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Moiré-driven electromagnetic responses and magic-angle in sandwiched hyperbolic metasurface

Yi Liu, Chunmei Ouyang, Quan Xu, Xiaoqiang Su, Quanlong Yang, Jiajun Ma, Yanfeng Li, Zhen tian, Jianqiang Gu, Liyuan Liu, Yunlong Shi, Jiaguang Han, and Weili Zhang

DOI: 10.1364/PRJ.462119 Received 27 Apr 2022; Accepted 01 Jul 2022; Posted 01 Jul 2022  View: PDF

Abstract: Recent moiré configurations provide a new platform for highly tunable and sensitive photonic responses, as their extremely enhanced light-matter interactions originate from the relative displacement or rotation angle in a staking bilayer or multilayer periodic array. However, pervious findings are mostly focused on atomically thin condensed matter, with limitations on the fabrication of multilayer structures and the control of rotation angles. Here, we design a novel moiré structure, which presents unprecedented capability in the manipulation of light-matter interactions. Based on the effective medium theory and S-parameter retrieval process, the rotation matrix is introduced into the dispersion relation to analyze the underlying physical mechanism, where the permittivity tensor transforms from a diagonal matrix to a fully populated one whereas the permeability tensor evolves from a unit matrix to a diagonal one and finally becomes fully filled, so that the electromagnetic responses change drastically as a result of stacking and rotation. Besides, the experimental and simulation results reveal hybridization of eigenmodes, drastic manipulation of surface states, and magic angle properties by controlling the mutual rotation angles between two isolated layers. Here, not only a more precisely controllable bilayer hyperbolic metasurface is introduced to moiré physics, the findings also open up a new avenue to realize flat bands at arbitrary frequencies, which shows great potential in active engineering of surface waves and designing multifunctional plasmonic devices.

A single optical microfiber enabled tactile sensor for simultaneous temperature and pressure measurement

Ni Yao, Xiaoyu Wang, Shuqi Ma, Xingda Song, Shan Wang, Zhangxing Shi, Jing Pan, Shipeng Wang, Jianliang Xiao, Haitao Liu, Longteng Yu, Yao Tang, Zhang Zhang, Xiong Li, Wei Fang, Lei Zhang, and Limin Tong

DOI: 10.1364/PRJ.461182 Received 14 Apr 2022; Accepted 29 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: The ability to sense heat and touch is essential for healthcare, robotics, and human-machine interfaces. By taking advantage of the engineerable waveguiding properties, we design and fabricate a flexible optical microfiber sensor for simultaneous temperature and pressure measurement based on theoretical calculation. The sensor exhibits a high temperature sensitivity of 1.2 nm/°C by measuring the shift of a high-order mode cutoff wavelength in the short-wavelength range. In the case of pressure sensing, the sensor shows a sensitivity of 4.5% per kilopascal with a fast temporal frequency response of 1000 Hz owing to the strong evanescent wave guided outside the microfiber. The crosstalk is negligible because the temperature and pressure signals are measured at different wavelengths based on different mechanisms. The properties of fast temporal response, high temperature and pressure sensitivity enable the sensor for real-time skin temperature and wrist pulse measurements, which is critical to the accurate analysis of pulse waveforms. We believe the sensor will have great potential in wearable optical devices ranging from healthcare to humanoid robots.

Experimental witnessing for entangled states with limited local measurements

Gaoyan Zhu, cheng-jie zhang, Kun Wang, Lei Xiao, and Peng Xue

DOI: 10.1364/PRJ.462212 Received 03 May 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: We experimentally demonstrate a method for detection of entanglement via construction of entanglement witnesses from a fixed limited set of local measurements (M). Such a method is suitable for a scenario where a full state tomography is not available, but the only resource is a limited set of M. We compare the performance of the method on pure two-qubit entangled states with that on mixed two-qubit entangled states, which emerge from the typical quantum noisy channels. The states we select are motivated by realistic experimental conditions and we confirm it works well for both cases. Furthermore, we also demonstrate both decomposable and indecomposable entanglement witnesses for higher-dimensional bipartite systems, which are constructed with the limited local measurements. Our experimental results show perfect validity of the method, indicate that even a limited set of local measurements can be used for quick entanglement detection and further provide a practical testbed for experiments with entanglement witnesses.

A scalable and highly-efficient approach for on-chip single photon source

Xingyu Chen, Rongbin Su, Jin Liu, Juntao Li, and Xue-Hua Wang

DOI: 10.1364/PRJ.462318 Received 26 Apr 2022; Accepted 28 Jun 2022; Posted 30 Jun 2022  View: PDF

Abstract: Integrated photonic circuits with quantum dots provide a promising route for scalable quantum chips with highly-efficient photonic sources. However, unpolarized emission photons in general sacrifice half efficiency when coupling to waveguide fundamental mode by cross polarization technique for suppressing the excitation laser, while the suspended waveguide photonics sources without polarization filters have poor scalability due to their mechanical fragility. Here, we propose a strategy for overcoming the challenge by coupling an elliptical Bragg resonator with waveguides on a solid-state base, featuring a near unity polarized efficiency and enabling on-chip pulsed resonant excitation without any polarization filters. We theoretically demonstrate that the proposed devices have outstanding performance of a single-photon source with 80% coupling efficiency into the on-chip planar waveguides and an ultra-small extinction ratio of 10^(-11), as well as robustness against quantum dot’s position deviation. Our design provides a promising method for scalable quantum chips with filter-free high-efficiency single-photon source.

Flat-field Superoscillation Metalens

Dingpeng Liao, Fengliang Dong, Kun Zhang, Yi Zhou, Gaofeng Liang, Zhihai Zhang, Zhongquan Wen, Zhengguo Shang, Gang Chen, Luru Dai, and Weiguo Chu

DOI: 10.1364/PRJ.462848 Received 02 May 2022; Accepted 28 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: Superoscillation metalenses have demonstrated promising prospects in breaking the theoretical diffraction limitations on the resolution of optical devices and systems. However, most reported superoscillation metalenses have a very small field-of-view of several tenth of a degree, which greatly limits their applications in imaging and microscopy. Meanwhile, it is of critical importance to achieve absolute high-resolution by increasing the numerical-aperture of optical devices and systems. Unfortunately, similar to the case in traditional optics, it is quite challenging to realize large field-of-view at high numerical-aperture, especially in the superoscillation regime. Up to date, no attempt has been made to achieve flat-field focusing in the superoscillation regime. Here, we demonstrate a high numerical-aperture superoscillation metalens with an entrance aperture stop, which is optimized for superoscillation performance with a comparatively large field-of-view. The proposed flat-field superoscillation metalens has an effective numerical-aperture as large as 0.89 and achieves superoscillation focusing within a field-of-view of 9°. Such a superoscillation metalens may offer a promising way toward superoscillation imaging and fast-scanning label-free far-field superoscillation microscopy.

An Ultrafast Miniaturized GaN-based Optoelectronic Proximity Sensor

Xiaoshuai An, Hongying Yang, YUMENG LUO, Zhiqin Chu, and Kwai Hei Li

DOI: 10.1364/PRJ.462933 Received 05 May 2022; Accepted 28 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: In this work, a novel ultrafast optoelectronic proximity sensor based on a submillimeter-sized GaN monolithic chip is presented. Fabricated through wafer-scale microfabrication processes, the on-chip units adopting identical InGaN/GaN diode structures can function as emitters and receivers. The optoelectronic properties of the on-chip units are thoroughly investigated and the ability of the receivers to respond to changes in light intensity from the emitter is verified, revealing that the sensor is suitable for operation in reflection mode. Through a series of dynamic measurements, the sensor is highly sensitive to object movement at sub-centimeter distances with high repeatability. The sensor exhibits ultrafast microsecond response, and its real-time monitoring capability is also demonstrated by applying it to detect slight motions of moving objects at different frequencies, including the human heart rate, the vibration of the rotary pump, the oscillation of the speaker diaphragm, and the speed of the rotating disk. The compact and elegant integration scheme presented herein opens a new avenue for realizing a chip-scale proximity sensing device, making it a promising candidate for widespread practical applications.

Heterogeneous integrated quantum-dot emitters efficiently driven by a quasi-BIC-supporting dielectric nanoresonator

li liu, Ruxue Wang, Xuyi Zhao, wen fu yu, Yi Jin, Qian Gong, and Aimin Wu

DOI: 10.1364/PRJ.462474 Received 29 Apr 2022; Accepted 27 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: Bound states in the continuum (BICs) can make subwavelength dielectric resonators sustain low radiation leakage, paving a new way to minimize the device size, enhance the photoluminescence, and even realize lasing. Here, we present a quasi-BIC-supporting GaAs nanodisk with embedded InAs quantum dots on a silicon substrate by heterogeneous integration, avoiding complex multilayered construction and subsequent mismatch and defects. The emitters are grown inside the nanodisk to match the mode field distribution for strong light-matter interaction. One fabricated sample demonstrates a photoluminescence peak sustaining a quality factor up to 68 enhanced by the quasi-BIC, and the emitting effect can be further promoted by improving the epi-layer quality and optimizing the layer-transferring process in the fabrication. This work provides a promising solution to build an ultra-compact optical source to be integrated on a silicon photonic chip for high-density integration.

Single-/fused-band Dual-mode Mid-infrared Imaging with Colloidal Quantum dot Triple-junction

Shuo Zhang, Ge Mu, jie CAO, Yuning Luo, Qun Hao, Menglu Chen, Yimei Tan, Pengfei Zhao, and Xin Tang

DOI: 10.1364/PRJ.458351 Received 15 Mar 2022; Accepted 27 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: Image data acquired with fused multispectral information can be used for effective identification and navigation owing to additional information beyond human vision, including thermal distribution, night vision and molecular composition. However, construction of photodetectors with such capabilities are hindered by the structural complexity arising from the integration of multiple semiconductor junctions with distinct energy gaps and lattice constants. In this work, we develop a colloidal quantum-dot dual-mode detector capable of detecting, separating and fusing photons from various wavelength ranges. Using three vertically stacked colloidal quantum-dot homojunctions with alternating polarity, single-band short-wave infrared imaging and fused-band imaging (short-wave and mid-wave infrared) can be achieved with the same detector by controlling bias polarity and magnitude. The dual-mode detectors show detectivity up to 8×10¹⁰ Jones @ fused-band mode and 3.1 ×10¹¹ Jones @ single-band mode, respectively. Without post processing or image algorithms, the dual-mode detectors could provide both night vision and thermal information-enhanced night vision imaging capability. The operation mode can be changed at high frequency up to 1.7 MHz, making it possible to achieve simultaneously dual-mode imaging and remote temperature sensing.

Flexible manipulation of plasmon dephasing time via adjustable Fano asymmetric dimer

Yang Xu, Yulu Qin, peng lang, Boyu Ji, Xiaowei Song, and Jingquan Lin

DOI: 10.1364/PRJ.460638 Received 07 Apr 2022; Accepted 27 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: It is highly desirable to flexibly and actively manipulate the dephasing time of a plasmon in many potential applications, however, it remains a challenge at present. In this work, with femtosecond time-resolved photoemission electron microscopy, we have experimentally demonstrated that Fano resonance mode in the asymmetric nanorod dimer can greatly extend the dephasing time of a femtosecond plasmon, whereas the non-Fano resonance results in a smaller dephasing time due to the large radiative damping, and flexible manipulation of the dephasing time can be realized by adjusting one of the nanorods in the Fano asymmetric dimer. Interestingly, it is found that plasmon resonance wavelengths both exhibit red-shifted as the length of the upper or lower nanorods increases individually, but the dephasing time varies differently. Furthermore, it also indicates that the dephasing time can also be prolonged with a smaller ascending rate by increasing the length of both the nanorods simultaneously while keeping the dimer asymmetry. Meanwhile, the roles of radiative damping and nonradiative damping in dephasing time are unveiled in the process of nanorod length variation. These results are well supported by numerical simulations and calculations.

High-Efficiency InGaN Red Micro-LEDs for Visible Light Communication

Hao-chung Kuo, Yu-Ming Huang, Chun-Yen Peng, WEN-CHIEN MIAO, Hsin Chiang, Tzu-Yi Lee, Yun-Han chang, Konthoujam James Singh, Ray-Hua Horng, Chi-Wai Chow, Chien-Chung Lin, Kazuhiro Ohkawa, Shih-Chen Chen, and Daisuke Iida

DOI: 10.1364/PRJ.462050 Received 22 Apr 2022; Accepted 27 Jun 2022; Posted 28 Jun 2022  View: PDF

Abstract: In this study, we present the high-efficiency InGaN red micro-LEDs were fabricated by the incorporation of superlattice (SL) structure, atomic layer deposition (ALD) passivation and distributed Bragg reflector (DBR), exhibiting the maximum external quantum efficiency (EQE) of 5.02% with a low efficiency droop corresponding to the injection current density of 112 A/cm2. The fast carrier dynamics in InGaN has been characterized by using time-resolved photoluminescence (TRPL), which is correlated to the high modulation bandwidth of 271 MHz achieved by 6х 25 µm-sized micro-LED array with the data transmission rate of 350 Mbit/s at the high injection current density of 2000 A/cm2. It holds great promise for the full-color micro-display as well as high-speed visible light communication (VLC) applications based on monolithic InGaN micro-LED technologies.

High-speed long-distance visible light communication based on multicolor series connection micro-LEDs and wavelength division multiplexing

Shijie Zhu, Pengjiang Qiu, Xinyi Shan, Runze Lin, zhou wang, Zuxin Jin, Xugao Cui, G.Q. Zhang, and Pengfei Tian

DOI: 10.1364/PRJ.459531 Received 28 Mar 2022; Accepted 23 Jun 2022; Posted 24 Jun 2022  View: PDF

Abstract: Multicolor series connection micro-LED arrays with emission wavelengths of violet, blue, green, and yellow were fabricated, and their optoelectronic properties and communication performances were investigated. The designed series connection micro-LED array exhibited the multiple mWs light output power while mostly keeping a slightly reduced modulation bandwidth, thus enabling a higher signal-to-noise ratio (SNR) compared to a single pixel and showing superior performance in the field of long-distance visible light communication (VLC). The achievable data rates of 400, 451, 509, and 556 nm micro-LED arrays using bit/power loading OFDM were 5.71, 4.86, 4.39, and 0.82 Gbps, respectively. The aggregate data rate of 15.78 Gbps was achieved for the proof-of-concept wavelength division multiplexing (WDM) system under a transmission distance of 13 m, which was the best data rate distance product performance for the LED-based VLC to the best of our knowledge. In addition, the long-distance VLC based on yellow micro-LED was also demonstrated for the first time in this work.

Observation of parity-time symmetry in time-division multiplexing pulsed optoelectronic oscillators within a single resonator

Yitang Dai, Hao Ding, Qizhuang Cen, Kun Xu, and Ming Li

DOI: 10.1364/PRJ.461637 Received 29 Apr 2022; Accepted 23 Jun 2022; Posted 24 Jun 2022  View: PDF

Abstract: In recent years, the study of PT-symmetry in optoelectronic systems has attracted increasing attention, due to its potential applications in lasers, sensors, topological networks, and other fields. We propose a time-division multiplexed (TDM) pulsed optoelectronic oscillator (OEO) to study the dynamics of the PT-symmetry system. We use two microwave pulses to realize the PT-symmetry in a single spatial resonator based on the temporal degrees of freedom. We can control the gain and loss of the microwave pulses and the coupling coefficient between them. We first demonstrate the phase diagram from PT-broken to PT-symmetry in the OEO system. We theoretically prove the perturbation of coupling-induced phase shift larger than (2π)·10¯² causes the disappearance of the PT-symmetry. The perturbation is less than (2π)·0.5×10¯² in our experiment, thus the phase transition of PT-symmetry is observed. In addition, multi pairs of PT-symmetry pulses indicate that pulsed OEO could be used to implement complex non-Hermi Hamilton systems. Therefore, it is confirmed that pulsed OEO is an excellent platform to explore the dynamics of PT-symmetry and other non-Hermitian Hamilton systems.

Mid-infrared broadband optical frequency comb generated in MgF₂ resonators

Wei Wu, Qibing Sun, Yi Wang, Yu Yang, Xianshun Ming, Lei Shi, Keyi Wang, Wei Zhao, and Leiran Wang

DOI: 10.1364/PRJ.459478 Received 25 Mar 2022; Accepted 21 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: Microresonator-based optical frequency combs are broadband light sources consisting of equally spaced and coherent narrow lines, which have an extremely promising for novel approaches to molecular spectroscopy and sensing in the mid-infrared (MIR) spectral region. There are still great challenges for exploring how to improve materials for microresonator fabrication, extend spectral bandwidth of parametric combs, as well as realize fully stabilized soliton MIR frequency combs. Here, we present an effective scheme for broadband MIR optical frequency comb generation in a MgF₂ crystalline microresonator pumped by the continuous tunable quantum cascade laser. The spectral evolution dynamics of the MIR Kerr frequency comb is numerically investigated, revealing the formation mechanism of microresonator soliton comb via scanning the pump-laser detuning. We also experimentally implement the modulation instability state MIR frequency comb generation in MgF₂ resonators coverage from 3380 nm to 7760 nm. This work proceeds microresonator-based comb technology toward a miniaturization MIR spectroscopic device that provide potential opportunities in many fields such as fundamental physics and metrology.

Single-shot ultrafast multiplexed coherent diffraction imaging

Mingying Sun, Yingming Xu, Xingchen Pan, Cheng Liu, Jianqiang Zhu, and Wenfeng Liu

DOI: 10.1364/PRJ.460948 Received 13 Apr 2022; Accepted 21 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: Classic interferometry was commonly adopted to realize ultrafast phase imaging using pulsed lasers, however the reference beam required makes the optical structure of imaging system very complex, and high temporal resolution was reached by sacrificing the spatial resolution. This study presents a kind of Single-shot Ultrafast Multiplexed Coherent Diffraction Imaging (SUM-CDI) technique to realize ultrafast phase imaging with both high spatial and temporal resolutions using simple optical setup, and temporal resolution of nanosecond to femtosecond can be realized by using lasers of different pulse durations. This technique applied multiplexed algorithm to avoid the data division in space domain or frequency domain and greatly improves the spatial resolution. The outstanding advantages of this proposed technique on both simple optical structure and high image quality were demonstrated by imaging the generation and evaluation of laser induced damage and accompanying phenomenon of laser filament and shock wave at spatial resolution higher than 6.96 µm and temporal resolution higher than 10ns.

Imaging Ultrafast Evolution of Subwavelength-sized Topography Using Single-probe Structured Illumination Microscopy

Jie Xu, Changjun Min, Yuquan Zhang, Jielei Ni, Gengwei Cao, Qianyi Wei, Jianjun Yang, and Xiaocong Yuan

DOI: 10.1364/PRJ.458613 Received 17 Mar 2022; Accepted 21 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: Imaging ultrafast process in femtosecond (fs) laser-material interactions such as fs laser ablation is very important to understand the physical mechanisms involved. To achieve this goal with both high resolutions in spatial and temporal domain, combination of optical pump-probe microscopy and structured illumination microscopy (SIM) could be a promising approach, but suffering from the multiple-frame method with phase shift that is inapplicable to irreversible ultrafast process like ablation. Here, we propose and build a wide-field single-probe structured illumination microscopy (SPSIM) to image the ultrafast three-dimensional topography evolution induced by fs laser, where only single imaging frame with single structured probe pulse is required for topography reconstruction, benefiting from the Fourier transform profilometry (FTP). The second harmonic of fs laser is used as the structured probe light to improve the spatial lateral resolution into the subwavelength region of ~478 nm, and the spatial axial and temporal resolutions are estimated to be ~22 nm and ~256 fs, respectively. With the SPSIM, we successfully image the ultrafast topography evolution of silicon wafer surface impacted by single and multiple fs pulses. Variable formation and evolution of the laser induced periodic surface structures (LIPSS) during the ultrashort time have been visualized and analyzed. We believe that the SPSIM will be a significant approach for revealing and understanding various ultrafast dynamics especially in fs laser ablation and material science.

Realizing fast temperature measurement and simulating Maxwell demon with nearly nondestructive detection in cold atoms

Yuan Sun, Xin Wang, and Liang Liu

DOI: 10.1364/PRJ.453159 Received 05 Jan 2022; Accepted 18 Jun 2022; Posted 22 Jun 2022  View: PDF

Abstract: Optical detection and manipulation of the thermal properties is an essential subject of cold atoms in the quantum era. For laser cooled alkali atoms, we have experimentally realized deterministic temperature measurement with time cost below 1 millisecond and effective filtering of colder atoms with temperature less than 1 micro-Kelvin, with the help of nearly nondestructive detection. The quick temperature measurement is accomplished by carefully resolving the diffusion dynamics of atoms with the information provided by a single probe laser pulse in the form of bucket detection, while suppressing the amplitude and phase noises of probe laser. The separation of colder atoms is attainable as the velocity differences of atoms translate into nontrivial position differences, when the diffusion sustains for a few tens of milliseconds. In particular, these efforts are based on a labeling process that distinguishes the cold atoms under study from the others by specific internal states, while the nearly nondestructive detection is implemented via driving a cycling transition with continuous optical pulses. Moreover, such a position-dependent labeling process can be further modified to become velocity-dependent, with which we have demonstrated a Maxwell demon type operation on cold atoms, as Maxwell demon's intricate abilities can be understood as measuring the velocity of an individual particle and then performing feedback according to a straightforward dichotomy of the velocity value.

67GHz light-trapping-structure germanium photodetector supports 240 Gbit/s PAM-4 Transmission

Daigao Chen, Hongguang Zhang, min liu, Yuguang Zhang, dingyi Wu, peiqi zhou, siyao chang, Lei Wang, Xi Xiao, and Xiao Hu

DOI: 10.1364/PRJ.455291 Received 04 Feb 2022; Accepted 16 Jun 2022; Posted 16 Jun 2022  View: PDF

Abstract: A light-trapping-structure vertical Ge PD is demonstrated. In the scheme, 3-μm-radius circular Ge is fabricated as the light-trapping structure, and light could be constrained in the circular absorption area. The proposed PD shows high opto-electronic bandwidth and high responsivity simultaneously. The measured 3-dB bandwidth is around 67GHz, and the responsivity of the PD is around 1.05 A/W when the wavelength shorter than 1560 nm. At 1580 nm, the responsivity could still be maintained above 0.78 A/W. Low dark current of 6.4 nA is also achieved with 2-V-bias. Base on this PD, clear eye diagrams of 100 GBaud PAM-4 is measured. 240 Gb/s PAM-4 signal transmission is achieved and the back to back (BTB) BER is 1.6×10-2. After 1km and 2-km fiber transmission, the highest bit rate is 0 Gb/s and 220 Gb/s separately.

Multi-channel multiplexing quantum teleportation based on the entangled sideband modes

yimiao Wu, Qingwei Wang, Long Tian, Xiaoli Zhang, Jiawei Wang, Shaoping shi, Yajun Wang, and Yaohui Zheng

DOI: 10.1364/PRJ.459889 Received 30 Mar 2022; Accepted 14 Jun 2022; Posted 16 Jun 2022  View: PDF

Abstract: Quantum teleportation is a key primitive across a number of quantum information tasks, and represents a fundamental ingredient for many quantum technologies. Channel capacity, other than the fidelity, becomesanother focus of quantum communication. Here, we present a 5-channel multiplexing continuous variable quantum teleportation protocol in the optical frequency comb system, exploiting five-order entangled sidebandmodes. In virtue of resonant electro-optical modulation (EOM) that is specifically designed, the fidelities of five channels are greater than 0.78, which are superior to the no-cloning limit 2/3. This work provides a feasiblescheme to implement efficient quantum information processing.

A Novel Three-Terminal Germanium-on-Silicon Avalanche Photodiode with Extended p-charge layer for Dark Current Reduction

Xiaobin Liu, Xuetong Li, Yingzhi Li, Yuxuan Li, Zihao Zhi, Min Tao, Baisong Chen, Lanxuan Zhang, Pengfei Guo, Guo-Qiang Lo, xueyan li, Fengli Gao, Bonan Kang, and Junfeng Song

DOI: 10.1364/PRJ.452004 Received 22 Dec 2021; Accepted 13 Jun 2022; Posted 13 Jun 2022  View: PDF

Abstract: Germanium-on-Silicon (Ge-on-Si) avalanche photodiode (APD) are widely used in near-infrared detection, laser ranging, free space communication, quantum communication and other fields. However, the existence of lattice defects at Ge/Si interface causes a high dark current in Ge-on-Si APD, degrading the device sensitivity and also increasing energy consumption in integrated circuits. In this work, we propose a novel surface illuminated Ge-on-Si APD architecture with three-terminal. Besides two electrodes on Si substrates, a third electrode is designed for Ge to regulate the control current and bandwidth, achieving multiple outputs of a single device and reducing the dark current of the device. When the voltage on Ge is -27.5 V, the proposed device achieves a dark current of 10 nA, responsivity of 2.48 A/W at -30 dBm input laser power at 1550 nm, and bandwidth of 150 MHz. The low dark current and improved responsivity can meet the requirements of autonomous driving and other applications demanding weak light detection. © 2021 Optical Society of America

Optical orbital angular momentum multiplexing communication via inverse-designed multi-phase plane light conversion

Xiaocong Yuan, Juncheng Fang, Zhenwei Xie, Ting Lei, Jinpei Li, Youpeng Xie, Aru Kong, and Chuxuan Lin

DOI: 10.1364/PRJ.458474 Received 15 Mar 2022; Accepted 12 Jun 2022; Posted 13 Jun 2022  View: PDF

Abstract: Multiplexing and demultiplexing of optical orbital angular momentum (OAM) are critical operations in mode-division multiplexing communications. Traditional Dammann gratings, spiral phase planes, and optical geometric transformations are regarded as convenient methods for OAM mode (de)multiplexing. However, crosstalk between the different modes and the difficulty of mode multiplexing greatly limit their application to mode division multiplexing communications. Here, using a set of inverse-designed phase planes, we demonstrate an OAM (de)multiplexer based on multi-phase plane light conversion that can enable perfect OAM multiplexing communication. The sorted patterns are Gaussian-like and can be coupled easily into single-mode fiber arrays. The inputs from the fiber array are turned into coaxial OAM modes after the phase planes. OAM mode crosstalk generated by the multiplexer is less than −20 dB, with insertion loss of less than −2.6 dB. OAM modes are sorted by the demultiplexer with mode crosstalk below −10 dB, and the sorting results spacing matches the fiber array. OAM modes carrying 10 Gbit/s on-off keying signals were transmitted in a 5 km few-mode fiber. The measured bit error rate curves have power penalties of less than 10 dB. The proposed configuration is highly efficient and convenient, and will be beneficial for potential applications in quantum information, information processing, and optical communications .

Repetition rate locked single soliton microcomb generation via rapid frequency sweep and sideband thermal cooling

Tian Jiang, runlin miao, Ke Yin, chenxi zhang, Xin Zheng, and Xiang'ai Cheng

DOI: 10.1364/PRJ.458472 Received 15 Mar 2022; Accepted 12 Jun 2022; Posted 13 Jun 2022  View: PDF

Abstract: Dissipative Kerr solitons (DKSs) with mode-locked pulse trains in high-Q optical microresonators possess low-noise and broadband parallelized comb lines, having already found plentiful cutting-edge applications. However, thermal bistability and thermal noise caused by the high microresonator power and large temperature exchange between microresonator and environment would prevent soliton microcombs formation and deteriorate the phase and frequency noise. Here, a novel method that combines rapid frequency sweep with optical sideband thermal cooling is presented, providing a simple and reliable way to get into the single soliton state. Meanwhile, it is shown that the phase and frequency noise of the generated soliton is greatly reduced. Moreover, by closing the locking loop, an in-loop repetition rate fractional instability of 5.5×10-15 at 1 s integration time and a triangular linear repetition rate sweep with 2.5 MHz could be realized. This demonstration offers an alternative knob for the generation, locking and tuning of a soliton microcomb, paving the way for the application of single soliton microcomb in low phase noise microwave generation and laser ranging.

High-Performance Ag2BiI5 Pb-Free Perovskite Photodetector

Zihao Shuang, Hai Zhou, Dingjun Wu, Xuhui Zhang, Boao Xiao, Jinxia Duan, and Hao Wang

DOI: 10.1364/PRJ.452883 Received 15 Mar 2022; Accepted 09 Jun 2022; Posted 10 Jun 2022  View: PDF

Abstract: Recently, lead-free all-inorganic halide perovskites have attracted great interest because they not only behave merits of the halide perovskite family, but also are non-toxic and good stable. However, the commercialization of the lead-free all-inorganic perovskites are restricted by their relatively low performances which are usually caused by the fabrication methods and the undesirable interfaces between the active layer and the carrier transport layers. Herein, we demonstrate a solution-processed route for the high-quality Ag2BiI5 lead-free perovskite film by adopting ideal electron transport material SnO2 and carbon electrode. By optimizing the fabrication process and tailoring the composition of the perovskite active layer, a high-performance photodetector (PD) with an FTO/SnO2/Ag2BiI5/Carbon structure PD is first fabricated, which shows good self-powered performance with a detectivity of as high as 5.3 × 1012 Jones and a linear dynamic range of up to 138 dB, which are better than the reported Pb-free perovskite PDs and comparable to the high-performance Pb-based perovskite PDs. In addition, our unpackaged PDs showed good light-, thermal- and storage stability in the air. Our results provided a special route for the development of lead-free perovskite devices in an environmentally friendly field.

Tailoring Magnetic Dipole Emission by Significantly Coupling to Magnetic Plasmonic Anapole States

Gui-Ming Pan, Li-Feng Yang, Fangzhou Shu, Yanlong Meng, Zhi Hong, and Zhong-Jian Yang

DOI: 10.1364/PRJ.461415 Received 19 Apr 2022; Accepted 09 Jun 2022; Posted 10 Jun 2022  View: PDF

Abstract: The interactions between magnetic quantum emitters and the local electromagnetic environment is a promising method to manipulate the spontaneous emission. However, it is severely limited by the weak interactions be-tween the magnetic component of light and the natural materials. Herein, we demonstrate that the special type of anapole states associated with the “one-fold” electric toroidal dipole moment can be excited by the strong interaction between magnetic dipole emitters and the silver oligomers. Based on magnetic anapole states, the radiative power is effectively suppressed without weakening the magnetic dipole transition in a silver nonamer, which physics provides an ideal playground for the study of non-radiative transitions. These findings not only add new members to the anapole family, but also open a door for the development of new high-performance magnetic-dipole based optoelectronic devices.

Multi-wavelength Injection Locked Comb Laser Epitaxially Grown on Silicon

jiajian chen, Wei Qi, Jiale Qin, Jing-Zhi Huang, bo yang, Zihao Wang, Ting Wang, and Jianjun Zhang

DOI: 10.1364/PRJ.455165 Received 28 Jan 2022; Accepted 08 Jun 2022; Posted 08 Jun 2022  View: PDF

Abstract: Quantum dot lasers on silicon have gained significant interest over the past decade due to their great potential as on-chip silicon photonic light source. Here, we demonstrate multi-wavelength injection locking of InAs/GaAs quantum dot fabry-perot (FP) lasers on silicon by optical self-injection via an external cavity. FP lasers by direct epitaxial growth on silicon substrates are self-injection locked as flat-top comb source with tunable free spectral range (FSR) from approximately 50 GHz to 750 GHz. The number of locked laser modes can be fully adjusted from single peak to multiple peaks by tuning wavelength-dependent phase and mode spacing of back-injected light through Lyot filter. The multi-wavelength injection-locked laser modes exhibit average optical linewidth of ~ 20 KHz, which are narrowed by approximately three orders of magnitude from their free-running condition. Furthermore, the multi-wavelength self-injection locking via external cavity exhibits flat-top optical spectral properties with approximately 30 stably locked channels under stable operation over time, where the frequency detuning is less than 700 MHz within 40 minutes. The reported results emphasize the great potential of multi-wavelength injection-locked lasers as tunable on-chip multi-wavelength light source.

Spectral Compressive Imaging Reconstruction Using Convolution and Contextual Transformer

Lishun Wang, Zongliang Wu, Yong Zhong, and Xin Yuan

DOI: 10.1364/PRJ.458231 Received 14 Mar 2022; Accepted 08 Jun 2022; Posted 08 Jun 2022  View: PDF

Abstract: Spectral compressive imaging (SCI) is able to encode the high-dimensional hyperspectral image into a two-dimensional (2D) measurement and then use algorithms to reconstruct the spatio-spectral data-cube. At present, the main bottleneck of SCI is the reconstruction algorithm, and state-of-the-art (SOTA) reconstruction methods generally face the problem of long reconstruction time and/or poor detail recovery. In this paper, we propose a novel hybrid network module, namely CCoT (Convolution and Contextual Transformer) block, which can simultaneously acquire the inductive bias ability of convolution and the powerful modeling ability of transformer, which is conducive to improving the quality of reconstruction to restore fine details. We integrate the proposed CCoT block into the physics-driven deep unfolding framework based on the generalized alternating projection algorithm, and further propose the GAP-CCoT network. Finally, we apply the GAP-CCoT algorithm into SCI reconstruction. Through experiments with extensive synthetic data and real data, our proposed model achieves higher reconstruction quality ($>$2dB in PSNR on simulated benchmark datasets) and shorter running time than existing SOTA algorithms by a large margin. The code and models are publicly available at ~\url{https://github.com/ucaswangls/GAP-CCoT}.

Unidirectional Bound States in the Continuum in Weyl Semimetal Nanostructures

chen zhao, Guangwei Hu, Yang Chen, qing zhang, Yongzhe Zhang, and Chengwei Qiu

DOI: 10.1364/PRJ.459383 Received 08 Apr 2022; Accepted 07 Jun 2022; Posted 08 Jun 2022  View: PDF

Abstract: Bound states in the continuum (BICs) are recently studied, which enables the perfect localization of light and enhances light-matter interactions despite that systems are optically open. It has found applications in numerous areas, including optical nonlinearity, light emitter and nano-sensor. However, its unidirectional nature in nonreciprocal devices is still elusive because such trapping states are easily destroyed when the symmetry of an optical system is broken. Herein, for the first time, we propose nonreciprocal and dynamically tunable BICs for unidirectional confinement of light and symmetry-protected BIC at Γ-point by introducing antiparallel-magnetism into the optical system. We demonstrate that such BICs could be achieved using topological magnetic Weyl semimetals near zero-index frequency without any structural asymmetry and largely tunable via modifying the Fermi level. Our results reveal a fundamentally new regime of extreme light manipulation and interaction with emerging quantum materials for various practical applications.

A bright single-photon source at telecom band by deterministically coupling single quantum dots to a hybrid circular Bragg resonator

Shi Xu, Yuming Wei, Rongbin Su, Xueshi Li, Peinian Huang, Shunfa Liu, Xiaoying Huang, Ying Yu, Jin Liu, and Xue-Hua Wang

DOI: 10.1364/PRJ.461034 Received 20 Apr 2022; Accepted 05 Jun 2022; Posted 06 Jun 2022  View: PDF

Abstract: High-performance solid-state quantum sources at telecom band are of paramount importance for long-distance quantum communications and quantum internet by taking advantages of the low-loss optical fiber network. Here, we demonstrate a bright telecom-wavelength single-photon source based on In(Ga)As/GaAs quantum dots (QDs) deterministically coupled to hybrid circular Bragg resonators (h-CBRs) by using a wide-field fluorescence imaging technique. The QD emissions are redshifted toward to the telecom O-band by using an ultra-low InAs growth rate and a InGaAs strain reducing layer. The single-photon emission under both continuous wave (CW) and pulsed operations are demonstrated, showing high-brightness with account rate of 1.14 MHz and 0.34 MHz under saturation powers and better single-photon purities of g^((2) ) (0)=0.11±0.02 (CW) and g^((2)) (0)=0.087±0.003 (pulsed) at low excitation powers. A Purcell factor of 4.2 with a collection efficiency of 10.6±0.5% at the first lens are extracted, suggesting the efficient coupling between the QD and the h-CBR. Our work contributes to the developments of highly-efficient single-photon sources at telecom band for fiber-based quantum communication and future distributed quantum network.

Toward high bandwidth yellow-green micro-LEDs utilizing nanoporous distributed Bragg reflectors for visible light communication

Hao-chung Kuo, WEI-TA HUANG, Chun-Yen Peng, Hsin Chiang, Yu-Ming Huang, Konthoujam James Singh, Wei-Bin Lee, Chi-Wai Chow, and Shih-Chen Chen

DOI: 10.1364/PRJ.462519 Received 28 Apr 2022; Accepted 05 Jun 2022; Posted 06 Jun 2022  View: PDF

Abstract: In this study, the high -3dB bandwidth yellow-green InGaN/GaN micro-LEDs grown on polar c-plane GaN substrates have been realized by using the nanoporous distributed Bragg reflectors that can increase the light extraction efficiency and serve as strain-relaxed buffers for mitigating the quantum-confined Stark effect, resulting the improved external quantum efficiency. Moreover, the atomic layer deposition technology was introduced for surface defect passivation thereby reducing the leakage current. As a result, the device exhibits the highest -3dB bandwidth up to 442 MHz, and a data transmission rate of 800 Mbit/s at the current density of 2.5 kA/cm2 with on-off keying modulation, which holds great promise for future high-speed visible light communication applications.

Tunable mechanical-mode coupling based on nanobeam-double optomechanical cavities

Qiancheng Xu, Kaiyu Cui, ning wu, Xue Feng, Fang Liu, Wei Zhang, and Yidong Huang

DOI: 10.1364/PRJ.447711 Received 10 Nov 2021; Accepted 02 Jun 2022; Posted 03 Jun 2022  View: PDF

Abstract: Tunable coupled mechanical resonators with nonequilibrium dynamic phenomena have attracted considerable attention in quantum simulations, quantum computations, and non-Hermitian systems. In this study, we proposed tunable mechanical-mode coupling based on nanobeam-double optomechanical cavities. The excited optical mode interacts with both the symmetric and antisymmetric mechanical supermodes and makes their couple with each other at a frequency of approximately 4.96 GHz. The mechanical-mode coupling was tuned through both optical spring and gain effects with observed reduced coupled frequency difference in non-Hermitian parameter space. These results benefit to the research on the microscopic mechanical parity-time symmetry for topology and on-chip high-sensitivity sensors.

Silicon Photonics for High-Capacity Data Communications

Yaocheng Shi, Yong Zhang, Yating Wan, Yuguang Zhang, Xiao Hu, Xi Xiao, Hongnan Xu, Yu Yu, long zhang, and Bingcheng Pan

DOI: 10.1364/PRJ.456772 Received 25 Feb 2022; Accepted 02 Jun 2022; Posted 03 Jun 2022  View: PDF

Abstract: In recent years, optical modulators, photodetectors, (de)multiplexers and hybrid integrated lasers based on silicon optical platform have been verified. The performance of some devices even surpasses the traditional III-V and PLC platforms, laying the foundation for large-scale photonic integration. Silicon photonic technology can overcome the limitations of traditional transceiver technology in high-speed transmission networks to support faster interconnection between data centers. In this article, we will review recent progresses for silicon PICs. The first part gives an overview of recent achievements in silicon PICs. The second part introduces the silicon photonic building blocks, including Low-loss waveguides, passive devices, modulators, photodetectors, hybrid integrated lasers etc. In the third part, the recent progress on high-capacity silicon photonic transceivers is discussed. In the fourth part, we give a review about high-capacity silicon photonic networks on-chip.

Ultrathin oxide controlled photocurrent generation through a metal-insulator-semiconductor heterojunction

Ning Liu, Xiao-hong Yan, Long Gao, Christophe Silien, SERGUEI BELOCHAPKINE, and Hong Wei

DOI: 10.1364/PRJ.450399 Received 03 Dec 2021; Accepted 24 May 2022; Posted 24 May 2022  View: PDF

Abstract: Recent advances in nanoscale lasers, amplifiers and nonlinear optical converters have demonstrated the unprecedented potentials of metal-insulator-semiconductor (MIS) structures as a versatile platform to realize integrated photonics at the nanoscale. While the electric field enhancement and confinement have been discussed intensively in MIS based plasmonic structures, little is known on the carrier redistribution across the heterojunction nor the photocurrent transport through the oxide. Herein, we investigate the photo-generated charge transport through single CdSe microbelt-Al2O3-Ag heterojunction with the oxide thickness varying from 3 nm to 5 nm. Combining photocurrent measurements with finite element simulations on electron (hole) redistribution across the heterojunction, we are able to explain the loss compensation observed in hybrid plasmonic waveguides at substantially reduced pump intensity based on the MIS geometry compared to its photonic counterpart. We also demonstrate that the MIS configuration offers a low-dark-current photodetection scheme, which can be further exploited for photodetection applications.

Stimulated generation of deterministic platicon frequency microcombs

Hao Liu, Shu-Wei Huang, Jinghui Yang, Wenting Wang, Mingbin Yu, Dim-Lee Kwong, Pierre Colman, and Chee Wei Wong

DOI: 10.1364/PRJ.459403 Received 28 Mar 2022; Accepted 15 May 2022; Posted 16 May 2022  View: PDF

Abstract: Dissipative Kerr soliton generation in chip-scale nonlinear resonators has recently observed remarkable advances, spanning from massively-parallel communications, self-referenced oscillators, to dual-comb spectroscopy. Often working in the anomalous dispersion regime, unique driving protocols and dispersion in these nonlinear resonators have been examined to achieve the soliton and soliton-like temporal pulse shapes and coherent frequency comb generation. The normal dispersion regime provides a complementary approach to bridge the nonlinear dynamical studies, including the possibility of square pulse formation with flat-top plateaus, or platicons. Here we report observations of square pulse formation in chip-scale frequency combs, through stimulated pumping at one free-spectral-range and in silicon nitride rings with +55 fs2/mm normal group velocity dispersion. Tuning of the platicon frequency comb via a varied sideband modulation frequency is examined in both spectral and temporal measurements. Determined by second-harmonic auto-correlation and cross-correlation, we observe bright square platicon pulse of 17 ps pulsewidth on a 19 GHz flat frequency comb. With auxiliary-laser-assisted thermal stabilization, we surpass the thermal bistable dragging and extend the mode-locking access to narrower 2 ps platicon pulse states, supported by nonlinear dynamical modeling and boundary limit discussions.

Lamb-dip Saturated-absorption CAvity Ring-down (SCAR) ro-vibrational molecular spectroscopy in the near-IR

Pasquale Maddaloni, roberto aiello, Valentina Di Sarno, maria giulia delli santi, Maurizio De Rosa, Iolanda Ricciardi, Giovanni Giusfredi, Paolo Natale, and Luigi Santamaria Amato

DOI: 10.1364/PRJ.456515 Received 22 Feb 2022; Accepted 10 May 2022; Posted 16 May 2022  View: PDF

Abstract: The high-detection-sensitivity saturated-absorption cavity ring-down (SCAR) technique is extended to Lamb-dip spectroscopy of ro-vibrational molecular transitions in the near-IR region. Frequency-comb-referenced sub-Doppler saturation measurements, performed on the acetylene (ν₁+ν₃+ν₄ <- ν₄) R(14)e line at 6562 cm-1, are analyzed by a SCAR global line profile (GLP) fitting routine, based on a specially developed theoretical model. Compared to a conventional cavity-ring-down evaluation, our approach yields dip profiles with a linewidth freed from saturation broadening effects, reduced by 40%, and a signal-to-noise ratio increased by 90%. Ultimately, an overall (statistical + systematic) fractional uncertainty as low as 7·10E-12 is achieved for the absolute line-center frequency. At the same time, our method is also able to accurately infer the linear (non-saturated) behaviour of the gas absorption, providing Lamb-dip-based linestrength measurements with a relative uncertainty of 5‰.

Tunable Non-Hermiticity through Reservoir Engineering

Xin Meng, Yanhong Xiao, Wei Yi, Xingda Lu, Zhiwei Hu, Xichang Zhang, Ying Hu, Wanxia Cao, and Li HaoWei

DOI: 10.1364/PRJ.450166 Received 01 Dec 2021; Accepted 04 May 2022; Posted 04 May 2022  View: PDF

Abstract: We experimentally demonstrate tunable non-Hermitian coupling in an atomic-vapour cell where atomic coherences in different optical channels are dissipatively coupled through atomic motion. Introducing a far-detuned light wall in the reservoir between the optical channels, we decorate the inter-channel coupling term so that it can be switched from dissipative to coherent. The tunable non-Hermiticity is then confirmed through measurements of the inter-channel light transport where the light-wall-induced phase shift is directly probed. Based on the tunable non-Hermiticity, we further discuss an exemplary scheme in which our setup can serve as a building block for the experimental study of exotic non-Hermitian criticality.

Dynamic bifunctional THz metasurface via dual-mode decoupling

Xuan Cong, Hongxin Zeng, Shiqi Wang, qiwu shi, Shix Liang, Jiandong Sun, Sen Gong, Feng Lan, Ziqiang Yang, and Yaxin Zhang

DOI: 10.1364/PRJ.453496 Received 13 Jan 2022; Accepted 21 Apr 2022; Posted 06 Jun 2022  View: PDF

Abstract: Metasurfaces have powerful light field manipulation capabilities and have been researched and developed extensively in various fields. With an increasing demand for diverse functionalities, terahertz (THz) metasurfaces are also expanding their domain. In particular, integrating different functionalities into a single device is a compelling domain in metasurfaces. In this work, we demonstrate a functionally decoupled THz metasurface, which can incorporate any two functions into one metasurface and switch dynamically through external excitation. This proposed metasurface is formed by the combination of split-ring resonators (SRR) and phase change material vanadium dioxide (VO2). It operates in single-ring resonant mode (S-mode) and double-ring resonant mode (D-mode) with the varying VO2 states in an insulating and a metallic, respectively. More importantly, the phase modulation is independent in two operating modes, and both coverage 360° cross-polarized phase with efficient polarization conversion. This characteristic makes it obtain arbitrary independent phase information on the metasurface with different modes to switch dual-function dynamically. Here, we experimentally demonstrate the functions of tunable focal length and large-angle focus deflection of the THz off-axis parabolic mirror to verify the dual-function switching characteristics of the functionally decoupled metasurface. The functionally decoupled metasurface developed in this work broadens the way for the research and application of multifunctional modulation devices in the THz band.

Total polarization conversion using subwavelength laser-induced periodic surface structures on stainless steel

Mahmoud Elshorbagy, Luis Miguel Sanchez-Brea, Jeronimo Buencuerpo, Jesus del Hoyo, Ángela Soria-García, Verónica Pastor-Villarrubia, Alejandro San Blas, Ainara Rodriguez, Santiago Olaizola, and Javier Alda

DOI: 10.1364/PRJ.454451 Received 25 Jan 2022; Accepted 18 Apr 2022; Posted 06 May 2022  View: PDF

Abstract: Stainless steel is a basic raw material in industry. It can be customized by generating laser-induced periodic surface structures (LIPSS) as subwavelength gratings. Here, we present the capabilities of LIPSS on stainless steel to modify the polarization state of the reflected radiation at the infrared . These structures have been modeled using the finite element method and fabricated by femtosecond laser processing. The Stokes parameters have been obtained experimentally and a model for the shape has been used to fit the simulated Stokes values to the experimental data. The birefringence of the LIPSS is analyzed to explain how they modify the polarization state of the incoming light. We find the geometry of the subwavelength grating that makes it work as an optical retarder that transforms a linearly polarized light into a circularly polarized wave. Also, the geometrical parameters of the LIPSS are tuned to absorb selectively one of the components of the incoming light, becoming a linear axial polarizer. An appropriate selection of the geometrical parameters and orientation of the fabricated LIPSS makes possible to obtain an arbitrary pure polarization state when illuminated by any other pure polarization state. The overall reflectance of these transformations reaches values up to 60% with respect to the incident intensity, that is the same reflectivity obtained for non nanostructured stainless steels flat surfaces.

Generalized Robust Training Scheme using Genetic Algorithm for Optical Neural Networks with Imprecise Components

Rui Shao, Gong Zhang, and Xiao Gong

DOI: 10.1364/PRJ.449570 Received 03 Dec 2021; Accepted 18 Mar 2022; Posted 18 Mar 2022  View: PDF

Abstract: One of the pressing issues for optical neural networks (ONNs) is the performance degradation introduced by parameter uncertainties in practical optical components. Hereby, we propose a novel two-step off-chip training scheme to configure phase shifts in a Mach-Zehnder interferometer (MZI)-based feedforward ONN, where a stochastic gradient descent algorithm followed by a genetic algorithm considering 4 types of practical imprecisions is employed. By doing so, the learning process features fast convergence and high computational efficiency, and the trained ONN is robust to varying degrees and types of imprecisions. We investigate the effectiveness of our scheme by using practical machine learning tasks including Iris and MNIST classifications, showing more than % accuracy improvement after training and comparable accuracy (90.8% in an imprecise ONN with 3 hidden layers and 224 tunable thermal-optic phase shifters) to the ideal one (92.0%).