacer, substrates, and coating. These [...] Read more.
Thermal noise in optical cavities sets a fundamental limit to the frequency instability of ultra-stable lasers. Numata et al. derived three equations based on strain energy and the fluctuation–dissipation theorem to estimate the thermal noise contributions of the spacer, substrates, and coating. These equations work well for cylindrical cavities. Extending from that, an expression for the thermal noise for a cubic spacer based on the fluctuation–dissipation theorem is derived, and the thermal noise in cubic optical cavities is investigated in detail by theoretical analysis and finite element simulation. The result shows that the thermal noise of the analytic estimate fits well with that of finite element analysis. Meanwhile, the influence of the compressive force Fp on the thermal noise in cubic optical cavities is analyzed for the first time. For a 50 mm long ultra-low expansion cubic cavity with fused silica substrates and GaAs/AlGaAs crystalline coating, the displacement noise contributed from every Fp of 100 N is about three times more than that of the substrate and coating. Full article
(This article belongs to the Section Lasers, Light Sources and Sensors)
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10 pages, 4756 KiB  
Article
Machine Learning Assisted Inverse Design for Ultrafine, Dynamic and Arbitrary Gain Spectrum Shaping of Raman Amplification
by Yuting Huang, Jiangbing Du, Yufeng Chen, Ke Xu and Zuyuan He
Photonics 2021, 8(7), 260; https://doi.org/10.3390/photonics8070260 - 6 Jul 2021
Cited by 9 | Viewed by 2933
Abstract
Distributed Raman amplifier (DRA) has been widely studied in recent decades because of its low noise figure and flexible gain. In this paper, we present a novel scheme of DRA with broadband amplified spontaneous emission(ASE) source as pump instead of discrete pump lasers. [...] Read more.
Distributed Raman amplifier (DRA) has been widely studied in recent decades because of its low noise figure and flexible gain. In this paper, we present a novel scheme of DRA with broadband amplified spontaneous emission(ASE) source as pump instead of discrete pump lasers. The broadband pump is optimized by machine learning based inverse design and shaped by programmable waveshaper, so as to realize the ultrafine, dynamic and arbitrary gain spectrum shaping of Raman amplification. For the target of flat gain spectrum, the maximum gain flatness of 0.1086 dB is realized based on the simulation results. For the target of arbitrary gain spectrum, we demonstrate four gain profiles with maximum root mean square error (RMSE) of 0.074 dB. To further measure the performance of arbitrary gain spectrum optimization, the probability density functions (PDF) of RMSE and Errormax are presented. Meanwhile, the numeral relationship between the bands of broadband pump and signal is also explored. Furthermore, this work has great application potential to compensate the gain distortion or dynamic change caused by other devices in communication systems. Full article
(This article belongs to the Special Issue Optical Amplifiers: Progress, Challenges, and Future Prospects)
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