Contents
2021
Volume: 9 Issue 4
41 Article(s)

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Special Issue on DEEP LEARNING IN PHOTONICS
Real-time deep learning design tool for far-field radiation profile
Jinran Qie, Erfan Khoram, Dianjing Liu, Ming Zhou, and Li Gao
The connection between Maxwell’s equations and artificial neural networks has revolutionized the capability and efficiency of nanophotonic design. Such a machine learning tool can help designers avoid iterative, time-consuming electromagnetic simulations and even allows long-desired inverse design. However, when we move from conventional design methods to machine-learning-based tools, there is a steep learning curve that is not as user-friendly as commercial simulation software. Here, we introduce a real-time, web-based design tool that uses a trained deep neural network (DNN) for accurate far-field radiation prediction, which shows great potential and convenience for antenna and metasurface designs. We believe our approach provides a user-friendly, readily accessible deep learning design tool, with significantly reduced difficulty and greatly enhanced efficiency. The web-based tool paves the way to present complicated machine learning results in an intuitive way. It also can be extended to other nanophotonic designs based on DNNs and replace conventional full-wave simulations with a much simpler interface.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 0400B104 (2021)
Sensing in the presence of strong noise by deep learning of dynamic multimode fiber interference
Linh V. Nguyen, Cuong C. Nguyen, Gustavo Carneiro, Heike Ebendorff-Heidepriem, and Stephen C. Warren-Smith
A new approach to optical fiber sensing is proposed and demonstrated that allows for specific measurement even in the presence of strong noise from undesired environmental perturbations. A deep neural network model is trained to statistically learn the relation of the complex optical interference output from a multimode optical fiber (MMF) with respect to a measurand of interest while discriminating the noise. This technique negates the need to carefully shield against, or compensate for, undesired perturbations, as is often the case for traditional optical fiber sensors. This is achieved entirely in software without any fiber postprocessing fabrication steps or specific packaging required, such as fiber Bragg gratings or specialized coatings. The technique is highly generalizable, whereby the model can be trained to identify any measurand of interest within any noisy environment provided the measurand affects the optical path length of the MMF’s guided modes. We demonstrate the approach using a sapphire crystal optical fiber for temperature sensing under strong noise induced by mechanical vibrations, showing the power of the technique not only to extract sensing information buried in strong noise but to also enable sensing using traditionally challenging exotic materials.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 0400B109 (2021)
Delay-weight plasticity-based supervised learning in optical spiking neural networks
Yanan Han, Shuiying Xiang, Zhenxing Ren, Chentao Fu, Aijun Wen, and Yue Hao
We propose a modified supervised learning algorithm for optical spiking neural networks, which introduces synaptic time-delay plasticity on the basis of traditional weight training. Delay learning is combined with the remote supervised method that is incorporated with photonic spike-timing-dependent plasticity. A spike sequence learning task implemented via the proposed algorithm is found to have better performance than via the traditional weight-based method. Moreover, the proposed algorithm is also applied to two benchmark data sets for classification. In a simple network structure with only a few optical neurons, the classification accuracy based on the delay-weight learning algorithm is significantly improved compared with weight-based learning. The introduction of delay adjusting improves the learning efficiency and performance of the algorithm, which is helpful for photonic neuromorphic computing and is also important specifically for understanding information processing in the biological brain.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 0400B119 (2021)
Free-space optical neural network based on thermal atomic nonlinearity
Albert Ryou, James Whitehead, Maksym Zhelyeznyakov, Paul Anderson, Cem Keskin, Michal Bajcsy, and Arka Majumdar
As artificial neural networks (ANNs) continue to make strides in wide-ranging and diverse fields of technology, the search for more efficient hardware implementations beyond conventional electronics is gaining traction. In particular, optical implementations potentially offer extraordinary gains in terms of speed and reduced energy consumption due to the intrinsic parallelism of free-space optics. At the same time, a physical nonlinearity—a crucial ingredient of an ANN—is not easy to realize in free-space optics, which restricts the potential of this platform. This problem is further exacerbated by the need to also perform the nonlinear activation in parallel for each data point to preserve the benefit of linear free-space optics. Here, we present a free-space optical ANN with diffraction-based linear weight summation and nonlinear activation enabled by the saturable absorption of thermal atoms. We demonstrate, via both simulation and experiment, image classification of handwritten digits using only a single layer and observed 6% improvement in classification accuracy due to the optical nonlinearity compared to a linear model. Our platform preserves the massive parallelism of free-space optics even with physical nonlinearity, and thus opens the way for novel designs and wider deployment of optical ANNs.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 0400B128 (2021)
Interfacing photonics with artificial intelligence: an innovative design strategy for photonic structures and devices based on artificial neural networks
Yihao Xu, Xianzhe Zhang, Yun Fu, and Yongmin Liu
Over the past decades, photonics has transformed many areas in both fundamental research and practical applications. In particular, we can manipulate light in a desired and prescribed manner by rationally designed subwavelength structures. However, constructing complex photonic structures and devices is still a time-consuming process, even for experienced researchers. As a subset of artificial intelligence, artificial neural networks serve as one potential solution to bypass the complicated design process, enabling us to directly predict the optical responses of photonic structures or perform the inverse design with high efficiency and accuracy. In this review, we will introduce several commonly used neural networks and highlight their applications in the design process of various optical structures and devices, particularly those in recent experimental works. We will also comment on the future directions to inspire researchers from different disciplines to collectively advance this emerging research field.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 0400B135 (2021)
On-demand design of spectrally sensitive multiband absorbers using an artificial neural network
Sunae So, Younghwan Yang, Taejun Lee, and Junsuk Rho
We report an approach assisted by deep learning to design spectrally sensitive multiband absorbers that work in the visible range. We propose a five-layered metal-insulator-metal grating structure composed of aluminum and silicon dioxide, and we design its structural parameters by using an artificial neural network (ANN). For a spectrally sensitive design, spectral information of resonant wavelengths is additionally provided as input as well as the reflection spectrum. The ANN facilitates highly robust design of a grating structure that has an average mean squared error (MSE) of 0.023. The optical properties of the designed structures are validated using electromagnetic simulations and experiments. Analysis of design results for gradually changing target wavelengths of input shows that the trained ANN can learn physical knowledge from data. We also propose a method to reduce the size of the ANN by exploiting observations of the trained ANN for practical applications. Our design method can also be applied to design various nanophotonic structures that are particularly sensitive to resonant wavelengths, such as spectroscopic detection and multi-color applications.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 0400B153 (2021)
Intelligent coding metasurface holograms by physics-assisted unsupervised generative adversarial network
Che Liu, Wen Ming Yu, Qian Ma, Lianlin Li, and Tie Jun Cui
Intelligent coding metasurface is a kind of information-carrying metasurface that can manipulate electromagnetic waves and associate digital information simultaneously in a smart way. One of its widely explored applications is to develop advanced schemes of dynamic holographic imaging. By now, the controlling coding sequences of the metasurface are usually designed by performing iterative approaches, including the Gerchberg–Saxton (GS) algorithm and stochastic optimization algorithm, which set a large barrier on the deployment of the intelligent coding metasurface in many practical scenarios with strong demands on high efficiency and capability. Here, we propose an efficient non-iterative algorithm for designing intelligent coding metasurface holograms in the context of unsupervised conditional generative adversarial networks (cGANs), which is referred to as physics-driven variational auto-encoder (VAE) cGAN (VAE-cGAN). Sharply different from the conventional cGAN with a harsh requirement on a large amount of manual-marked training data, the proposed VAE-cGAN behaves in a physics-driving way and thus can fundamentally remove the difficulties in the conventional cGAN. Specifically, the physical operation mechanism between the electric-field distribution and metasurface is introduced to model the VAE decoding module of the developed VAE-cGAN. Selected simulation and experimental results have been provided to demonstrate the state-of-the-art reliability and high efficiency of our VAE-cGAN. It could be faithfully expected that smart holograms could be developed by deploying our VAE-cGAN on neural network chips, finding more valuable applications in communication, microscopy, and so on.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 0400B159 (2021)
Learning to recognize misaligned hyperfine orbital angular momentum modes
Xiao Wang, Yufeng Qian, JingJing Zhang, Guangdong Ma, Shupeng Zhao, RuiFeng Liu, Hongrong Li, Pei Zhang, Hong Gao, Feng Huang, and Fuli Li
Orbital angular momentum (OAM)-carrying beams have received extensive attention due to their high-dimensional characteristics in the context of free-space optical communication. However, accurate OAM mode recognition still suffers from reference misalignment of lateral displacement, beam waist size, and initial phase. Here we propose a deep-learning method to exquisitely recognize OAM modes under misalignment by using an alignment-free fractal multipoint interferometer. Our experiments achieve 98.35% recognizing accuracy when strong misalignment is added to hyperfine OAM modes whose Bures distance is 0.01. The maximum lateral displacement we added with respect to the perfectly on-axis beam is about ±0.5 beam waist size. This work offers a superstable proposal for OAM mode recognition in the application of free-space optical communication and allows an increase of the communication capacity.
Photonics Research
  • Publication Date: Mar. 12, 2021
  • Vol.9 Issue, 4 04000B81 (2021)
Experimental study of neuromorphic node based on a multiwaveband emitting two-section quantum dot laser
George Sarantoglou, Menelaos Skontranis, Adonis Bogris, and Charis Mesaritakis
In this work, we present experimental results concerning excitability in a multiband emitting quantum-dot-based photonic neuron. The experimental investigation revealed that the same two-section quantum dot laser can be tuned through a simple bias adjustment to operate either as a leaky integrate and fire or as a resonate and fire neuron. Furthermore, by exploiting the inherent multiband emission of quantum-dot devices revealed by the existence of multiple lasing thresholds, a significant enhancement in the neurocomputational capabilities, such as spiking duration and firing rate, is observed. Spike firing rate increased by an order of magnitude that leads to an enhancement in processing speed and, more importantly, neural spike duration was suppressed to the picosecond scale, which corresponds to a significant temporal resolution enhancement. These new regimes of operation, when combined with thermal insensitivity, silicon cointegration capability, and the fact that these multiband mechanisms are also present in miniaturized quantum-dot devices, render these neuromorphic nodes a proliferating platform for large-scale photonic spiking neural networks.
Photonics Research
  • Publication Date: Mar. 22, 2021
  • Vol.9 Issue, 4 04000B87 (2021)
Engineering of multiple bound states in the continuum by latent representation of freeform structures
Ronghui Lin, Zahrah Alnakhli, and Xiaohang Li
Photonics Research
  • Publication Date: Mar. 25, 2021
  • Vol.9 Issue, 4 04000B96 (2021)
Research Articles
Fiber Optics and Optical Communications
Characterization of a VLC system in real museum scenario using diffusive LED lighting of artworks
Marco Seminara, Marco Meucci, Fabio Tarani, Cristiano Riminesi, and Jacopo Catani
Visible light communication (VLC) is currently recognized as a relevant technology for a wealth of possible application scenarios. New classes of services can be designed in both outdoor and indoor environments, exploiting the directionality of the optical channel and the low attainable latencies. Such features allow VLC to offer both spatial localization of users and wireless communication by using widespread high-power LEDs as simultaneous illumination and information sources. In the indoor scenario, one of the most promising deployments is expected in museums, where digital data can be cast by the specific illumination system of each artwork and received by visitors placed nearby. This would enable a full set of services, aiming, e.g., at an immersive experience in the augmented reality approach or at real-time localization of visitors. In this work, we characterize for the first time the performance of a photodiode-based VLC system in a real museum environment, performing an extensive measurement campaign on several masterpieces (wall, canvas, and wood paintings) in the Basilica of Santa Maria Novella in Florence, Italy. In particular, we demonstrate the possibility of using indirect (diffused) illumination light to deliver specific information on each artwork to a visitor. We characterize the quality of such non-line-of-sight VLC links by performing packet error rate measurements as a function of angle and distance from the artwork, and we measure the effective field of view (FoV) of our receiving stage, as well as the influence of side displacements of the receiver on the transmission quality, demonstrating that diffusive VLC links can also be used for efficient localization of users in front of each artwork in museum applications. With observed baud rates up to 28 kbaud and FoV values up to 60° for realistic distances up to 6 m, we believe our work could pave the way for future studies involving VLC in a wealth of indoor applications, beyond the cultural heritage sector.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000548 (2021)
Integrated Optics
Robust hybrid laser linewidth reduction using Si3N4-based subwavelength hole defect assisted microring reflector
Jiachen Li, Baoyu Zhang, Sigang Yang, Hongwei Chen, and Minghua Chen
We demonstrate a hybrid laser with a low intrinsic linewidth of 34.2 Hz and a high fiber-coupled output power of 11.7 dBm, by coupling a Si3N4-based subwavelength hole defect assisted microring reflector (SHDA-MRR) to a commercially available distributed feedback semiconductor laser. The proposed SHDA-MRR structure features an accurately controlled reflection response, with the manipulated modal coupling between two degenerate counterpropagating modes induced by a subwavelength hole defect embedded in the microring waveguide. With further joint optimization of cavity parameters, this Si3N4 SHDA-MRR structure is expected to reduce the laser intrinsic linewidth to a sub-hertz level. This work explores a low-cost and robust linewidth reduction scheme for the applications of high-speed coherent optical communications systems and high-resolution optical metrology.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000558 (2021)
Thermo-optically tunable spectral broadening in a nonlinear ultra-silicon-rich nitride Bragg grating | On the Cover
Yanmei Cao, Ezgi Sahin, Ju Won Choi, Peng Xing, George F. R. Chen, D. K. T. Ng, Benjamin J. Eggleton, and Dawn T. H. Tan
Spectral tunability methods used in optical communications and signal processing leveraging optical, electrical, and acousto-optic effects typically involve spectral truncation that results in energy loss. Here we demonstrate temperature tunable spectral broadening using a nonlinear ultra-silicon-rich nitride device consisting of a 3-mm-long cladding-modulated Bragg grating and a 7-mm-long nonlinear channel waveguide. By operating at frequencies close to the grating band edge, in an apodized Bragg grating, we access strong grating-induced dispersion while maintaining low losses and high transmissivity. We further exploit the redshift in the Bragg grating stopband due to the thermo-optic effect to achieve tunable dispersion, leading to varying degrees of soliton-effect compression and self-phase-modulation-induced spectral broadening. We observe an increase in the bandwidth of the output pulse spectrum from 69 to 106 nm as temperature decreases from 70°C to 25°C, in good agreement with simulated results using the generalized nonlinear Schrödinger equation. The demonstrated approach provides a new avenue to achieve on-chip laser spectral tuning without loss in pulse energy.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000596 (2021)
80 GHz germanium waveguide photodiode enabled by parasitic parameter engineering
Yang Shi, De Zhou, Yu Yu, and Xinliang Zhang
A high-speed germanium (Ge) waveguide photodiode (PD) is one of the key components of an integrated silicon photonics platform for large-capacity data communication applications, but the parasitic parameters limit the increase of its bandwidth. Several studies have been reported to reduce parasitic parameters, at the cost of compromising other performances. Here, we propose and investigate a bandwidth-boosting technique by comprehensively engineering the parasitic parameters. Experimentally, a bandwidth up to 80 GHz is realized for vertical positive-intrinsic-negative (PIN) Ge PDs without decreasing the responsivity and dark current, indicating that parasitic parameter engineering is a promising method to promote high-speed performance of Ge PDs.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000605 (2021)
Characterization of field-effect mobility at optical frequency by microring resonators
Wei-Che Hsu, Erwen Li, Bokun Zhou, and Alan X. Wang
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000615 (2021)
Lasers and Laser Optics
Effects of background spectral noise in the phase-modulated single-frequency seed laser on high-power narrow-linewidth fiber amplifiers
Wei Liu, Jiaxin Song, Pengfei Ma, Hu Xiao, and Pu Zhou
In this work, we analyze the effects of the background spectral noise in phase-modulated single-frequency seed lasers on the spectral purity of high-power narrow-linewidth fiber amplifiers. Through demonstrating the spectral evolution of the phase-modulated single-frequency part and the background spectral noise in a narrow-linewidth fiber amplifier, the mechanism for the spectral wing broadening effect is clarified and design strategies to maintain high spectral purity are given. Specifically, the background spectral noise in phase-modulated single-frequency seed lasers could lead to obvious spectral wing broadening and degeneration of spectral purity in narrow-linewidth fiber amplifiers through the four-wave-mixing effect. Notably, the spectral wing broadening effect could be suppressed by filtering out the background spectral noise in the seed laser or applying a counter-pumped configuration in the fiber amplifier. We have also conducted contrast experiments, which have verified the validity of the theoretical model and the design strategies for high-spectral-purity operation.
Photonics Research
  • Publication Date: Mar. 11, 2021
  • Vol.9 Issue, 4 04000424 (2021)
Direct generation of watt-level yellow Dy3+-doped fiber laser
Jinhai Zou, Tianran Li, Yanbo Dou, Jin Li, Nan Chen, Yikun Bu, and Zhengqian Luo
Yellow lasers (~565–590 nm) are of tremendous interest in biomedicine, astronomy, spectroscopy, and display technology. So far, yellow lasers still have relied heavily on nonlinear frequency conversion of near-infrared lasers, precluding compact and low-cost yellow laser systems. Here, we address the challenge through demonstrating, for the first time, to the best of our knowledge, watt-level high-power yellow laser generation directly from a compact fiber laser. The yellow fiber laser simply consists of a Dy3+-doped ZBLAN fiber as gain medium, a fiber end-facet mirror with high reflectivity at yellow and a 450-nm diode laser as the pump source. We comprehensively investigated the dependence of the yellow laser performance on the output coupler reflectivity and the gain fiber length and demonstrated that the yellow fiber laser with an output coupler reflectivity of 4% and a gain fiber length of ~1.8 m yields a maximum efficiency of 33.6%. A maximum output power of 1.12 W at 575 nm was achieved at a pump power of 4.20 W. This work demonstrated the power scaling of yellow Dy3+-doped ZBLAN fiber lasers, showing their promise for applications in ophthalmology, astronomical exploration, and high-resolution spectroscopy.
Photonics Research
  • Publication Date: Mar. 22, 2021
  • Vol.9 Issue, 4 04000446 (2021)
Pulse combination and compression in hollow-core fiber for few-cycle intense mid-infrared laser generation
Junyu Qian, Pengfei Wang, Yujie Peng, Yanyan Li, Beijie Shao, Hongpeng Su, Xinlin Lv, Ding Wang, Yuxin Leng, and Ruxin Li
The generation of high-peak-power, few-cycle mid-infrared (MIR) pulses using coherent beam combination and nonlinear pulse compression techniques simultaneously is demonstrated. The two pulses, with identical pulse energy of 2.8 mJ and pulse duration of 160 fs, are coherently combined at the input end of a krypton-filled hollow-core fiber (HCF), and then the bandwidth of the combined pulse is broadened to near an optical octave due to strong phase modulations, and the temporal width is compressed into a few-cycle regime. Finally, a 2.7 mJ, 22.9 fs, 20 Hz laser at 4 μm can be obtained, and the pulse peak power is greatly enhanced compared with that of conventional single-channel optical parametric chirped pulse-amplification systems. Furthermore, the peak power generated from this system has the prospect of further scaling up through use of more channels of coherent combination, which can pave a way to generate higher peak power ultra-intense MIR pulses for strong-field physics.
Photonics Research
  • Publication Date: Mar. 25, 2021
  • Vol.9 Issue, 4 04000477 (2021)
Observation of transition between multimode Q-switching and spatiotemporal mode locking
Kewei Liu, Xiaosheng Xiao, and Changxi Yang
We report experimental observation of multimode Q-switching and spatiotemporal mode locking in a multimode fiber laser. A typical steady Q-switching state is achieved with a 1.88 μs pulse duration, a 70.14 kHz repetition rate, and a 215.8 mW output power, corresponding to the single pulse energy of 3.08 μJ. We find weak spatial filtering is essential to obtain stable Q-switched pulses, in contrast to the relatively stronger spatial filtering for spatiotemporal mode locking. Furthermore, a reversible transition process, as well as a critical bistable state, between multimode Q-switching and spatiotemporal mode locking, is achieved with specific spatial coupling and waveplates sets. We believe the results will not only contribute to understanding the complicated nonlinear dynamics in multimode, fiber-based platforms, but also benefit the development of promising high-pulse energy lasers.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000530 (2021)
On the upper limit of laser intensity attainable in nonideal vacuum
Yitong Wu, Liangliang Ji, and Ruxin Li
The upper limit of the laser field strength in a perfect vacuum is usually considered as the Schwinger field, corresponding to ~1029 W/cm2. We investigate such limitations under realistic nonideal vacuum conditions and find that intensity suppression appears starting from 1025 W/cm2, showing an upper threshold at 1026 W/cm2 level if the residual electron density in chamber surpasses 109 cm-3. This is because the presence of residual electrons triggers the avalanche of quantum electrodynamics cascade that creates copious electron and positron pairs. The leptons are further trapped within the driving laser field due to radiation reaction, which significantly depletes the laser energy. The relationship between the attainable intensity and the vacuity is given according to particle-in-cell simulations and theoretical analysis. These results answer a critical problem on the achievable light intensity based on present vacuum conditions and provide a guideline for future hundreds of petawatt class laser development.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000541 (2021)
Broadband mid-infrared supercontinuum generation in dispersion-engineered As2S3-silica nanospike waveguides pumped by 2.8 μm femtosecond laser
Pan Wang, Jiapeng Huang, Shangran Xie, Johann Troles, and Philip St.J. Russell
Broadband mid-infrared (IR) supercontinuum laser sources are essential for spectroscopy in the molecular fingerprint region. Here, we report generation of octave-spanning and coherent mid-IR supercontinua in As2S3-silica nanospike hybrid waveguides pumped by a custom-built 2.8 μm femtosecond fiber laser. The waveguides are formed by pressure-assisted melt-filling of molten As2S3 into silica capillaries, allowing the dispersion and nonlinearity to be precisely tailored. Continuous coherent spectra spanning from 1.1 μm to 4.8 μm (30 dB level) are observed when the waveguide is designed so that 2.8 μm lies in the anomalous dispersion regime. Moreover, linearly tapered millimeter-scale As2S3-silica waveguides are fabricated and investigated for the first time, to the best of our knowledge, showing much broader supercontinua than uniform waveguides, with improved spectral coherence. The waveguides are demonstrated to be long-term stable and water-resistant due to the shielding of the As2S3 by the fused silica sheath. They offer an alternative route to generating broadband mid-IR supercontinua, with applications in frequency metrology and molecular spectroscopy, especially in humid and aqueous environments.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000630 (2021)
Nonlinear Optics
Low-energy-threshold deep-ultraviolet generation in a small-mode-area hollow-core fiber | Spotlight on Optics
Daiqi Xiong, Jiaqi Luo, Muhammad Rosdi Abu Hassan, Xu Wu, and Wonkeun Chang
We demonstrate the generation of wavelength-tunable deep-ultraviolet pulses in a small-mode-area hollow-core fiber fabricated by tapering a nodeless tubular-type hollow-core fiber. Down-scaling of the cross-sectional geometry reduces the pump energy requirement for inducing sufficient nonlinear effects, presenting a unique opportunity for staging low-energy-threshold gas-based nonlinear optics. We report the onset of the ultraviolet light with the pump pulse energy as low as 125 nJ. Our numerical analysis shows that the frequency conversion arises due to soliton phase matching, and therefore shot-to-shot coherence of the ultraviolet emission is well-preserved. It offers a promising platform for a compact ultraviolet frequency comb source.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000590 (2021)
Optically addressed spatial light modulator based on nonlinear metasurface | Editors' Pick
Shengchao Gong, Mengxin Ren, Wei Wu, Wei Cai, and Jingjun Xu
Spatial light modulators (SLMs) are devices for modulating amplitude, phase, or polarization of light beams on demand. Such devices are regarded as the backbone for optical information parallel processing and future optical computers. Currently, SLMs are mainly operated in an electrical addressing manner, wherein the optical beams are modulated by electrical signals. However, future all-optical information processing systems prefer to control light directly by light (i.e., optically addressed, OA) without electro-optical conversion. Here, we present an OASLM based on a metasurface (MS-OASLM), whose operation principle relies on nonlinear polarization control of read light by another write light at the nanoscale. Its resolution is more than 10 times higher than a typical commercial SLM and achieves 500 line pairs per millimeter (corresponding to a pixel size of only 1 μm). The MS-OASLM shows unprecedented compactness and is only 400 nm in thickness. Such MS-OASLMs could provide opportunities to develop next generation all-optical information processing and high resolution display technologies.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000610 (2021)
Tailoring spatial structure of Brillouin spectra via spiral phase precoding
Hongwei Li, Bo Zhao, Jipeng Ni, and Wei Gao
Brillouin spectroscopy is an important topic and powerful tool in modern optics, as the acquisitions of acoustic velocities and elastic moduli are one of the keys to investigate and analyze the contents of material science and condensed matter physics. Although stimulated Brillouin spectroscopy based on the pump-probe technique has striking advantages that include higher spectral resolution and signal-to-noise ratio, it is challenging to accomplish high-speed acquisition in the presence of pump background noise. In this paper, we propose a method for signal–noise separation through spiral phase precoding of the Brillouin spectrum signal. We achieve on-demand tailoring spatial distribution of the signal, and hence the signal can be separated from the background noise. Furthermore, this approach has little energy loss due to phase-only modulation, and retains the advantages of high efficiency and high gain in Brillouin interaction. The proof-of-principle demonstration provides a practical way to reshape the spatial structure of Brillouin spectra, and shows the potential in quasi-noise-free nonlinear interactions.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000637 (2021)
Rogue wave light bullets of the three-dimensional inhomogeneous nonlinear Schrödinger equation
Jingsong He, Yufeng Song, C. G. L. Tiofack, and M. Taki
We discover single and homocentric optical spheres of the three-dimensional inhomogeneous nonlinear Schr?dinger equation (NLSE) with spherical symmetry, which is a novel model of light bullets that can present a three-dimensional rogue wave. The isosurface of this light bullet oscillates along the radius direction and does not travel with the evolution of time. The localized nature of rogue wave light bullets both in space and in time, which is in complete contrast to the traveling character of the usual light bullets, is due to the localization of the rogue wave in the one-dimensional NLSE. We present also an investigation of the stability of the optical sphere solutions. The lower modes of perturbation are found to display transverse instabilities that break the spherical symmetry of the system. For the higher modes, the optical sphere solutions can be classified as stable solutions.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000643 (2021)
Optical Devices
Realization of directional single-mode lasing by a GaN-based warped microring
Shengnan Zhang, Yufeng Li, Peng Hu, Zhenhuan Tian, Qiang Li, Aixing Li, Ye Zhang, and Feng Yun
Multimode and random directionalities are major issues restricting the application of whispering gallery mode microcavity lasers. We demonstrated a 40 μm diameter microring with an off-centered embedded hole and warped geometry from strained III-nitride quantum well multilayers. Single-mode directional whispering gallery mode lasing was achieved by the warped structure and high-order mode suppression induced by the off-centered hole. In addition, the introduction of the off-centered hole reduced the lasing threshold from 3.24 to 2.79 MW/cm2 compared with the warped microdisk without an embedded hole while maintaining a high-quality factor of more than 4000. Directional light emission in 3D was achieved and attributed to the warped structure, which provides a vertical component of the light emission, making it promising for building multifunctional coherent light sources in optoelectronic integration.
Photonics Research
  • Publication Date: Mar. 12, 2021
  • Vol.9 Issue, 4 04000432 (2021)
All-silicon dual-cavity fiber-optic pressure sensor with ultralow pressure-temperature cross-sensitivity and wide working temperature range
Xue Wang, Junfeng Jiang, Shuang Wang, Kun Liu, and Tiegen Liu
Pressure-temperature cross-sensitivity and its accompanying temperature-related stability is a nerve-wracking obstruction for pressure sensor performance in a wide temperature range. To solve this problem, we propose a novel (to the best of our knowledge) all-silicon dual-cavity optical Fabry–Perot interferometer (FPI) pressure sensor. The all-silicon structure has high intrinsic reflectivity and is able to eliminate the influence of thermal-expansion-mismatch-induced stress and chemical-reaction-induced gas generation, and therefore, in essence, enhances measurement accuracy. From the experiment results, the pressure-temperature cross-sensitivity is reduced to be ~5.96 Pa/°C, which presents the lowest pressure-temperature cross-sensitivity among the FPI pressure sensors with the capability of surviving high temperatures up to 700°C thereby opening the way for high-precision pressure monitoring in various harsh and remote environments.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000521 (2021)
Optoelectronics
Phosphor-free microLEDs with ultrafast and broadband features for visible light communications
Zhenhuan Tian, Qiang Li, Xuzheng Wang, Mingyin Zhang, Xilin Su, Ye Zhang, Yufeng Li, Feng Yun, and S. W. Ricky Lee
Modulation bandwidth and the emission region are essential features for the widespread use of visible light communications (VLC). This paper addresses the contradictory requirements to achieve broadband and proposes ultrafast, asymmetric pyramids grown on adjacent deep concave holes via lateral overgrowth. Multicolor emission with an emission region between 420 nm and 600 nm is obtained by controlling the growth rate at different positions on the same face, which also can provide multiple subcarrier frequency points for the employment of wavelength division multiplexing technology. The spontaneous emission rate distinction is narrowed by lowering the number of the crystal plane, ensuring a high modulation bandwidth over broadband. More importantly, the residual stress and dislocation density were minimized by employing a patterned substrate, and lateral overgrowth resulted in a further enhancement of the recombination rate. Finally, the total modulation bandwidth of multiple subcarriers of the asymmetric pyramids is beyond GHz. These ultrafast, multicolor microLEDs are viable for application in VLC systems and may also enable applications for intelligent lighting and display.
Photonics Research
  • Publication Date: Mar. 22, 2021
  • Vol.9 Issue, 4 04000452 (2021)
Physical Optics
Free-space local nonseparability dynamics of vector modes
Xiao-Bo Hu, Benjamin Perez-Garcia, Valeria Rodríguez-Fajardo, Raul I. Hernandez-Aranda, Andrew Forbes, and Carmelo Rosales-Guzmán
One of the most prominent features of quantum entanglement is its invariability under local unitary transformations, which implies that the degree of entanglement or nonseparability remains constant during free-space propagation, true for both quantum and classically entangled modes. Here we demonstrate an exception to this rule using a carefully engineered vectorial light field, and we study its nonseparability dynamics upon free-space propagation. We show that the local nonseparability between the spatial and polarization degrees of freedom dramatically decays to zero while preserving the purity of the state and hence the global nonseparability. We show this by numerical simulations and corroborate it experimentally. Our results evince novel properties of classically entangled modes and point to the need for new measures of nonseparability for such vectorial fields, while paving the way for novel applications for customized structured light.
Photonics Research
  • Publication Date: Mar. 12, 2021
  • Vol.9 Issue, 4 04000439 (2021)
Sensitivity of topological edge states in a non-Hermitian dimer chain
Zhiwei Guo, Tengzhou Zhang, Juan Song, Haitao Jiang, and Hong Chen
Photonic topological edge states in one-dimensional dimer chains have long been thought to be robust to structural perturbations by mapping the topological Su–Schrieffer–Heeger model of a solid-state system. However, the edge states at the two ends of a finite topological dimer chain will interact as a result of near-field coupling. This leads to deviation from topological protection by the chiral symmetry from the exact zero energy, weakening the robustness of the topological edge state. With the aid of non-Hermitian physics, the splitting frequencies of edge states can be degenerated again, with topological protection recovered by altering the gain or loss strength of the structure. This point of coalescence is known as the exceptional point (EP). The intriguing physical properties of EPs in topological structures give rise to many fascinating and counterintuitive phenomena. In this work, based on a finite non-Hermitian dimer chain composed of ultra-subwavelength resonators, we propose theoretically and verify experimentally that the sensitivity of topological edge states is greatly affected when the system passes through the EP. Using the EP of a non-Hermitian dimer chain, we realize a new sensor that is sensitive to perturbation of on-site frequency at the end of the structure and yet topologically protected from internal perturbation of site-to-site couplings. Our demonstration of a non-Hermitian topological structure with an EP paves the way for the development of novel sensors that are not sensitive to internal manufacturing errors but are highly sensitive to changes in the external environment.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000574 (2021)
Fractal topological band-gap structure induced by singularities in the one-dimensional Thue–Morse system
Yu Zhang, Langlang Xiong, Meng Zhang, and Xunya Jiang
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000622 (2021)
Quantum Optics
High-fidelity, low-latency polarization quantum state transmissions over a hollow-core conjoined-tube fiber at around 800 nm
Xinyu Chen, Wei Ding, Ying-Ying Wang, Shou-Fei Gao, Feixiang Xu, Huichao Xu, Yi-Feng Hong, Yi-Zhi Sun, Pu Wang, Yan-Qing Lu, and Lijian Zhang
Hollow-core fiber (HCF) promises to unify air-borne light propagation and non-line-of-sight transmission, thus holding great potential for versatile photonics-based quantum information applications. The early version of HCF based on photonic-bandgap guidance has not proven itself a reliable quantum channel because of the poor modal purity in both spatial and polarization domains, as well as significant difficulty in fabrication when the wavelength shifts to the visible region. In this work, based on the polarization degree of freedom, we demonstrate high-fidelity (~0.98) single-photon transmission and distribution of entangled photons over a 36.4 m hollow-core conjoined-tube fiber (CTF) by using commercial silicon single-photon avalanche photodiodes. Our CTF realizes the combined merits of low loss, high spatial modal purity, low polarization degradation, and low chromatic dispersion. We also demonstrate single-photon low-latency (~99.96% speed of light in vacuum) transmission, paving the way for extensive uses of HCF links in versatile photonics-based quantum information processing.
Photonics Research
  • Publication Date: Mar. 22, 2021
  • Vol.9 Issue, 4 04000460 (2021)
Silicon Photonics
Mode-conversion-based silicon photonic modulator loaded by a combination of lateral and interleaved p-n junctions
Omid Jafari, Sasan Zhalehpour, Wei Shi, and Sophie LaRochelle
Photonics Research
  • Publication Date: Mar. 25, 2021
  • Vol.9 Issue, 4 04000471 (2021)
30 GHz GeSn photodetector on SOI substrate for 2 µm wavelength application
Xiuli Li, Linzhi Peng, Zhi Liu, Zhiqi Zhou, Jun Zheng, Chunlai Xue, Yuhua Zuo, Baile Chen, and Buwen Cheng
We report the demonstration of a normal-incidence p-i-n germanium-tin (Ge0.951Sn0.049) photodetector on silicon-on-insulator substrate for 2 μm wavelength application. The DC and RF characteristics of the devices have been characterized. A dark current density under -1 V bias of approximately 125 mA/cm2 is achieved at room temperature, and the optical responsivity of 14 mA/W is realized for illumination wavelength of 2 μm under -1 V reverse bias. In addition, a 3 dB bandwidth (f3 dB) of around 30 GHz is achieved at -3 V, which is the highest reported value among all group III–V and group IV photodetectors working in the 2 μm wavelength range. This work illustrates that a GeSn photodetector has great prospects in 2 μm wavelength optical communication.
Photonics Research
  • Publication Date: Mar. 25, 2021
  • Vol.9 Issue, 4 04000494 (2021)
High-speed silicon photonic Mach–Zehnder modulator at 2 μm
Xi Wang, Weihong Shen, Wenxiang Li, Yingjie Liu, Yong Yao, Jiangbing Du, Qinghai Song, and Ke Xu
Recently, 2-μm wave band has gained increasing interest due to its potential application for next-generation optical communication. But the development of 2-μm optical communications is substantially hampered by the modulation speed due to the device bandwidth constraints. Thus, a high-speed modulator is highly demanded at 2 μm. Motivated by this prospect, we demonstrate a high-speed silicon Mach–Zehnder modulator for a 2-μm wave band. The device is configured as a single-ended push–pull structure with waveguide electrorefraction via the free carrier plasma effect. The modulator was fabricated via a multiproject wafer shuttle run at a commercial silicon photonic foundry. The modulation efficiency of a single arm is measured to be 1.6 V·cm. The high-speed characterization is also performed, and the modulation speed can reach 80 Gbit/s with 4-level pulse amplitude modulation (PAM-4) formats.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000535 (2021)
Surface Optics and Plasmonics
Chirality-selective transparency induced by lattice resonance in bilayer metasurfaces
Shuxia Zhao, Lei Shao, Jianfang Wang, Hai-Qing Lin, and Wei Zhang
Chiral optical responses of bilayer metasurfaces made of twisted metallic nanorods are investigated in detail with focus on the collective effect due to lattice resonance (LR). Using an analytical approach based on the coupled dipole method (supported by full wave simulation), we find optical chirality is dramatically increased by the coupling between localized surface plasmon resonances and LR. The collective effect results in significant chiral signal even for metasurfaces made of achiral unit cells. The interlayer coupling generally destroys the Wood’s anomaly and the associated transparency. While making use of Pancharatnam–Berry (PB) phase and propagation phase, one can modulate the optical activity effectively and achieve chirality-selective transparency induced by LR in a designed structure with a g-factor of absorption as high as 1.99 (close to the upper limit of 2). Our studies not only reveal a new mechanism of modulating chiral optical response by combination effects from PB phase, propagation phase, and LR, but also give a quantitative relationship between the geometry configuration and chiral optical properties, thus providing helpful guidance for device design.
Photonics Research
  • Publication Date: Mar. 25, 2021
  • Vol.9 Issue, 4 04000484 (2021)
Augmenting photoluminescence of monolayer MoS2 using high order modes in a metal dimer-on-film nanocavity
Shiyin Cao, Liping Hou, Qifa Wang, Chenyang Li, Weixing Yu, Xuetao Gan, Kaihui Liu, Malin Premaratne, Fajun Xiao, and Jianlin Zhao
Plasmonic particle-on-film nanocavities, supporting gap modes with ultra-small volume, provide a great solution to boost light–matter interactions at the nanoscale. In this work, we report on the photoluminescence (PL) enhancement of monolayer MoS2 using high order modes of an Au nanosphere dimer-on-film nanocavity (DoFN). The high order plasmon modes, consisting of two bonding quadrupoles in the dimer and their images in the Au film, are revealed by combining the polarization-resolved scattering spectra with the numerical simulations. Further integrating the monolayer MoS2 into the DoFN, these high order modes are used to enhance PL intensity through simultaneously boosting the absorption and emission processes, producing a 1350-fold enhancement factor. It opens an avenue to enhance the light–matter interaction with high order plasmon modes and may find applications in future optoelectronics and nanophotonics devices.
Photonics Research
  • Publication Date: Mar. 25, 2021
  • Vol.9 Issue, 4 04000501 (2021)
Ultrafast spatiotemporal control of directional launching of surface plasmon polaritons in a plasmonic nano coupler
Yulu Qin, Boyu Ji, Xiaowei Song, and Jingquan Lin
Ultrafast spatiotemporal control of a surface plasmon polariton (SPP) launch direction is a prerequisite for ultrafast information processing in plasmonic nanocircuit components such as ultrafast on–off of plasmonic switching and information recording. Here we realize for the first time, to the best of our knowledge, ultrafast spatiotemporal control of the preferential launch direction of an SPP at the nano-femtosecond scale via a plasmonic nano directional coupler. The spatiotemporal switching of the SPP field was revealed using time-resolved photoemission electron microscopy (TR-PEEM). Experimental results show that the extinction ratio of the SPP directional coupler can be substantially optimized by properly selecting the amplitude and time delay of the two incident light pulses in the experiment. More importantly, we demonstrate a solution for the launch direction of the SPP field, switched in a plasmonic nano directional coupler on the femtosecond timescale, by adjusting the instantaneous polarization state of the excitation light. The TR-PEEM images are supported by finite-difference time-domain (FDTD) simulations. We believe the results of this study can be used to develop high-speed, miniaturized signal processing systems.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000514 (2021)
Circular dichroism-like response of terahertz wave caused by phase manipulation via all-silicon metasurface
Jie Li, Chenglong Zheng, Guocui Wang, Jitao Li, Hongliang Zhao, Yue Yang, Zhang Zhang, Maosheng Yang, Liang Wu, Jining Li, Yating Zhang, Yan Zhang, and Jianquan Yao
Chiral metasurfaces based on asymmetric meta-atoms have achieved artificial circular dichroism (CD), spin-dependent wavefront control, near-field imaging, and other spin-related electromagnetic control. In this paper, we propose and experimentally verify a scheme for achieving high-efficiency chiral response similar to CD of terahertz (THz) wave via phase manipulation. By introducing the geometric phase and dynamic phase in an all-silicon metasurface, the spin-decoupled terahertz transmission is obtained. The giant circular dichroism-like effect in the transmission spectrum is observed by using a random phase distribution for one of the circular polarization components. More importantly, the effect can be adjusted when we change the area of the metasurface illuminated by an incident terahertz beam. In addition, we also demonstrate the spin-dependent arbitrary wavefront control of the transmitted terahertz wave, in which one of the circularly polarized components is scattered, while the other forms a focused vortex beam. Simulated and experimental results show that this method provides a new idea for spin selective control of THz waves.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000567 (2021)
High efficiency all-dielectric pixelated metasurface for near-infrared full-Stokes polarization detection | Editors' Pick
Chong Zhang, Jingpei Hu, Yangeng Dong, Aijun Zeng, Huijie Huang, and Chinhua Wang
Pixelated metasurfaces integrating both the functions of linear polarization and circular polarization filters on a single platform can achieve full-Stokes polarization detection. At present, the pixelated full-Stokes metasurfaces mainly face the following problems: low transmission, low circular dichroism (CD) of circular polarization filters, and high requirements in fabrication and integration. Herein, we propose high performance ultracompact all-dielectric pixelated full-Stokes metasurfaces in the near-infrared band based on silicon-on-insulator, which is compatible with the available semiconductor industry technologies. Circular polarization filters with high CD are achieved by using simple two-dimensional chiral structures, which can be easily integrated with the linear polarization filters on a single chip. In addition, the dielectric materials have higher transmission than metal materials with intrinsic absorption. We experimentally demonstrated the circular polarization filter with maximum CD up to 70% at a wavelength of 1.6 μm and average transmission efficiency above 80% from 1.48 μm to 1.6 μm. Therefore, our design is highly desirable for many applications, such as target detection, clinical diagnosis, and polarimetric imaging and sensing.
Photonics Research
  • Publication Date: Apr. 06, 2021
  • Vol.9 Issue, 4 04000583 (2021)

About the Cover

The figure shows thermo-optically tuned spectral broadening in a nonlinear Ultra-Silicon-Rich Nitride (USRN) grating. Bragg soliton dynamics and the large thermo-optic coefficient in USRN underpin the observed tunable spectral broadening.