Contents
2021
Volume: 9 Issue 6
26 Article(s)
Special Issue on DEEP LEARNING IN PHOTONICS
Genetic-algorithm-based deep neural networks for highly efficient photonic device design
Yangming Ren, Lingxuan Zhang, Weiqiang Wang, Xinyu Wang, Yufang Lei, Yulong Xue, Xiaochen Sun, and Wenfu Zhang
While deep learning has demonstrated tremendous potential for photonic device design, it often demands a large amount of labeled data to train these deep neural network models. Preparing these data requires high-resolution numerical simulations or experimental measurements and cost significant, if not prohibitive, time and resources. In this work, we present a highly efficient inverse design method that combines deep neural networks with a genetic algorithm to optimize the geometry of photonic devices in the polar coordinate system. The method requires significantly less training data compared with previous inverse design methods. We implement this method to design several ultra-compact silicon photonics devices with challenging properties including power splitters with uncommon splitting ratios, a TE mode converter, and a broadband power splitter. These devices are free of the features beyond the capability of photolithography and generally in compliance with silicon photonics fabrication design rules.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 0600B247 (2021)
Research Articles
Imaging Systems, Microscopy, and Displays
Single-sweep volumetric optoacoustic tomography of whole mice
Sandeep Kumar Kalva, Xose Luis Dean-Ben, and Daniel Razansky
Applicability of optoacoustic imaging in biology and medicine is determined by several key performance characteristics. In particular, an inherent trade-off exists between the acquired field-of-view (FOV) and temporal resolution of the measurements, which may hinder studies looking at rapid biodynamics at the whole-body level. Here, we report on a single-sweep volumetric optoacoustic tomography (sSVOT) system that attains whole body three-dimensional mouse scans within 1.8 s with better than 200 μm spatial resolution. sSVOT employs a spherical matrix array transducer in combination with multibeam illumination, the latter playing a critical role in maximizing the effective FOV and imaging speed performance. The system further takes advantage of the spatial response of the individual ultrasound detection elements to mitigate common image artifacts related to limited-view tomographic geometry, thus enabling rapid acquisitions without compromising image quality and contrast. We compare performance metrics to the previously reported whole-body mouse imaging implementations and alternative image compounding and reconstruction strategies. It is anticipated that sSVOT will open new venues for studying large-scale biodynamics, such as accumulation and clearance of molecular agents and drugs across multiple organs, circulation of cells, and functional responses to stimuli.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 6 06000899 (2021)
Optical and Photonic Materials
Theoretical study on residual infrared absorption of Ti:sapphire laser crystals
Qiaorui Gong, Chengchun Zhao, Yilun Yang, Qiannan Fang, Shanming Li, Min Xu, and Yin Hang
Residual infrared absorption is a key problem affecting the laser emission efficiency of Ti:sapphire crystal. In this paper, the origin of residual infrared absorption of Ti:sapphire crystal is systematically studied by using the first principles method. According to the contact conditions of O octahedron in the crystal structure of Al2O3, four Ti3+-Ti3+ ion pair models and three Ti4+-Ti3+ ion pair models were defined and constructed. For what we believe is the first time, the near-infrared absorption spectra consistent with the experimental results were obtained in specific theoretical models. The electronic structures, absorption spectra, and charge distributions calculated show that the line-contact Ti3+-Ti3+ ion pair with antiferromagnetic coupling and the face-contact Ti4+-Ti3+ ion pair are two main contributors to the residual infrared absorption of Ti:sapphire, while some other ion pair models provide a basis to explain more complex residual infrared absorption.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 6 06000909 (2021)
Fiber Optics and Optical Communications
Characterization of dynamic distortion in LED light output for optical wireless communications
Anton Alexeev, Jean-Paul M. G. Linnartz, Kumar Arulandu, and Xiong Deng
Light-emitting diodes (LEDs) are widely used for data transmission in emerging optical wireless communications (OWC) systems. This paper analyzes the physical processes that limit the bandwidth and cause nonlinearities in the light output of modern, high-efficiency LEDs. The processes of carrier transport, as well as carrier storage, recombination, and leakage in the active region appear to affect the communications performance, but such purely physics-based models are not yet commonly considered in the algorithms to optimize OWC systems. Using a dynamic modeling of these phenomena, we compile a (invertable) signal processing model that describes the signal distortion and a parameter estimation procedure that is feasible in an operational communications link. We combine multiple approaches for steady-state and dynamic characterization to estimate such LED parameters. We verify that, for a high-efficiency blue GaN LED, the models become sufficiently accurate to allow digital compensation. We compare the simulation results using the model against optical measurements of harmonic distortion and against measurements of the LED response to a deep rectangular current modulation. We show how the topology of the model can be simplified, address the self-calibration techniques, and discuss the limits of the presented approach. The model is suitable for the creation of improved nonlinear equalizers to enhance the achievable bit rate in LED-based OWC systems and we believe it is significantly more realistic than LED models commonly used in communications systems.
Photonics Research
  • Publication Date: May. 13, 2021
  • Vol.9 Issue, 6 06000916 (2021)
Ultrafast Optics
All-optical sampling of few-cycle infrared pulses using tunneling in a solid | Editors' Pick
Yangyang Liu, Shima Gholam-Mirzaei, John E. Beetar, Jonathan Nesper, Ahmed Yousif, M. Nrisimhamurty, and Michael Chini
Recent developments in ultrafast laser technology have resulted in novel few-cycle sources in the mid-infrared. Accurately characterizing the time-dependent intensities and electric field waveforms of such laser pulses is essential to their applications in strong-field physics and attosecond pulse generation, but this remains a challenge. Recently, it was shown that tunnel ionization can provide an ultrafast temporal “gate” for characterizing high-energy few-cycle laser waveforms capable of ionizing air. Here, we show that tunneling and multiphoton excitation in a dielectric solid can provide a means to measure lower-energy and longer-wavelength pulses, and we apply the technique to characterize microjoule-level near- and mid-infrared pulses. The method lends itself to both all-optical and on-chip detection of laser waveforms, as well as single-shot detection geometries.
Photonics Research
  • Publication Date: May. 13, 2021
  • Vol.9 Issue, 6 06000929 (2021)
Nanophotonics and Photonic Crystals
Phonon lasing in a hetero optomechanical crystal cavity
Kaiyu Cui, Zhilei Huang, Ning Wu, Qiancheng Xu, Fei Pan, Jian Xiong, Xue Feng, Fang Liu, Wei Zhang, and Yidong Huang
Micro- and nanomechanical resonators have emerged as promising platforms for sensing a broad range of physical properties, such as mass, force, torque, magnetic field, and acceleration. The sensing performance relies critically on the motional mass, mechanical frequency, and linewidth of the mechanical resonator. Herein, we demonstrate a hetero optomechanical crystal (OMC) cavity based on a silicon nanobeam structure. The cavity supports phonon lasing in a fundamental mechanical mode with a frequency of 5.91 GHz, an effective mass of 116 fg, and a mechanical linewidth narrowing in the range from 3.3 MHz to 5.2 kHz, while the optomechanical coupling rate is as high as 1.9 MHz. With this phonon laser, on-chip sensing can be predicted with a resolution of δλ/λ=1.0×10-8. The use of a silicon-based hetero OMC cavity that harnesses phonon lasing could pave the way toward high-precision sensors that allow silicon monolithic integration and offer unprecedented sensitivity for a broad range of physical sensing applications.
Photonics Research
  • Publication Date: May. 19, 2021
  • Vol.9 Issue, 6 06000937 (2021)
Optical Devices
Polarization-robust mid-infrared carpet cloak with minimized lateral shift
Yao Huang, Jingjing Zhang, Jinhui Zhou, Bo Qiang, Zhengji Xu, Lin Liu, Jifang Tao, Nicolas Kossowski, Qijie Wang, and Yu Luo
With the advent and rapid development of the transformation optics and metamaterials, invisibility cloaks have captivated much attention in recent years. While most cloaking schemes suffer from limited bandwidth, the carpet cloak, which can hide an object on a reflecting plane, can operate over a broadband frequency range. However, the carpet cloaks experimentally realized thus far still have several limitations. For example, the quasi-conformal mapping carpet cloak leads to a lateral shift of the reflected light ray, while the birefringent carpet cloak only works for a specific polarization. In this work, we propose a conformal transformation scheme to tackle these two problems simultaneously. As an example, we design a mid-infrared carpet cloak in a silicon platform and demonstrate its polarization-insensitive property as well as the minimized lateral shift over a broad frequency band from 24 to 28.3 THz.
Photonics Research
  • Publication Date: May. 19, 2021
  • Vol.9 Issue, 6 06000944 (2021)
Optical and Photonic Materials
Cylindrical vector beam revealing multipolar nonlinear scattering for superlocalization of silicon nanostructures
Bin Wang, Ying Che, Xiangchao Zhong, Wen Yan, Tianyue Zhang, Kai Chen, Yi Xu, Xiaoxuan Xu, and Xiangping Li
The resonant optical excitation of dielectric nanostructures offers unique opportunities for developing remarkable nanophotonic devices. Light that is structured by tailoring the vectorial characteristics of the light beam provides an additional degree of freedom in achieving flexible control of multipolar resonances at the nanoscale. Here, we investigate the nonlinear scattering of subwavelength silicon (Si) nanostructures with radially and azimuthally polarized cylindrical vector beams to show a strong dependence of the photothermal nonlinearity on the polarization state of the applied light. The resonant magnetic dipole, selectively excited by an azimuthally polarized beam, enables enhanced photothermal nonlinearity, thereby inducing large scattering saturation. In contrast, radially polarized beam illumination shows no observable nonlinearity owing to off-resonance excitation. Numerical analysis reveals a difference of more than 2 orders of magnitude in photothermal nonlinearity under two types of polarization excitations. Nonlinear scattering and the unique doughnut-shaped focal spot generated by the azimuthally polarized beam are demonstrated as enabling far-field high-resolution localization of nanostructured Si with an accuracy approaching 50 nm. Our study extends the horizons of active Si photonics and holds great potential for label-free superresolution imaging of Si nanostructures.
Photonics Research
  • Publication Date: May. 20, 2021
  • Vol.9 Issue, 6 06000950 (2021)
Quantum Optics
Steering paradox for Einstein–Podolsky–Rosen argument and its extended inequality
Tianfeng Feng, Changliang Ren, Qin Feng, Maolin Luo, Xiaogang Qiang, Jing-Ling Chen, and Xiaoqi Zhou
The Einstein–Podolsky–Rosen (EPR) paradox is one of the milestones in quantum foundations, arising from the lack of a local realistic description of quantum mechanics. The EPR paradox has stimulated an important concept of “quantum nonlocality,” which manifests itself in three types: quantum entanglement, quantum steering, and Bell’s nonlocality. Although Bell’s nonlocality is more often used to show “quantum nonlocality,” the original EPR paradox is essentially a steering paradox. In this work, we formulate the original EPR steering paradox into a contradiction equality, thus making it amenable to experimental verification. We perform an experimental test of the steering paradox in a two-qubit scenario. Furthermore, by starting from the steering paradox, we generate a generalized linear steering inequality and transform this inequality into a mathematically equivalent form, which is friendlier for experimental implementation, i.e., one may measure the observables only in the x, y, or z axis of the Bloch sphere, rather than other arbitrary directions. We also perform experiments to demonstrate this scheme. Within the experimental errors, the experimental results coincide with theoretical predictions. Our results deepen the understanding of quantum foundations and provide an efficient way to detect the steerability of quantum states.
Photonics Research
  • Publication Date: May. 20, 2021
  • Vol.9 Issue, 6 06000992 (2021)
Imaging Systems, Microscopy, and Displays
Non-iterative complex wave-field reconstruction based on Kramers–Kronig relations
Cheng Shen, Mingshu Liang, An Pan, and Changhuei Yang
A non-iterative and non-interferometric computational imaging method to reconstruct a complex wave field called synthetic aperture imaging based on Kramers–Kronig relations (KKSAI) is reported. By collecting images through a modified microscope system with pupil modulation capability, we show that the phase and amplitude profile of the sample at pupil limited resolution can be extracted from as few as two intensity images by using Kramers–Kronig (KK) relations. It is established that as long as each subaperture’s edge crosses the pupil center, the collected raw images are mathematically analogous to off-axis holograms. This in turn allows us to adapt a recently reported KK-relations-based phase recovery framework in off-axis holography for use in KKSAI. KKSAI is non-iterative, free of parameter tuning, and applicable to a wider range of samples. Simulation and experiment results have proved that it has much lower computational burden and achieves the best reconstruction quality when compared with two existing phase imaging methods.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001003 (2021)
Integrated Optics
Free-spectral-range-free filters with ultrawide tunability across the S + C + L band
Chunlei Sun, Chuyu Zhong, Maoliang Wei, Hui Ma, Ye Luo, Zequn Chen, Renjie Tang, Jialing Jian, Hongtao Lin, and Lan Li
Optical filters are essential parts of advanced optical communication and sensing systems. Among them, the ones with an ultrawide free spectral range (FSR) are especially critical. They are promising to provide access to numerous wavelength channels highly desired for large-capacity optical transmission and multipoint multiparameter sensing. Present schemes for wide-FSR filters either suffer from limited cavity length or poor fabrication tolerance or impose an additional active-tuning control requirement. We theoretically and experimentally demonstrate a filter that features FSR-free operation capability, subnanometer optical bandwidth, and acceptable fabrication tolerance. Only one single deep dip within a record-large waveband (S+C+L band) is observed by appropriately designing a side-coupled Bragg-grating-assisted Fabry–Perot filter, which has been applied as the basic sensing unit for both the refractive index and temperature measurement. Five such basic units are also cascaded in series to demonstrate a multichannel filter. This work provides a new insight to design FSR-free filters and opens up a possibility of flexible large-capacity integration using more wavelength channels, which will greatly advance integrated photonics in optical communication and sensing.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001013 (2021)
Physical Optics
Spin-decoupled metalens with intensity-tunable multiple focal points | On the Cover
Bingshuang Yao, Xiaofei Zang, Yang Zhu, Dahai Yu, Jingya Xie, Lin Chen, Sen Han, Yiming Zhu, and Songlin Zhuang
The control of spin electromagnetic (EM) waves is of great significance in optical communications. Although geometric metasurfaces have shown unprecedented capability to manipulate the wavefronts of spin EM waves, it is still challenging to independently manipulate each spin state and intensity distribution, which inevitably degrades metasurface-based devices for further applications. Here we propose and experimentally demonstrate an approach to designing spin-decoupled metalenses based on pure geometric phase, i.e., geometric metasurfaces with predesigned phase modulation possessing functionalities of both convex lenses and concave lenses. Under the illumination of left-/right-handed circularly polarized (LCP or RCP) terahertz (THz) waves, these metalenses can generate transversely/longitudinally distributed RCP/LCP multiple focal points. Since the helicity-dependent multiple focal points are locked to the polarization state of incident THz waves, the relative intensity between two orthogonal components can be controlled with different weights of LCP and RCP THz waves, leading to the intensity-tunable functionality. This robust approach for simultaneously manipulating orthogonal spin states and energy distributions of spin EM waves will open a new avenue for designing multifunctional devices and integrated communication systems.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001019 (2021)
Ultrafast Optics
Complex Swift Hohenberg equation dissipative soliton fiber laser | Editors' Pick
Ankita Khanolkar, Yimin Zang, and Andy Chong
Complex Swift Hohenberg equation (CSHE) has attracted intensive research interest over the years, as it enables realistic modeling of mode-locked lasers with saturable absorbers by adding a fourth-order term to the spectral response. Many researchers have reported a variety of numerical solutions of CSHE which reveal interesting pulse patterns and structures. In this work, we have demonstrated a CSHE dissipative soliton fiber laser experimentally using a unique spectral filter with a complicated transmission profile. The behavior and performance of the laser agree qualitatively with the numerical simulations based on CSHE. Our findings bring insight into dissipative soliton dynamics and make our mode-locked laser a powerful testbed for observing dissipative solitons of CSHE, which may open a new course in ultrafast fiber laser research.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001033 (2021)
Spectroscopy
Lamellar hafnium ditelluride as an ultrasensitive surface-enhanced Raman scattering platform for label-free detection of uric acid
Yang Li, Haolin Chen, Yanxian Guo, Kangkang Wang, Yue Zhang, Peilin Lan, Jinhao Guo, Wen Zhang, Huiqing Zhong, Zhouyi Guo, Zhengfei Zhuang, and Zhiming Liu
The development of two-dimensional (2D) transition metal dichalcogenides has been in a rapid growth phase for the utilization in surface-enhanced Raman scattering (SERS) analysis. Here, we report a promising 2D transition metal tellurides (TMTs) material, hafnium ditelluride (HfTe2), as an ultrasensitive platform for Raman identification of trace molecules, which demonstrates extraordinary SERS activity in sensitivity, uniformity, and reproducibility. The highest Raman enhancement factor of 2.32×106 is attained for a rhodamine 6G molecule through the highly efficient charge transfer process at the interface between the HfTe2 layered structure and the adsorbed molecules. At the same time, we provide an effective route for large-scale preparation of SERS substrates in practical applications via a facile stripping strategy. Further application of the nanosheets for reliable, rapid, and label-free SERS fingerprint analysis of uric acid molecules, one of the biomarkers associated with gout disease, is performed, which indicates arresting SERS signals with the limits of detection as low as 0.1 mmol/L. The study based on this type of 2D SERS substrate not only reveals the feasibility of applying TMTs to SERS analysis, but also paves the way for nanodiagnostics, especially early marker detection.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001039 (2021)
Lasers and Laser Optics
Arbitrary cylindrical vector beam generation enabled by polarization-selective Gouy phase shifter
Junliang Jia, Kepeng Zhang, Guangwei Hu, Maping Hu, Tong Tong, Quanquan Mu, Hong Gao, Fuli Li, Cheng-Wei Qiu, and Pei Zhang
Cylindrical vector beams (CVBs), which possess polarization distribution of rotational symmetry on the transverse plane, can be developed in many optical technologies. Conventional methods to generate CVBs contain redundant interferometers or need to switch among diverse elements, thus being inconvenient in applications containing multiple CVBs. Here we provide a passive polarization-selective device to substitute interferometers and simplify generation setup. It is accomplished by reversing topological charges of orbital angular momentum based on a polarization-selective Gouy phase. In the process, tunable input light is the only condition to generate a CVB with arbitrary topological charges. To cover both azimuthal and radial parameters of CVBs, we express the mapping between scalar Laguerre–Gaussian light on a basic Poincaré sphere and CVB on a high-order Poincaré sphere. The proposed device simplifies the generation of CVBs enormously and thus has potential in integrated devices for both quantum and classic optical experiments.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001048 (2021)
Experimental demonstration of pyramidal neuron-like dynamics dominated by dendritic action potentials based on a VCSEL for all-optical XOR classification task
Yahui Zhang, Shuiying Xiang, Xingyu Cao, Shihao Zhao, Xingxing Guo, Aijun Wen, and Yue Hao
We experimentally and numerically demonstrate an approach to optically reproduce a pyramidal neuron-like dynamics dominated by dendritic Ca2+ action potentials (dCaAPs) based on a vertical-cavity surface-emitting laser (VCSEL) for the first time. The biological pyramidal neural dynamics dominated by dCaAPs indicates that the dendritic electrode evoked somatic spikes with current near threshold but failed to evoke (or evoked less) somatic spikes for higher current intensity. The emulating neuron-like dynamics is performed optically based on the injection locking, spiking dynamics, and damped oscillations in the optically injected VCSEL. In addition, the exclusive OR (XOR) classification task is examined in the VCSEL neuron equipped with the pyramidal neuron-like dynamics dominated by dCaAPs. Furthermore, a single spike or multiple periodic spikes are suggested to express the result of the XOR classification task for enhancing the processing rate or accuracy. The experimental and numerical results show that the XOR classification task is achieved successfully in the VCSEL neuron enabled to mimic the pyramidal neuron-like dynamics dominated by dCaAPs. This work reveals valuable pyramidal neuron-like dynamics in a VCSEL and offers a novel approach to solve XOR classification task with a fast and simple all-optical spiking neural network, and hence shows great potentials for future photonic spiking neural networks and photonic neuromorphic computing.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001055 (2021)
Optical Devices
Integrating the optical tweezers and spanner onto an individual single-layer metasurface
Tianyue Li, Xiaohao Xu, Boyan Fu, Shuming Wang, Baojun Li, Zhenlin Wang, and Shining Zhu
Optical tweezers (OTs) and optical spanners (OSs) are powerful tools of optical manipulation, which are responsible for particle trapping and rotation, respectively. Conventionally, the OT and OS are built using bulky three-dimensional devices, such as microscope objectives and spatial light modulators. Recently, metasurfaces are proposed for setting up them on a microscale platform, which greatly miniaturizes the systems. However, the realization of both OT and OS with one identical metasurface is posing a challenge. Here, we offer a metasurface-based solution to integrate the OT and OS. Using the prevailing approach based on geometric and dynamic phases, we show that it is possible to construct an output field, which promises a high-numerical-aperture focal spot, accompanied with a coaxial vortex. Optical trapping and rotation are numerically demonstrated by estimating the mechanical effects on a particle probe. Moreover, we demonstrate an on-demand control of the OT-to-OS distance and the topological charge possessed by the OS. By revealing the OT–OS metasurfaces, our results may empower advanced applications in on-chip particle manipulation.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001062 (2021)
Image Processing and Image Analysis
Preconditioned deconvolution method for high-resolution ghost imaging
Zhishen Tong, Zhentao Liu, Chenyu Hu, Jian Wang, and Shensheng Han
Ghost imaging (GI) can nonlocally image objects by exploiting the fluctuation characteristics of light fields, where the spatial resolution is determined by the normalized second-order correlation function g(2). However, the spatial shift-invariant property of g(2) is distorted when the number of samples is limited, which hinders the deconvolution methods from improving the spatial resolution of GI. In this paper, based on prior imaging systems, we propose a preconditioned deconvolution method to improve the imaging resolution of GI by refining the mutual coherence of a sampling matrix in GI. Our theoretical analysis shows that the preconditioned deconvolution method actually extends the deconvolution technique to GI and regresses into the classical deconvolution technique for the conventional imaging system. The imaging resolution of GI after preconditioning is restricted to the detection noise. Both simulation and experimental results show that the spatial resolution of the reconstructed image is obviously enhanced by using the preconditioned deconvolution method. In the experiment, 1.4-fold resolution enhancement over Rayleigh criterion is achieved via the preconditioned deconvolution. Our results extend the deconvolution technique that is only applicable to spatial shift-invariant imaging systems to all linear imaging systems, and will promote their applications in biological imaging and remote sensing for high-resolution imaging demands.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001069 (2021)
Optoelectronics
Harmonic injection locking of high-power mid-infrared quantum cascade lasers | Spotlight on Optics
F. Wang, S. Slivken, and M. Razeghi
High-power, high-speed quantum cascade lasers (QCLs) with stable emission in the mid-infrared regime are of great importance for applications in metrology, telecommunication, and fundamental tests of physics. Owing to the intersubband transition, the unique ultrafast gain recovery time of the QCL with picosecond dynamics is expected to overcome the modulation limit of classical semiconductor lasers and bring a revolution for the next generation of ultrahigh-speed optical communication. Therefore, harmonic injection locking, offering the possibility to fast modulate and greatly stabilize the laser emission beyond the rate limited by cavity length, is inherently adapted to QCLs. In this work, we demonstrate for the first time the harmonic injection locking of a mid-infrared QCL with an output power over 1 W in continuous-wave operation at 288 K. Compared with an unlocked laser, the intermode spacing fluctuation of an injection-locked QCL can be considerably reduced by a factor above 1×103, which permits the realization of an ultrastable mid-infrared semiconductor laser with high phase coherence and frequency purity. Despite temperature change, this fluctuation can be still stabilized to hertz level by a microwave modulation up to ~18 GHz. These results open up the prospect of the applications of mid-infrared QCL technology for frequency comb engineering, metrology, and the next-generation ultrahigh-speed telecommunication. It may also stimulate new schemes for exploring ultrafast mid-infrared pulse generation in QCLs.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001078 (2021)
Image Processing and Image Analysis
Generalized framework for non-sinusoidal fringe analysis using deep learning
Shijie Feng, Chao Zuo, Liang Zhang, Wei Yin, and Qian Chen
Phase retrieval from fringe images is essential to many optical metrology applications. In the field of fringe projection profilometry, the phase is often obtained with systematic errors if the fringe pattern is not a perfect sinusoid. Several factors can account for non-sinusoidal fringe patterns, such as the non-linear input–output response (e.g., the gamma effect) of digital projectors, the residual harmonics in binary defocusing projection, and the image saturation due to intense reflection. Traditionally, these problems are handled separately with different well-designed methods, which can be seen as “one-to-one” strategies. Inspired by recent successful artificial intelligence-based optical imaging applications, we propose a “one-to-many” deep learning technique that can analyze non-sinusoidal fringe images resulting from different non-sinusoidal factors and even the coupling of these factors. We show for the first time, to the best of our knowledge, a trained deep neural network can effectively suppress the phase errors due to various kinds of non-sinusoidal patterns. Our work paves the way to robust and powerful learning-based fringe analysis approaches.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001084 (2021)
Surface Optics and Plasmonics
Ultrafast all-optical terahertz modulation based on an inverse-designed metasurface
Weibao He, Mingyu Tong, Zhongjie Xu, Yuze Hu, Xiang’ai Cheng, and Tian Jiang
Metasurface plays a key role in various terahertz metadevices, while the designed terahertz metasurface still lacks flexibility and variety. On the other hand, inverse design has drawn plenty of attention due to its flexibility and robustness in the application of photonics. This provides an excellent opportunity for metasurface design as well as the development of multifunctional, high-performance terahertz devices. In this work, we demonstrate that, for the first time, a terahertz metasurface supported by the electromagnetically induced transparency (EIT) effect can be constructed by inverse design, which combines the particle swarm optimization algorithm with the finite-difference time-domain method. Incorporating germanium (Ge) film with inverse-designed metasurface, an ultrafast EIT modulation on the picosecond scale has been experimentally verified. The experimental results suggest a feasibility to build the terahertz EIT effect in the metasurface through an optimization algorithm of inverse design. Furthermore, this method can be further utilized to design multifunctional and high-performance terahertz devices, which is hard to accomplish in a traditional metamaterial structure. In a word, our method not only provides a novel way to design an ultrafast all-optical terahertz modulator based on artificial metamaterials but also shows the potential applications of inverse design on the terahertz devices.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001099 (2021)
Physical Optics
Band dynamics accompanied by bound states in the continuum at the third-order Γ point in leaky-mode photonic lattices
Sun-Goo Lee, Seong-Han Kim, and Chul-Sik Kee
Bound states in the continuum (BICs) and Fano resonances in planar photonic lattices, including metasurfaces and photonic-crystal slabs, have been studied extensively in recent years. Typically, the BICs and Fano resonances are associated with the second stop bands open at the second-order Γ point. This paper addresses the fundamental properties of the fourth stop band accompanied by BICs at the third-order Γ point in one-dimensional leaky-mode photonic lattices. At the fourth stop band, one band edge mode suffers radiation loss, thereby generating a Fano resonance, while the other band edge mode becomes a nonleaky BIC. The fourth stop band is controlled primarily by the Bragg processes associated with the first, second, and fourth Fourier harmonic components of the periodic dielectric constant modulation. The interplay between these three major processes closes the fourth band gap and induces a band flip whereby the leaky and BIC edges transit across the fourth band gap. At the fourth stop band, an accidental BIC is formed owing to the destructive interplay between the first and second Fourier harmonics. When the fourth band gap closes with strongly enhanced radiative Q factors, Dirac cone dispersions can appear at the third-order Γ point.
Photonics Research
  • Publication Date: May. 27, 2021
  • Vol.9 Issue, 6 06001109 (2021)
Optoelectronics
Electrical and optical characteristics of highly transparent MOVPE-grown AlGaN-based tunnel heterojunction LEDs emitting at 232 nm
Frank Mehnke, Christian Kuhn, Martin Guttmann, Luca Sulmoni, Verena Montag, Johannes Glaab, Tim Wernicke, and Michael Kneissl
We present the growth and electro-optical characteristics of highly transparent AlGaN-based tunnel heterojunction light-emitting diodes (LEDs) emitting at 232 nm entirely grown by metalorganic vapor phase epitaxy (MOVPE). A GaN:Si interlayer was embedded into a highly Mg- and Si-doped Al0.87Ga0.13N tunnel junction to enable polarization field enhanced tunneling. The LEDs exhibit an on-wafer integrated emission power of 77 μW at 5 mA, which correlates to an external quantum efficiency (EQE) of 0.29% with 45 μW emitted through the bottom sapphire substrate and 32 μW emitted through the transparent top surface. After depositing a highly reflective aluminum reflector, a maximum emission power of 1.73 mW was achieved at 100 mA under pulsed mode operation with a maximum EQE of 0.35% as collected through the bottom substrate.
Photonics Research
  • Publication Date: Jun. 01, 2021
  • Vol.9 Issue, 6 06001117 (2021)
Fiber Optics and Optical Communications
Covert wireless communication using massive optical comb channels for deep denoising
Xianglei Yan, Xihua Zou, Peixuan Li, Wei Pan, and Lianshan Yan
Covert wireless communications are unprecedentedly vital for security and privacy of individuals, government, and military bodies. Besides encryption, hiding signal transmission deeply under noise background highly proliferates the covertness in the physical layer. A deep signal hiding leads to a low interception probability at the interceptor but a poor data recovery at the receiver. To ensure both high covertness and high-fidelity recovery, massive and dense optical comb channels are utilized for deep denoising through the analog spectrum convolution. Using an external modulation-based optical frequency comb (OFC) and a single detection branch, the available optical comb channels can sustainably scale up by breaking or greatly mitigating physical bottlenecks on immense hardware and spectrum requirements. Thus, a striking signal-to-noise ratio (SNR) rise can be achieved for deep denoising. Combination of 1024 comb channels (the first parallel comb channel number beyond 1000) and the analog spectrum convolution enable a record SNR enhancement of 29 dB for a microwave signal with a 10.24 GHz bandwidth and a 10 Mbit/s data rate, which is deeply hidden below the in-band noises by 18 dB or even 30 dB in both the frequency and time domains. This method opens a new avenue for covert communications.
Photonics Research
  • Publication Date: Jun. 01, 2021
  • Vol.9 Issue, 6 06001124 (2021)
Quantum Optics
Effect of dispersion on indistinguishability between single-photon wave-packets
Yun-Ru Fan, Chen-Zhi Yuan, Rui-Ming Zhang, Si Shen, Peng Wu, He-Qing Wang, Hao Li, Guang-Wei Deng, Hai-Zhi Song, Li-Xing You, Zhen Wang, You Wang, Guang-Can Guo, and Qiang Zhou
With propagating through a dispersive medium, the temporal–spectral profile of optical pulses should be inevitably modified. Although such dispersion effect has been well studied in classical optics, its effect on a single-photon wave-packet has not yet been entirely revealed. In this paper, we investigate the effect of dispersion on indistinguishability between single-photon wave-packets through the Hong–Ou–Mandel (HOM) interference. By dispersively manipulating two weak coherent single-photon wave-packets which are prepared by attenuating mode-locked laser pulses before interfering with each other, we observe that the difference of the second-order dispersion between two optical paths of the HOM interferometer can be mapped to the interference curve, indicating that (i) with the same amount of dispersion effect in both paths, the HOM interference curve must be only determined by the intrinsic indistinguishability between the wave-packets, i.e., dispersion cancellation due to the indistinguishability between Feynman paths; and (ii) unbalanced dispersion effect in two paths cannot be canceled and will broaden the interference curve thus providing a way to measure the second-order dispersion coefficient. Our results suggest a more comprehensive understanding of the single-photon wave-packet and pave ways to explore further applications of the HOM interference.
Photonics Research
  • Publication Date: Jun. 01, 2021
  • Vol.9 Issue, 6 06001134 (2021)
Reviews
Optoelectronics
Lead–halide perovskites for next-generation self-powered photodetectors: a comprehensive review
Chandrasekar Perumal Veeramalai, Shuai Feng, Xiaoming Zhang, S. V. N. Pammi, Vincenzo Pecunia, and Chuanbo Li
Metal halide perovskites have aroused tremendous interest in optoelectronics due to their attractive properties, encouraging the development of high-performance devices for emerging application domains such as wearable electronics and the Internet of Things. Specifically, the development of high-performance perovskite-based photodetectors (PDs) as an ultimate substitute for conventional PDs made of inorganic semiconductors such as silicon, InGaAs, GaN, and germanium-based commercial PDs, attracts great attention by virtue of its solution processing, film deposition technique, and tunable optical properties. Importantly, perovskite PDs can also deliver high performance without an external power source; so-called self-powered perovskite photodetectors (SPPDs) have found eminent application in next-generation nanodevices operating independently, wirelessly, and remotely. Earlier research reports indicate that perovskite-based SPPDs have excellent photoresponsive behavior and wideband spectral response ranges. Despite the high-performance perovskite PDs, their commercialization is hindered by long-term material instability under ambient conditions. This review aims to provide a comprehensive compilation of the research results on self-powered, lead–halide perovskite PDs. In addition, a brief introduction is given to flexible SPPDs. Finally, we put forward some perspectives on the further development of perovskite-based self-powered PDs. We believe that this review can provide state-of-the-art current research on SPPDs and serve as a guide to improvising a path for enhancing the performance to meet the versatility of practical device applications.
Photonics Research
  • Publication Date: May. 20, 2021
  • Vol.9 Issue, 6 06000968 (2021)