Volume: 8 Issue 7
26 Article(s)

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Research Articles
Imaging Systems, Microscopy, and Displays
Computational 4D imaging of light-in-flight with relativistic effects
Yue Zheng, Ming-Jie Sun, Zhi-Guang Wang, and Daniele Faccio
Light-in-flight imaging enables the visualization and characterization of light propagation, which provides essential information for the study of the fundamental phenomena of light. A camera images an object by sensing the light emitted or reflected from it, and interestingly, when a light pulse itself is to be imaged, the relativistic effects, caused by the fact that the distance a pulse travels between consecutive frames is of the same scale as the distance that scattered photons travel from the pulse to the camera, must be accounted for to acquire accurate space–time information of the light pulse. Here, we propose a computational light-in-flight imaging scheme that records the projection of light-in-flight on a transverse x?y plane using a single-photon avalanche diode camera, calculates z and t information of light-in-flight via an optical model, and therefore reconstructs its accurate (x, y, z, t) four-dimensional information. The proposed scheme compensates the temporal distortion in the recorded arrival time to retrieve the accurate time of a light pulse, with respect to its corresponding spatial location, without performing any extra measurements. Experimental light-in-flight imaging in a three-dimensional space of 375 mm×75 mm×50 mm is performed, showing that the position error is 1.75 mm, and the time error is 3.84 ps despite the fact that the camera time resolution is 55 ps, demonstrating the feasibility of the proposed scheme. This work provides a method to expand the recording and measuring of repeatable transient events with extremely weak scattering to four dimensions and can be applied to the observation of optical phenomena with ps temporal resolution.
Photonics Research
  • Publication Date: Jun. 03, 2020
  • Vol.8 Issue, 7 07001072 (2020)
Great enhancement of image details with high fidelity in a scintillator imager using an optical coding method
Huijuan Xia, Yanqing Wu, Lei Zhang, Yuanhe Sun, Zhongyang Wang, and Renzhong Tai
High-resolution lens-coupled indirect X-ray scintillator imagers are required by many imaging applications. However, the severe weakening of image details prevents its further performance improvement. Through our research, this image degradation is attributed to the broadband loss of the high-spatial-frequency information caused by the high refractive index. A technique known as high-spatial-frequency spectrum enhanced reconstruction is thus proposed to retrieve this information. A two-dimensional high-density array is covered on the scintillator’s exit surface and operates as an encoder based on which high-frequency information can be shifted to the low-frequency region to improve the signal-to-noise ratio. The experimental results show that the middle-high-frequency signal intensities can be increased by an order of magnitude or more, up to ~50 times. Therefore, the image details can be effectively enhanced to break through the performance bottleneck of such widely used X-ray imagers for synchrotron radiation facilities or tabletop X-ray tubes.
Photonics Research
  • Publication Date: Jun. 03, 2020
  • Vol.8 Issue, 7 07001079 (2020)
Portable quantitative phase microscope for material metrology and biological imaging
Mengxuan Niu, Gang Luo, Xin Shu, Fuyang Qu, Shuang Zhou, Yi-Ping Ho, Ni Zhao, and Renjie Zhou
Quantitative phase microscopy (QPM) has emerged as an important tool for material metrology and biological imaging. For broader adoption in those applications, we have proposed and demonstrated a new portable off-axis QPM method, which works in both transmission and reflection modes to meet different sample measurement requirements. The temporal and spatial sensitivities of our system, as quantified by optical path-length difference values, are 0.65 nm and 1.04 nm, respectively. To demonstrate its applicability for a wide range of applications, we deployed our system for profiling transistor gold electrode samples, observing red blood cell membrane fluctuations, imaging living cells flowing in a microfluidic chip, etc. Our portable QPM system has a low-cost design and involves a simple and robust phase-retrieval algorithm that we envision will allow for broader deployment at different environmental settings, including in resource-limited sites and integration with other metrology or imaging modalities.
Photonics Research
  • Publication Date: Jun. 30, 2020
  • Vol.8 Issue, 7 07001253 (2020)
Lasers and Laser Optics
Widely tunable ultra-narrow-linewidth dissipative soliton generation at the telecom band
Chang Kyun Ha, Ki Sang Lee, Dohyeon Kwon, and Myeong Soo Kang
Ultra-narrow-linewidth mode-locked lasers with wide wavelength tunability can be versatile light sources for a variety of newly emergent applications. However, it is very challenging to achieve the stable mode locking of substantially long, anomalously dispersive fiber laser cavities employing a narrowband spectral filter at the telecom band. Here, we show that a nearly dispersion-insensitive dissipative mode-locking regime can be accessed through a subtle counterbalance among significantly narrowband spectral filtering, sufficiently deep saturable absorption, and moderately strong in-fiber Kerr nonlinearity. This achieves ultra-narrow-linewidth (a few gigahertz) nearly transform-limited self-starting stable dissipative soliton generation at low repetition rates (a few megahertz) without cavity dispersion management over a broad tuning range of wavelengths covering the entire telecom C-band. This unique laser may have immediate application as an idealized pump source for high-efficiency nonlinear frequency conversion and nonclassical light generation in dispersion-engineered tightly light-confining microphotonic/nanophotonic systems.
Photonics Research
  • Publication Date: Jun. 03, 2020
  • Vol.8 Issue, 7 07001100 (2020)
Ultralow-quantum-defect Raman laser based on the boson peak in phosphosilicate fiber
Yang Zhang, Jiangming Xu, Jun Ye, Jiaxin Song, Tianfu Yao, and Pu Zhou
Quantum defects (QDs) have always been a key factor of the thermal effect in high-power fiber lasers. Much research on low-QD fiber lasers has been reported in the past decades, but most of it is based on active fibers. Besides, Raman fiber lasers based on the stimulated Raman scattering effect in passive fiber are also becoming an important kind of high-power fiber laser for their unique advantages, such as their significantly broader wavelength-tuning range and being free of photon darkening. In this paper, we demonstrate an ultralow-QD Raman fiber laser based on phosphosilicate fiber. There is a strong boson peak located at a frequency shift of 3.65 THz in the Raman gain spectrum of the phosphosilicate fiber we employed. By utilizing this boson peak to provide Raman gain and adopting an amplified spontaneous emission source at 1066 nm as the pump source, 1080 nm Stokes light is generated, corresponding to a QD of 1.3%. The spectral purity at 1080 nm can be up to 96.03%, and the output power is 12.5 W, corresponding to a conversion efficiency of 67.2%. Moreover, by increasing the pump wavelength to 1072 nm, the QD is reduced to 0.74%, and the output power at 1080 nm is 10.7 W, with a spectral purity of 82.82%. To the best of our knowledge, this is the lowest QD ever reported for Raman fiber lasers. This work proposes a promising way of achieving high-power, high-efficiency Raman fiber lasers.
Photonics Research
  • Publication Date: Jun. 15, 2020
  • Vol.8 Issue, 7 07001155 (2020)
Real-time observation of vortex mode switching in a narrow-linewidth mode-locked fiber laser
Jiafeng Lu, Fan Shi, Linghao Meng, Longkun Zhang, Linping Teng, Zhengqian Luo, Peiguang Yan, Fufei Pang, and Xianglong Zeng
Temporal and spatial resonant modes are always possessed in physical systems with energy oscillation. In ultrafast fiber lasers, enormous progress has been made toward controlling the interactions of many longitudinal modes, which results in temporally mode-locked pulses. Recently, optical vortex beams have been extensively investigated due to their quantized orbital angular momentum, spatially donut-like intensity, and spiral phase front. In this paper, we have demonstrated the first to our knowledge observation of optical vortex mode switching and their corresponding pulse evolution dynamics in a narrow-linewidth mode-locked fiber laser. The spatial mode switching is achieved by incorporating a dual-resonant acousto-optic mode converter in the vortex mode-locked fiber laser. The vortex mode-switching dynamics have four stages, including quiet-down, relaxation oscillation, quasi mode-locking, and energy recovery prior to the stable mode-locking of another vortex mode. The evolution dynamics of the wavelength shifting during the switching process are observed via the time-stretch dispersion Fourier transform method. The spatial mode competition through optical nonlinearity induces energy fluctuation on the time scale of ultrashort pulses, which plays an essential role in the mode-switching dynamic process. The results have great implications in the study of spatial mode-locking mechanisms and ultrashort laser applications.
Photonics Research
  • Publication Date: Jun. 29, 2020
  • Vol.8 Issue, 7 07001203 (2020)
Medical Optics and Biotechnology
Distance-controllable and direction-steerable opto-conveyor for targeting delivery
Zhen Che, Wenguo Zhu, Yaoming Huang, Yu Zhang, Linqing Zhuo, Pengpeng Fan, Zhibin Li, Huadan Zheng, Wenjin Long, Wentao Qiu, Yunhan Luo, Jun Zhang, Jinghua Ge, Jianhui Yu, and Zhe Chen
Opto-conveyors have attracted widespread interest in various fields because of their non-invasive and non-contact delivery of micro/nanoparticles. However, the flexible control of the delivery distance and the dynamic steering of the delivery direction, although very desirable in all-optical manipulation, have not yet been achieved by opto-conveyors. Here, using a simple and cost-effective scheme of an elliptically focused laser beam obliquely irradiated on a substrate, a direction-steerable and distance-controllable opto-conveyor for the targeting delivery of microparticles is implemented. Theoretically, in the proposed scheme of the opto-conveyor, the transverse and longitudinal resultant forces of the optical gradient force and the optical scattering force result in the transverse confinement and the longitudinal transportation of microparticles, respectively. In this study, it is experimentally shown that the proposed opto-conveyor is capable of realizing the targeting delivery for microparticles. Additionally, the delivery distance of microparticles can be flexibly and precisely controlled by simply adjusting the irradiation time. By simply rotating the cylindrical lens, the proposed opto-conveyor is capable of steering the delivery direction flexibly within a large range of azimuthal angles, from ?75° to 75°. This study also successfully demonstrated the real-time dynamic steering of the delivery direction from ?45° to 45° with the dynamical rotation of the cylindrical lens. Owing to its simplicity, flexibility, and controllability, the proposed method is capable of creating new opportunities in bioassays as well as in drug delivery.
Photonics Research
  • Publication Date: Jun. 04, 2020
  • Vol.8 Issue, 7 07001124 (2020)
Optical and Photonic Materials
Ultrapure and highly efficient green light emitting devices based on ligand-modified CsPbBr3 quantum dots
Dongdong Yan, Shuangyi Zhao, Huaxin Wang, and Zhigang Zang
All inorganic CsPbBr3 perovskite quantum dots (QDs) have been recognized as promising optical materials to fabricate green light emission devices because of their excellent optical performance. However, regular CsPbBr3 QDs with an oleic acid (OA) ligand show poor stability, which limits their practical application. We replaced the OA ligand in CsPbBr3 QDs with a 2-hexyldecanoic acid (DA) ligand and, in the synthesis, found that the new material has better optical properties than regular CsPbBr3 QDs (CsPbBr3-OA QDs). Due to the strong binding energy between the DA ligand and QDs, the ligand-modified CsPbBr3 QDs (CsPbBr3-DA QDs) show a high photoluminescence quantum yield (PLQY) of 96%, while the PLQY of CsPbBr3-OA QDs is 84%. Subsequently, the CsPbBr3 QDs coated on the blue light-emitting diode (LED) chips as green phosphors are demonstrated. The color conversion from blue to pure green is achieved by adding the CsPbBr3-OA QDs solution up to 60 μL, while the pure green emission devices only need 18 μL CsPbBr3-DA QDs solution under the same concentration. The ultrapure, highly efficient green light-emitting devices based on CsPbBr3-DA QDs exhibit a luminous efficiency of 43.6 lm/W with a CIE (0.2086, 0.7635) under a 15.3 mA driving current. In addition, the green emission wavelength of the devices based on CsPbBr3-DA QDs almost has no shift, even under a high injection current. These results highlight the promise of DA ligand-modified CsPbBr3 QDs for light-emitting devices and enrich the application field of ligand-modified CsPbBr3 QDs.
Photonics Research
  • Publication Date: Jun. 03, 2020
  • Vol.8 Issue, 7 07001086 (2020)
MXene-based high-performance all-optical modulators for actively Q-switched pulse generation
Qing Wu, Yunzheng Wang, Weichun Huang, Cong Wang, Zheng Zheng, Meng Zhang, and Han Zhang
Q-switched fiber lasers are integral tools in science, industry, and medicine due to their advantages of flexibility, compactness, and reliability. All-optical strategies to generate ultrashort pulses have obtained considerable attention as they can modulate the intracavity Q factors without employing costly and complex electrically driven devices. Here, we propose a high-performance all-optical modulator for actively Q-switched pulse generation based on a microfiber knot resonator deposited with V2CTx MXene. Experimental results show that the obtained Q-switching pulses exhibit a wide adjustment range of repetition rate from 1 kHz to 20 kHz, a high signal-to-background contrast ratio of ~55 dB, and a narrow pulse width of 8.82 μs, indicating great potentials of providing a simple and viable solution in photonic applications.
Photonics Research
  • Publication Date: Jun. 11, 2020
  • Vol.8 Issue, 7 07001140 (2020)
Phosphor-free single chip GaN-based white light emitting diodes with a moderate color rendering index and significantly enhanced communications bandwidth | Editors' Pick
Rongqiao Wan, Xiang Gao, Liancheng Wang, Shuo Zhang, Xiongbin Chen, Zhiqiang Liu, Xiaoyan Yi, Junxi Wang, Junhui Li, Wenhui Zhu, and Jinmin Li
To achieve high quality lighting and visible light communication (VLC) simultaneously, GaN based white light emitting diodes (WLEDs) oriented for lighting in VLC has attracted great interest. However, the overall bandwidth of conventional phosphor converted WLEDs is limited by the long lifetime of phosphor, the slow Stokes transfer process, the resistance-capacitance (RC) time delay, and the quantum-confined Stark effect (QCSE). Here by adopting a self-assembled InGaN quantum dots (QDs) structure, we have fabricated phosphor-free single chip WLEDs with tunable correlated color temperature (CCT, from 1600 K to 6000 K), a broadband spectrum, a moderate color rendering index (CRI) of 75, and a significantly improved modulation bandwidth (maximum of 150 MHz) at a low current density of 72 A/cm2. The broadband spectrum and high modulation bandwidth are ascribed to the capture of carriers by different localized states of InGaN QDs with alleviative QCSE as compared to the traditional InGaN/GaN quantum well (QW) structures. We believe the approach reported in this work will find its potential application in GaN WLEDs and advance the development of semiconductor lighting-communication integration.
Photonics Research
  • Publication Date: Jun. 03, 2020
  • Vol.8 Issue, 7 07001110 (2020)
64 Gb/s low-voltage waveguide SiGe avalanche photodiodes with distributed Bragg reflectors | Editors' Pick
Binhao Wang, Zhihong Huang, Yuan Yuan, Di Liang, Xiaoge Zeng, Marco Fiorentino, and Raymond G. Beausoleil
We demonstrate low-voltage waveguide silicon-germanium avalanche photodiodes (APDs) integrated with distributed Bragg reflectors (DBRs). The internal quantum efficiency is improved from 60% to 90% at 1550 nm assisted with DBRs while still achieving a 25 GHz bandwidth. A low breakdown voltage of 10 V and a gain bandwidth product of near 500 GHz are obtained. APDs with DBRs at a data rate of 64 Gb/s pulse amplitude modulation with four levels (PAM4) show a 30%–40% increase in optical modulation amplitude (OMA) compared to APDs with no DBR. A sensitivity of around -13 dBm at a data rate of 64 Gb/s PAM4 and a bit error rate of 2.4×10-4 is realized for APDs with DBRs, which improves the sensitivity by ~2 dB compared to APDs with no DBR.
Photonics Research
  • Publication Date: Jun. 04, 2020
  • Vol.8 Issue, 7 07001118 (2020)
Polarization-enhanced AlGaN solar-blind ultraviolet detectors
Ke Jiang, Xiaojuan Sun, Zi-Hui Zhang, Jianwei Ben, Jiamang Che, Zhiming Shi, Yuping Jia, Yang Chen, Shanli Zhang, Wei Lv, and Dabing Li
AlGaN solar-blind ultraviolet detectors have great potential in many fields, although their performance has not fully meet the requirements until now. Here, we proposed an approach to utilize the inherent polarization effect of AlGaN to improve the detector performance. AlGaN heterostructures were designed to enhance the polarization field in the absorption layer, and a high built-in field and a high electron mobility conduction channel were formed. As a result, a high-performance solar-blind ultraviolet detector with a peak responsivity of 1.42 A/W at 10 V was achieved, being 50 times higher than that of the nonpolarization-enhanced one. Moreover, an electron reservoir structure was proposed to further improve the performance. A higher peak responsivity of 3.1 A/W at 30 V was achieved because the electron reservoir structure could modulate the electron concentration in the conduction channel. The investigation presented here provided feasible approaches to improve the performance of the AlGaN detector by taking advantage of its inherent property.
Photonics Research
  • Publication Date: Jun. 30, 2020
  • Vol.8 Issue, 7 07001243 (2020)
Quantum Optics
Multiphoton synthetic lattices in multiport waveguide arrays: synthetic atoms and Fock graphs
Konrad Tschernig, Roberto de J. León-Montiel, Armando Pérez-Leija, and Kurt Busch
Activating transitions between internal states of physical systems has emerged as an appealing approach to create lattices and complex networks. In such a scheme, the internal states or modes of a physical system are regarded as lattice sites or network nodes in an abstract space whose dimensionality may exceed the systems’ apparent (geometric) dimensionality. This introduces the notion of synthetic dimensions, thus providing entirely novel pathways for fundamental research and applications. Here, we analytically show that the propagation of multiphoton states through multiport waveguide arrays gives rise to synthetic dimensions where a single waveguide system generates a multitude of synthetic lattices. Since these synthetic lattices exist in photon-number space, we introduce the concept of pseudo-energy and demonstrate its utility for studying multiphoton interference processes. Specifically, the spectrum of the associated pseudo-energy operator generates a unique ordering of the relevant states. Together with generalized pseudo-energy ladder operators, this allows for representing the dynamics of multiphoton states by way of pseudo-energy term diagrams that are associated with a synthetic atom. As a result, the pseudo-energy representation leads to concise analytical expressions for the eigensystem of N photons propagating through M nearest-neighbor coupled waveguides. In the regime where N≥2 and M≥3, nonlocal coupling in Fock space gives rise to hitherto unknown all-optical dark states that display intriguing nontrivial dynamics.
Photonics Research
  • Publication Date: Jun. 15, 2020
  • Vol.8 Issue, 7 07001161 (2020)
Silicon Photonics
Wavelength-selective 2 × 2 optical switch based on a Ge2Sb2Te5-assisted microring
Changping Zhang, Ming Zhang, Yiwei Xie, Yaocheng Shi, Rajesh Kumar, Roberto R. Panepucci, and Daoxin Dai
A novel wavelength-selective 2×2 optical switch based on a Ge2Sb2Te5 (GST)-assisted microring-resonator (MRR) is proposed. The present GST-assisted MRR consists of two access optical waveguides and an MRR coupled with a bent GST-loaded silicon photonic waveguide. The 2×2 optical switch is switched ON or OFF by modifying the GST state to be crystalline or amorphous. In particular, the microring waveguide and the bent GST-loaded waveguide are designed to satisfy the phase-matching condition when the GST is crystalline. As a result, the MRR becomes highly lossy and the resonance peak is depressed significantly. On the other hand, when it is off, there is little coupling due to the significant phase mismatching. Consequently, one has a low-loss transmission at the drop port for the resonance wavelength. In this paper, the simulation using the three-dimensional finite-difference method shows that the extinction ratio of the designed photonic switch is ~20 dB at the resonance wavelength, while the excess losses at the through port and drop port are 0.9 dB and 2 dB. In particular, the resonance wavelength changes little between the ON and OFF states, which makes it suitable for multichannel wavelength-division-multiplexing systems.
Photonics Research
  • Publication Date: Jun. 15, 2020
  • Vol.8 Issue, 7 07001171 (2020)
Subwavelength thick ultrahigh-Q terahertz disc microresonators
Dominik Walter Vogt, Angus Harvey Jones, Thomas Alan Haase, and Rainer Leonhardt
Photonics Research
  • Publication Date: Jun. 19, 2020
  • Vol.8 Issue, 7 07001183 (2020)
All-optical PtSe2 silicon photonic modulator with ultra-high stability
Kangkang Wei, Delong Li, Zhitao Lin, Zhao Cheng, Yuhan Yao, Jia Guo, Yunzheng Wang, Yupeng Zhang, Jianji Dong, Han Zhang, and Xinliang Zhang
Photonics Research
  • Publication Date: Jun. 22, 2020
  • Vol.8 Issue, 7 07001189 (2020)
Surface Optics and Plasmonics
Polarization-independent highly efficient generation of Airy optical beams with dielectric metasurfaces | On the Cover
Binbin Yu, Jing Wen, Lei Chen, Leihong Zhang, Yulong Fan, Bo Dai, Saima Kanwal, Dangyuan Lei, and Dawei Zhang
Airy optical beams have emerged to hold enormous theoretical and experimental research interest due to their outstanding characteristics. Conventional approaches suffer from bulky and costly systems, as well as poor phase discretization. The newly developed metasurface-based Airy beam generators have constraints of polarization dependence or limited generation efficiency. Here, we experimentally demonstrate a polarization-independent silicon dielectric metasurface for generation of high-efficiency Airy optical beams. In our implementation, rather than synchronous manipulation of the amplitude and phase by plasmonic or Huygens’ metasurfaces, we employ and impose a 3/2 phase-only manipulation to the dielectric metasurface, consisting of an array of silicon nanopillars with an optimized transmission efficiency as high as 97%. The resultant Airy optical beams possess extraordinarily large deflection angles and relatively narrow beam widths. Our validated scheme will open up a fascinating doorway to broaden the application scenarios of Airy optical beams on ultracompact photonic platforms.
Photonics Research
  • Publication Date: Jun. 11, 2020
  • Vol.8 Issue, 7 07001148 (2020)
Reconfigurable nano-kirigami metasurfaces by pneumatic pressure
Shanshan Chen, Wei Wei, Zhiguang Liu, Xing Liu, Shuai Feng, Honglian Guo, and Jiafang Li
Tunable/reconfigurable metasurfaces that can actively control electromagnetic waves upon external stimuli are of great importance for practical applications of metasurfaces. Here, we demonstrate a reconfigurable nano-kirigami metasurface driven by pneumatic pressure operating in the near-infrared wavelength region. The metasurfaces consist of combined Archimedean spirals and are fabricated in a free-standing gold/silicon nitride nanofilm by employing focused ion beam (FIB) lithography. The deformable spirals are instantly transformed from two dimensional (2D) to three-dimensional (3D) by the FIB-based nano-kirigami process. The 2D–to–3D transformation induces a dramatic irreversible change of the plasmonic quadruple modes and results in significant modulation in reflection by 137%. The suspended porous nano-kirigami metasurface is further integrated with an optofluidics device, with which the optical resonance is reversibly modulated by the pneumatic pressure. This work provides a strategy for tunable/reconfigurable metasurfaces, which are useful to build a promising lab-on-a-chip platform for microfluidics, biological diagnostics, chemical sensing, and pressure monitoring.
Photonics Research
  • Publication Date: Jun. 16, 2020
  • Vol.8 Issue, 7 07001177 (2020)
Low-loss hybrid plasmonic TM-pass polarizer using polarization-dependent mode conversion
Ruixuan Chen, Bowen Bai, and Zhiping Zhou
A low-loss hybrid plasmonic transverse magnetic (TM)-pass polarizer has been demonstrated utilizing polarization-dependent mode conversion. Taking advantage of the silicon hybrid plasmonic slot waveguide (HPSW), the unwanted transverse electric (TE) fundamental mode can be efficiently converted first to a TM higher-order mode and then suppressed by a power combiner, while the retained TM fundamental mode can pass through with negligible influence. Since the HPSW feature both strong structural asymmetry and a small interaction area in the cross-section between the metal and optical field, the optimized insertion loss of the device is as low as 0.4 dB. At the wavelength of 1550 nm, the extinction ratio is 28.3 dB with a moderate footprint of 2.38 μm×10 μm. For the entire C band, the average reflection of the TE mode is suppressed below ?14 dB, and the extinction ratio is over 18.6 dB. This work provides another more efficient and effective approach for better on-chip polarizers.
Photonics Research
  • Publication Date: Jun. 29, 2020
  • Vol.8 Issue, 7 07001197 (2020)
Multitask deep-learning-based design of chiral plasmonic metamaterials
Eric Ashalley, Kingsley Acheampong, Lucas V. Besteiro, Peng Yu, Arup Neogi, Alexander O. Govorov, and Zhiming M. Wang
The field of chiral plasmonics has registered considerable progress with machine-learning (ML)-mediated metamaterial prototyping, drawing from the success of ML frameworks in other applications such as pattern and image recognition. Here, we present an end-to-end functional bidirectional deep-learning (DL) model for three-dimensional chiral metamaterial design and optimization. This ML model utilizes multitask joint learning features to recognize, generalize, and explore in detail the nontrivial relationship between the metamaterials’ geometry and their chiroptical response, eliminating the need for auxiliary networks or equivalent approaches to stabilize the physically relevant output. Our model efficiently realizes both forward and inverse retrieval tasks with great precision, offering a promising tool for iterative computational design tasks in complex physical systems. Finally, we explore the behavior of a sample ML-optimized structure in a practical application, assisting the sensing of biomolecular enantiomers. Other potential applications of our metastructure include photodetectors, polarization-resolved imaging, and circular dichroism (CD) spectroscopy, with our ML framework being applicable to a wider range of physical problems.
Photonics Research
  • Publication Date: Jun. 30, 2020
  • Vol.8 Issue, 7 07001213 (2020)
Plasmonic resonance-linewidth shrinkage to boost biosensing
Min Gao, Weimin Yang, Zhengying Wang, Shaowei Lin, Jinfeng Zhu, and Zhilin Yang
Coupling effects of surface plasmon resonance (SPR) induce changes in the wavelength, intensity, and linewidth of plasmonic modes. Here, inspired by coupling effects, we reveal an abrupt linewidth-shrinking effect in 2D gold nanohole arrays at the azimuthal angle of 45° arising from the interference of two degenerate SPR modes. We further demonstrate the biosensing capability under various excitation conditions for detecting the critical molecular biomarker of prostatic carcinoma, and achieve the maximum sensitivity at this angle. Our study not only enhances the understanding toward plasmonic resonance-linewidth shrinking, but also provides a promising strategy to greatly improve biosensing performance by light manipulation on plasmonic nanostructures.
Photonics Research
  • Publication Date: Jun. 30, 2020
  • Vol.8 Issue, 7 07001226 (2020)
Fiber Optics and Optical Communications
Phase demodulation method based on a dual-identical-chirped-pulse and weak fiber Bragg gratings for quasi-distributed acoustic sensing
Guanhua Liang, Junfeng Jiang, Kun Liu, Shuang Wang, Tianhua Xu, Wenjie Chen, Zhe Ma, Zhenyang Ding, Xuezhi Zhang, Yongning Zhang, and Tiegen Liu
A phase demodulation method for quasi-distributed acoustic sensing (DAS) systems based on a dual-identical-chirped-pulse and weak fiber Bragg gratings (WFBGs) is proposed. Compared to the use of Rayleigh backscattering light in optical fibers, the implementation of WFBGs can contribute to obtaining an optical signal with a higher signal-to-noise ratio (SNR). The dual-identical-chirped-pulse is generated by a time-delay fiber, and the sinusoidal carrier is generated by the interference between the two chirped pulses reflected by adjacent WFBGs. The phase of the sinusoidal carrier represents the dynamic strain change posed on the sensing fiber. Discrete Fourier transform is used to directly retrieve the phase information. The performance of the phase demodulation from interference signals under different sinusoidal carrier frequencies and SNRs is numerically investigated. The piezoelectric transducer is employed to emulate the sound in the experiment to verify the effectiveness of our method. It is shown that the dynamic strain can be well reconstructed at the end of a 101.64 km fiber when the signal SNR is down to 3.234 dB. Our proposed method enables the application of the long-distance sensing in quasi-DAS systems.
Photonics Research
  • Publication Date: Jun. 03, 2020
  • Vol.8 Issue, 7 07001093 (2020)
High-sensitivity, high-spatial-resolution distributed strain sensing based on a poly(methyl methacrylate) chirped fiber Bragg grating
Chengang Lyu, Ziqi Liu, Ziqiang Huo, Chunfeng Ge, Xin Cheng, and Haw-Yaw Tam
In this study, a high-sensitivity, high-spatial-resolution distributed strain-sensing approach based on a poly(methyl methacrylate) chirped fiber Bragg grating (CFBG) is proposed and experimentally demonstrated. Linearly chirped FBGs in a polymer optical fiber provide an alternative to the silica fiber owing to the lower Young’s modulus, which can yield a higher stress sensitivity under the same external force. According to the spatial wavelength-encoded characteristic of the CFBG, a fully distributed strain measurement can be achieved by optical frequency-domain reflectometry. Through time-/space-resolved short-time Fourier transform, the applied force can be located by the beat frequency originated from the space-induced time delay and measured by the differential frequency offset originated from the strain-induced dispersion time delay. In a proof-of-concept experiment, a high spatial resolution of 1 mm over a gauge length of 40 mm and a strain resolution of 0.491 Hz/με were achieved.
Photonics Research
  • Publication Date: Jun. 04, 2020
  • Vol.8 Issue, 7 07001134 (2020)
Mode-division multiplexed transmission of wavelength-division multiplexing signals over a 100-km single-span orbital angular momentum fiber | Spotlight on Optics
Junwei Zhang, Junyi Liu, Lei Shen, Lei Zhang, Jie Luo, Jie Liu, and Siyuan Yu
We experimentally demonstrate mode-division multiplexed (MDM) transmission using eight orbital angular momentum (OAM) modes over a single span of 100-km low-attenuation and low-crosstalk ring-core fiber (RCF). Each OAM mode channel carries 10 wavelength-division multiplexing (WDM) signal channels in the C band, with each WDM channel in turn transmitting 16-GBaud quadrature phase-shift keying signal. An aggregate capacity of 2.56 Tbit/s and an overall spectral efficiency of 10.24 bit/(s · Hz) are realized. The capacity-distance product of 256 (Tbit/s) · km is the largest reported so far for OAM fiber communications systems to the best of our knowledge. Exploiting the low crosstalk between the OAM mode groups in the RCF, the scheme only requires the use of modular 4×4 multiple-input multiple-output processing, and it can therefore be scaled up in the number of MDM channels without increasing the complexity of signal processing.
Photonics Research
  • Publication Date: Jun. 30, 2020
  • Vol.8 Issue, 7 07001236 (2020)
Optical Devices
Ultrabroadband and sensitive cavity optomechanical magnetometry
Bei-Bei Li, George Brawley, Hamish Greenall, Stefan Forstner, Eoin Sheridan, Halina Rubinsztein-Dunlop, and Warwick P. Bowen
Magnetostrictive optomechanical cavities provide a new optical readout approach to room-temperature magnetometry. Here we report ultrasensitive and ultrahigh bandwidth cavity optomechanical magnetometers constructed by embedding a grain of the magnetostrictive material Terfenol-D within a high quality (Q) optical microcavity on a silicon chip. By engineering their physical structure, we achieve a peak sensitivity of 26 pT/Hz comparable to the best cryogenic microscale magnetometers, along with a 3 dB bandwidth as high as 11.3 MHz. Two classes of magnetic response are observed, which we postulate arise from the crystallinity of the Terfenol-D. This allows single crystalline and polycrystalline grains to be distinguished at the level of a single particle. Our results may enable applications such as lab-on-chip nuclear magnetic spectroscopy and magnetic navigation.
Photonics Research
  • Publication Date: Jun. 04, 2020
  • Vol.8 Issue, 7 07001064 (2020)