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Mimicking gravitational effect with graded index lens in geometrical optics

Wen Xiao, Sicen Tao, and Huanyang Chen

DOI: 10.1364/PRJ.418787 Received 05 Jan 2021; Accepted 19 Apr 2021; Posted 19 Apr 2021  View: PDF

Abstract: General Relativity establishes the equality between matter–energy density and the Riemann curvature of spacetime. Therefore, light or matter will be bent or trapped when passing near the massive celestial objects and the Newton’s second law fails to explain it. The gravitational effects have not only been extensively studied in astronomy, but also attracts a great interestin the field of optics. In the past decades, people in optics have mimicked the black holes, Einstein’s ring, deSitter Space and other fascinating effects. Lots of optical applications emerged inspired by the these mimicking including wave-front shaping waveguide, light absorbers. Most of them are presented in wave optics, rarely in geometrical optics. Here, based on opticalmechanical analogy, in geometrical optics regime, with a graded index lens, we mimic the Schwarzschild precession in the orbit of the star S2 near the Galactic Centre massive black hole, which was first detected by European Southern Observatory recently. We also find that another series of graded index lenses can be used to mimic the possible Reissner-Nordström metric of Einstein’s field equation and the dark matter particles motions, where light or matter paths in the graded lenses will be closed in some cases while in others will be trapped by the center or keep travelling around the center. Our work provides an efficient way to investigate the complex celestial behaviors which is difficult to directly detect using astronomical tools and enriches the family of absolute optical instruments due to the closed trajectories as well.

Nanohole Array Structured GaN-based White-LEDs with improved Modulation Bandwidth via Plasmon Resonance and Non-Radiative Energy Transfer

Rongqiao Wan, Guoqiang Li, Xiang Gao, Zhiqiang Liu, Junhui Li, Xiaoyan Yi, Nan Chi, and Liancheng Wang

DOI: 10.1364/PRJ.421366 Received 09 Feb 2021; Accepted 18 Apr 2021; Posted 19 Apr 2021  View: PDF

Abstract: Commercial white LEDs (WLEDs) are generally limited in modulation bandwidth due to slow stokes process, long lifetime of phosphors and quantum-confined Stark effect (QCSE). Here we report a novel plasmonic WLED by infiltrating nanohole LED (H-LED) with quantum dots (QDs) and Ag nanoparticles (NPs) together (M-LED). This decreased QW-QD distance would open extra non-radiative energy transfer (NRET) channel and thus enhance stokes transfer efficiency. The presence of Ag NPs enhances the spontaneous emission rate significantly. Compared with the H-LED filled with QDs (QD-LED), the optimized M-LED demonstrates a maximum CRI of 91.2, a 43% increase in optical power at 60mA, and a lowered CCT. Simultaneously, M-LED exhibits a date rate of 2.21Gbps at low current density of 96A/cm2 (60mA), which is 77% higher than QD-LED. This is mainly due to the higher optical power and modulation bandwidth of M-LED under the influence of plasmon, resulting to a higher date rate and higher signal-to-noise ratio (SNR) under the forward error correction (FEC). We believe the approach reported in this work should contribute to WLED light source with increased modulation bandwidth for higher speed VLC (Visible Light Communication) application.

Broad-intensity-range optical nonreciprocity based on feedback-induced Kerr nonlinearity

Lei Tang, Jiangshan Tang, Haodong Wu, Jing Zhang, Min Xiao, and Keyu Xia

DOI: 10.1364/PRJ.413286 Received 23 Oct 2020; Accepted 16 Apr 2021; Posted 19 Apr 2021  View: PDF

Abstract: Nonreciprocal light propagation plays an important role in modern optical systems, from photonic networks to integrated photonics. We propose a nonreciprocal system based on a resonance-frequency-tunable cavity and intensity-adaptive feedback control. Because the feedback-induced Kerr nonlinearity in the cavity is dependent on the incident direction of light, the system exhibits nonreciprocal transmission with a high isolation contrast of 0.90 and a very low insertion loss of 0.09 dB. By utilizing the intensity-adaptive feedback control, the operating intensity range of the nonreciprocal system is broadened to 20.0 dB, which relaxes the limitation of the operating intensity range for nonlinear nonreciprocal systems. Our protocol paves the way to realize high-performance nonreciprocal propagation in optical systems and can also be extended to microwave systems.

A generalized framework for non-sinusoidal fringe analysis using deep learning

Shijie Feng, Chao Zuo, Liang Zhang, Wei Yin, and Qian Chen

DOI: 10.1364/PRJ.420944 Received 02 Feb 2021; Accepted 13 Apr 2021; Posted 13 Apr 2021  View: PDF

Abstract: Phase retrieval from fringe images is essential to many optical metrology applications. In the field of fringe projection profilometry, the phase is often obtained with systematic errors if the fringe pattern is not perfect sinusoidal. Several factors can account for non-sinusoidal fringe patterns, such as the nonlinear input-output response (e.g., the gamma effect) of digital projectors, the residual harmonics in binary defocusing projection, and the image saturation due to intense reflection. Traditionally, these problems are handled separately with different well-designed methods, which can be seen as “one-to-one” strategies. Inspired by recent successful artificial intelligence-based optical imaging applications, we propose a “one-to-many” deep learning technique that can analyze non-sinusoidal fringe images resulting from different non-sinusoidal factors and even the coupling of these factors. We show for the first time, to the best of our knowledge, a trained deep neural network can effectively suppress the phase errors due to various kinds of non-sinusoidal patterns. Our work paves the way to robust and powerful learning based fringe analysis approaches.

Efficient and Wideband Acousto-Optic Modulation on Thin-Film Lithium Niobate for Microwave to Photonic Conversion

Ahmed Hassanien, Steffen Link, Yansong Yang, Edmond Chow, Lynford Goddard, and Songbin Gong

DOI: 10.1364/PRJ.421612 Received 02 Feb 2021; Accepted 13 Apr 2021; Posted 13 Apr 2021  View: PDF

Abstract: Microwave photonics, a field that crosscuts microwave/millimeter-wave engineering with optoelectronics, has sparked great interest from research and commercial sectors. This multidisciplinary fusion can achieve ultrawide bandwidth and ultrafast speed that were considered impossible in conventional chip-scale microwave/mm-wave systems. Conventional microwave-to-photonic converters, based on the resonant acousto-optic modulation produce highly efficient modulation but sacrifice bandwidth and limit their applicability for most real-world microwave signal processing applications. In this article, we build highly efficient and wideband microwave-to-photonic modulators using the acousto-optic effect on suspended lithium niobate thin films. A wideband microwave signal is first piezoelectrically transduced using interdigitated electrodes into Lamb acoustic waves, which directly propagates across an optical waveguide and causes refractive index perturbation through the photo-elastic effect. This approach is power efficient, with phase shifts up to 0.0166 rad/√mW over a 45 µm modulation length and with bandwidth up to 140 MHz at a center frequency of 1.9 GHz. Compared to the state-of-the-art, a 9× more efficient modulation has been achieved by optimizing the acoustic and optical modes and their interactions.

Ultrafast all-optical terahertz modulation based on inverse-designed metasurface

Weibao He, Tong mingyu, Zhongjie Xu, Yuze Hu, Xiang'ai Cheng, and Tian Jiang

DOI: 10.1364/PRJ.423119 Received 18 Feb 2021; Accepted 10 Apr 2021; Posted 13 Apr 2021  View: PDF

Abstract: Metasurface plays a key role in various terahertz metadevices, while the designed terahertz metasurface still lacks flexibility and variety. On the other hand, inverse design has drawn a plenty of attentions due to its flexibility and robustness in the application of photonics. This provides an excellent opportunity for metasurface design as well as the development of multifunctional, high-performance terahertz devices. In this work, we demonstrate that, for the first time, a terahertz metasurface supported by electromagnetically induced transparency (EIT) effect can be constructed by inverse design, which combines particle swam optimization (PSO) algorithm with finite-difference time-domain (FDTD) method. Incorporating germanium (Ge) film with inverse-designed metasurface, an ultrafast EIT modulation on the picosecond scale has been experimentally verified. The experimental results suggest a feasibility to build terahertz EIT effect in the metasurface through optimization algorithm of inverse design. Furthermore, this method can be further utilized to design multifunctional and high-performance terahertz devices, which is hard to accomplish in traditional metamaterial structure. In a word, our method not only provides a novel way to design ultrafast all-optical terahertz modulator based on artificial metamaterials, but also shows the potential applications of inverse design on the terahertz devices.

Superior performances of 2 kHz Nd:YAG pulse laser with a gradient dopant crystal

Haihe Jiang, Mengen Wei, Tingqing Cheng, Renqin Dou, and Qing-li Zhang

DOI: 10.1364/PRJ.424989 Received 12 Mar 2021; Accepted 10 Apr 2021; Posted 13 Apr 2021  View: PDF

Abstract: The improvement of high-repetition-rate Nd:YAG laser performances is still long-standing challenge, although many novel architectures of the laser have been developed. The key difficulty is that the uniform Nd3+-doped YAG crystals adsorb pumping light with a remarkable decrease of pumping strength along axis direction, leading to an uneven pumping distribution with unsatisfactory laser performance. Herein, we report a home-made new Nd:YAG crystal rod that contains a gradient dopant of 0.39-0.80 at.% Nd3+ from end to end, achieving superior performances of 2 kHz Nd:YAG pulse laser. The optical-optical conversion efficiency reached 53.8 %, and the maximal output power of laser was 24.2 W, enhanced by 35.9 % comparing with a uniform crystal rod with the same total concentration of Nd3+. Significantly, our experiments revealed that the gradient concentration crystal produced a relatively even pumping distribution along the rod axis, greatly reducing the temperature gradient as well as the smaller thermal effect. The smoothing pump and thermal distribution obviously improved the features of laser oscillation and output.

Effect of dispersion on indistinguishability between single-photon wave-packets

Yunru Fan, Chenzhi yuan, Ruiming Zhang, Si Shen, Peng Wu, Heqing Wang, Hao Li, Guangwei Deng, Hai-Zhi Song, Lixing YOU, Zhen Wang, You WANG, Guang-can Guo, and Qiang Zhou

DOI: 10.1364/PRJ.421180 Received 02 Feb 2021; Accepted 09 Apr 2021; Posted 13 Apr 2021  View: PDF

Abstract: With propagating through a dispersive medium, the temporal-spectral profile of laser pulses should be inevitably modified. Although such dispersion effect has been well studied in classical optics, its effect on a single-photon wave-packet, i.e., the matter wave of a single-photon, has not yet been entirely revealed. In this paper, we investigate the effect of dispersion on indistinguishability between single-photon wave-packets through the Hong-Ou-Mandel (HOM) interference. By dispersively manipulating two indistinguishable single-photon wave-packets before interfering with each other, we observe that the difference of the second-order dispersion between two optical paths of the HOM interferometer can be mapped to the interference curve, indicating that (1) with the same amount of dispersion effect in both paths, the HOM interference curve must be only determined by the intrinsic indistinguishability between the wave-packets, i.e., dispersion cancellation due to the indistinguishability between Feynman paths; (2) unbalanced dispersion effect in two paths cannot be cancelled and will broaden the interference curve thus providing a way to measure the second-order dispersion coefficient. Our results suggest a more comprehensive understanding of the single-photon wave-packet and pave ways to explore further applications of the HOM interference.

Band dynamics accompanied by bound states in the continuum at the third-order Γ point in leaky-mode photonic lattices

Sun-Goo Lee, seong-han kim, and Chul-Sik Kee

DOI: 10.1364/PRJ.417150 Received 09 Dec 2020; Accepted 08 Apr 2021; Posted 13 Apr 2021  View: PDF

Abstract: Bound states in the continuum (BICs) and Fano resonances in planar photonic lattices, including metasurfaces and photonic crystal slabs, have been studied extensively in recent years. Typically, the BICs and Fano resonances are associated with the second stop bands open at the second-order Γ point. This paper address the fundamental properties of the fourth stop band accompanied by BICs at the third-order Γ point in one-dimensional leaky-mode photonic lattices. At the fourth stop band, one band edge mode suffers radiation loss, thereby generating a Fano resonance, while the other band edge mode becomes a nonleaky BIC. The fourth stop band is controlled primarily by the Bragg processes associated with the first, second, and fourth Fourier harmonic components of the periodic dielectric constant modulation. The interplay between these three major processes closes the fourth band gap and induces a band flip whereby the leaky and BIC edges transit across the fourth band gap. At the fourth stop band, a new type of BIC is formed owing to the destructive interplay between the first and second Fourier harmonics. When the fourth band gap closes with strongly enhanced Q factors, Dirac cone dispersions can appear at the third-order Γ point.

Covert wireless communication using massive optical comb channels for deep denoising

Xianglei Yan, Xihua Zou, Peixuan Li, Wei Pan, and Lianshan Yan

DOI: 10.1364/PRJ.419605 Received 13 Jan 2021; Accepted 08 Apr 2021; Posted 13 Apr 2021  View: PDF

Abstract: Covert wireless communications are unprecedentedly vital for security and privacy of individuals, government and military bodies. Besides encryption, hiding signal transmission deeply under noise background highly proliferates the covertness in physical layer. A deep signal hiding leads to a low interception probability at interceptor but a poor data recovery at receiver. To ensure both high covertness and high-fidelity recovery, massive and dense optical comb channels are utilized for deep denoising. The available optical comb channels can scale up without physical bottlenecks on immense hardware and spectrum requirements. Thus a striking signal-to-noise ratio (SNR) rise can be achieved for deep denoising. Combination of massive comb channels (a record high of 1024) through analog spectrum convolution enables a 29-dB SNR enhancement, even with in-band noises by 18dB in both frequency and time domains. This method opens a new avenue for covert communications.

Preconditioned deconvolution method for high-resolution ghost imaging

Zhishen Tong, Zhentao Liu, Chenyu Hu, Jian Wang, and Shensheng Han

DOI: 10.1364/PRJ.420326 Received 19 Jan 2021; Accepted 05 Apr 2021; Posted 05 Apr 2021  View: PDF

Abstract: Ghost imaging (GI) can nonlocally image objects by exploiting the fluctuation characteristics of light fields, where the spatial resolution is determined by the normalized second-order correlation function $g^{(2)}$. However, the spatial shift-invariant property of $g^{(2)}$ is distorted when the number of samples is limited, which hinders the deconvolution methods from improving the spatial resolution of GI. In this paper, based on the prior of transfer functions of GI, we propose a preconditioned deconvolution method to improve the imaging resolution of GI by refining the mutual coherence of sampling matrix in GI. Our theoretical analysis shows that the preconditioned deconvolution method actually extends the deconvolution technique to GI, and it regresses into the classical deconvolution technique for the conventional imaging system. The imaging resolution of GI after preconditioning is restricted to the detection noise. Both simulated and experimental results exhibit that the spatial resolution of the reconstructed image is obviously enhanced by using the preconditioned deconvolution method. In the experiment, $1.4$-fold resolution enhancement over the Rayleigh's criterion is achieved via the preconditioned deconvolution method. Our results extend the deconvolution technique that is only applicable to spatial shift-invariant imaging systems to all linear imaging systems, and will promote their applications in biological imaging and remote sensing for high-resolution imaging demands.

Focus-tunable microscope for imaging small neuronal processes in freely moving animals

Arutyun Bagramyan, Loïc Tabourin, Ali Rastqarfarajzadeh, Narges Karimi, Frédéric Bretzner, and Tigran Galstian

DOI: 10.1364/PRJ.418154 Received 21 Dec 2020; Accepted 04 Apr 2021; Posted 05 Apr 2021  View: PDF

Abstract: Miniature 1-photon microscopes have been widely used to image neuronal assemblies in the brain of freely moving animals over the last decade. However, these systems have important limitations for imaging in-depth fine neuronal structures. We present a novel subcellular imaging 1-photon device that uses an electrically tunable liquid crystal lens to enable a motion-free depth scan in the search of such structures. Our miniaturized microscope is compact (10 mm x 17 mm x 12 mm), lightweight (≈ 1.4 g), provides fast acquisition rate (30-50 frames/second), high magnification (8.7x) and high resolution (1.4 µm) that allow imaging of calcium activity of fine neuronal processes in deep brain regions during a wide range of behavioural tasks of freely moving mice.

Harmonic injection locking of high-power mid-infrared quantum cascade lasers

Feihu Wang, Steven Slivken, and Manijeh Razeghi

DOI: 10.1364/PRJ.423573 Received 24 Feb 2021; Accepted 03 Apr 2021; Posted 05 Apr 2021  View: PDF

Abstract: High-power, high-speed quantum cascade lasers (QCLs) with stable emission in the mid-infrared regime are of great importance for applications in metrology, telecommunication, and fundamental tests of physics. Owing to the inter-sub-band transition, the unique ultrafast gain recovery time of the QCL with picosecond dynamics is expected to overcome the modulation limit of classical semiconductor lasers and bring a revolution for the next generation of ultrahigh-speed optical communication. Therefore, harmonic injection locking, offering the possibility to fast modulate and greatly stabilize the laser emission beyond the rate limited by cavity length, is inherently adapted to QCLs. In this work, we demonstrate for the first time the harmonic injection locking of a mid-infrared QCL with an output power over 1 watt in continuous-wave operation at 288 K. Compared with an unlocked laser, the inter-mode spacing fluctuation of an injection locked QCL can be considerably reduced by a factor of 10^3, which permits the realization of an ultra-stable mid-infrared semiconductor laser with high phase coherence and frequency purity. Despite temperature change, this fluctuation can be still stabilized to hertz level by a microwave modulation up to 18 GHz. These results open up the prospect of the applications of mid-infrared QCL technology for frequency comb engineering, metrology and the next generation ultrahigh-speed telecommunication. It may also stimulate new schemes for exploring ultrafast mid-infrared pulse generation in QCLs.

Integrating the optical tweezers and spanner onto individual single-layer metasurfaces

Tianyue Li, Boyan Fu, Xiaohao Xu, Shuming Wang, Baojun Li, Zhenlin Wang, and Shining Zhu

DOI: 10.1364/PRJ.421121 Received 28 Jan 2021; Accepted 01 Apr 2021; Posted 02 Apr 2021  View: PDF

Abstract: Optical tweezers (OT) and optical spanner (OS) are powerful tools of optical manipulation, which are responsible for particle trapping and rotation, respectively. Conventionally, the OT and OS are built using bulky three-dimensional devices, such as microscope objectives and spatial light modulators. Recently, metasurfaces are proposed for setting up them on a microscale platform, which greatly miniaturizing the systems. However, the realization of both OT and OS with one identical metasurface is posing a challenge. Here, we offer a metasurface-based solution to integrating the OT and OS. We show that, by utilizing the interplay between the geometric and dynamic phases, it is possible to construct an output field, which promises a high-numerical-aperture focal spot, accompanied with a coaxial vortex. Optical trapping and rotation are numerically demonstrated by estimating the mechanical effects on a particle probe. Moreover, we demonstrate an on-demand control of the OT-to-OS distance and the topological charge possessed by the OS. By revealing the OT-OS metasurfaces, our results may empower advanced applications in on-chip particle manipulation.

Complex Swift Hohenberg equation dissipative solitonfiber laser

Ankita Khanolkar, Yimin Zang, and Andy Chong

DOI: 10.1364/PRJ.419686 Received 12 Jan 2021; Accepted 30 Mar 2021; Posted 30 Mar 2021  View: PDF

Abstract: Complex Swift Hohenberg Equation (CSHE) has attracted intensive research interest over the years as it enables modeling of mode-locked lasers with saturable absorber more realistic by adding a fourth order term to the spectral response. Multiple researchers have reported a variety of numerical solutions of the CSHE model which reveal interesting pulse patterns and structures. In this work, we have demonstrated a CSHE dissipative soliton fiber laser experimentally using a unique spectral filter with a complicatedtransmission profile. The behavior and performance of the laser agree qualitatively with the numerical simulations. Our findings bring insight into dissipative soliton dynamics and make our mode-locked laser a powerful testbed for observing dissipative solitons of CSHE, which may open a new course in ultrafast fiber lasers research.

Modeling of a SiGeSn Quantum Well Laser

Bahareh Marzban, Daniela Stange, Denis Rainko, Zoran Ikonic, Dan Buca, and Jeremy Witzens

DOI: 10.1364/PRJ.416505 Received 02 Dec 2020; Accepted 30 Mar 2021; Posted 31 Mar 2021  View: PDF

Abstract: We present comprehensive modeling of a SiGeSn multi-quantum-well laser that has been previously experimentally shown to feature an order of magnitude reduction in optical pump threshold compared with bulk lasers. We combine experimental material data obtained over the last few years with k·p theory in order to adapt transport, optical gain and optical loss models to this material system (drift-diffusion, thermionic emission, gain calculations, free carrier absorption and intervalence band absorption). Good consistency is obtained with experimental data and the main mechanisms limiting the laser performance are discussed. In particular, modeling results indicate a low non-radiative lifetime, in the 100 ps range for the investigated material stack, and lower than expected Γ-L energy separation to play a dominant role in the device properties. Moreover, they further indicate that these lasers emit in the transverse magnetic polarization at higher temperatures due to lower intervalence band absorption losses. To the best of our knowledge, this is the first comprehensive modeling of experimentally realized SiGeSn lasers taking the wealth of experimental material data accumulated over the past years into account. The methods described in this paper pave the way to predictive modeling of new SiGeSn laser device concepts.

Arbitrary cylindrical vector beam generation enabled by polarization-selective Gouy phase shifter

Junliang Jia, Kepeng Zhang, Guangwei Hu, Maping Hu, Tong Tong, Quanquan Mu, Hong Gao, fuli li, Chengwei Qiu, and Pei Zhang

DOI: 10.1364/PRJ.419368 Received 12 Jan 2021; Accepted 29 Mar 2021; Posted 30 Mar 2021  View: PDF

Abstract: Cylindrical vector beams (CVBs), which possesses polarization distribution of rotational symmetry on the transverse plane, can be developed in many optical technologies. Conventional methods to generate CVBs contain redundant interferometers or need to switch among diverse elements, thus being inconvenient in applications containing multiple CVBs. Here we provide a passive polarization-selective device to substitute interferometers and simplify generation setup. It is accomplished by reversing topological charges of orbital angular momentum based on polarization-selective Gouy phase. In the process, tunable input light is the only condition to generate CVB with arbitrary topological charges. To cover both azimuthal and radial parameters of CVBs, we express the mapping between scalar Laguerre-Gaussian light on basic Poincaré sphere and CVB on high-order Poincaré sphere. The proposed device simplifies the generation of CVBs enormously, and thus has potentials in integrated devices for both quantum and classic optical experiments.

Experimental demonstration of pyramidal neuron-like dynamics dominated by dendritic action potentials based on a VCSEL for all-optical XOR classification task

Shuiying Xiang, Yahui Zhang, xing cao, ShiHao Zhao, Xingxing Guo, Aijun Wen, and Yue Hao

DOI: 10.1364/PRJ.422628 Received 15 Feb 2021; Accepted 29 Mar 2021; Posted 30 Mar 2021  View: PDF

Abstract: We experimentally and numerically demonstrate an approach to optically reproduce a pyramidal neuron-like dynamics dominated by dendritic Ca2+ action potentials (dCaAPs) based on a vertical-cavity surface-emitting laser(VCSEL) for the first time. The biological pyramidal neural dynamics dominated by dCaAPs indicate that the dendritic electrode evoked somatic spikes with current near threshold but failed to evoke (or evoked less) somatic spikes for higher current intensity. The emulating neuron-like dynamics are performed optically based on the injection locking, spiking dynamics, and damped oscillations in the optically injected VCSEL. Besides, the exclusive OR (XOR) classification task is examined in the VCSEL neuron equipped with the pyramidal neuron-like dynamics dominated by dCaAPs. Furthermore, a single spike or multiple periodic spikes are suggested to express the result of the XOR classification task for enhancing the processing rate or accuracy. The experimental and numerical results show that the XOR classification task is achieved successfully in the VCSEL neuron enabled to mimic the pyramidal neuron-like dynamics dominated by dCaAPs. This work reveals valuable pyramidal neuron-like dynamics in a VCSEL and offers a novel approach to solve XOR classification task with a fast and simple all-optical spiking neural network, hence shows great potentials for future photonic spiking neural networks and photonic neuromorphic computing.

Solving the century-old problem of incoherent imaging systems with synthetic aperture using a single opening instead of two

Angika Bulbul and Joseph Rosen

DOI: 10.1364/PRJ.422381 Received 09 Feb 2021; Accepted 25 Mar 2021; Posted 30 Mar 2021  View: PDF

Abstract: Imaging with an optical incoherent synthetic aperture (SA) means that the incoherent light from observed objects is processed over time from various points of view to obtain a resolution equivalent to single-shot imaging by the SA larger than the actual physical aperture. The operation of such systems has always been based on two-wave interference where the beams propagate through two separate channels. This limitation of two channels at a time is removed in the present study with the proposed SA where the two beams pass through the same single channel at any given time. The system is based on a newly developed self-interference technique named coded aperture correlation holography. At any given time, the recorded intensity is obtained from interference between two waves co-propagating through the same physical channel. One wave oriented in a particular polarization is modulated by a pseudorandom coded phase mask and the other one oriented orthogonally passes through an open subaperture. Both subapertures are multiplexed at the same physical window. The system is calibrated by a point spread hologram synthesized from the responses of a guidestar. All the measurements are digitally processed to achieve a final image with a resolution higher than obtained by the limited physical aperture. This unique configuration can eliminate the dependency of current cumbersome systems composed of far apart optical channels in the large optical astronomical interferometers and paves the way to a SA system with a single less-expensive compact light collector in an incoherent optical regime that may be utilized for the future ground-based or space telescopes.

Inflection point: a new perspective on photonic nanojets

Guoqiang Gu, Pengcheng Zhang, SiHai Chen, Yi Zhang, and Hui Yang

DOI: 10.1364/PRJ.419106 Received 06 Jan 2021; Accepted 25 Mar 2021; Posted 30 Mar 2021  View: PDF

Abstract: When light propagates through the edge or middle part of microparticle’s incoming interface, there is a basic rule that light converges and diverges rapidly or slowly at the output port. These two parts are referred to as region of rapid change (RRC) and region of slow change (RSC), respectively. Finding the boundary point between RRC and RSC is the key to reveal and expound this rule scientifically. Based on the correlation between light convergence-divergence and the slope of emergent light, combined with the relationship between natural logarithm and growth in physical reality and the second derivative of a function in practical significance, we determine the boundary point between RRC and RSC, namely the inflection point. From such perspective, photonic nanojet (PNJ) and near-field focusing by light irradiation on RSC and RRC, as well as the position of the inflection point under different refractive index contrast and the field distribution of light-focusing, are studied with finite-element-method-based numerical simulation and ray-optics-based theoretical analysis. By illuminating light of different field intensity ratios to the regions divided by the inflection point, we demonstrate the generation of photonic hook (PH) and the modulation of PNJ/PH in a new manner.

Genetic Algorithm based Deep Neural Networks for High-efficient photonic devices design

Yangming Ren, Lingxuan Zhang, Weiqiang Wang, Xinyu Wang, Yufang Lei, Yulong Xue, xiaochen sun, and Wenfu Zhang

DOI: 10.1364/PRJ.416294 Received 01 Dec 2020; Accepted 25 Mar 2021; Posted 30 Mar 2021  View: PDF

Abstract: Deep-learning have already demonstrated their tremendous potential for photonic structure design. Neural networks often need to prepare a large amount of labeled data to complete the training of thousands to millions of learning parameters. However, generating data requires physical simulations or experimental measurements. Collecting a massive dataset is time consuming, expensive and even not always practical. In order to reduce the burden of data collection, a highly efficient inverse design method of photonic devices using a deep learning approach is presented. The method combines neural networks with genetic algorithm, and optimizes the geometry of a photonic device in the polar coordinate system. The method requires significantly less training data compared with previous deep neural network inverse design methods. It presents great flexibility and high efficiency in designing ultra-compact photonic devices with challenging properties. We apply this method for designing several silicon photonics devices including power splitters with specific splitting ratios, a TE mode converter, and a broadband power splitter. These devices present good performance at micrometers footprint in full compliance with fabrication design rules.

Towards simple, generalizable neural networks with universal training for low SWaP hybrid vision

Baurzhan Muminov, Altai Perry, Rakib Hyder, Salman Asif, Luat Vuong, and Gavin OMalley

DOI: 10.1364/PRJ.416614 Received 07 Dec 2020; Accepted 25 Mar 2021; Posted 19 Apr 2021  View: PDF

Abstract: We demonstrate generalizable image reconstruction with the simplest of hybridmachine vision systems: fixed, linear optical preprocessors combined with no-hidden-layer,"small-brain" neural networks. Such neural network are capable of learning the reconstruction ofa "universal data training set" composed of Fourier, random, and vortex singularities. Modelsthat are trained with sinusoidal or random patterns uniformly distribute errors around the image,whereas models trained with datasets with vortices detect edges and corners more sharply.Although the spectral methods shown here are intuitive, the outcomes are not obvious from theneural network architectures. Reconstructed images carry a background behind objects thatlimit accuracy, which can be seen in the reconstruction of CIFAR images. With thresholdingand objects, we achieve consistent accuracy of<5%with the reconstruction of various disjointdatasets, including MNIST and Fashion MNIST. Our work is favorable for future real-timemachine vision systems: we reconstruct images on a 15W laptop CPU with 15k fps: faster by afactor of 3 than previously reported results and 3 orders of magnitude faster than convolutional neural networks.

FSR-free filters with ultra-wide tunability across S+C+L band

Lan Li, Chunlei Sun, Chuyu Zhong, Maoliang Wei, Hui Ma, Ye Luo, Zequn Chen, Renjie Tang, Jialing Jian, and Hongtao Lin

DOI: 10.1364/PRJ.420005 Received 15 Jan 2021; Accepted 24 Mar 2021; Posted 25 Mar 2021  View: PDF

Abstract: Optical filters are essential parts of advanced optical communication and sensing systems. Among them, the ones with an ultra-wide free-spectral range (FSR) are especially critical. They are promising to provide access to numerous wavelength channels highly desired for large-capacity optical transmission and multi-point multi-parameter sensing. Present schemes for wide-FSR filters either suffer from limited cavity length or poor fabrication tolerance or impose an additional active-tuning control requirement. We theoretically and experimentally demonstrate a filter that features FSR-free operation capability, sub-nanometer optical bandwidth, and acceptable fabrication tolerance. Only one single deep dip within a record-large waveband (S+C+L band) is observed by appropriately designing a side-coupled Bragg grating assisted Fabry-Perot (F-P) filter, which has been applied as the basic sensing unit for both the refractive index and temperature measurement. Five such basic units are also cascaded in series to demonstrate a multi-channel filter. This work provides a new insight to design FSR-free filters and opens up a possibility of flexible large-capacity integration using more wavelength channels, which will greatly advance integrated photonics in optical communication and sensing.

Spin-decoupled metalens with intensity-tunable multiple focal points

Bingshuang Yao, XiaoFei Zang, Yang Zhu, dahai yu, Jingya Xie, Lin Chen, Sen Han, Yiming Zhu, and Songlin Zhuang

DOI: 10.1364/PRJ.420665 Received 22 Jan 2021; Accepted 23 Mar 2021; Posted 25 Mar 2021  View: PDF

Abstract: The control of spin electromagnetic (EM) waves is of great significance in optical communications. Although geometric metasurfaces have shown unprecedented capability to manipulate the wavefronts of spin EM waves, they are still challenging to independently manipulate each spin state and intensity distributions, which inevitably degrades metasurface-based devices for further applications. Here, we propose and experimentally demonstrate an approach to designing spin-decoupled metalenses based on pure geometric phase, i.e. geometric metasurfaces with predesigned phase modulation possessing functionalities of both convex lenses and concave lenses. Under the illumination of left/right-handed circularly polarized (LCP (or RCP)) terahertz waves, these metalenses can generate transversely/longitudinally distributed RCP/LCP multiple focal points. Since the helicity-dependent multiple focal points are locked to the polarization state of incident THz waves, the relative intensity can be controlled with different weights of LCP and RCP THz waves, leading to the intensity-tunable functionality. This robust approach for simultaneously manipulating orthogonal spin states and energy distributions of spin EM waves will open a new avenue for designing multifunctional devices and integrated communication systems.

Superconducting microstrip single-photon detector with system detection efficiency over 90% at 1550 nm

Guangzhao Xu, Weijun Zhang, Lixing YOU, Jiamin Xiong, xingqu sun, HAO HUANG, Xin Ou, Yiming Pan, chaolin lv, Hao Li, Zhen Wang, and Xiaoming Xie

DOI: 10.1364/PRJ.419514 Received 11 Jan 2021; Accepted 23 Mar 2021; Posted 23 Mar 2021  View: PDF

Abstract: Generally, a superconducting nanowire single-photon detector (SNSPD) is composed of wires with a typical width of ~100 nm. Recent studies have found that superconducting stripes with a micrometer-scale width can also detect single photons. Compared with the SNSPD, the superconducting microstrip single-photon detector (SMSPD) have smaller kinetic inductance, higher working current, and lower requirement in fabrication accuracy, providing potential applications in the development of ultra-large active area detectors. However, the study on SMSPD is still in its infancy, and the realization of its high-performance and practical use remains an opening question. This study demonstrates a NbN SMSPD with a saturated system detection efficiency (SDE) of ~92.2% at a dark count rate of ~200 cps, a polarization sensitivity of ~1.03, and a timing jitter of ~48 ps, at the telecom wavelength of 1550 nm when coupled with a single mode fiber and operated at 0.84 K. Furthermore, the detector’s SDE is over 70% when operated at a 2.1-K closed-cycle cryocooler.

Non-iterative complex wave-field reconstruction based on Kramers-Kronig relations

Cheng Shen, Mingshu Liang, An Pan, and Changhuei Yang

DOI: 10.1364/PRJ.419886 Received 19 Jan 2021; Accepted 23 Mar 2021; Posted 25 Mar 2021  View: PDF

Abstract: A non-iterative and non-interferometric computational imaging method, synthetic aperture imaging based on Kramers-Kronig relations (KKSAI), to reconstruct complex wave-field is reported. By collecting images through a modified microscope system with pupil modulation capability, we show that the phase and amplitude profile of the sample at pupil limited resolution can be extracted from as few as two intensity images by using Kramers-Kronig (KK) relations. It is established that as long as each sub-aperture’s edge crosses the pupil center, the collected raw images are mathematically analogous to off-axis holograms. This in turn allows us to adapt a recently reported KK relations based phase recovery framework in off-axis holography for use in KKSAI. Since KKSAI is non-iterative, free of parameter tuning and is applicable to a wide range of samples, simulation and experiment results have proved that it has much lower computational burden and achieves the best reconstruction quality when compared with two existing phase imaging methods.

Theoretical study on residual infrared absorption of Ti: sapphire laser crystals

Qiaorui Gong, Yilun Yang, Qiannan Fang, Shanming Li, Min Xu, Chengchun Zhao, and Yin Hang

DOI: 10.1364/PRJ.418395 Received 23 Dec 2020; Accepted 22 Mar 2021; Posted 22 Mar 2021  View: PDF

Abstract: Residual infrared absorption is a key problem affecting the laser emission efficiency of Ti: sapphire crystal. However, the origin of this absorption is still controversial. In this paper, the origin of residual infrared absorption of Ti: sapphire crystal is systematically studied by using first principles method. According to the contact conditions of O octahedron in the crystal structure of Al2O3, four Ti3+-Ti3+ ion pair models and three Ti4+-Ti3+ ion pair models were defined and constructed. For the first time, the near infrared absorption spectra consistent well with the experimental results were obtained in specific theoretical models. The electronic structures and absorption spectra calculated show that the line-contact Ti3+-Ti3+ ion pair with antiferromagnetic coupling and the face-contact Ti4+-Ti3+ ion pair are two main contributors to the residual infrared absorption of Ti: sapphire, while some other ion pair models provide a basis for explaining more complex residual infrared absorption.

Lead-halide perovskites for next-generation self-powered photodetectors: A comprehensive review

Chandrasekar veeramalai, shuai feng, XIAOMING ZHANG, S.V.N Pammi, Vincenzo Pecunia, and Chuanbo li

DOI: 10.1364/PRJ.418450 Received 05 Feb 2021; Accepted 22 Mar 2021; Posted 22 Mar 2021  View: PDF

Abstract: Metal halide perovskites have aroused tremendous interest in optoelectronics due to their attractive properties, encouraging the development of high-performance devices for emerging application domains such as wearable electronics and Internet of Things. Specifically, the development of high-performance perovskite-based photodetectors, as an ultimate substitute for conventional photodetectors made up of inorganic semiconductors such as silicon, InGaAs, GaN and germanium based-commercial photodetectors, attracts great attention by virtue of its solution processing, film deposition technique and tunable optical properties. Importantly, perovskite photodetectors can also deliver high performance without external power source, so called self-powered perovskite photodetectors (SPPDs), and have found eminent application in next generation nanodevices operate independently, wirelessly and remotely. Earlier research reports clearly indicate that perovskite-based SPPDs have excellent photo responsive behavior and wide band spectral response ranges. This review aims to give a comprehensive summary of the research results on self-powered, lead-halide perovskite photodetectors. In spite of the high-performance perovskite photodetectors, their commercialization is hindered by long-term material stability under ambient conditions. So, commercial viability of perovskite photodetectors among the existing conventional photodetectors is highlighted. In addition, a brief introduction has been given on flexible, self-powered perovskite photodetectors. Finally, we put forward some perspective on the further development of perovskite based self-powered photodetector. We believe that this review can provide the state-of-the-art current research on self-powered perovskite photodetector and guide to the improvising path for enhancing the performance to meet the versatility of practical device application.

All-optical sampling of few-cycle infrared pulses using tunneling in a solid

Yangyang Liu, Shima Gholam Mirzaeimoghadar, John Beetar, Jonathan Nesper, Ahmed Yousif, Nrisimhamurty Madugula, and Michael Chini

DOI: 10.1364/PRJ.420916 Received 26 Jan 2021; Accepted 21 Mar 2021; Posted 22 Mar 2021  View: PDF

Abstract: Recent developments in ultrafast laser technology have resulted in novel few-cycle sources in the mid-infrared. Accurately characterizing the time-dependent intensities and electric field waveforms of such laser pulses is essential to their applications in strong-field physics and attosecond pulse generation, but this remains a challenge. Recently, it was shown that tunnel ionization can provide an ultrafast temporal “gate” for characterizing high-energy few-cycle laser waveforms capable of ionizing air. Here, we show that tunneling and multiphoton excitation in a dielectric solid can provide a means to measure lower-energy and longer-wavelength pulses, and we apply the technique to characterize microjoule-level near- and mid-infrared pulses. The method lends itself to both all-optical and on-chip detection of laser waveforms, as well as single-shot detection geometries.

Fully transparent MOVPE-grown AlGaN-based tunnel heterojunction LEDs emitting at 2 nm

Frank Mehnke, Christian Kuhn, Martin Guttmann, Luca Sulmoni, Verena Montag, Johannes Glaab, Tim Wernicke, and M Kneissl

DOI: 10.1364/PRJ.414315 Received 09 Nov 2020; Accepted 21 Mar 2021; Posted 22 Mar 2021  View: PDF

Abstract: We present the growth and electro-optical characteristics of fully transparent AlGaN-based tunnel heterojunction light emitting diodes (LEDs) emitting at 2 nm entirely grown by metalorganic vapor phase epitaxy. A GaN:Si interlayer was embedded into a highly Mg- and Si-doped Al₀.₈₇Ga₀.₁₃N tunnel junction to enable polarization field enhanced tunneling. The LEDs exhibit an on-wafer integrated emission power of 77 μW at 5 mA which correlates to an external quantum efficiency (EQE) of 0.29% with 45 μW emitted through the bottom sapphire substrate and 32 μW emitted through the transparent top surface. After depositing a highly reflective aluminum reflector, a maximum emission power of 1.73 mW was achieved at 100 mA under pulsed mode operation with a maximum EQE of 0.35% as collected through the bottom substrate.

Non-suspended optomechanical crystal cavities using As2S3 chalcogenide glass

Renduo Qi, Qiancheng Xu, ning wu, Kaiyu Cui, Wei Zhang, and Yidong Huang

DOI: 10.1364/PRJ.417933 Received 18 Dec 2020; Accepted 21 Mar 2021; Posted 22 Mar 2021  View: PDF

Abstract: An optomechanical crystal cavity with non-suspended structure using As2S3 material is proposed. The principle of mode confinement in the non-suspended cavity is analyzed, and two different types of optical and acoustic defect modes are calculated through appropriate design of the cavity structure. An optomechanical coupling rate of 82.3 kHz is obtained in the proposed cavity, and the designed acoustic frequency is 3.44 GHz. The acoustic mode coupling between two non-suspended optomechanical crystal cavities is also demonstrated, showing that the proposed cavity structure has great potential for realizing further optomechanical applications in multi-cavity systems.

Lamellar hafnium ditelluride as an ultrasensitive surface-enhanced Raman scattering platform for label-free detection of uric acid

Yang Li, Haolin Chen, Yanxian Guo, Kangkang Wang, Yue Zhang, Peilin Lan, Jinhao Guo, Wen Zhang, Huiqing Zhong, Zhou Guo, Zhengfei Zhuang, and ZhinMing Liu

DOI: 10.1364/PRJ.421415 Received 04 Feb 2021; Accepted 20 Mar 2021; Posted 23 Mar 2021  View: PDF

Abstract: The development of two-dimensional (2D) transition metal dichalcogenides has been in rapid growth phase for the utilization in surface-enhanced Raman scattering (SERS) analysis. Herein, we report a promising 2D transition metal tellurides (TMTs) material, hafnium ditelluride (HfTe₂), as ultrasensitive platform for Raman identification of trace molecules, which demonstrates extraordinary SERS activity in sensitivity, uniformity and reproducibility. The highest Raman enhancement factor of 2.32 × 10⁶ is attained for rhodamine 6G molecule through the highly efficient charge transfer process at the interface between the HfTe₂ layered structure and the adsorbed molecules. At the same time, we provide an effective route for large-scale preparation of SERS substrates in practical applications via a facile stripping strategy. Further application of the nanosheets for reliable, rapid and label-free SERS fingerprint analysis of uric acid molecules, one of the biomarker associated with gout disease, is performed, which indicates arresting SERS signals with the limits of detection as low as 0.1 mM. The study based on this type of 2D SERS substrate not only reveals the great feasibility of applying TMTs to SERS analysis, also paves the way for nanodiagnostics, especially early marker detection.

Free-space Realization of Tunable Pin-like Optical Vortex Beams

domenico bongiovanni, Denghui Li, Michail Goutsoulas, Hao Wu, Yi Hu, Daohong song, Roberto Morandotti, Nikolaos Efremidis, and Zhigang Chen

DOI: 10.1364/PRJ.420872 Received 27 Jan 2021; Accepted 19 Mar 2021; Posted 19 Mar 2021  View: PDF

Abstract: We demonstrate, both analytically and experimentally, free-space ultra-long pin-like optical vortex beams. Such angular-momentum-carrying beams feature tunable peak intensity and undergo robust anti-diffracting propagation, realized by judiciously modulating both the amplitude and the phase profile of a standard laser beam. Specifically, they are generated by impressing a radially-symmetric power-law phase that adds an orbital angular momentum term carrying the intrinsic topological charge. During propagation, these vortex beams initially exhibit autofocusing dynamics in free-space. Subsequently, their amplitude patterns morph into a high-order Bessel-like profile, characterized by a hollow-core and an annular main-lobe with a constant or tunable width during propagation. In contrast with numerous previous endeavors on Bessel beams, our work represents the first demonstration of long-distance free-space generation of optical vortex “pins”, with their peak intensity evolution controlled by the impressed amplitude structure. Both the Poynting vectors and the optical radiation forces associated with these beams are also numerically analyzed, revealing fascinating properties that may be useful for a wide range of applications.

Characterisation of Dynamic Distortion in LED Light Output for Optical Wireless Communication

Anton Alexeev, Jean-Paul Linnartz, Kumar Arulandu, and Xiong Deng

DOI: 10.1364/PRJ.416269 Received 01 Dec 2020; Accepted 19 Mar 2021; Posted 19 Mar 2021  View: PDF

Abstract: Light Emitting Diodes (LEDs) are widely used for data transmission in emerging Optical Wireless Communication (OWC) systems. This paper analyses the physical processes limiting the bandwidth and causing nonlinearities in the light output of the modern high efficiency LEDs.The processes of carrier transport, as well as carrier storage, recombination and leakage in the active-region appear to affect communication performance, but such purely physics-based models are not yet commonly considered in the algorithms to optimize OWC systems.Using a dynamic modeling of these phenomena, we compile an (invertable) signal processing model that describes signal distortion and a parameter estimation procedure that is feasible in an operational communication link. We combine multipleapproaches for steady-state and dynamic LED characterization to estimate such parameters. %This ensures that the model is independent of boundary conditions.We verify that for a high-efficiency blue GaN LED, the models become sufficiently accurate to allow digital compensation. We compare the simulation results using the model against optical measurements of harmonic distortion and against measurements of the LED response to a deep rectangular current modulation. We show how the topology of the model can be simplified, and we address self-calibration techniques and discuss the limits of the presented approach. The model is suitable for creation of improved non-linear equalizers to enhance the achievable bit rate in LED-based OWC systems and is significantly more realistic than LED models commonly used in communication systems.

A Steering Paradox for Einstein-Podolsky-Rosen Argument and its Extended Inequality

Xiaoqi Zhou, Tianfeng Feng, Qin Feng, Changliang Ren, Maolin Luo, Xiaogang Qiang, and Jing-Ling Chen

DOI: 10.1364/PRJ.411033 Received 06 Oct 2020; Accepted 17 Mar 2021; Posted 19 Mar 2021  View: PDF

Abstract: The Einstein-Podolsky-Rosen (EPR) paradox is one of the milestones in quantum foundations, arising from the lack of local realistic description of quantum mechanics. The EPR paradox has stimulated an important concept of "quantum nonlocality", which manifests itself by three different types: quantum entanglement, quantum steering, and Bell nonlocality. Although Bell nonlocality is more often used to show the ``quantum nonlocality', the original EPR paradox is essentially a steering paradox. In this work, we formulate the original EPR steering paradox into a contradiction equality "k=1", thus making it amenable to an experimental verification. We perform an experimental test of the steering paradox "2=1" in a two-qubit scenario. Furthermore, by starting from the steering paradox "k=1', we generate a generalized linear steering inequality and transform this inequality into a mathematically equivalent form, which is more friendly for experimental implementation, i.e., one may only measure the observables in x-, y-, or z-axis, rather than other arbitrary directions. We also perform experiments to demonstrate this scheme. Within the experimental errors, the experimental results coincide with the theoretical predictions. Our results deepen the understanding of quantum foundations and provide an efficient way to detect the steerability of quantum states.

Single-sweep volumetric optoacoustic tomography of whole mice

XOSÉ DEÁN-BEN, Daniel Razansky, and Sandeep Kumar Kalva

DOI: 10.1364/PRJ.418591 Received 04 Jan 2021; Accepted 15 Mar 2021; Posted 16 Mar 2021  View: PDF

Abstract: Applicability of optoacoustic imaging in biology and medicine is largely determined by a number of key performance characteristics. In particular, an inherent trade-off exists between the acquired field-of-view (FOV) and temporal resolution of the measurements, which may hinder studies looking at rapid biodynamics at the whole-body level. Here, we report on a single-sweep volumetric optoacoustic tomography (sSVOT) system that attains whole body 3D mouse scans with better than 200µm spatial resolution and an unprecedented scanning speed of only 1.8 seconds. The system employs a spherical matric array transducer in conjunction with uniform multi-beam illumination delivery, the latter playing a critical role in facilitating the optimal FOV and imaging speed performance. The system further takes advantage of the spatial response of the individual ultrasound detection elements in order to mitigate common image artifacts related to limited-view acquisitions, thus enabling ultrafast acquisitions without compromising image quality and contrast. We compare performance metrics to the previous spiral volumetric optoacoustic tomography implementations and alternative image compounding and reconstruction strategies. It is anticipated that sSVOT opens new venues for studying large-scale biodynamics, such as accumulation and clearance of molecular agents and drugs across multiple organs, circulation of cells, as well as functional responses to stimuli.

High-resolution two-photon transcranial imaging of brain using direct wavefront sensing

Congping Chen, Zhongya Qin, Sicong He, Shaojun Liu, Shun-Fat Lau, Wanjie Wu, Dan Zhu, Nancy Ip, and Jianan Qu

DOI: 10.1364/PRJ.420220 Received 20 Jan 2021; Accepted 14 Mar 2021; Posted 16 Mar 2021  View: PDF

Abstract: Imaging of the brain in its native state at high resolution poses major challenges to visualization techniques. Two-photon microscopy integrated with the thinned-skull or optical clearing skull technique provides a minimally invasive tool for in vivo imaging of the cortex of mice without activating immune response and inducing brain injury. However, the imaging contrast and resolution are severely compromised by the optical heterogeneity of the skull, limiting the imaging depth to the superficial layer. In this work, an optimized configuration of adaptive optics two-photon microscope system and new wavefront sensing algorithm are proposed for accurate correction for the aberrations induced by the skull window and brain tissue. Using this system, we achieved high-resolution transcranial imaging of layer 5 pyramidal neurons up to 700 µm below pia in living mice. In addition, we investigated microglia-plaque interaction in living brain of Alzheimer’s disease and demonstrated high-precision laser dendrotomy and single-spine ablation.

Nonreciprocal transition between two nondegenerate energy levels

xun wei xu, Yanjun Zhao, Hui Wang, Aixi Chen, and Yuxi Liu

DOI: 10.1364/PRJ.412904 Received 10 Nov 2020; Accepted 10 Mar 2021; Posted 12 Mar 2021  View: PDF

Abstract: Stimulated emission and absorption are two fundamental processes of light-matter interaction, and the coefficients of the two processes should be equal in general. However, we will describe a generic method to realize significant difference between the stimulated emission and absorption coefficients of two nondegenerate energy levels, which we refer to as 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 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.

Quantifying quantum coherence of optical cat states

Miao Zhang, Haijun Kang, Meihong Wang, Fengyi Xu, Xiaolong Su, and Kunchi Peng

DOI: 10.1364/PRJ.418417 Received 23 Dec 2020; Accepted 10 Mar 2021; Posted 12 Mar 2021  View: PDF

Abstract: Optical cat state plays an essential role in quantum computation and quantum metrology. Here, we experimentally quantify quantum coherence of an optical cat state by means of relative entropy and $l_{1}$ norm of coherence in Fock basis based on the prepared optical cat state at rubidium D1 line. By transmitting the optical cat state through a lossy channel, we also demonstrate the robustness of quantum coherence of optical cat state in the presence of loss, which is different from the decoherence properties of fidelity and Wigner function negativity of the optical cat state. Our results confirm that quantum coherence of optical cat states is robust against loss and pave the way for the application with optical cat states.

Cylindrical vector beam revealing multipolar nonlinear scattering for super-localization of silicon nanostructures

Bin Wang, Ying Che, Xiangchao Zhong, Wen Yan, Tianyue Zhang, Kai Chen, Yi Xu, Xiaoxuan Xu, and Xiangping Li

DOI: 10.1364/PRJ.419300 Received 08 Jan 2021; Accepted 10 Mar 2021; Posted 25 Mar 2021  View: PDF

Abstract: The resonant optical excitation of dielectric nanostructures offers unique opportunities for developing remarkable nanophotonic devices. Light that is structured by tailoring the vectoral characteristics of the light beam provides an additional degree of freedom in achieving flexible control of multipolar resonances at the nanoscale. Here, we investigate the nonlinear scattering of subwavelength silicon nanostructures with radially and azimuthally polarized cylindrical vector beams to show a strong dependence of the photothermal nonlinearity on the polarization state of the applied light. The resonant magnetic dipole, selectively excited by an azimuthally polarized beam, enables enhanced photothermal nonlinearity, thereby inducing large scattering saturation. In contrast, radially polarized beam illumination shows no observable nonlinearity owing to off-resonance excitation. Numerical analysis reveals a difference of more than two orders of magnitude in photothermal nonlinearity under two types of polarization excitations. Nonlinear scattering and the unique doughnut-shaped focal spot generated by the azimuthally polarized beam are demonstrated as enabling far-field high-resolution localization of nanostructured Si with an accuracy approaching 50 nm. Our study extends the horizons of active silicon photonics and holds great potential for label-free super-resolution imaging of silicon nanostructures.

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, Yong-min Jung, Shaif-Ul Alam, and David Richardson

DOI: 10.1364/PRJ.412342 Received 12 Oct 2020; Accepted 09 Mar 2021; Posted 09 Mar 2021  View: PDF

Abstract: 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 (SLM) 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 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.

Immensely Enhanced Color-Adjustable Up-conversion Fluorescence in Electron Donor-Acceptor Exciplex Chromophores Doped with Fluorescent Emitters

Zhen Chen, Qian Zhou, Huitian Du, Yuan Yu, Chuang Zhang, Shenghao Han, and Zhiyong Pang

DOI: 10.1364/PRJ.412860 Received 19 Oct 2020; Accepted 09 Mar 2021; Posted 09 Mar 2021  View: PDF

Abstract: Two-photon excited fluorescence (TPEF) materials usually suffer from inefficient two-photon absorption (TPA) and non-radiative excited states. Here, up-conversion fluorescence in electron donor-acceptor (D-A) exciplex doped with fluorescent emitters are systemitically investigated. It has been found that the un-doped D-A exciplex exhibits enhancements of ~129% and ~365% in up-conversion fluorescence compared with donor- and acceptor-only systems, respectively. Interestingly, photoluminescence quantum yields (PLQY) upto ~98.65% were measured and immensely enhanced up-conversion fluorescence was observed after doping various fluorescent emitters into D-A exciplex. Our results reveal the existence of two-photon excited energy harvesting in thermally activated delayed fluorescence (TADF) D-A exciplex doped with fluorescent emitters, via reverse intersystem crossing (RISC) followed by rapid Förster resonance energy transfer (FRET). Moreover, additional gain mechanism related to intermolecular CT interaction occurs at TPA stage is found in TADF D-A exciplex system.

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, Aiping Luo, Pu Zhou, Kenneth Kin-Yip Wong, Wen-Cheng Xu, and Zhi-Chao Luo

DOI: 10.1364/PRJ.419273 Received 07 Jan 2021; Accepted 09 Mar 2021; Posted 09 Mar 2021  View: PDF

Abstract: 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 ultra-high 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 0 fs could be achieved. The development of dissipative soliton seed laser represents the first step in achieving the high-energy chirped pulse amplification (CPA) system at 1.7-μm waveband, which would find potential applications in fields such as biomedical imaging and material processing.

Breakdown of Maxwell Garnett theory due to evanescent fields at deep-subwavelength scale

Ting Dong, jie luo, Hongchen Chu, Xiang Xiong, and Lai Yun

DOI: 10.1364/PRJ.409248 Received 02 Sep 2020; Accepted 08 Mar 2021; Posted 09 Mar 2021  View: PDF

Abstract: 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 utilizing two- and three-dimensional dielectric composite materials with inhomogeneities at the scale of λ/100, we show that local evanescent fields generally occur nearby 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 made a correction to the Maxwell Garnett theory, such 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. 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.

Plasmonic Anapole States of Active Metamolecules

Gui-Mian Pan, Fang-Zhou Shu, Le Wang, liping Shi, and Andrey Evlyukhin

DOI: 10.1364/PRJ.416256 Received 10 Dec 2020; Accepted 08 Mar 2021; Posted 08 Mar 2021  View: PDF

Abstract: Anapole state, accompanied by strong suppression of light scattering, has attracted extensive attention in recent years due to its supreme performances 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 touched the topic of plasmonic anapole states. Here we demonstrate theoretically and numerically that the 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 plasmon system by introducing of a gain material, 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 increasing of metamolecule components is discussed.

Research ArticlePhotonics Research1One-step implementation of Rydberg-antiblockedSWAP and controlled-SWAP gates with modifiedrobustness

Jin-Lei Wu, Yan Wang, Jin xuan Han, Yu-kun Feng, Shi-Lei Su, Yan Xia, Yongyuan Jiang, and Jie Song

DOI: 10.1364/PRJ.415795 Received 24 Nov 2020; Accepted 07 Mar 2021; Posted 08 Mar 2021  View: PDF

Abstract: The prevalent fashion of executing Rydberg-mediated two- and multi-qubit quantum gates in neutral atomic systems is to pump Rydberg excitations using multi-step piecewise pulses in the Rydberg blockade regime. Here we propose to synthesize Λ-type Rydberg antiblockade~(RAB) of two neutral atoms using periodic fields, which facilitates one-step implementations of SWAP and Fredkin 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 Fredkin gates, and may pave a way to enhance robustness of RAB-based gates.

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

DOI: 10.1364/PRJ.418445 Received 06 Jan 2021; Accepted 04 Mar 2021; Posted 12 Mar 2021  View: PDF

Abstract: Being invisible at will is a long-standing dream for centuries, epitomized by numerous legends, where human have never stopped their exploration steps to realize the dream. Especially, recent years have witnessed a breakthrough due to the advent of transformation optics, metamaterials and metasurfaces. However, the previous metasurface cloaks typically work in a reflection manner that rely on a high-reflection background, thus limiting the applications. Here, we propose a facile yet viable approach to realize the transmitted metasurface cloak, composed of two planar metasurfaces to hide an object inside, such as a cat. To tackle the hard-to-convergence issue caused by non-uniqueness phenomenon, we deploy a tandem neural network to efficiently streamline the inverse design. Once pre-trained, the tandem neural network can work for a customer-desired electromagnetic response in one-single forward computation, saving a great amount of time. Our work provides a new avenue to realize transparent invisibility cloak, meriting a broad range of applications. And the tandem neural network can also inspire the inverse design of other meta-devices.

Ultralarge Rabi splitting and broadband Strong Coupling in spherical hyperbolic metamaterial cavity

Ping Gu, Jing Chen, Siyu Chen, Chun Yang, Zuxing Zhang, Wei Du, Zhendong Yan, Chaojun Tang, and zhuo chen

DOI: 10.1364/PRJ.417648 Received 18 Dec 2020; Accepted 04 Mar 2021; Posted 08 Mar 2021  View: PDF

Abstract: 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, the 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 of as large as 788 meV arising from the SC between 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 by 7 alternating layers of silver/ dielectric materials. In addition, the new hybrid modes formed by the SC are demonstrated to exhibit 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 ultrabroad range from visible to near-infrared by simply changing the dielectric core sizes. These findings may find potential applications in bright single photon source.

Extremely regular periodic surface structures in a large area efficiently induced on silicon by temporally shaped femtosecond laser

Yuchan Zhang, Q.L J, Kaiqiang Cao, Tianqi Chen, Chengke No Last Name, shi zhang, Donghai Feng, Tianqing Jia, Zhenrong Sun, and Jianrong Qiu

DOI: 10.1364/PRJ.418937 Received 11 Jan 2021; Accepted 04 Mar 2021; Posted 08 Mar 2021  View: PDF

Abstract: 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 LSFL are efficiently fabricated on silicon. The scan velocity for fabricating regular LSFL is 2.3 times faster, while the LSFL depth is two times deeper, and the diffraction efficiency is three 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.

Polarization-robust mid-infrared carpet cloak with minimized lateral shift

Yao Huang, Jingjing Zhang, Jinhui Zhou, Bo Qiang, Zhengji XU, Lin Liu, Jifang Tao, Nicolas Kossowski, Qijie Wang, and Yu Luo

DOI: 10.1364/PRJ.414437 Received 12 Nov 2020; Accepted 04 Mar 2021; Posted 25 Mar 2021  View: PDF

Abstract: With the advent and rapid development of the transformation optics and metamaterials, invisibility cloaks have captivated much attention in recent years. While most cloaking schemes suffer from limited bandwidth, the carpet cloak which can hide an object on a reflecting plane can operate over a broadband frequency range. However, carpet cloaks experimentally realized thus far still have several limitations. For example, the quasi-conformal mapping carpet cloak leads to a lateral shift of the reflected light ray, while the birefringent carpet cloak only works for a specific polarization. In this work, we propose a conformal transformation scheme to tackle these two problems, simultaneously. As an example, we design a mid-infrared carpet cloak in a silicon platform and demonstrate its polarization-insensitive property as well as the minimized lateral shift over a broad frequency band from 24 THz to 28.3 THz.

Concise and efficient direct-view generation of arbitrary cylindrical vector beams by vortex half-wave plate

Junli Qi, Weihua Wang, Bo Shi, Hui Zhang, yanan shen, Haifei Deng, Wenjing pu, Xin Liu, Huihui Shan, Xiaomin Ma, Lianqing Zhang, Wei Lu, Meicheng Fu, and Xiujian Li

DOI: 10.1364/PRJ.419561 Received 11 Jan 2021; Accepted 03 Mar 2021; Posted 03 Mar 2021  View: PDF

Abstract: 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 vortex half-wave plate (VHP). Six kinds of first-order and other high-order CV beams, such as azimuthally polarized (AP) beam, even mode polarized beam and 3-order AP beam, are formed by simply rotating a half-wave plate. The Stokes parameters and double-slit interference of multi-type 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. Besides, 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 for and for , where P is the polarization order number, and the fringe tilt offset is positively related to the topological charge number l of CVV 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 the practical performance, which shows the potential to be implemented as a commercial device.

Distributed static and dynamic detection of acoustic wave in Brillouin random fiber laser

Zichao Zhou, Haiyang Wang, Yuan Wang, Liang Chen, and Xiaoyi Bao

DOI: 10.1364/PRJ.415747 Received 23 Nov 2020; Accepted 01 Mar 2021; Posted 03 Mar 2021  View: PDF

Abstract: The interaction of random laser and random medium is important to understand the noise origin in random fiber lasers. Here, using the optical time domain reflectometry (OTDR) method, the time resolved distributed acoustic dynamic grating in a Brillouin random fiber laser (BRFL) is characterized which reflects 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 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 position are 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 random medium, giving us a new perspective to understand the fundamental physics of the random lasing process and its noise property.

Accurate Inverse Design of Fabry–Pérot-Cavity-Based Color Filters far beyond sRGB via a Bidirectional Artificial Neural Network

Peng Dai, Yasi Wang, Yueqiang Hu, C. de Groot, Otto Muskens, Huigao Duan, and Ruomeng Huang

DOI: 10.1364/PRJ.415141 Received 26 Nov 2020; Accepted 01 Mar 2021; Posted 01 Mar 2021  View: PDF

Abstract: Structural color based on Fabry–Pérot (F-P) cavity enables a wide color gamut with high resolution at sub-microscopic 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 ΔE2000 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. A 100,000 times improvement in the design efficiency has been demonstrated by comparing the neural network to the meta-heuristic optimization technique using evolutionary algorithm. 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.

High-speed and high-power germanium photodetector with lateral silicon nitride waveguide

Xiao Hu, dingyi Wu, Hongguang Zhang, Weizhong Li, Daigao Chen, Lei Wang, Xi Xiao, and Shaohua Yu

DOI: 10.1364/PRJ.417601 Received 15 Dec 2020; Accepted 01 Mar 2021; Posted 01 Mar 2021  View: PDF

Abstract: Up to now, the optical coupling configuration of waveguide-integrated germanium-on-silicon photodetector (Ge-on-Si PD) can be divided into three main categories: butt-coupling, bottom-coupling, and top-coupling. 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 be served as the fourth waveguide-integrated optical coupling configuration: lateral-coupling. The Ge-on-Si PD with double lateral Si3N4 waveguide features uniform optical field distribution in Ge region, which is very beneficial to improving the operation speed for high input optical power. The comprehensively characterizing the proposed lateral-coupling Si3N4 waveguide Ge-on-Si PD is presented. Under 25 mW input optical power, the typical internal responsivity is evaluated to be 0.52 A/W at 1550 nm. 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 (RF) measurements, under 4 mA photocurrent, a 60 GHz bandwidth operating at -3 V bias voltage is demonstrated. When the photocurrent up to 12 mA, the 3-dB OE bandwidth still has 36 GHz. With 1 mA photocurrent, the 70 Gbit/s, 80 Gbit/s, 90 Gbit/s, and 100 Gbit/s non-return-to-zero (NRZ), and 100 Gbit/s, 120 Gbit/s, 140 Gbit/s, and 150 Gbit/s four-level pulse amplitude modulation (PAM-4) clear opening of eye diagrams are experimentally obtained without utilizing any offline digital signal processing (DSP) at receiver (RX) side. In order to verify the high-power handling performance in high-speed data transmission, we investigate the eye diagram variations with the increasing of photocurrent. The clear open electrical eye diagrams of 60 Gbit/s NRZ under 20 mA photocurrent is also obtained. Overall, the proposed lateral Si3N4 waveguide structure is a flexibly extendable to optical coupling configuration of PD, which makes it very attractive for developing high-performance silicon photonic integrated circuits (PICs) in the future.

The first demonstration of on-chip quadplexer for passive optical network systems

Dajian Liu, long zhang, Hexin Jiang, and Daoxin Dai

DOI: 10.1364/PRJ.420545 Received 22 Jan 2021; Accepted 01 Mar 2021; Posted 01 Mar 2021  View: PDF

Abstract: An on-chip quadplexer is proposed and demonstrated with four wavelength-channels of 1270, 1310, 1490, and 1577 nm for the first time. The present quadplexer consists of four cascaded multimode-waveguide-grating (MWG)-based filters, which compose of a two-mode (de)multiplexer and an MWG. For the fabricated quadplexer on silicon, all four wavelength-channels have flat-top responses with low excess losses of <0.5 dB as well as the desired bandwidths, which are about 16, 38, 19, and 6 nm, respectively. The crosstalk for both upstream channels and downstream channels is less than −24 dB. Moreover, the data transmission of 10 Gb/s of the present silicon quadplexer is also demonstrated successfully.

Phonon laser sensing in a hetero optomechanical crystal cavity

Kaiyu Cui, Zhilei Huang, Qiancheng Xu, Fei Pan, Jian Xiong, Xue Feng, Fang Liu, Wei Zhang, Yidong Huang, and ning wu

DOI: 10.1364/PRJ.403833 Received 30 Jul 2020; Accepted 01 Mar 2021; Posted 23 Mar 2021  View: PDF

Abstract: Micro- and nanomechanical resonators have emerged as promising platforms for sensing a broad range of physical properties, such as mass, force, torque, magnetic field, and acceleration. The sensing performance relies critically on the motional mass, the mechanical frequency, and the linewidth of the mechanical resonator. Herein, we demonstrate a hetero optomechanical crystal (OMC) cavity based on a silicon nanobeam structure. The cavity supports phonon lasing in a fundamental mechanical mode with a frequency of 5.91 GHz, an effective mass of 116 fg, and a mechanical linewidth narrowing in the range from 3.3 MHz to 5.2 kHz, while the optomechanical coupling rate is as high as 1.9 MHz. With this phonon laser, on-chip sensing can be attained with a resolution of δλ/λ = 1.0×10⁻⁸, which is at least two orders of magnitude larger than that obtained with conventional silicon-based sensors. The use of a silicon-based hetero OMC cavity that harnesses phonon lasing could pave the way toward high-precision sensors that allow silicon monolithic integration and offer unprecedented sensitivity for a broad range of physical sensing applications.

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, Sun Xiaojuan, and Dabing Li

DOI: 10.1364/PRJ.418813 Received 05 Jan 2021; Accepted 23 Feb 2021; Posted 23 Feb 2021  View: PDF

Abstract: 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 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 the illumination intensity for the light source becomes increased.

Singlemode spatiotemporal soliton attractor in multimode GRIN fibers.

Mario Zitelli, Mario Ferraro, Fabio Mangini, and Stefan Wabnitz

DOI: 10.1364/PRJ.419235 Received 06 Jan 2021; Accepted 23 Feb 2021; Posted 23 Feb 2021  View: PDF

Abstract: Experimental and numerical studies of spatiotemporal femtosecond soliton propagation over up to 1 km spans of parabolic graded-index (GRIN) fibers reveal that initial multimode soliton pulses naturally and irreversibly evolve into a singlemode soliton. This is carried by the fundamental mode of the fiber, which acts as a dynamical attractor of the multimode system for up to the record value (for multimode fibers) of 5600 chromatic dispersion distances. This experimental evidence of soliton beam self-cleaning invalidates the use of variational approaches, which intrinsically require that the initial multimode propagation of a self-imaging soliton is indefinitely maintained.

Spin-selective corner reflector for retro-reflection and absorption by circular dichroitic manner

he wang, Yao Jing, Yongfeng li, Lingling Huang, maochang feng, Qi Yuan, Jiafu Wang, Jieqiu Zhang, and Shaobo Qu

DOI: 10.1364/PRJ.422509 Received 11 Feb 2021; Accepted 21 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: Recent years have witnessed an extraordinary spurt in attention toward manipulating electromagnetic waves by metasurfaces. Particularly, tailoring circular polarization attracts great amounts of interest in both microwave and optics regimes. Circular dichroism, an exotic chiroptical effect of natural molecules, has aroused widely discussion in this issue yet it is still in its infancy. Herein, we initiate circular dichroism followed by controlling spin-selective wave-fronts 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 in microwave frequency. Encouragingly, our effort provides an alternative approach for tailoring electromagnetic waves in a circular dichroitic manner and may also find applications in multi-functional systems in optics and microwave regimes.

FourierCam: A camera for video spectrum acquisition in single-shot

Chengyang Hu, Honghao Huang, Minghua Chen, Sigang Yang, and Hongwei Chen

DOI: 10.1364/PRJ.412491 Received 13 Oct 2020; Accepted 21 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: 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 single-shot through optical coding. Compared with 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-light imaging, 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.

Controllable one-step doping synthesis for white-light emission of cesium copper iodide perovskites

Ranran Fan, SHAOFAN FANG, Chengchuan Liang, Zhaoxing Liang, and Haizhe Zhong

DOI: 10.1364/PRJ.415015 Received 13 Nov 2020; Accepted 19 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: 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-emission with CIE coordinates of (0.3397, 0.3325) and a CRI reaching up to 90 was realized in a convenient way. Through doing impurities into the Cs3Cu2I5 system, high efficiency and stable CsCu2I3 weresynthesized, strikingly, blue-emitting Cs3Cu2I5 and yellow-emitting CsCu2I3 could coexist, and their respective luminescence was not interacted in the compound powder, which was beneficial to acquire 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.

Incoherent imaging through highly dynamic and optically thick turbid media based on neural network

Shanshan Zheng, Hao Wang, Shi Dong, Fei Wang, and Guohai Situ

DOI: 10.1364/PRJ.416246 Received 01 Dec 2020; Accepted 18 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: Imaging through dynamic scattering media is one of the major challenges in optics, which is encountered in imaging through dense fog or turbid water and many other situations. Here, we propose a single-shot incoherent imaging through highly dynamic 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 a strong interference noise. We calibrate the optical thickness of the tank of turbid media is as high as 16 and the signal-to-interference ratio (SIR) 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 chaotic environment.

Spatio-spectral Transformation of Non-collimated LightBeam Diffracted by Ultrasound in Birefringent Crystals

Alexander Machikhin, Alexey Gorevoy, Grigoriy Martynov, and Vitold Pozhar

DOI: 10.1364/PRJ.417992 Received 17 Dec 2020; Accepted 17 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: Spatio-spectral structure of wave phase-matching in birefringent crystals has a strong dependence onthe geometry of the acousto-optic interaction and incident light spectrum. This dependence defines de-tails of light beam profile transformation. It is especially important for imaging applications related toa large angular aperture and wide spectral bandwidth of the incident light. In this paper, we demonstrateaccurate three-dimensional plotting of light transmission pattern without small birefringence approxima-tion. Rather complicated shape of phase-matching locus in spatio-spectral domain inevitably leads tothe residual spatially non-uniform chromatic aberrations in the spectral image. Theoretical considerationand computational modeling are confirmed by the experiments on Bragg diffraction in paratellurite crys-tal. The results are especially important for the development of imaging acousto-optical devices and laserbeam shaping technologies.

Direct demonstration of carrier distribution and recombination within step-bunched UV-LED

Houqiang Xu, Jiean Jiang, Li Chen, Jason Hoo, long yan, Shiping Guo, Cai Shen, Yanping Wei, Zi Hui Zhang, Wei Guo, Jichun Ye, and Hua Shao

DOI: 10.1364/PRJ.411832 Received 06 Oct 2020; Accepted 16 Feb 2021; Posted 02 Mar 2021  View: PDF

Abstract: 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 counterpart. One of the inherent reasons is due to 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 high-misoriented substrates proves to be useful, yet the carrier distribution and recombination mechanism in such structures were seldom reported. In this paper, 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 micro-scale current distribution and surface morphology was provided for the first time, directly demonstrating that current prefer to flow through the step edges of the epitaxial layers. Experimental results were further supported by numerical simulation, in which case efficient radiative recombination centers are formed in the inclined quantum well regions. A schematic 3-dimensional energy band structure of the MQWs across the step was proposed, helps in further understanding on the luminescence behavior of LEDs grown on misoriented substrates. Finally, a general technique to achieve carrier localization with enhanced output power of optoelectronic devices is proposed, which is valid for all ternary Ⅲ-Ⅴ semiconductors exhibiting the ability of phase separation, including Ⅲ-arsenic.

Dry-etched ultra-high-Q silica microdisk resonators on a silicon chip

Xiaoshun Jiang, Jiaxi Gu, Xinyu Cheng, Guanyu Li, Ziqi Bai, Qi Shi, and Min Xiao

DOI: 10.1364/PRJ.412840 Received 19 Oct 2020; Accepted 11 Feb 2021; Posted 12 Feb 2021  View: PDF

Abstract: We demonstrate the fabrication of ultra-high quality (Q) factor silica microdisk resonators on a silicon chip by inductively-coupled plasma (ICP) etching. We achieve a dry-etched optical microresonator with an intrinsic Q-factor as high as 1.94×108 from a 1-mm-diameter silica microdisk with a thickness of 4 μm. Our work provides a chip-based microresonator platform operating in the ultra-high-Q region that will be useful in nonlinear photonics such as Brillouin lasers and Kerr microcombs.

Stimulated emission at 1.54 µm from Erbium/Oxygen-doped silicon-based light emitting diodes

Jin Hong, Huimin Wen, Jiajing He, Jing Liu, Yaping Dan, Jens Tomm, Fangyu Yue, Duan Chungang, and Junhao Chu

DOI: 10.1364/PRJ.417090 Received 16 Dec 2020; Accepted 07 Feb 2021; Posted 08 Feb 2021  View: PDF

Abstract: 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 doped by ion implantation with erbium ions (Er3+) is considered a promising approach, but 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 disclose the complex carrier transfer dynamics from the silicon to Er3+ by relaxing electrons from the indirect conduction band of the silicon. This picture differs from the frequently-assumed energy transfer by 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 a way for fabricating superluminescent diodes or efficient LDs at communication wavelengths based on rare-earth doped silicon.

1.3 GHz E-O bandwidth quantum dot micro-LED for multi-gigabit visible light communication

Zixian Wei, Lei Wang, Chien Ju Chen, Lai Wang, Hongyan Fu, Li Zhang, KAI-CHIA Chen, Meng-Chyi Wu, Yuhan Dong, zhibiao hao, and YI LUO

DOI: 10.1364/PRJ.411863 Received 05 Oct 2020; Accepted 02 Feb 2021; Posted 02 Feb 2021  View: PDF

Abstract: 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 carrier recombination rate on E-O bandwidth in conventional c-plane LEDs based on InGaN quantum wells (QWs), a blue micro-LED with a single layer of InGaN quantum dots (QDs) active region is designed, fabricated, and packaged to realize a high-speed VLC system. The E-O bandwidth of the QD 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 (OOK) signal for a 3-meters real-time VLC. In addition, a 4-Gbps VLC using quadrature phase shift keying-orthogonal frequency division multiplexing (QPSK-OFDM) 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.

Towards smart optical focusing: Deep learning-empowered dynamic wavefront shaping in nonstationary scattering media

yunqi luo, Suxia Yan, Huanhao Li, Puxiang Lai, and Yuanjin Zheng

DOI: 10.1364/PRJ.415590 Received 20 Nov 2020; Accepted 21 Jan 2021; Posted 22 Jan 2021  View: PDF

Abstract: Optical focusing through scattering media is of great significance yet challenging in lots of scenarios, including biomedical imaging, optical communication, cybersecurity, and 3D displays etc. Wavefront shaping is a promising approach to solve this problem, but most implementations thus far have only dealt with static media which, however, deviates from realistic applications. Herein, we put forward a deep learning-empowered adaptive framework which is specifically implemented by a proposed Timely-Focusing-Optical-Transformation-Net (TFOTNet), and it effectively tackles the grand challenge of real-time light focusing and refocusing through time-variant media without complicated computation. The introduction of recursive fine-tuning allows timely focusing recovery, and the adaptive adjustment of hyperparameters of TFOTNet on the basis of medium changing speed efficiently handles the spatiotemporal non-stationarity of the medium. Simulation and experimental results demonstrate that the adaptive recursive algorithm with the proposed network significantly improves light focusing and tracking performance over traditional methods, permitting rapid recovery of an optical focus from degradation. It is believed that the proposed deep learning-empowered framework delivers a promising platform towards smart optical focusing implementations requiring dynamic wavefront control.

A Novel 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, Xin Ding, Yating Zhang, Jian-Quan Yao, and Silei Wang

DOI: 10.1364/PRJ.416580 Received 02 Dec 2020; Accepted 20 Jan 2021; Posted 02 Mar 2021  View: PDF

Abstract: 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 firstly 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 bias voltage of −2 V, which is the champion responsivity to date 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 remarkably advantages for the fabrication of high-performance PPDs and can provide a reference for the other similar research work.

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