Contents
2021
Volume: 9 Issue 5
37 Article(s)

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Special Issue on DEEP LEARNING IN PHOTONICS
Deep-learning based denoising and reconstruction of super-resolution structured illumination microscopy images
Zafran Hussain Shah, Marcel Müller, Tung-Cheng Wang, Philip Maurice Scheidig, Axel Schneider, Mark Schüttpelz, Thomas Huser, and Wolfram Schenck
Super-resolution structured illumination microscopy (SR-SIM) provides an up to twofold enhanced spatial resolution of fluorescently labeled samples. The reconstruction of high-quality SR-SIM images critically depends on patterned illumination with high modulation contrast. Noisy raw image data (e.g., as a result of low excitation power or low exposure time), result in reconstruction artifacts. Here, we demonstrate deep-learning based SR-SIM image denoising that results in high-quality reconstructed images. A residual encoding–decoding convolutional neural network (RED-Net) was used to successfully denoise computationally reconstructed noisy SR-SIM images. We also demonstrate the end-to-end deep-learning based denoising and reconstruction of raw SIM images into high-resolution SR-SIM images. Both image reconstruction methods prove to be very robust against image reconstruction artifacts and generalize very well across various noise levels. The combination of computational image reconstruction and subsequent denoising via RED-Net shows very robust performance during inference after training even if the microscope settings change.
Photonics Research
  • Publication Date: Apr. 19, 2021
  • Vol.9 Issue, 5 0500B168 (2021)
Deep learning in nano-photonics: inverse design and beyond
Peter R. Wiecha, Arnaud Arbouet, Christian Girard, and Otto L. Muskens
Deep learning in the context of nano-photonics is mostly discussed in terms of its potential for inverse design of photonic devices or nano-structures. Many of the recent works on machine-learning inverse design are highly specific, and the drawbacks of the respective approaches are often not immediately clear. In this review we want therefore to provide a critical review on the capabilities of deep learning for inverse design and the progress which has been made so far. We classify the different deep-learning-based inverse design approaches at a higher level as well as by the context of their respective applications and critically discuss their strengths and weaknesses. While a significant part of the community’s attention lies on nano-photonic inverse design, deep learning has evolved as a tool for a large variety of applications. The second part of the review will focus therefore on machine learning research in nano-photonics “beyond inverse design.” This spans from physics-informed neural networks for tremendous acceleration of photonics simulations, over sparse data reconstruction, imaging and “knowledge discovery” to experimental applications.
Photonics Research
  • Publication Date: Apr. 19, 2021
  • Vol.9 Issue, 5 0500B182 (2021)
All-optical neuromorphic binary convolution with a spiking VCSEL neuron for image gradient magnitudes
Yahui Zhang, Joshua Robertson, Shuiying Xiang, Matěj Hejda, Julián Bueno, and Antonio Hurtado
All-optical binary convolution with a photonic spiking vertical-cavity surface-emitting laser (VCSEL) neuron is proposed and demonstrated experimentally for the first time, to the best of our knowledge. Optical inputs, extracted from digital images and temporally encoded using rectangular pulses, are injected in the VCSEL neuron, which delivers the convolution result in the number of fast (<100 ps long) spikes fired. Experimental and numerical results show that binary convolution is achieved successfully with a single spiking VCSEL neuron and that all-optical binary convolution can be used to calculate image gradient magnitudes to detect edge features and separate vertical and horizontal components in source images. We also show that this all-optical spiking binary convolution system is robust to noise and can operate with high-resolution images. Additionally, the proposed system offers important advantages such as ultrafast speed, high-energy efficiency, and simple hardware implementation, highlighting the potentials of spiking photonic VCSEL neurons for high-speed neuromorphic image processing systems and future photonic spiking convolutional neural networks.
Photonics Research
  • Publication Date: Apr. 19, 2021
  • Vol.9 Issue, 5 0500B201 (2021)
Imaging through unknown scattering media based on physics-informed learning
Shuo Zhu, Enlai Guo, Jie Gu, Lianfa Bai, and Jing Han
Imaging through scattering media is one of the hotspots in the optical field, and impressive results have been demonstrated via deep learning (DL). However, most of the DL approaches are solely data-driven methods and lack the related physics prior, which results in a limited generalization capability. In this paper, through the effective combination of the speckle-correlation theory and the DL method, we demonstrate a physics-informed learning method in scalable imaging through an unknown thin scattering media, which can achieve high reconstruction fidelity for the sparse objects by training with only one diffuser. The method can solve the inverse problem with more general applicability, which promotes that the objects with different complexity and sparsity can be reconstructed accurately through unknown scattering media, even if the diffusers have different statistical properties. This approach can also extend the field of view (FOV) of traditional speckle-correlation methods. This method gives impetus to the development of scattering imaging in practical scenes and provides an enlightening reference for using DL methods to solve optical problems.
Photonics Research
  • Publication Date: Apr. 19, 2021
  • Vol.9 Issue, 5 0500B210 (2021)
Incoherent imaging through highly nonstatic and optically thick turbid media based on neural network
Shanshan Zheng, Hao Wang, Shi Dong, Fei Wang, and Guohai Situ
Imaging through nonstatic scattering media is one of the major challenges in optics, and encountered in imaging through dense fog, turbid water, and many other situations. Here, we propose a method to achieve single-shot incoherent imaging through highly nonstatic and optically thick turbid media by using an end-to-end deep neural network. In this study, we use fat emulsion suspensions in a glass tank as a turbid medium and an additional incoherent light to introduce strong interference noise. We calibrate that the optical thickness of the tank of turbid media is as high as 16, and the signal-to-interference ratio is as low as -17 dB. Experimental results show that the proposed learning-based approach can reconstruct the object image with high fidelity in this severe environment.
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 0500B220 (2021)
Realizing transmitted metasurface cloak by a tandem neural network
Zheng Zhen, Chao Qian, Yuetian Jia, Zhixiang Fan, Ran Hao, Tong Cai, Bin Zheng, Hongsheng Chen, and Erping Li
Being invisible at will has been a long-standing dream for centuries, epitomized by numerous legends; humans have never stopped their exploration steps to realize this dream. Recent years have witnessed a breakthrough in this search due to the advent of transformation optics, metamaterials, and metasurfaces. However, the previous metasurface cloaks typically work in a reflection manner that relies on a high-reflection background, thus limiting the applications. Here, we propose an easy yet viable approach to realize the transmitted metasurface cloak, just composed of two planar metasurfaces to hide an object inside, such as a cat. To tackle the hard-to-converge issue caused by the nonuniqueness phenomenon, we deploy a tandem neural network (T-NN) to efficiently streamline the inverse design. Once pretrained, the T-NN can work for a customer-desired electromagnetic response in one single forward computation, saving a great amount of time. Our work opens a new avenue to realize a transparent invisibility cloak, and the tandem-NN can also inspire the inverse design of other metamaterials and photonics.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 0500B229 (2021)
Accurate inverse design of Fabry–Perot-cavity-based color filters far beyond sRGB via a bidirectional artificial neural network
Peng Dai, Yasi Wang, Yueqiang Hu, C. H. de Groot, Otto Muskens, Huigao Duan, and Ruomeng Huang
Structural color based on Fabry&ndash;Perot (F-P) cavity enables a wide color gamut with high resolution at submicroscopic scale by varying its geometrical parameters. The ability to design such parameters that can accurately display the desired color is therefore crucial to the manufacturing of F-P cavities for practical applications. This work reports the first inverse design of F-P cavity structure using deep learning through a bidirectional artificial neural network. It enables the production of a significantly wider coverage of color space that is over 215% of sRGB with extremely high accuracy, represented by an average &Delta;E2000 value below 1.2. The superior performance of this structural color-based neural network is directly ascribed to the definition of loss function in the uniform CIE 1976-Lab color space. Over 100,000 times improvement in the design efficiency has been demonstrated by comparing the neural network to the metaheuristic optimization technique using an evolutionary algorithm when designing the famous painting of “Haystacks, end of Summer” by Claude Monet. Our results demonstrate that, with the correct selection of loss function, deep learning can be very powerful to achieve extremely accurate design of nanostructured color filters with very high efficiency.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 0500B236 (2021)
Research Articles
Fiber Optics and Optical Communications
Over 255 mW single-frequency fiber laser with high slope efficiency and power stability based on an ultrashort Yb-doped crystal-derived silica fiber
Ying Wan, Jianxiang Wen, Chen Jiang, Fengzai Tang, Jing Wen, Sujuan Huang, Fufei Pang, and Tingyun Wang
A single-frequency distributed Bragg reflector (DBR) fiber laser at 1030 nm has been demonstrated using a 0.7 cm long homemade Yb:YAG crystal-derived silica fiber (YCDSF). The absorption and emission cross sections of the YCDSF are 4.93×10-20 cm2 at 980 nm and 1.1×10-20 cm2 at 1030 nm. Using this gain fiber, an over 255 mW continuous-wave lasing in the constructed laser has been obtained at single transverse and longitudinal mode operation. The slope efficiency and the pump threshold of the fiber laser were up to ~35% and as low as 25 mW, respectively. The fiber laser also demonstrated an optical signal-to-noise ratio of 79 dB and a beam quality factor of 1.016 in two orthogonal directions. Its power fluctuation at 210.5 mW was less than 0.85% of the average power within 13 h. Moreover, the relative intensity noise and linewidth of the laser are also investigated at the different pump powers. The results indicate that the single-frequency DBR fiber laser has potential applications in high-quality seed sources and high-precision optical sensing.
Photonics Research
  • Publication Date: Apr. 19, 2021
  • Vol.9 Issue, 5 05000649 (2021)
Single-mode spatiotemporal soliton attractor in multimode GRIN fibers
M. Zitelli, M. Ferraro, F. Mangini, and S. Wabnitz
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000741 (2021)
Distributed static and dynamic detection of an acoustic wave in a Brillouin random fiber laser
Zichao Zhou, Haiyang Wang, Yuan Wang, Liang Chen, and Xiaoyi Bao
The interaction of random laser and gain medium is important to understand the noise origin in random fiber lasers. Here, using the optical time domain reflectometry method, the time-resolved distributed acoustic wave generated by a Brillouin random fiber laser (BRFL) is characterized. The dynamic property of the acoustic wave reflects the gain dynamics of the BRFL. The principle is based on the polarization-decoupled stimulated Brillouin scattering (SBS)-enhanced four-wave mixing process, where the probe light experiences maximum reflection when the phase match condition is satisfied. Static measurements present exponentially depleted Brillouin gain along the gain medium in the BRFL, indicating the localized random SBS frequency change in the maximum local gain region, which varies with time to contribute random laser noise as revealed in the dynamic measurement. The SBS-induced birefringence change in the Brillouin gain fiber is approximately 10-7 to 10-6. The phase noise of the BRFL is observed directly inside the random laser gain medium for the first time via time and spatially varied acoustic wave intensity. By counting the temporal intensity statistical distribution, optical rogue waves are detected near the lasing threshold of the BRFL. Different temporal intensity statistical distribution at high and low gain positions is found, which is caused by the SBS nonlinear transfer function and localized gain. The distributed characterization methods in the paper provide a new platform to study the interaction of random lasers and gain medium, giving us a new perspective to understand the fundamental physics of the random lasing process and its noise property.
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000772 (2021)
Imaging Systems, Microscopy, and Displays
Optical Vernier sampling using a dual-comb-swept laser to solve distance aliasing
Seongjin Bak, Gyeong Hun Kim, Hansol Jang, and Chang-Seok Kim
Optical interferometry using comb-swept lasers has the advantage of efficiently reducing the acquisition bandwidth for high-speed and long-range detection. However, in general, the use of a comb-swept laser involves a critical limitation in that the absolute distance cannot be measured, and, thus, multiple layers cannot be distinguished when measuring each position. This is because of the distance ambiguity induced by optical aliasing, in which there is periodic repetition of the frequency of an interferometric signal owing to discrete spectral sweeping, which does not occur in conventional optical interferometry that uses a continuous swept laser. In this paper, we introduce an optical Vernier sampling method using a dual-comb-swept laser to measure the absolute distances in a multi-layer target. For this, we designed a new type of dual-comb-swept laser to include two different free spectral ranges (FSRs) in separated wavelength bands to provide a stable lasing condition. Using a principle similar to that of a Vernier caliper for length measurement, the two different FSRs can be used to recover a higher frequency of an optical interferometric signal to measure longer distances from different layers in a target. Using the dual-comb-swept laser in optical interferometry, we solved the optical aliasing issue and measured the absolute distances of three layers separated over 83 mm using a point-scanning imaging setup and the simultaneous absolute distance of the top surfaces separated over 45 mm using a full-field imaging setup at 14 and 8 times lower acquisition bandwidth than a conventional continuous swept laser that is based on optical interferometry.
Photonics Research
  • Publication Date: Apr. 19, 2021
  • Vol.9 Issue, 5 05000657 (2021)
FourierCam: a camera for video spectrum acquisition in a single shot
Chengyang Hu, Honghao Huang, Minghua Chen, Sigang Yang, and Hongwei Chen
The novel camera architecture facilitates the development of machine vision. Instead of capturing frame sequences in the temporal domain as traditional video cameras, FourierCam directly measures the pixel-wise temporal spectrum of the video in a single shot through optical coding. Compared to the classic video cameras and time-frequency transformation pipeline, this programmable frequency-domain sampling strategy has an attractive combination of characteristics for low detection bandwidth, low computational burden, and low data volume. Based on the various temporal filter kernel designed by FourierCam, we demonstrated a series of exciting machine vision functions, such as video compression, background subtraction, object extraction, and trajectory tracking.
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000701 (2021)
Integrated Optics
Lasers and Laser Optics
1.7-μm dissipative soliton Tm-doped fiber laser
Ji-Xiang Chen, Xiang-Yue Li, Ti-Jian Li, Ze-Yu Zhan, Meng Liu, Can Li, Ai-Ping Luo, Pu Zhou, Kenneth K.-Y. Wong, Wen-Cheng Xu, and Zhi-Chao Luo
We report on the dissipative soliton generation in a 1.7-μm net-normal dispersion Tm-doped fiber laser by nonlinear polarization rotation technique. An intra-cavity bandpass filter was employed to suppress the long-wavelength emission, while the cavity dispersion was compensated by a segment of ultrahigh numerical aperture (UHNA4) fiber. The dissipative soliton with a central wavelength of 1746 nm was obtained, covering a spectral range from 1737 nm to 1754 nm. The de-chirped duration and energy of the dissipative soliton were 370 fs and 0.2 nJ, respectively. In addition, the dynamics of multiple dissipative solitons were also investigated. Through optimization of the cavity dispersion, the 50 nm broadband dissipative soliton with de-chirped pulse duration of 230 fs could be achieved. The development of dissipative soliton seed laser represents the first step in achieving the chirped pulse amplification system at the 1.7-μm wave band, which would find potential applications in fields such as biomedical imaging and material processing.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000873 (2021)
Nanophotonics and Photonic Crystals
All-dielectric photonic crystal with unconventional higher-order topology | Editors' Pick
Shiqiao Wu, Bin Jiang, Yang Liu, and Jian-Hua Jiang
Photonic crystals (PhCs) have been demonstrated as a versatile platform for the study of topological phenomena. The recent discovery of higher-order topological insulators introduces new aspects of topological PhCs that are yet to be explored. Here, we propose an all-dielectric PhC with an unconventional higher-order band topology. Besides the conventional spectral features of gapped edge states and in-gap corner states, topological band theory predicts that the corner boundary of the higher-order topological insulator hosts a 2/3 fractional charge. We demonstrate that in the PhC such a fractional charge can be verified from the local density-of-states of photons, through the concept of local spectral charge as an analog of the local electric charge due to the band filling anomaly in electronic systems. Furthermore, we show that by introducing a disclination in the proposed PhC, localized states and a 2/3 fractional spectral charge emerge around the disclination core. The emergence of the fractional spectral charges and topological boundary modes here, however, is distinct from the known cases; particularly by the 2/3 fractional spectral charges and the unique topological indices. The predicted effects can be readily observed in the state-of-the-art experiments and may lead to potential applications in integrated and quantum photonics.
Photonics Research
  • Publication Date: Apr. 19, 2021
  • Vol.9 Issue, 5 05000668 (2021)
Nonsuspended optomechanical crystal cavities using As2S3 chalcogenide glass
Renduo Qi, Qiancheng Xu, Ning Wu, Kaiyu Cui, Wei Zhang, and Yidong Huang
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000893 (2021)
Optical and Photonic Materials
Principles to tailor the saturable and reverse saturable absorption of epsilon-near-zero material
Hao Ma, Yuanan Zhao, Yuchen Shao, Yafei Lian, Weili Zhang, Guohang Hu, Yuxin Leng, and Jianda Shao
Indium tin oxide (ITO) films have recently emerged as a new class of functional materials for nonlinear optical (NLO) devices due to their exotic properties around epsilon-near-zero (ENZ) wavelength. Here, we experimentally investigated and tailored the NLO absorption properties of ITO films. The NLO absorption response of ITO films is investigated by using the femtosecond Z-scan measurement technique at two different wavelengths of 1030 nm (out of ENZ region) and 1440 nm (within ENZ region). Interestingly, we observed conversion behavior from saturable absorption (SA) to reverse saturable absorption (RSA) at 1030 nm with the increasing incident laser intensity, whereas only SA behavior was observed at 1440 nm. We demonstrate that SA behavior was ascribed to ground-state free electrons bleaching in the conduction band, and RSA was attributed to three-photon absorption. Moreover, results reveal that ITO film shows more excellent SA performance at 1440 nm with a nonlinear absorption coefficient of ~-23.2 cm/GW and a figure of merit of ~1.22×10-16 esu·cm. Furthermore, we tailored the SA and RSA behaviors of ITO films at 1030 and 1440 nm wavelengths via post-annealing treatment. The modulatable NLO absorption was ascribed to the changing of free-carrier concentration in ITO films via annealing treatment. The experimental findings offered an inroad for researchers to tailor its NLO absorption properties by changing the free-carrier concentration through chemical modification such as annealing, oxidation, or defect implantation. The superior and tunable nonlinear optical response suggests that ITO film might be employed as a new class material with potential applications in novel optical switches or optical limiters to realize the all-optical information process.
Photonics Research
  • Publication Date: May. 24, 2021
  • Vol.9 Issue, 5 05000678 (2021)
Controllable one-step doping synthesis for the white-light emission of cesium copper iodide perovskites
Ranran Fan, Shaofan Fang, Chengchuan Liang, Zhaoxing Liang, and Haizhe Zhong
In this paper, a controllable one-step doping method has been successfully adopted in the cesium copper iodide perovskite’s luminescence, a high-quality white-light emission with Commission Internationale de l′Eclairage (CIE) coordinates of (0.3397, 0.3325), and a color rendering index (CRI) reaching up to 90 was realized in a convenient way. Through adding impurities into the Cs3Cu2I5 system, high efficiency and stable CsCu2I3 was synthesized, and the coexistence of varied high luminescence phases realized the white lighting. Strikingly, blue-emitting Cs3Cu2I5 and yellow-emitting CsCu2I3 could coexist, and their respective luminescence was not interacted in the compound, which was beneficial for acquiring a single emission and highly efficient white lighting. This work carried out a deep exploration of the Cu-based metal halides, and would be favorable to the applications of lead-free perovskites.
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000694 (2021)
Spin-selective corner reflector for retro-reflection and absorption by a circular dichroitic manner
He Wang, Yao Jing, Yongfeng Li, Lingling Huang, Maochang Feng, Qi Yuan, Jiafu Wang, Jieqiu Zhang, and Shaobo Qu
Recently, we have witnessed an extraordinary spurt in attention toward manipulating electromagnetic waves by metasurfaces. Particularly, tailoring of circular polarization has attracted great amounts of interest in both microwave and optics regimes. Circular dichroism, an exotic chiroptical effect of natural molecules, has aroused discussion about this issue, yet it is still in its infancy. Herein, we initiate circular dichroism followed by controlling spin-selective wavefronts via chiral metasurfaces. An N-shaped chiral resonator loaded with two lumped resistors is proposed as the meta-atom producing an adequate phase gradient. Assisted by the ohmic dissipation of the introduced resistors, the effect of differential absorption provides an auxiliary degree of freedom for developing circularly polarized waves with a designated spin state. A planar corner reflector that can achieve retro-reflection and absorption for right- and left-handed circularly polarized incidence is theoretically simulated and experimentally observed at microwave frequency. Thus, our effort provides an alternative approach to tailoring electromagnetic waves in a circular dichroitic manner and may also find applications in multi-functional systems in optics and microwave regimes.
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000726 (2021)
Immensely enhanced color-adjustable upconversion fluorescence in electron donor-acceptor exciplex chromophores doped with fluorescent emitters | Editors' Pick
Zhen Chen, Qian Zhou, Huitian Du, Yuan Yu, Chuang Zhang, Shenghao Han, and Zhiyong Pang
Two-photon excited fluorescence materials usually suffer from inefficient two-photon absorption (TPA) and nonradiative excited states. Here, upconversion fluorescence in an electron donor-acceptor (DA) exciplex doped with fluorescent emitters are systematically investigated. It has been found that the undoped DA exciplex exhibits enhancements of ~129% and ~365% in upconversion fluorescence compared to donor- and acceptor-only systems, respectively. Interestingly, photoluminescence quantum yields (PLQYs) up to ~98.65% were measured and immensely enhanced upconversion fluorescence was observed after doping various fluorescent emitters into the DA exciplex. Our results reveal the existence of two-photon excited energy harvesting in a thermally activated delayed fluorescence (TADF) DA exciplex doped with fluorescent emitters, via reverse intersystem crossing followed by rapid F?rster resonance energy transfer. Moreover, the additional gain mechanism related to intermolecular CT interaction that occurs at the TPA stage is found in the TADF DA exciplex system.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000865 (2021)
Optical Devices
Spatiospectral transformation of noncollimated light beams diffracted by ultrasound in birefringent crystals
Alexey V. Gorevoy, Alexander S. Machikhin, Grigoriy N. Martynov, and Vitold E. Pozhar
Spatiospectral structure of wave phase matching in birefringent crystals has a strong dependence on the geometry of the acousto-optic interaction and incident light spectrum. This dependence defines details of light beam profile transformation. It is especially important for imaging applications related to a large angular aperture and a wide spectral bandwidth of the incident light. In this paper, we demonstrate accurate three-dimensional plotting of a light transmission pattern without small birefringence approximation. The rather complicated shape of the phase-matching locus in the spatiospectral domain inevitably leads to residual spatially nonuniform chromatic aberrations in the spectral image. Theoretical consideration and computational modeling are confirmed by the experiments on Bragg diffraction in paratellurite crystal. The results are especially important for the development of acousto-optical imaging devices and laser beam shaping technologies.
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000687 (2021)
Polarization assisted self-powered GaN-based UV photodetector with high responsivity
Jiaxing Wang, Chunshuang Chu, Kangkai Tian, Jiamang Che, Hua Shao, Yonghui Zhang, Ke Jiang, Zi-Hui Zhang, Xiaojuan Sun, and Dabing Li
In this work, a self-powered GaN-based metal-semiconductor-metal photodetector (MSM PD) with high responsivity has been proposed and fabricated. The proposed MSM PD forms an asymmetric feature by using the polarization effect under one electrode, such that we adopt an AlGaN/GaN heterojunction to produce the electric field, and by doing so, an asymmetric energy band between the two electrodes can be obtained even when the device is unbiased. The asymmetric feature is proven by generating the asymmetric current-voltage characteristics both in the dark and the illumination conditions. Our results show that the asymmetric energy band enables the self-powered PD, and the peak responsivity wavelength is 240 nm with the responsivity of 0.005 A/W. Moreover, a high responsivity of 13.56 A/W at the applied bias of 3 V is also achieved. Thanks to the very strong electric field in the charge transport region, when compared to the symmetric MSM PD, the proposed MSM PD can reach an increased photocurrent of 100 times larger than that for the conventional PD, even if the illumination intensity for the light source becomes increased.
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000734 (2021)
Optoelectronics
Direct demonstration of carrier distribution and recombination within step-bunched UV-LEDs
Houqiang Xu, Jiean Jiang, Li Chen, Jason Hoo, Long Yan, Shiping Guo, Cai Shen, Yanping Wei, Hua Shao, Zi-Hui Zhang, Wei Guo, and Jichun Ye
AlGaN-based solid state UV emitters have many advantages over conventional UV sources. However, UV-LEDs still suffer from numerous challenges, including low quantum efficiency compared to their blue LED counterparts. One of the inherent reasons is a lack of carrier localization effect inside fully miscible AlGaN alloys. In the pursuit of phase separation and carrier localization inside the active region of AlGaN UV-LED, utilization of highly misoriented substrates proves to be useful, yet the carrier distribution and recombination mechanism in such structures has seldom been reported. In this paper, a UV-LED with step-bunched surface morphology was designed and fabricated, and the internal mechanism of high internal quantum efficiency was studied in detail. The correlation between microscale current distribution and surface morphology was provided, directly demonstrating that current prefers to flow through the step edges of the epitaxial layers. Experimental results were further supported by numerical simulation. It was found that efficient radiative recombination centers were formed in the inclined quantum well regions. A schematic three-dimensional energy band structure of the multiple quantum wells (MQWs) across the step was proposed and helps in further understanding the luminescence behavior of LEDs grown on misoriented substrates. Finally, a general principle to achieve carrier localization was proposed, which is valid for most ternary III-V semiconductors exhibiting phase separation.
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000764 (2021)
Environment-friendly antisolvent tert-amyl alcohol modified hybrid perovskite photodetector with high responsivity
Tengteng Li, Qingyan Li, Xin Tang, Zhiliang Chen, Yifan Li, Hongliang Zhao, Silei Wang, Xin Ding, Yating Zhang, and Jianquan Yao
The preparation of high-quality perovskite films with optimal morphologies is important for achieving high-performance perovskite photodetectors (PPDs). An effective strategy to optimize the morphologies is to add antisolvents during the spin-coating steps. In this work, a novel environment-friendly antisolvent tert-amyl alcohol (TAA) is employed first to improve the quality of perovskite films, which can effectively regulate the formation of an intermediate phase staged in between a liquid precursor phase and a solid perovskite phase due to its moderate polarity and further promote the homogeneous nucleation and crystal growth, thus leading to the formation of high-quality perovskite films and enhanced photodetector performance. As a result, the responsivity of the PPD reaches 1.56 A/W under the illumination of 532 nm laser with the power density of 6.37 μW/cm2 at a bias voltage of -2 V, which is good responsivity for PPDs with the vertical structure and only CH3NH3PbI3 perovskite as the photosensitive material. The corresponding detectivity reaches 1.47×1012 Jones, while the linear dynamic range reaches 110 dB. These results demonstrate that our developed green antisolvent TAA has remarkable advantages for the fabrication of high-performance PPDs and can provide a reference for similar research work.
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000781 (2021)
1.3 GHz E-O bandwidth GaN-based micro-LED for multi-gigabit visible light communication | Editors' Pick
Lei Wang, Zixian Wei, Chien-Ju Chen, Lai Wang, H. Y. Fu, Li Zhang, Kai-Chia Chen, Meng-Chyi Wu, Yuhan Dong, Zhibiao Hao, and Yi Luo
The data rate of a visible light communication (VLC) system is basically determined by the electrical-to-optical (E-O) bandwidth of its light-emitting diode (LED) source. In order to break through the intrinsic limitation of the carrier recombination rate on E-O bandwidth in conventional c-plane LEDs based on InGaN quantum wells, a blue micro-LED with an active region of nano-structured InGaN wetting layer is designed, fabricated, and packaged to realize a high-speed VLC system. The E-O bandwidth of the micro-LED can reach up to 1.3 GHz. Based on this high-speed micro-LED, we demonstrated a data rate of 2 Gbps with a bit error rate (BER) of 1.2×10-3 with simple on-off keying signal for a 3-m real-time VLC. In addition, a 4-Gbps VLC system using quadrature phase shift keying-orthogonal frequency-division multiplexing with a BER of 3.2×10-3 is also achieved for the same scenario. Among all the point-to-point VLC systems based on a single-pixel LED, this work has the highest distance-bandwidth product of 3 GHz·m and the highest distance-rate product of 12 Gbps·m.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000792 (2021)
Physical Optics
Concise and efficient direct-view generation of arbitrary cylindrical vector beams by a vortex half-wave plate
Junli Qi, Weihua Wang, Bo Shi, Hui Zhang, Yanan Shen, Haifei Deng, Wenjing Pu, Xin Liu, Huihui Shan, Xiaomin Ma, Lianqiang Zhang, Wei Lu, Meicheng Fu, and Xiujian Li
A concise, efficient, and practical direct-view scheme is presented to generate arbitrary cylindrical vector (CV) beams, including CV beams, vortex beams, and cylindrical vector vortex (CVV) beams, by a vortex half-wave plate (VHP). Six kinds of first-order and other high-order CV beams, such as azimuthally polarized (AP) beams, antivortex radial polarization mode beams, and three-order AP beams, are formed by simply rotating a half-wave plate. The Stokes parameters and double-slit interference of multitype CV beams are investigated in detail. The polarization parameters, including degree of polarization, polarization azimuth, and ellipticity, are obtained, which demonstrates the efficient generation of CV beams. In addition, the double-slit interference experiment is introduced in the setup, and fringe misplacement and tilt appear for CVV beams, in which the misplacement number M is 2P+1 for P≤2 and 2P-1 for P≥3, where P is the polarization order number, and the fringe tilt offset is positively related to the topological charge number l of CVV beams. In addition, new types of VHPs can be formed by cascading two or more VHPs when the types of available VHPs are limited, assisting in more flexible generation of multitype CV beams. It is experimentally demonstrated that arbitrary CV beams with high quality are effectively achieved by the proposed setup, and the double-slit interference method can be utilized to determine and analyze CV beams rapidly and concisely by practical performance, which shows the potential to be implemented as a commercial device.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000803 (2021)
Breakdown of Maxwell Garnett theory due to evanescent fields at deep-subwavelength scale
Ting Dong, Jie Luo, Hongchen Chu, Xiang Xiong, Ruwen Peng, Mu Wang, and Yun Lai
Deep-subwavelength all-dielectric composite materials are believed to tightly obey the Maxwell Garnett effective medium theory. Here, we demonstrate that the Maxwell Garnett theory could break down due to evanescent fields in deep-subwavelength dielectric structures. By using two- and three-dimensional dielectric composite materials with inhomogeneities at a scale of λ/100, we show that local evanescent fields generally occur near the dielectric inhomogeneities. When tiny absorptive constituents are placed there, the absorption and transmission of the whole composite will show strong dependence on the positions of the absorptive constituents. The Maxwell Garnett theory fails to predict such position-dependent characteristics because it averages out the evanescent fields. By taking the distribution of the evanescent fields into consideration, we have made a correction to the Maxwell Garnett theory so that the position-dependent characteristics become predictable. We reveal not only the breakdown of the Maxwell Garnett theory, but also a unique phenomenon of “invisible” loss induced by the prohibition of electric fields at deep-subwavelength scales. We believe our work promises a route to control the macroscopic properties of composite materials without changing their composition, which is beyond the traditional Maxwell Garnett theory.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000848 (2021)
High-power, electronically controlled source of user-defined vortex and vector light beams based on a few-mode fiber amplifier
Di Lin, Joel Carpenter, Yutong Feng, Yongmin Jung, Shaif-ul Alam, and David J. Richardson
Optical angular momentum (OAM)-based structured light beams provide an additional degree of freedom for practical applications ranging from optical communication to laser-based material processing. Many techniques exist for generating such beams within laser sources and these primarily rely upon the use of specially designed optical components that limit laser power scaling and ready tunability of the topological charge and polarization of the output OAM beams. Here we show that some of these limitations can be overcome by employing a computer controlled reflective phase-only spatial light modulator to adaptively tailor the input (and subsequent output) beam wavefront and polarization in a few-mode fiber amplifier. In this way modal-coupling-induced beam distortion within the fiber amplifier can be mitigated and we are able to generate at will any desired supported spatial mode guided in the fiber, including conventional linearly polarized (LP) modes, scalar OAM modes, and cylindrical vector modes, at average powers >10 W and with a peak power of >11 kW. Our results pave the way to the realization of practical high-power structured laser sources with tunable chirality and polarization.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000856 (2021)
Quantum Optics
One-step implementation of Rydberg-antiblockade SWAP and controlled-SWAP gates with modified robustness
Jin-Lei Wu, Yan Wang, Jin-Xuan Han, Yu-Kun Feng, Shi-Lei Su, Yan Xia, Yongyuan Jiang, and Jie Song
The prevalent fashion of executing Rydberg-mediated two- and multi-qubit quantum gates in neutral atomic systems is to pump Rydberg excitations using multistep piecewise pulses in the Rydberg blockade regime. Here, we propose to synthesize a Λ-type Rydberg antiblockade (RAB) of two neutral atoms using periodic fields, which facilitates one-step implementations of SWAP and controlled-SWAP (CSWAP) gates with the same gate time. Besides, the RAB condition is modified so as to circumvent the sensitivity of RAB-based gates to infidelity factors, including atomic decay, motional dephasing, and interatomic distance deviation. Our work makes up the absence of one-step schemes of Rydberg-mediated SWAP and CSWAP gates and may pave a way to enhance the robustness of RAB-based gates.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000814 (2021)
Nonreciprocal transition between two nondegenerate energy levels
Xunwei Xu, Yanjun Zhao, Hui Wang, Aixi Chen, and Yu-Xi Liu
Stimulated emission and absorption are two fundamental processes of light–matter interaction, and the coefficients of the two processes should be equal. However, we will describe a generic method to realize the significant difference between the stimulated emission and absorption coefficients of two nondegenerate energy levels, which we refer to as a nonreciprocal transition. As a simple implementation, a cyclic three-level atom system, comprising two nondegenerate energy levels and one auxiliary energy level, is employed to show a nonreciprocal transition via a combination of synthetic magnetism and reservoir engineering. Moreover, a single-photon nonreciprocal transporter is proposed using two one-dimensional semi-infinite coupled-resonator waveguides connected by an atom with nonreciprocal transition effect. Our work opens up a route to design atom-mediated nonreciprocal devices in a wide range of physical systems.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000879 (2021)
Quantifying quantum coherence of optical cat states
Miao Zhang, Haijun Kang, Meihong Wang, Fengyi Xu, Xiaolong Su, and Kunchi Peng
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000887 (2021)
Silicon Photonics
Stimulated emission at 1.54 μm from erbium/oxygen-doped silicon-based light-emitting diodes
Jin Hong, Huimin Wen, Jiajing He, Jingquan Liu, Yaping Dan, Jens W. Tomm, Fangyu Yue, Junhao Chu, and Chungang Duan
Silicon-based light sources, including light-emitting diodes (LEDs) and laser diodes (LDs) for information transmission, are urgently needed for developing monolithic integrated silicon photonics. Silicon with erbium ions (Er3+) doped by ion implantation is considered a promising approach, but it suffers from an extremely low quantum efficiency. Here we report an electrically pumped superlinear emission at 1.54 μm from Er/O-doped silicon planar LEDs, which are produced by applying a new deep cooling process. Stimulated emission at room temperature is realized with a low threshold current of ~6 mA (~0.8 A/cm2). Time-resolved photoluminescence and photocurrent results have revealed the complex carrier transfer dynamics by relaxing electrons from the Si conduction band to the Er3+ ion. This picture differs from the frequently assumed energy transfer via electron–hole pair recombination of the silicon host. Moreover, the amplified emission from the LEDs is likely due to a quasi-continuous Er/O-related donor band created by the deep cooling technique. This work paves the way for fabricating superluminescent diodes or efficient LEDs at communication wavelengths based on rare-earth-doped silicon.
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000714 (2021)
High-speed and high-power germanium photodetector with a lateral silicon nitride waveguide
Xiao Hu, Dingyi Wu, Hongguang Zhang, Weizhong Li, Daigao Chen, Lei Wang, Xi Xiao, and Shaohua Yu
Up to now, the light coupling schemes of germanium-on-silicon photodetectors (Ge-on-Si PDs) could be divided into three main categories: (1) vertical (or normal-incidence) illumination, which can be from the top or back of the wafer/chip, and waveguide-integrated coupling including (2) butt coupling and (3) evanescent coupling. In evanescent coupling the input waveguide can be positioned on top, at the bottom, or lateral to the absorber. Here, to the best of our knowledge, we propose the first concept of Ge-on-Si PD with double lateral silicon nitride (Si3N4) waveguides, which can serve as a novel waveguide-integrated coupling configuration: double lateral coupling. The Ge-on-Si PD with double lateral Si3N4 waveguides features uniform optical field distribution in the Ge region, which is very beneficial to improving the operation speed for high input power. The proposed Ge-on-Si PD is comprehensively characterized by static and dynamic measurements. The typical internal responsivity is evaluated to be 0.52 A/W at an input power of 25 mW. The equivalent circuit model and theoretical 3 dB opto-electrical (OE) bandwidth investigation of Ge-on-Si PD with lateral coupling are implemented. Based on the small-signal (S21) radio-frequency measurements, under 4 mA photocurrent, a 60 GHz bandwidth operating at -3 V bias voltage is demonstrated. When the photocurrent is up to 12 mA, the 3 dB OE bandwidth still has 36 GHz. With 1 mA photocurrent, the 70, 80, 90, and 100 Gbit/s non-return-to-zero (NRZ) and 100, 120, 140, and 150 Gbit/s four-level pulse amplitude modulation clear openings of eye diagrams are experimentally obtained without utilizing any offline digital signal processing at the receiver side. In order to verify the high-power handling performance in high-speed data transmission, we investigate the eye diagram variations with the increase of photocurrents. The clear open electrical eye diagrams of 60 Gbit/s NRZ under 20 mA photocurrent are also obtained. Overall, the proposed lateral Si3N4 waveguide structure is flexibly extendable to a light coupling configuration of PDs, which makes it very attractive for developing high-performance silicon photonic integrated circuits in the future.
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000749 (2021)
First demonstration of an on-chip quadplexer for passive optical network systems | Spotlight on Optics
Dajian Liu, Long Zhang, Hexin Jiang, and Daoxin Dai
Photonics Research
  • Publication Date: Apr. 26, 2021
  • Vol.9 Issue, 5 05000757 (2021)
Surface Optics and Plasmonics
Plasmonic anapole states of active metamolecules
Gui-Ming Pan, Fang-Zhou Shu, Le Wang, Liping Shi, and Andrey B. Evlyukhin
Anapole states, accompanied by strong suppression of light scattering, have attracted extensive attention in recent years due to their supreme performance in enhancing both linear and nonlinear optical effects. Although both low- and high-order anapole states are observed in the dielectric particles with high refractive index, so far few studies have touched on the topic of plasmonic anapole states. Here we demonstrate theoretically and numerically that the ideal plasmonic anapole states (strong suppression of electric dipole scattering) can be achieved in metallic metamolecules via increasing the coupling strength between Cartesian electric dipole and toroidal dipole moments of the system. The increasing coupling is based on compensation of ohmic losses in a plasmon system by introducing of a gain material, the influence of which is well described by the extended coupled oscillator model. Due to suppression of dipole radiation losses, the excitation of anapole states in plasmonic systems can result in enhancement of the near fields in subwavelength spatial regions outside of nanoparticles. That is especially important for developments of nonlinear nanophotonic and plasmonic devices and active functional metamaterials, which provide facilities for strong light energy concentration at the nanoscale. Development of the considered anapole effect with increase of metamolecule components is discussed.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000822 (2021)
Ultralarge Rabi splitting and broadband strong coupling in a spherical hyperbolic metamaterial cavity
Ping Gu, Jing Chen, Siyu Chen, Chun Yang, Zuxing Zhang, Wei Du, Zhengdong Yan, Chaojun Tang, and Zhuo Chen
Strong coupling (SC) between two resonant plasmon modes can result in the formation of new hybrid modes exhibiting Rabi splitting with strong energy exchange at the nanoscale. However, normal Rabi splitting is often limited to ~50–320 meV due to the short lifetime of the plasmon mode. Here, we theoretically demonstrate a record Rabi splitting energy as large as 805 meV arising from the SC between the high-Q plasmonic whispering gallery mode and high-Q cavity plasmon resonance supported by a spherical hyperbolic metamaterial cavity, which consists of a dielectric nanosphere core wrapped in 7 alternating layers of silver/dielectric materials. In addition, the new hybrid modes formed by the SC are shown to exhibit an extralong lifetime of up to 71.9–81.6 fs, with the large electric field intensity enhancement at both the dielectric core and the dielectric layers. More importantly, the spectral ranges of SC can be tuned across an ultrabroad range from the visible to the near-IR by simply changing the dielectric core size. These findings may have potential applications in bright single-photon sources.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000829 (2021)
Ultrafast Optics
Extremely regular periodic surface structures in a large area efficiently induced on silicon by temporally shaped femtosecond laser | On the Cover
Yuchan Zhang, Qilin Jiang, Kaiqiang Cao, Tianqi Chen, Ke Cheng, Shian Zhang, Donghai Feng, Tianqing Jia, Zhenrong Sun, and Jianrong Qiu
Femtosecond laser-induced periodic surface structures (LIPSS) have several applications in surface structuring and functionalization. Three major challenges exist in the fabrication of regular and uniform LIPSS: enhancing the periodic energy deposition, reducing the residual heat, and avoiding the deposited debris. Herein, we fabricate an extremely regular low-spatial-frequency LIPSS (LSFL) on a silicon surface by a temporally shaped femtosecond laser. Based on a 4f configuration zero-dispersion pulse shaping system, a Fourier transform limit (FTL) pulse is shaped into a pulse train with varying intervals in the range of 0.25–16.2 ps using periodic π-phase step modulation. Under the irradiation of the shaped pulse with an interval of 16.2 ps, extremely regular LSFLs are efficiently fabricated on silicon. The scan velocity for fabricating regular LSFL is 2.3 times faster, while the LSFL depth is 2 times deeper, and the diffraction efficiency is 3 times higher than those of LSFL using the FTL pulse. The formation mechanisms of regular LSFL have been studied experimentally and theoretically. The results show that the temporally shaped pulse enhances the excitation of surface plasmon polaritons and the periodic energy deposition while reducing the residual thermal effects and avoiding the deposition of the ejected debris, eventually resulting in regular and deeper LSFL on the silicon surface.
Photonics Research
  • Publication Date: May. 07, 2021
  • Vol.9 Issue, 5 05000839 (2021)