Photonics

Open AccessEditor’s ChoiceArticle

Electron Spill-Out Effect in Singular Metasurfaces

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Fan Yang

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Kun Ding

Photonics 2021, 8(5), 154; https://doi.org/10.3390/photonics8050154 - 05 May 2021

Cited by 1 | Viewed by 2022

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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Open AccessEditor’s ChoiceArticle

Low Noise Short Wavelength Infrared Avalanche Photodetector Using SB-Based Strained Layer Superlattice

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Arash Dehzangi

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Jiakai Li

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Manijeh Razeghi

Photonics 2021, 8(5), 148; https://doi.org/10.3390/photonics8050148 - 30 Apr 2021

Cited by 3 | Viewed by 3248

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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Open AccessEditor’s ChoiceArticle

Akinetic Swept-Source Master–Slave-Enhanced Optical Coherence Tomography

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Photonics 2021, 8(5), 141; https://doi.org/10.3390/photonics8050141 - 24 Apr 2021

Cited by 3 | Viewed by 2351

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 m m

), 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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Open AccessEditor’s ChoiceReview

Metasurface Holography in the Microwave Regime

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Photonics 2021, 8(5), 135; https://doi.org/10.3390/photonics8050135 - 22 Apr 2021

Cited by 17 | Viewed by 6123

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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Open AccessEditor’s ChoiceReview

Nonreciprocal and Topological Plasmonics

by

Kunal Shastri

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Mohamed Ismail Abdelrahman

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Francesco Monticone

Photonics 2021, 8(4), 133; https://doi.org/10.3390/photonics8040133 - 20 Apr 2021

Cited by 14 | Viewed by 3420

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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Open AccessEditor’s ChoiceArticle

A New Type of Shape-Invariant Beams with Structured Coherence: Laguerre-Christoffel-Darboux Beams

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Rosario Martínez-Herrero

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Massimo Santarsiero

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Gemma Piquero

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Juan Carlos González de Sande

Photonics 2021, 8(4), 134; https://doi.org/10.3390/photonics8040134 - 20 Apr 2021

Cited by 9 | Viewed by 1821

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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Open AccessEditor’s ChoiceArticle

High-Sensitivity Optical-Resolution Photoacoustic Microscopy with an Optical-Acoustic Combiner Based on an Off-Axis Parabolic Acoustic Mirror

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Photonics 2021, 8(4), 127; https://doi.org/10.3390/photonics8040127 - 18 Apr 2021

Cited by 6 | Viewed by 2035

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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Open AccessFeature PaperEditor’s ChoiceArticle

Microshape Measurement Method Using Speckle Interferometry Based on Phase Analysis

by

Yasuhiko Arai

Photonics 2021, 8(4), 112; https://doi.org/10.3390/photonics8040112 - 08 Apr 2021

Cited by 4 | Viewed by 2220

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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Open AccessEditor’s ChoiceArticle

Joule-Level Twelve-Pass LD End-Pumped Bonded Neodymium Glass Laser Amplifier

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Photonics 2021, 8(4), 96; https://doi.org/10.3390/photonics8040096 - 30 Mar 2021

Cited by 1 | Viewed by 1950

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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Open AccessEditor’s ChoiceCommunication

Quest for New Data: Ionizing Radiation Metrology in the Presence of Laser-Assisted Scattering Processes

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Davide Bianco

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Filomena Loffredo

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Maria Quarto

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Luigi Santamaria Amato

Photonics 2021, 8(4), 94; https://doi.org/10.3390/photonics8040094 - 27 Mar 2021

Viewed by 1552

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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Open AccessEditor’s ChoiceArticle

Wide Field-of-View, High-Resolution Endoscopic Lens Design with Low F-Number for Disposable Endoscopy

by

Dongmok Kim

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Sehui Chang

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Hyuk-Sang Kwon

Photonics 2021, 8(4), 89; https://doi.org/10.3390/photonics8040089 - 24 Mar 2021

Cited by 4 | Viewed by 3088

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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Open AccessEditor’s ChoiceCommunication

Coherence Stokes Parameters in the Description of Electromagnetic Coherence

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Tero Setälä

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Kimmo Saastamoinen

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Ari T. Friberg

Photonics 2021, 8(3), 85; https://doi.org/10.3390/photonics8030085 - 22 Mar 2021

Cited by 3 | Viewed by 2205

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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Open AccessEditor’s ChoiceArticle

The Overlap Factor Model of Spin-Polarised Coupled Lasers

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Photonics 2021, 8(3), 83; https://doi.org/10.3390/photonics8030083 - 20 Mar 2021

Cited by 1 | Viewed by 1895

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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Open AccessEditor’s ChoiceArticle

Propagation of a Modified Complex Lorentz–Gaussian-Correlated Beam in a Marine Atmosphere

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Photonics 2021, 8(3), 82; https://doi.org/10.3390/photonics8030082 - 19 Mar 2021

Cited by 5 | Viewed by 1770

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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Open AccessEditor’s ChoiceArticle

Optical Machine Learning Using Time-Lens Deep Neural NetWorks

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Photonics 2021, 8(3), 78; https://doi.org/10.3390/photonics8030078 - 15 Mar 2021

Cited by 4 | Viewed by 4207

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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Open AccessEditor’s ChoiceArticle

Engineering of TiO₂ or ZnO—Graphene Oxide Nanoheterojunctions for Hybrid Solar Cells Devices

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Photonics 2021, 8(3), 75; https://doi.org/10.3390/photonics8030075 - 12 Mar 2021

Cited by 6 | Viewed by 2545

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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Open AccessEditor’s ChoiceArticle

Computational Modeling and Simulation to Increase Laser Shooting Accuracy of Autonomous LEO Trackers

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Jose M. Gambi

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Maria L. Garcia del Pino

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Jonathan Mosser

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Ewa B. Weinmüller

Photonics 2021, 8(2), 55; https://doi.org/10.3390/photonics8020055 - 18 Feb 2021

Cited by 2 | Viewed by 1560

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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Open AccessEditor’s ChoiceArticle

Mie Resonance Engineering in Two Disks

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Evgeny Bulgakov

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Konstantin Pichugin

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Almas Sadreev

Photonics 2021, 8(2), 49; https://doi.org/10.3390/photonics8020049 - 13 Feb 2021

Cited by 10 | Viewed by 2618

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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Open AccessEditor’s ChoiceArticle

Accurate Power-Efficient Format-Scalable Multi-Parallel Optical Digital-to-Analogue Conversion

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Moshe Nazarathy

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Ioannis Tomkos

Photonics 2021, 8(2), 38; https://doi.org/10.3390/photonics8020038 - 04 Feb 2021

Cited by 5 | Viewed by 2610

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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Open AccessEditor’s ChoiceReview

Inverted p-down Design for High-Speed Photodetectors

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Photonics 2021, 8(2), 39; https://doi.org/10.3390/photonics8020039 - 04 Feb 2021

Cited by 7 | Viewed by 2695

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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Open AccessEditor’s ChoiceArticle

Topological-Insulator-Based Gap-Surface Plasmon Metasurfaces

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Andreas Aigner

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Stefan A. Maier

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Haoran Ren

Photonics 2021, 8(2), 40; https://doi.org/10.3390/photonics8020040 - 04 Feb 2021

Cited by 3 | Viewed by 4160

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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Open AccessEditor’s ChoiceArticle

Proposal of Highly Efficient Quantum Well Microring Resonator-Loaded Optical Phase Modulator Integrated with Antenna-Coupled Electrodes for Radio-over-Fiber

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Hiro Kamada

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Taro Arakawa

Photonics 2021, 8(2), 37; https://doi.org/10.3390/photonics8020037 - 03 Feb 2021

Cited by 3 | Viewed by 2599

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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Open AccessEditor’s ChoiceCommunication

Effect of Si Doping on the Performance of GaN Schottky Barrier Ultraviolet Photodetector Grown on Si Substrate

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Photonics 2021, 8(2), 28; https://doi.org/10.3390/photonics8020028 - 23 Jan 2021

Cited by 4 | Viewed by 2096

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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Open AccessEditor’s ChoiceArticle

Image Deconvolution with Hybrid Reweighted Adaptive Total Variation (HRATV) for Optoacoustic Tomography

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Photonics 2021, 8(2), 25; https://doi.org/10.3390/photonics8020025 - 20 Jan 2021

Cited by 1 | Viewed by 1922

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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Open AccessEditor’s ChoiceArticle

Spiral Caustics of Vortex Beams

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Photonics 2021, 8(1), 24; https://doi.org/10.3390/photonics8010024 - 19 Jan 2021

Cited by 12 | Viewed by 2946

Abstract

We discuss the nonparaxial focusing of laser light into a three-dimensional (3D) spiral distribution. For calculating the tangential and normal components of the electromagnetic field on a preset curved surface we propose an asymptotic method, using which we derive equations for calculating stationary [...] Read more.

We discuss the nonparaxial focusing of laser light into a three-dimensional (3D) spiral distribution. For calculating the tangential and normal components of the electromagnetic field on a preset curved surface we propose an asymptotic method, using which we derive equations for calculating stationary points and asymptotic relations for the electromagnetic field components in the form of one-dimensional (1D) integrals over a radial component. The results obtained through the asymptotic approach and the direct calculation of the Kirchhoff integral are identical. For a particular case of focusing into a ring, an analytical relation for stationary points is derived. Based on the electromagnetic theory, we design and numerically model the performance of diffractive optical elements (DOEs) to generate field distributions shaped as two-dimensional (2D) and 3D light spirals with the variable angular momentum. We reveal that under certain conditions, there is an effect of splitting the longitudinal electromagnetic field component. Experimental results obtained with the use of a spatial light modulator are in good agreement with the modeling results. Full article

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Open AccessEditor’s ChoiceReview

Diagnosis of Glioma Molecular Markers by Terahertz Technologies

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Photonics 2021, 8(1), 22; https://doi.org/10.3390/photonics8010022 - 16 Jan 2021

Cited by 17 | Viewed by 4724

Abstract

This review considers glioma molecular markers in brain tissues and body fluids, shows the pathways of their formation, and describes traditional methods of analysis. The most important optical properties of glioma markers in the terahertz (THz) frequency range are also presented. New metamaterial-based [...] Read more.

This review considers glioma molecular markers in brain tissues and body fluids, shows the pathways of their formation, and describes traditional methods of analysis. The most important optical properties of glioma markers in the terahertz (THz) frequency range are also presented. New metamaterial-based technologies for molecular marker detection at THz frequencies are discussed. A variety of machine learning methods, which allow the marker detection sensitivity and differentiation of healthy and tumor tissues to be improved with the aid of THz tools, are considered. The actual results on the application of THz techniques in the intraoperative diagnosis of brain gliomas are shown. THz technologies’ potential in molecular marker detection and defining the boundaries of the glioma’s tissue is discussed. Full article

(This article belongs to the Special Issue Terahertz Biophotonics)

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Open AccessFeature PaperEditor’s ChoiceArticle

Neural Network DPD for Aggrandizing SM-VCSEL-SSMF-Based Radio over Fiber Link Performance

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Photonics 2021, 8(1), 19; https://doi.org/10.3390/photonics8010019 - 14 Jan 2021

Cited by 21 | Viewed by 3687

Abstract

This paper demonstrates an unprecedented novel neural network (NN)-based digital predistortion (DPD) solution to overcome the signal impairments and nonlinearities in Analog Optical fronthauls using radio over fiber (RoF) systems. DPD is realized with Volterra-based procedures that utilize indirect learning architecture (ILA) and [...] Read more.

This paper demonstrates an unprecedented novel neural network (NN)-based digital predistortion (DPD) solution to overcome the signal impairments and nonlinearities in Analog Optical fronthauls using radio over fiber (RoF) systems. DPD is realized with Volterra-based procedures that utilize indirect learning architecture (ILA) and direct learning architecture (DLA) that becomes quite complex. The proposed method using NNs evades issues associated with ILA and utilizes an NN to first model the RoF link and then trains an NN-based predistorter by backpropagating through the RoF NN model. Furthermore, the experimental evaluation is carried out for Long Term Evolution 20 MHz 256 quadraturre amplitude modulation (QAM) modulation signal using an 850 nm Single Mode VCSEL and Standard Single Mode Fiber to establish a comparison between the NN-based RoF link and Volterra-based Memory Polynomial and Generalized Memory Polynomial using ILA. The efficacy of the DPD is examined by reporting the Adjacent Channel Power Ratio and Error Vector Magnitude. The experimental findings imply that NN-DPD convincingly learns the RoF nonlinearities which may not suit a Volterra-based model, and hence may offer a favorable trade-off in terms of computational overhead and DPD performance. Full article

(This article belongs to the Special Issue Radio over Fiber)

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Open AccessEditor’s ChoiceCommunication

InP-Components for 100 GBaud Optical Data Center Communication

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Photonics 2021, 8(1), 18; https://doi.org/10.3390/photonics8010018 - 13 Jan 2021

Cited by 6 | Viewed by 3519

Abstract

Externally modulated DFB lasers (EML) and vertically illuminated photodetectors are presented. Because of their excellent high-speed behavior and operation wavelength of 1310 nm, the devices are of interest for intra-data center communication. Since the EML and the photodetector chips are compatible with current [...] Read more.

Externally modulated DFB lasers (EML) and vertically illuminated photodetectors are presented. Because of their excellent high-speed behavior and operation wavelength of 1310 nm, the devices are of interest for intra-data center communication. Since the EML and the photodetector chips are compatible with current systems, these devices are candidates for upgrading existing transceivers to higher baud rates. Therefore, a proof of concept for 100 GBaud data transmission with the presented components is demonstrated. Even without predistortion, the experiments show clearly open eye diagrams. Full article

(This article belongs to the Special Issue Advanced Ultra High Speed Optoelectronic Devices)

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Open AccessEditor’s ChoiceArticle

Study of Phase Transition in MOCVD Grown Ga₂O₃ from κ to β Phase by Ex Situ and In Situ Annealing

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Photonics 2021, 8(1), 17; https://doi.org/10.3390/photonics8010017 - 13 Jan 2021

Cited by 15 | Viewed by 3540

Abstract

We report the post-growth thermal annealing and the subsequent phase transition of Ga₂O₃ grown on c-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). We demonstrated the post-growth thermal annealing at temperatures higher than 900 °C under N₂ [...] Read more.

We report the post-growth thermal annealing and the subsequent phase transition of Ga₂O₃ grown on c-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). We demonstrated the post-growth thermal annealing at temperatures higher than 900 °C under N₂ ambience, by either in situ or ex situ thermal annealing, can induce phase transition from nominally metastable κ- to thermodynamically stable β-phase. This was analyzed by structural characterizations such as high-resolution scanning transmission electron microscopy and x-ray diffraction. The highly resistive as-grown Ga₂O₃ epitaxial layer becomes conductive after annealing at 1000 °C. Furthermore, we demonstrate that in situ annealing can lead to a crack-free β-Ga₂O₃. Full article

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Open AccessEditor’s ChoiceReview

Recent Advances in High Speed Photodetectors for eSWIR/MWIR/LWIR Applications

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Baile Chen

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Yaojiang Chen

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Zhuo Deng

Photonics 2021, 8(1), 14; https://doi.org/10.3390/photonics8010014 - 11 Jan 2021

Cited by 17 | Viewed by 4684

Abstract

High speed photodetectors operating at a telecommunication band (from 1260 to 1625 nm) have been well studied with the development of an optical fiber communication system. Recent innovations of photonic systems have raised new requirements on the bandwidth of photodetectors with cutoff wavelengths [...] Read more.

High speed photodetectors operating at a telecommunication band (from 1260 to 1625 nm) have been well studied with the development of an optical fiber communication system. Recent innovations of photonic systems have raised new requirements on the bandwidth of photodetectors with cutoff wavelengths from extended short wavelength infrared (eSWIR) to long wavelength infrared (LWIR). However, the frequency response performance of photodetectors in these longer wavelength bands is less studied, and the performances of the current high-speed photodetectors in these bands are still not comparable with those in the telecommunication band. In this paper, technical routes to achieve high response speed performance of photodetectors in the extended short wavelength infrared/mid wavelength infrared/long wavelength infrared (eSWIR/MWIR/LWIR) band are discussed, and the state-of-the-art performances are reviewed. Full article

(This article belongs to the Special Issue Advanced Ultra High Speed Optoelectronic Devices)

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Open AccessEditor’s ChoiceArticle

Function of Porous Carbon Electrode during the Fabrication of Multiporous-Layered-Electrode Perovskite Solar Cells

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Photonics 2020, 7(4), 133; https://doi.org/10.3390/photonics7040133 - 18 Dec 2020

Cited by 8 | Viewed by 3083

Abstract

We demonstrate the effect of sheet conductivity and infiltration using the example of two graphite types, showing that, in general, the graphite type is very important. Amorphous and pyrolytic graphite were applied to carbon electrodes in fully printable carbon-based multiporous-layered-electrode perovskite solar cells [...] Read more.

We demonstrate the effect of sheet conductivity and infiltration using the example of two graphite types, showing that, in general, the graphite type is very important. Amorphous and pyrolytic graphite were applied to carbon electrodes in fully printable carbon-based multiporous-layered-electrode perovskite solar cells (MPLE-PSCs): <glass/F-doped SnO₂/compact-TiO₂/porous-TiO₂+perovskite/porous-ZrO₂+perovskite/porous-carbon+perovskite>. The power conversion efficiency (PCE) using amorphous graphite-based carbon (AGC) electrode was only 5.97% due to the low short-circuit photocurrent density (J_sc) value, which was due to the low incident photon-to-current efficiency (IPCE) in the short wavelength region caused by the poor perovskite filling into the porous TiO₂-ZrO₂ layers. Conversely, using pyrolytic graphite-based carbon (PGC) electrode, J_sc, open-circuit photovoltage (V_oc), fill factors (FF), and PCE values of 21.09 mA cm⁻², 0.952 V, 0.670, and 13.45%, respectively, were achieved in the champion device. PGC had poorer wettability and a small specific surface area as compared with AGC, but it had better permeability of the perovskite precursor solution into the porous TiO₂/ZrO₂ layers, and therefore a denser filling and crystallization of the perovskite within the porous TiO₂/ZrO₂ layers than AGC. It is confirmed that the permeability of the precursor solution depends on the morphology and structure of the graphite employed in the carbon electrode. Full article

(This article belongs to the Special Issue Photovoltaic Materials and Devices)

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Open AccessEditor’s ChoiceArticle

Electromagnetic Scattering Analysis of SHDB Objects Using Surface Integral Equation Method

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Beibei Kong

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Pasi Ylä-Oijala

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Ari Sihvola

Photonics 2020, 7(4), 134; https://doi.org/10.3390/photonics7040134 - 18 Dec 2020

Cited by 1 | Viewed by 2107

Abstract

A surface integral equation (SIE) method is applied in order to analyze electromagnetic scattering by bounded arbitrarily shaped three-dimensional objects with the SHDB boundary condition. SHDB is a generalization of SH (Soft-and-Hard) and DB boundary conditions (at the DB boundary, the normal components [...] Read more.

A surface integral equation (SIE) method is applied in order to analyze electromagnetic scattering by bounded arbitrarily shaped three-dimensional objects with the SHDB boundary condition. SHDB is a generalization of SH (Soft-and-Hard) and DB boundary conditions (at the DB boundary, the normal components of the D and B flux densities vanish). The SHDB boundary condition is a general linear boundary condition that contains two scalar equations that involve both the tangential and normal components of the electromagnetic fields. The multiplication of these scalar equations with two orthogonal vectors transforms them into a vector form that can be combined with the tangential field integral equations. The resulting equations are discretized and converted to a matrix equation with standard method of moments (MoM). As an example of use of the method, we investigate scattering by an SHDB circular disk and demonstrate that the SHDB boundary allows for an efficient way to control the polarization of the wave that is reflected from the surface. We also discuss perspectives into different levels of materialization and realization of SHDB boundaries. Full article

(This article belongs to the Special Issue Advances in Complex Media Electromagnetics)

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Open AccessEditor’s ChoiceArticle

Wavelength-Dependent Features of Photoelectron Spectra from Nanotip Photoemission

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Photonics 2020, 7(4), 129; https://doi.org/10.3390/photonics7040129 - 11 Dec 2020

Cited by 5 | Viewed by 3217

Abstract

If a metal nanotip is irradiated with the light of a wavelength much larger than the nanotip’s radius of curvature, optical near-fields become excited. These fields are responsible for distinct strong-field electron dynamics, due to both the field enhancement and spatial localization. By [...] Read more.

If a metal nanotip is irradiated with the light of a wavelength much larger than the nanotip’s radius of curvature, optical near-fields become excited. These fields are responsible for distinct strong-field electron dynamics, due to both the field enhancement and spatial localization. By classical trajectory, Monte Carlo (CTMC) simulation, and the integration of the time-dependent Schrödinger equation (TDSE), we find that the photoelectron spectra for nanotip strong-field photoemission, irradiated by mid-infrared laser pulses, present distinctive wavelength-dependent features, especially in the mid- to high-electron energy regions, which are different from the well known ones. By extracting the electron trajectories from the CTMC simulation, we investigate these particular wavelength-dependent features. Our theoretical results contribute to understanding the photoemission and electron dynamics at nanostructures, and pave new pathways for designing high-energy nanometer-sized ultrafast electron sources. Full article

(This article belongs to the Special Issue Strong Light Fields Coupled with Plasmonic Nano-Structures)

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Open AccessEditor’s ChoiceArticle

Scaling of Beam Collective Effects with Bunch Charge in the CompactLight Free-Electron Laser

Photonics 2020, 7(4), 125; https://doi.org/10.3390/photonics7040125 - 04 Dec 2020

Cited by 3 | Viewed by 2816

Abstract

The CompactLight European consortium is designing a state-of-the-art X-ray free-electron laser driven by radiofrequency X-band technology. Rooted in experimental data on photo-injector performance in the recent literature, this study estimates analytically and numerically the performance of the CompactLight delivery system for bunch charges [...] Read more.

The CompactLight European consortium is designing a state-of-the-art X-ray free-electron laser driven by radiofrequency X-band technology. Rooted in experimental data on photo-injector performance in the recent literature, this study estimates analytically and numerically the performance of the CompactLight delivery system for bunch charges in the range 75–300 pC. Space-charge forces in the injector, linac transverse wakefield, and coherent synchrotron radiation in bunch compressors are all taken into account. The study confirms efficient lasing in the soft X-rays regime with pulse energies up to hundreds of microjoules at repetition rates as high as 1 kHz. Full article

(This article belongs to the Special Issue Photonics, Optics and Laser Technology)

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Open AccessEditor’s ChoiceArticle

Coherent Beam Combining Using an Internally Sensed Optical Phased Array of Frequency-Offset Phase Locked Lasers

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Photonics 2020, 7(4), 118; https://doi.org/10.3390/photonics7040118 - 28 Nov 2020

Cited by 6 | Viewed by 2895

Abstract

Coherent beam combining can be used to scale optical power and enable mechanism-free beam steering using an optical phased array. Coherently combining multiple free-running lasers in a leader-follower laser configuration is challenging due to the need to measure and stabilize large and highly [...] Read more.

Coherent beam combining can be used to scale optical power and enable mechanism-free beam steering using an optical phased array. Coherently combining multiple free-running lasers in a leader-follower laser configuration is challenging due to the need to measure and stabilize large and highly dynamic phase differences between them. We present a scalable technique based on frequency-offset phase locking and digitally enhanced interferometry to clone the coherence of multiple lasers without the use of external sampling optics, which has the potential to support both coherent and spectral beam combining, and alleviates issues of voltage wrapping associated with actuating feedback control using electro-optic modulators. This technique was demonstrated experimentally using a tiled-aperture optical phased array in which the relative output phase of three free-running lasers was stabilized with an RMS output phase stability of

λ / 104

. Full article

(This article belongs to the Special Issue Photonics, Optics and Laser Technology)

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Open AccessEditor’s ChoiceArticle

Multiwavelength Frequency Modulated CW Ladar: The Effect of Refractive Index

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Mariano Barbieri

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Deborah Katia Pallotti

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Mario Siciliani de Cumis

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Luigi Santamaria Amato

Photonics 2020, 7(4), 90; https://doi.org/10.3390/photonics7040090 - 08 Oct 2020

Cited by 3 | Viewed by 4291

Abstract

Frequency modulated continuous wave (FMCW) laser detection and ranging is a technique for absolute distance measurements with high performances in terms of resolution, non-ambiguity range, accuracy and fast detection. It is based on a simple experimental setup, thus resulting in cost restraint with [...] Read more.

Frequency modulated continuous wave (FMCW) laser detection and ranging is a technique for absolute distance measurements with high performances in terms of resolution, non-ambiguity range, accuracy and fast detection. It is based on a simple experimental setup, thus resulting in cost restraint with potential wide spread, not only limited to research institutions. The technique has been widely studied and improved both in terms of experimental setup by absolute reference or active stabilization and in terms of data analysis. Very recently a multi-wavelength approach has been exploited, demonstrating high precision and non ambiguity range. The variability of refractive index along the path was not taken into account with consequent degradation of range accuracy. In this work we developed a simple model able to take into account refractive index effect in multi-wavelength FMCW measurement. We performed a numerical simulation in different atmospheric conditions of temperature, pressure, humidity and CO₂ concentration showing a net improvement of range accuracy when refractive index modeling is used. Full article

(This article belongs to the Special Issue Spectroscopy, Metrology and Quantum Technology for Space Science and Astrophysics)

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Open AccessEditor’s ChoiceArticle

A Metamaterial-Inspired Approach to Mitigating Radio Frequency Blackout When a Plasma Forms Around a Reentry Vehicle

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Bruce A. Webb

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Richard W. Ziolkowski

Photonics 2020, 7(4), 88; https://doi.org/10.3390/photonics7040088 - 06 Oct 2020

Cited by 6 | Viewed by 2284

Abstract

Radio frequency (RF) blackout and attenuation have been observed during atmospheric reentry since the advent of space exploration. The effects range from severe attenuation to complete loss of communications and can last from 90 s to 10 min depending on the vehicle’s trajectory. [...] Read more.

Radio frequency (RF) blackout and attenuation have been observed during atmospheric reentry since the advent of space exploration. The effects range from severe attenuation to complete loss of communications and can last from 90 s to 10 min depending on the vehicle’s trajectory. This paper examines a way of using a metasurface to improve the performance of communications during reentry. The technique is viable at low plasma densities and matches a split-ring resonator (SRR)-based mu-negative (MNG) sheet to the epsilon-negative (ENG) plasma region. Considering the MNG metasurface as a window to the exterior of a reentry vehicle, its matched design yields high transmission of an electromagnetic plane wave through the resulting MNG-ENG metastructure into the region beyond it. A varactor-based SRR design facilitates tuning the MNG layer to ENG layers with different plasma densities. Both simple and Huygens dipole antennas beneath a matched metastructure are then employed to demonstrate the consequent realization of significant signal transmission through it into free space beyond the exterior ENG plasma layer. Full article

(This article belongs to the Special Issue Advances in Complex Media Electromagnetics)

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Open AccessFeature PaperEditor’s ChoiceArticle

Tunable THz Pulses Generation in Non-Equilibrium Magnetized Plasma: The Role of Plasma Kinetics

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Anna V. Bogatskaya

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Nelli E. Gnezdovskaia

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Alexander M. Popov

Photonics 2020, 7(4), 82; https://doi.org/10.3390/photonics7040082 - 24 Sep 2020

Cited by 1 | Viewed by 2190

Abstract

In this paper the theoretical model to consider the influence of kinetic properties of nonequilibrium two-color plasma during the THz pulses generation in the presence of static magnetic field is developed. It is shown that applying a static magnetic field on a gas [...] Read more.

In this paper the theoretical model to consider the influence of kinetic properties of nonequilibrium two-color plasma during the THz pulses generation in the presence of static magnetic field is developed. It is shown that applying a static magnetic field on a gas along the direction of propagation of an ionizing two-color laser pulse allows one to produce two-frequency emissions in THz range with tunable central frequency and bandwidth, which are strongly dependent on electron velocity distribution function (EVDF) formed in the plasma as well as relations between collisional, plasma and cyclotron frequencies. Full article

(This article belongs to the Special Issue Photonics, Optics and Laser Technology)

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Open AccessEditor’s ChoiceArticle

Near- and Far-Field Excitation of Topological Plasmonic Metasurfaces

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Paloma Arroyo Huidobro

Photonics 2020, 7(4), 81; https://doi.org/10.3390/photonics7040081 - 24 Sep 2020

Cited by 6 | Viewed by 2838

Abstract

The breathing honeycomb lattice hosts a topologically non-trivial bulk phase due to the crystalline-symmetry of the system. Pseudospin-dependent edge states, which emerge at the interface between trivial and non-trivial regions, can be used for the directional propagation of energy. Using the plasmonic metasurface [...] Read more.

The breathing honeycomb lattice hosts a topologically non-trivial bulk phase due to the crystalline-symmetry of the system. Pseudospin-dependent edge states, which emerge at the interface between trivial and non-trivial regions, can be used for the directional propagation of energy. Using the plasmonic metasurface as an example system, we probe these states in the near- and far-field using a semi-analytical model. We provide the conditions under which directionality was observed and show that it is source position dependent. By probing with circularly-polarised magnetic dipoles out of the plane, we first characterise modes along the interface in terms of the enhancement of source emissions due to the metasurface. We then excite from the far-field with non-zero orbital angular momentum beams. The position-dependent directionality holds true for all classical wave systems with a breathing honeycomb lattice. Our results show that a metasurface in combination with a chiral two-dimensional material, could be used to guide light effectively on the nanoscale. Full article

(This article belongs to the Special Issue Plasmonic Metasurfaces)

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Open AccessEditor’s ChoiceArticle

Structural, Optical and Electrical Characterizations of Midwave Infrared Ga-Free Type-II InAs/InAsSb Superlattice Barrier Photodetector

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Photonics 2020, 7(3), 76; https://doi.org/10.3390/photonics7030076 - 18 Sep 2020

Cited by 11 | Viewed by 3188

Abstract

In this paper, a full set of structural, optical and electrical characterizations performed on midwave infrared barrier detectors based on a Ga-free InAs/InAsSb type-II superlattice, grown by molecular beam epitaxy (MBE) on a GaSb substrate, are reported and analyzed. a Minority carrier lifetime [...] Read more.

In this paper, a full set of structural, optical and electrical characterizations performed on midwave infrared barrier detectors based on a Ga-free InAs/InAsSb type-II superlattice, grown by molecular beam epitaxy (MBE) on a GaSb substrate, are reported and analyzed. a Minority carrier lifetime value equal to 1 µs at 80 K, carried out on dedicated structure showing photoluminescence peak position at 4.9 µm, is extracted from a time resolved photoluminescence measurement. Dark current density as low as 3.2 × 10⁻⁵ A/cm² at 150 K is reported on the corresponding device exhibiting a 50% cut-off wavelength around 5 µm. A performance analysis through normalized spectral response and dark current density-voltage characteristics was performed to determine both the operating bias and the different dark current regimes. Full article

(This article belongs to the Special Issue Photonics, Optics and Laser Technology)

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Open AccessEditor’s ChoiceReview

The Interband Cascade Laser

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Photonics 2020, 7(3), 75; https://doi.org/10.3390/photonics7030075 - 15 Sep 2020

Cited by 57 | Viewed by 7028

Abstract

We review the history, development, design principles, experimental operating characteristics, and specialized architectures of interband cascade lasers for the mid-wave infrared spectral region. We discuss the present understanding of the mechanisms limiting the ICL performance and provide a perspective on the potential for [...] Read more.

We review the history, development, design principles, experimental operating characteristics, and specialized architectures of interband cascade lasers for the mid-wave infrared spectral region. We discuss the present understanding of the mechanisms limiting the ICL performance and provide a perspective on the potential for future improvements. Such device properties as the threshold current and power densities, continuous-wave output power, and wall-plug efficiency are compared with those of the quantum cascade laser. Newer device classes such as ICL frequency combs, interband cascade vertical-cavity surface-emitting lasers, interband cascade LEDs, interband cascade detectors, and integrated ICLs are reviewed for the first time. Full article

(This article belongs to the Section Lasers, Light Sources and Sensors)

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