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Accepted papers to appear in an upcoming issue

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Optomechanical cooling and self-stabilization of a waveguide coupled to a whispering-gallery-mode resonator

Riccardo Pennetta, Shangran Xie, Richard Zeltner, Jonas Hammer, and Philip Russell

DOI: 10.1364/PRJ.380151 Received 10 Oct 2019; Accepted 27 Mar 2020; Posted 27 Mar 2020  View: PDF

Abstract: Laser cooling of mechanical degrees of freedom is one of the most significant achievements in the field of optomechanics. Here we report for the first time efficient passive optomechanical cooling of the motion of a free-standing waveguide coupled to a whispering-gallery-mode (WGM) resonator. The waveguide is an 8 mm long glass-fibre nanospike, which has a fundamental flexural resonance at Ω/2π = 2.5 kHz and a Q-factor of 1.2×10^5. Upon launching ~250 μW laser power at an optical frequency close to the WGM resonant frequency, we observed cooling of the nanospike resonance from room temperature down to 1.8 K. Simultaneous cooling of the first higher order mechanical mode is also observed. The strong suppression of the overall Brownian motion of the nanospike, observed as an 11.6 dB reduction in its mean square displacement, indicates strong optomechanical stabilization of linear coupling between the nanospike and the cavity mode. The results are of direct relevance in the many applications of WGM resonators, including atom physics, optomechanics and sensing.

Spectrally-resolved Hong-Ou-Mandel interferometry for Quantum-Optical Coherence Tomography

Pablo Daniel Yepiz Graciano, Ali Angulo, Dorilian Lopez-Mago, HECTOR CRUZ-RAMIREZ, and Alfred U'Ren

DOI: 10.1364/PRJ.388693 Received 21 Jan 2020; Accepted 24 Mar 2020; Posted 26 Mar 2020  View: PDF

Abstract: In this paper, we revisit the well-known Hong-Ou-Mandel (HOM) effect in which two photons, which meet at a beamsplitter, can interfere destructively, leading to null in coincidence counts. In a standard HOM measurement, the coincidence counts across the two output ports of the beamsplitter are monitored as the temporal delay between the two photons prior to the beamsplitter is varied, resulting in the well-known HOM dip. We show, both theoretically and experimentally, that by leaving the delay fixed at a particular value while relying on spectrally-resolved coincidence photon-counting, we can reconstruct the HOM dip, which would have been obtained through a standard delay-scanning, non-spectrally-resolved HOM measurement. We show that our numerical reconstruction procedure exhibits a novel dispersion cancellation effects, to all orders. We discuss how our present work can lead to a drastic reduction in the time required to acquire a HOM interferogram, and specifically discuss how this could be of particular importance for the implementation of efficient quantum-optical coherence tomography devices.

Frequency comb swept laser with a high-Q micro-ring filter

Dongmei HUANG, Feng Li, Shang Chao, Zihao Cheng, Sai Tak Chu, and Ping Kong Wai

DOI: 10.1364/PRJ.386900 Received 30 Dec 2019; Accepted 24 Mar 2020; Posted 24 Mar 2020  View: PDF

Abstract: Frequency comb swept laser is the enabling technology of circular interferometric imaging, which was proposed to break the bottleneck of data acquisition and processing in the optical coherence tomography (OCT) at video rate. In this paper, we propose and demonstrate a highly coherent frequency comb swept laser by using a high-quality (high-Q) micro-ring comb filter to discretize a Fourier domain mode-locked (FDML) laser. The micro-ring filter has a Q-factor of ~2×10⁶ and a linewidth of ~90 MHz. To demonstrate the improvement in performance, a Fabry-Pérot comb filter with a Q-factor of 6.4×10⁴ and a linewidth of 2.8 GHz is also used in experiment for comparison. Both comb filters have free spectral ranges (FSRs) of ~50 GHz for consistence. Stable and clearly discretized temporal waveforms and frequency comb spectra with 50 GHz FSR are observed. Adoption of the high-Q micro-ring filter narrows the instantaneous linewidth of the FDML laser down to 1.5 GHz because of the ultra-narrow linewidth of the comb filter. The OCT system with the frequency comb swept laser source with micro-ring filter demonstrates an ultra-long imaging depth, which has a 6-, 10- and 15-dB sensitivity roll off length of ~49, ~70 and over 100 mm, respectively.

Sub-nanosecond-speed frequency-reconfigurable photonic RF switch using a silicon modulator

Yiwei Xie, Leimeng Zhuang, pengcheng jiao, and Daoxin Dai

DOI: 10.1364/PRJ.387480 Received 08 Jan 2020; Accepted 22 Mar 2020; Posted 24 Mar 2020  View: PDF

Abstract: Radio frequency (RF) switches are essential for implementing routing of RF signals. However, the increasing demand for RF signal frequency and bandwidth is posing a challenge of switching speed to the conventional solutions, i.e. the capability of operating at a speed of sub-nanosecond or faster. In addition, signal frequency reconfigurability is also a desirable feature to facilitate new innovations of flexible system functions. Utilizing microwave photonics as an alternative path, we present here a photonic implementation of a RF switch providing not only the capability of switching at a sub-nanosecond speed, but also options of frequency doubling of the input RF signals, allowing for flexible output waveforms. The core device is a travelling-wave silicon modulator with a device size of 0.2 × 1.8 mm2 and a modulation bandwidth of 10 GHz. Using microwave frequencies, i.e. 15 GHz and 20 GHz, as two simultaneous RF input signals, we experimentally demonstrated amplitude and frequency switching of them as well as the doubled frequencies, i.e. 30 GHz and 40 GHz, at a switching frequency of 5 GHz. The results of this work point to a solution for creating fast-speed RF switches with high compactness and flexibility.

Applying a mixed light field generated from a two-level atomic ensemble to two-photon interference

Shuyu Zhou, Shanchao Zhang, Ying Wang, and Yu-zhu Wang

DOI: 10.1364/PRJ.386557 Received 23 Dec 2019; Accepted 19 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: A mixed light field generated from a two-level atomic ensemble can be used for two-photon interference. In this mixed light field, correlated paired-photons generated from four-wave mixing process provide a signal of two-photon interference, while Rayleigh scattered photons of the pump laser provide a stable reference to calibrate the normalized second-order correlation function. We demonstrate two-photon interference using Hong-Ou-Mandel (HOM) and Hanbury Brown-Twiss (HBT) interferometers. A direct quantitative comparison between theoretical predictions and experimental data is performed under perturbed experimental conditions, which reveal this kind of light source has potential application for quantum metrology.

TM mode Enhancement and droop Reduction by Nanoporous n-AlGaN Under-layer in 290-nm UV-LED

Yufeng Li, Chenyu Wang, Ye Zhang, Peng Hu, shengnan zhang, Mengqi Du, Xilin Su, Li Qiang, and Feng Yun

DOI: 10.1364/PRJ.387607 Received 13 Jan 2020; Accepted 18 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: A full structure 290-nm UV-LED with a nanoporous n-AlGaN under layer was fabricated by top via holes formation followed by high voltage electrochemical etching. Nano size pores in the range of 20 to 120 nm were prepared in the regular doped n-AlGaN by adjusting the etching voltages. A comparison of Raman measurement and photoluminescence wavelength shows a clear relaxation of the biaxial stress in the nanoporous sample. The photoluminescence enhancement was found highly dependent on the size of the pores. Not only was the top emitting TE mode photon extraction efficiency increased, but also the TM mode photon emitting from the side was greatly enhanced. This leads to the polarization change in the side-emitting light from -0.08 to -0.242. The intensity of the electroluminescence was increased by 36.5% at 100 mA and the efficiency droop at high current was found to reduce from 61% to 31%.

Chip-scale Full-Stokes Spectropolarimeter in Silicon Photonic Circuits

Zhongjin Lin, Tigran Dadalyan, Simon Bélanger-de Villers, Tigran Galstian, and Wei Shi

DOI: 10.1364/PRJ.385008 Received 03 Dec 2019; Accepted 18 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: Wavelength-dependent polarization state of light carries crucial information about light-matter interactions. However, its measurement is limited to bulky, energy-consuming devices, which prohibits many modern, portable applications. Here, we propose and demonstrate a chip-scale spectropolarimeter implemented using a CMOS-compatible silicon photonics technology. Four compact Vernier microresonator spectrometers are monolithically integrated with a broadband polarimeter consisting of a 2D nanophotonic antenna and a polarimetric circuit to achieve full-Stokes spectropolarimetric analysis. The proposed device offers a solid-state spectropolarimetry solution with a small footprint of 1 × 0.6 mm² and low power consumption of 360 mW. Full-Stokes spectral detection across a broad spectral range of 50 nm with a resolution of 1 nm is demonstrated in characterizing a material possessing structural chirality. The proposed device may enable a broader application of spectropolarimetry in the fields ranging from biomedical diagnostics and chemical analysis to observational astronomy.

Cavity-based Photoconductive sources for real time terahertz imaging

Jacques Hawecker, Valentino Pistore, Amalya Minasyan, Kenneth Maussang, José Palomo, Isabelle Sagnes, Raffaele Colombelli, Jean-Michel Manceau, Jerome Tignon, Juliette Mangeney, and Sukhdeep Dhillon

DOI: 10.1364/PRJ.388219 Received 16 Jan 2020; Accepted 18 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: Optically-driven photoconductive switches are one of the predominant sources currently used in terahertz imaging systems. However, owing to their low average powers, only raster-based images can be taken, resulting in slow acquisition times. In this work, we show that by placing a photoconductive switch within a cavity, we are able to generate absolute average THz powers of 181 µW, with the frequency of the THz emission centered at 1.5THz, specifications ideally adapted to applications such as non-destructive imaging. The cavity is based on a metal-insulator-metal structure that permits an enhancement of the average power by almost one order of magnitude compared to a standard structure, whilst conserving a broadband spectral response. We demonstrate proof-of-principle real-time imaging using this source, with the broadband spectrum permitting to eliminate strong diffraction artefacts.

Dual-layered metasurfaces for asymmetric focusing

XiaoFei Zang, Bingshuang Yao, Zhen li, Lin Chen, Yiming Zhu, Songlin Zhuang, and Jingya Xie

DOI: 10.1364/PRJ.387672 Received 09 Jan 2020; Accepted 17 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: Asymmetric transmission, defined as the difference in the total transmission between forward and backward propagations, enables a plethora of applications toward on-chip integration and telecommunications. However, traditional methods for asymmetric transmission can only control the energy flux, hindering the further applications. Metasurfaces, two-dimensional metamaterials, have shown unprecedented ability to manipulate the amplitude, phase and polarization of electromagnetic waves. Here, we propose and experimentally demonstrate a metasurface-based directional device consisting of a geometric metasurface with spatially rotated microrods and metallic gratings, that can simultaneously control the phase, polarization and energy flux, resulting in the asymmetric focusing in terahertz region. The dual-layered metasurfaces for asymmetric focusing can work in a wide bandwidth ranging from 0.6 THz to 1.1 THz. The flexible and robust approach for designing broadband asymmetric focusing may open a new avenue for compact devices with potential applications in encryption, information processing and communication.

High-speed and high-efficiency three-dimensional shape measurement based on Gray-coded light

Qican Zhang, Zhoujie Wu, Wenbo Guo, Yueyang Li, and Yihang Liu

DOI: 10.1364/PRJ.389076 Received 23 Jan 2020; Accepted 17 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: Fringe projection profilometry has been increasingly sought and applied in dynamic three-dimensional (3D) shape measurement. In this work, a robust and high-efficiency 3D measurement based on Gray-code light is proposed. Unlike the traditional method, a novel tripartite phase unwrapping method is proposed to avoid the jump errors on the boundary of code words, which are mainly caused by the defocusing of the projector and the motion of the tested object. Subsequently, the time-overlapping coding strategy is presented to greatly increase the coding efficiency, decreasing the projected number in each group from 7 (i.e. 3 + 4) to 4 (i.e. 3 + 1) for one restored 3D frame. Combination of two proposed techniques allows to reconstruct a pixel-wise and unambiguous 3D geometry of dynamic scenes with strong noise using every 4 projected patterns. To our knowledge, the presented techniques preserve the high anti-noise ability of Gray-coded-based method while overcoming the drawbacks of jump errors and low coding efficiency for the first time. Experiments have demonstrated that the proposed method can achieve the robust and high-efficiency 3D shape measurement of high-speed dynamic scenes even polluted by strong noise.

Revealing the surface electronic structures of AlGaN deep ultraviolet multiple-quantum-wells with lateral polarity domains

Wei Guo, Li Chen, Houqiang Xu, Yingda Qian, MOHEB SHEIKHI, Jason Hoo, Shiping Guo, Liang Xu, Jianzhe Liu, Feras Alqatari, Xiaohang Li, Kaiyan He, Zhe Chuan Feng, and Jichun Ye

DOI: 10.1364/PRJ.387700 Received 10 Jan 2020; Accepted 17 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: We report on the carrier dynamic and electronic structure investigations on AlGaN-based deep-ultraviolet multiple-quantum-wells (DUV-MQWs) with lateral polarity domains. Localized potential maximum is predicted near the domain boundaries by first-principle calculation, suggesting carrier localization and efficient radiative recombination. More importantly, lateral band diagrams of the MQWs are proposed based on electron affinities and valance band levels calculated from ultraviolet photoelectron spectroscopy (UPS). The proposed lateral band diagram is further demonstrated by surface potential distribution collected by Kelvin Probe Microscopy (KFM) and density-of-state (DOS) calculation of energy bands. This work illustrates that lateral polarity structures are playing essential roles in the electronic properties of III-nitride photonic devices, and may provide novel perspective in the realization of high-efficiency UV emitters

Measuring high orbital angular momentum of vortex beams with improved multipoint interferometer

Qi Zhao, Miao Dong, Yihua Bai, and Yuanjie Yang

DOI: 10.1364/PRJ.384925 Received 03 Dec 2019; Accepted 16 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: A multipoint interferometer (MI), uniformly distributed pointlike pinholes in a circle, was proposed to measure the orbital angular momentum (OAM) of vortex beams recently [Phys. Rev. Lett. 101, 100801 (2008)], which can be used for measuring OAM of light from astronomical sources. This is a simple and robust method, while, it is noted that this method is only available for low topological charge, because the diffracted intensity patterns for vortex beams with higher OAM will repeat periodically. Here, we propose an improved multipoint interferometer (IMI) for measuring the OAM of optical vortex with high topological charge. The structure of our IMI is almost the same as the MI, but the size of the each pinhole is bigger rather than a point in MI. Such a little change enable each pinhole get more phase information of the incident beams, accordingly, the IMI can distinguish any vortex beams with different OAM. We demonstrate its viability both theoretically and experimentally.

Effects of third-order dispersion on temporal soliton compression in dispersion engineered silicon photonic crystal waveguides

Jiali Liao, Yang Gao, yanling sun, Lin Ma, Zhenzhong Lu, and Xiujian Li

DOI: 10.1364/PRJ.381371 Received 19 Nov 2019; Accepted 14 Mar 2020; Posted 16 Mar 2020  View: PDF

Abstract: We design a silicon photonic crystal waveguide with wideband low anomalous dispersion. Temporal soliton evolution in the designed waveguide is investigated numerically, with remarkable high order temporal soliton compression being achieved with ultralow (<19 pJ) pulse energy, at telecom wavelengths. Particularly, we report the first demonstration of the dual opposite effects of third-order dispersion (TOD) on temporal soliton compression, which are strengthening or weakening, not only depending on the sign of TOD but also the relative magnitude of TOD to group velocity dispersion. Free carrier dispersion (FCD) counteracts the effects of negative TOD on the soliton compression, while it reinforces the effects of positive TOD on the soliton compression. In addition to the physical mechanism of the effects of TOD and TOD-FCD interaction on the temporal soliton compression unveiled here, these results provide an approach to realize superior on-chip temporal pulse compression.

Build-up dynamics in bidirectional soliton fibre laser

Maria Chernysheva, Srikanth Sugavanam, and Igor Kudelin

DOI: 10.1364/PRJ.388988 Received 27 Jan 2020; Accepted 14 Mar 2020; Posted 16 Mar 2020  View: PDF

Abstract: Bidirectional ultrafast fibre lasers present an attractive solution, enabling the generation of two mutually coherent ultrashort pulse trains in a simple and turnkey system. Still, the lack of a comprehensive numerical model describing steady-state bidirectional generation, and even less ultrafast soliton breakdowns and collisions, is obstructing the achievement of the performance compared with unidirectional lasers.In this paper, we have experimentally investigated real-time build-up dynamics of counter-propagating solitons in ultrafast ring Er-doped fibre laser via the dispersive Fourier transform methodology. We demonstrate that counter-propagating pulses experience independent build-up dynamics from modulation instability, undergoing breathing dynamics and diverging sub-ordinate pulse structures formation and annihilation to a stable bidirectional pulse train. Yet, interaction of pulses in the cavity presents the key underlying phenomenon driving formation evolution distinct form unidirectional pulse build-up. Our findings will provide physical foundations for bidirectional ultrafast fibre laser design to carry forward their application.

Enhanced Non-linear Instabilities in Photonic Circuits with Exceptional Point Degeneracies

Tsampikos Kottos, Rodion Kononchuk, Andrey Chabanov, Ilya Vitebskiy, Nicholaos Limberopoulos, and Suwun Suwunnarat

DOI: 10.1364/PRJ.385780 Received 11 Dec 2019; Accepted 11 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: We propose a conceptual design of optical power limiters, with abrupt limiting action and enhanced power-handling capabilities, which is based on exceptional point degeneracies (EPDs). The photonic circuit consists of two coupled cavities with differential Q-factors. One of the cavities includes a Kerr-like non-linear material. The underlying mechanism that triggers an abrupt transmittance suppression, relies on the interplay between a nonlinear instability, and an abrupt destruction of EPDs due to a resonance detuning occurred when the incident power exceeds a critical value. Our proposal opens up possibilities for the use of EPDs in optical power switching, Q-switching, routing, etc.

Highly luminescent and stable lead-free cesium copper halide perovskite powders for UV pumped phosphor converted light-emitting diodes

lingling xie, Bingkun Chen, Fa Zhang, Ziheng Zhao, Xinxin Wang, lijie shi, yue liu, Lingling Huang, Bingsuo ZOU, Yongtian Wang, and ruibin liu

DOI: 10.1364/PRJ.387707 Received 14 Jan 2020; Accepted 10 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: Lead halide perovskites have drawn extensive attention over recent decades owing to their outstanding photoelectric performances. However, their toxicity and instability as big issues need to be solved for further commercialization. Herein, we adopt a facile dry ball milling method to synthesize lead-free Cs3Cu2X5 (X=I, Cl) perovskites with photoluminescence quantum yield (PL QY) up to 60%. The optical features including broad emission spectrum, large Stokes shift, and long PL lifetime attribute to self-trapped excitons recombination. The as-synthesized blue emissive Cs3Cu2I5 and green emissive Cs3Cu2Cl5 lead-free perovskite powders have good thermal stability and photostability. Furthermore, UV pumped phosphor converted light-emitting diodes (UV pumped pc-LEDs) were obtained by using Cs3Cu2I5 and Cs3Cu2Cl5 asphosphors.

Carrier lifetime of GeSn measured by spectrally resolved picosecond photoluminescence spectroscopy

Brian Julsgaard, Nils von den Driesch, Peter Tidemand-Lichtenberg, Christian Pedersen, Zoran Ikonic, and Dan Buca

DOI: 10.1364/PRJ.385096 Received 03 Dec 2019; Accepted 09 Mar 2020; Posted 09 Mar 2020  View: PDF

Abstract: We present an experimental setup capable of time-resolved photoluminescence spectroscopy for photon energies in the range of 0.51 eV to 0.56 eV with an instrument time jitter of 75 ps. The detection system is based on optical parametric three-wave mixing, operates at room temperature, has spectral resolving power, and is shown to be well-suited for investigating dynamical processes in germanium-tin alloys. In particular, the carrier lifetime of a direct-bandgap Ge(1-x)Sn(x) film with concentration x = 12.5 % and biaxial strain -0.55 % is determined to be 217 +/- 15 ps at a temperature of 20 K.

Excellent light capture capability of trilobal SiNW for ultra-high JSC in single-nanowire solar cells

Zhongliang Gao, Guilu lin, Yupeng Zheng, Na Sang, Yingfeng Li, Lei Chen, and Meicheng Li

DOI: 10.1364/PRJ.385867 Received 13 Dec 2019; Accepted 09 Mar 2020; Posted 09 Mar 2020  View: PDF

Abstract: Single-nanowire solar cells with unique light-concentration property are expected to exceed the Shockley-Queisser limit. The architecture of single-nanowire is an important factor to regulate its optical performance. We designed a trilobal silicon nanowire (SiNW) with two equivalent scales, which possesses superior light absorption efficiency in the whole wavelength range and shows good tolerance to incident angle. The electric field distribution in this geometry is concentrated in the blade with small equivalent scale and pivot with large equivalent scale respectively in the short wavelength range and long wavelength range. Corresponding good light absorption of trilobal SiNW in the two wavelength ranges leads to stronger total light absorption capacity than that of cylindrical SiNW. Trilobal single-nanowire solar cells can obtain a short-circuit current density (JSC) of 647 mA·cm-2, which provides a new choice for designing single-nanowire with excellent light capture capability.

Zero-order-free meta-holograms in a broadband visible range

Guoxing Zheng, Rao Fu, Liangui Deng, Zhiqiang Guan, Sheng Chang, Jin Tao, and Zile Li

DOI: 10.1364/PRJ.387397 Received 06 Jan 2020; Accepted 08 Mar 2020; Posted 09 Mar 2020  View: PDF

Abstract: The unwanted zero-order light, accompanied by the birth of diffractive optical elements and caused mainly by fabrication errors and wavelength variations, is a key factor that deteriorates the performance of diffraction related optical devices such as holograms, gratings, beam shapers, beam splitters, optical diffusers and diffractive microlenses. Here, inspired by the unique characteristic of nano-polarizers based metasurface for both positive and negative amplitude modulation of incident light, we propose a general design paradigm to eliminate zero-order diffraction without torturing the metasurface design and fabrication. The experimentally demonstrated meta-hologram, which projects a holographic image with a wide angle of 70° × 70° in the far field, presents a very low zero-order intensity (only 0.7% of the total energy of the reconstructed image). More importantly, the zero-order-free meta-hologram has a large tolerance limit for wavelength variations, under a broadband illumination from 520 nm to 660 nm, which brings important technical advances. The strategy proposed could significantly relieve fabrication difficulty of metasurfaces and be viable for various diffractive optics related applications including holography, laser beam shaping, optical data storage, vortex beam generation and so on.

Correlative cross-cumulant and radiality microscopy for multicolor super-resolution subcellular imaging

Zhiping Zeng, Jing Ma, and Xu Canhua

DOI: 10.1364/PRJ.387582 Received 08 Jan 2020; Accepted 07 Mar 2020; Posted 09 Mar 2020  View: PDF

Abstract: Fluorescence fluctuation-based super-resolution techniques can achieve fast super-resolution imaging on a cost-effective wide-field platform at a low light level with reduced phototoxicity. However, the current methods exhibit certain imaging deficiencies that misinterpret nanoscale features reconstructed from fluctuating image sequences, thus degrading the reconstructed image fidelity and quality. Here we propose correlative cross-cumulant and radiality microscopy (COXCURM) that employs the cross-cumulant analysis in tandem with radiality processing. We demonstrated that COXCURM can significantly improve the spatial resolution at a low light level whilst eliminate the misinterpretations of nanoscale features of the existing fluctuation-based super-resolution methods. In the experiment, we further verified the superior performance of COXCURM over the current methods through performing multicolor super-resolution imaging of subcellular microtubule networks and clathrin-coated pits (CCPs) as well as the high-precision reconstruction of densely packed RNA transcripts.

Photonic engineering of superbroadband near-infrared emission in nano-glass composites containing hybrid metal and dielectric nanocrystals

Jing Ren, Zhigang Gao, Haibo Zhu, Bochao Sun, yingke ji, Xiaosong Lu, Hao Tian, Shu Guo, Jianzhong Zhang, Jun Yang, Xiangeng Meng, and Katsuhisa Tanaka

DOI: 10.1364/PRJ.379662 Received 03 Oct 2019; Accepted 06 Mar 2020; Posted 06 Mar 2020  View: PDF

Abstract: Photonic media containing hybrid noble metal-dielectric nanocrystals (NCs) represent a wonderland of nanophotonics with a myriad of uncharted optical functions yet to be explored. Capitalizing on the unique phase separation and self-reduction of Au3+ ions in a gallosilicate glass, we fabricated Ni2+-doped transparent nano-glass composites (GCs) containing Au-metal/γ-Ga2O3-dielectric NCs. Compared with GCs free of Au-metal NCs, the superbroadband near-infrared (NIR) emission of Ni2+ with a full width of half maximum over 280 nm is enhanced twice in the Au-metal/γ-Ga2O3 dual-phase GCs. A comparison is given as to the spontaneous emission (SPE) properties of Ni2+ in the dual-phase GCs when pumped resonantly and off-resonantly with the localized surface plasmon resonance band of the Au-metal NCs. The important role of the Au-metal NCs in the SPE enhancement is revealed by theoretical simulation based on the finite-element method. Combining the photonic engineering effect of hybrid Au-metal/γ-Ga2O3 NCs and the sensitization effect of Yb3+ on Ni2+, a record-high enhancement factor over 10 of the Ni2+ NIR emission is achieved, and optical gain is demonstrated in the GCs at the fiber communication wavelength.

Reshaping arbitrarily the impulse ring profile for perfect vortex beams

Junjie Yu, Chaofeng Miao, Jun Wu, and Changhe Zhou

DOI: 10.1364/PRJ.387527 Received 09 Jan 2020; Accepted 06 Mar 2020; Posted 06 Mar 2020  View: PDF

Abstract: Perfect optical vortices (POVs) provide an enabling solution to address the predicament induced by the strong dependence of classical optical vortices on theirs carried topological charges. However, the traditional POVs are all shaped in bright rings with a single mainlobe along the radial direction. Here, we proposed a method for reshaping arbitrarily the impulse ring profile for POVs based on modulated circular sine/cosine radial functions. Especially, a type of “absolute” dark POVs surrounded by two bright ringlobes in each side was presented, which provides a perfect annular potential well along those dark impulse rings for trapping steadily low-index particles, cells, or quantum gas, etc. In further, several POVs with different impulse ring profiles, including conventional POVs with bright rings, dark POVs mentioned above, and also POVs with controllable impulse ring profile, were demonstrated. This work opens up new possibilities to reshape arbitrarily the impulse ring profile for perfect vortices, and this type of novel POVs will enrich functions of optical vortices and it should be of high interest for its potential applications in optical manipulation, both quantum and classical optical communications, enhanced optical imaging, and also novel structured pumping lasers, etc.

Full color micro-LED display with high color stability using semipolar (20-21) InGaN LED and quantum dot photoresist

Sung-Wen Huang Chen, Yu-Ming Huang, Konthoujam James Singh, Yu-Chien Hsu, Fang-Jyun Liou, JIE Song, Joowon Choi, Po-Tsung Lee, Chien-Chung Lin, Zhong Chen, Jung Han, Tingzhu Wu, and Hao-chung Kuo

DOI: 10.1364/PRJ.388958 Received 22 Jan 2020; Accepted 05 Mar 2020; Posted 05 Mar 2020  View: PDF

Abstract: Red-green-blue (RGB) full-color micro light-emitting diodes (µ-LEDs) fabricated from (20-21) semipolar wafers, with a quantum dot photoresist color-conversion layer, were demonstrated. The (20-21) semipolar InGaN/GaN µ-LEDs were fabricated on large (4”) patterned sapphire substrates by orientation-controlled epitaxy. The semipolar µ-LEDs showed a 3.2 nm peak wavelength-shift and a 14.7% efficiency droop under 200 A/cm2 injected current density, indicating significant amelioration of the quantum-confined Stark effect. Because of the semipolar µ-LEDs’ emission-wavelength stability, the RGB pixel showed little color shift with current density and achieved a wide color gamut (114.4% NTSC space and 85.4% Rec. 2020).

Low threshold random lasing in dye-doped and strongly disordered chiral liquid crystals

Xinzheng Zhang, Shaohua Gao, Jiayi Wang, Wenhua Li, Xuanyi Yu, Xiao Song, Andrey Iljin, Irena Drevensek-Olenik, Romano Rupp, and Jingjun Xu

DOI: 10.1364/PRJ.388706 Received 22 Jan 2020; Accepted 05 Mar 2020; Posted 06 Mar 2020  View: PDF

Abstract: Random lasing was experimentally investigated in pyrromethene 597-doped strongly disordered chiral liquid crystals (CLCs) composed of the nematic liquid crystal SLC1717 and the chiral agent CB15. The concentration of the chiral agent tuned the band gap, and disordered CLC micro-domains were achieved by fast quenching of the mixture from the isotropic to the cholesteric phase. Random lasing and band edge lasing were observed synchronously, and their behavior changed with the spectral location of the band gap. The emission band for band edge lasing shifted with the change of the band gap, while the emission band for random lasing remained practically constant. The results show that the threshold for random lasing sharply decreases if the CLC selective reflection band overlaps with the fluorescence peak of the dye molecules and if the band edge coincides at the same time with the excitation wavelength.

Deep learning assists optimization algorithm: a two-stage phase control method for coherent laser arrays enables high-power, all-electronic flexibly mode-programmable orbital angular momentum beams generation

Tianyue Hou, Yi An, Qi Chang, Pengfei Ma, Jun Li, Liangjin Huang, Dong Zhi, Jian Wu, Rongtao Su, Yanxing Ma, and Pu Zhou

DOI: 10.1364/PRJ.388551 Received 22 Jan 2020; Accepted 03 Mar 2020; Posted 04 Mar 2020  View: PDF

Abstract: High-power, mode-programmable orbital angular momentum (OAM) beams have received substantial attentions in recent years and are widely used in optical communication, nonlinear frequency conversion and laser processing applications. To overcome the power limitation of a single beam, coherent beam combining (CBC) of laser arrays opens up a promising way. However, in specific CBC systems for generating structured light with complex wavefront, eliminating phase noise and realizing flexible phase modulation present critical challenges. In this paper, we propose and demonstrate a two-stage phase control method that can generate OAM beams with different orders from a CBC system. During the phase control process, the phase noise could be preliminarily compensated by a deep learning network and further eliminated by an optimization algorithm. Moreover, by modulating the expected relative phase vector and the cost function, the all-electronic flexibly programmable switching of OAM-mode could be realized. Results indicate that the proposed method combines the characteristic of deep learning in undesired convergent phases avoidance and the advantage of the optimization algorithm in accuracy improvement, thereby ensuring high mode purity of the generated OAM beams. This work could provide a valuable reference on future implementation of high-power, fast switchable structured light generation and manipulation.

Stimulated Raman scattering signal generation in scattering medium using self-reconstructing Bessel beams

Xueli Chen, Xinyu Wang, Lin Wang, PENG LIN, Yonghua Zhan, and Ji-Xin Cheng

DOI: 10.1364/PRJ.384604 Received 20 Dec 2019; Accepted 02 Mar 2020; Posted 24 Mar 2020  View: PDF

Abstract: Scattering is a big challenge for microscopic imaging. Current Gaussian beam-based stimulated Raman scattering (SRS) microscopy does not work well in observing the scattering media, because the tight focus of Gaussian beam is destroyed after propagating through a certain distance. Having the self-reconstructing property, the Bessel beams may bring a solution to this problem. By combining Bessel beams with the SRS microscopy, we can probe SRS signal from a scattering medium. In this paper, using the beam propagation method, we first simulated the propagation of Bessel beam as well as the generation and self-reconstruction of SRS signals. By adding glass beads onto the beam propagation path to simulate scatters, the propagation of the Bessel beams and the generation of the SRS signals will change. We designed a series of simulations to investigate the influence of the size, position, number, and distribution of the added glass beads on the generation of SRS signals. We then carried out a preliminary experiment to confirm the simulation results. Results demonstrated that the SRS signals can generate or be recovered at the certain depth in scattering media, and such signals are greatly affected by parameters of the scatters.

Gap-surface plasmon metasurfaces for linear-polarization conversion, focusing and beam splitting

Fei Ding, Yiting Chen, and Sergey Bozhevolnyi

DOI: 10.1364/PRJ.386655 Received 26 Dec 2019; Accepted 01 Mar 2020; Posted 04 Mar 2020  View: PDF

Abstract: Gap-surface plasmon (GSP) metasurfaces have attracted progressively increasing attention due to their planar configurations, ease of fabrication, and unprecedented capabilities in manipulating the reflected fields that enable integrating diverse bulk-optic-based optical components into a single ultrathin flat element. In this work, we design and experimentally demonstrate multifunctional metalenses that perform simultaneous linear-polarization conversion, focusing and beam-splitting, reproducing thereby combined functionalities of conventional half-wave plates, parabolic reflectors, and beam splitters. The fabricated single-focal metalens incorporates properly configured distinct half-wave-plate-like GSP meta-atoms and exhibits good performance under linearly polarized incidence in terms of orthogonal linear-polarization conversion (> 75%) and focusing (overall efficiency > 22%) in the wavelength spectrum ranging from 800 to 950 nm. To further extend the combined functionalities, we demonstrate a dual-focal metalens that splits and focuses a linearly-polarized incident beam into two focal spots, while maintaining the capability of orthogonal linear-polarization conversion. Furthermore, the power distribution between two split beams can readily be controlled by judiciously positioning the incident beam. The demonstrated multifunctional GSP-based metalenses mimic the combined functionalities of a sequence of discrete bulk optical components, eliminating thereby the need of their mutual alignment, and open new perspectives in the development of ultra-compact and integrated photonic devices.

Strain Enhancement for MoS2-on-GaN photodetector with Al2O3 stress liner grown by atomic layer deposition

Zhiwen Li, Jiangliu luo, Shenqun Hu, qiang liu, WENJIE YU, Youming Lu, and xinke liu

DOI: 10.1364/PRJ.385885 Received 19 Dec 2019; Accepted 29 Feb 2020; Posted 16 Mar 2020  View: PDF

Abstract: Strain engineering as an effective way to enhance the photoelectric properties of Two-dimensional transition metal dichalcogenides (TMDs) has been employed to improve the performance of MoS2-based photodetectors in this work. A tensile strain has been introduced into multilayer MoS2 by depositing 3nm Al2O3 stress liner. The temperature-dependent Raman spectrum shows that the thermal stability of MoS2 is improved after applying the Al2O3 stress liner. Theoretical simulation has been used to verify the existence of tensile strain on MoS2, and the electron effective mass of multilayer MoS2 are decreased due to tensile strain, which results in an increase of electron mobility. Due to the tensile strain effect, the photodetector with a Al2O3 stress liner shows a better performance under the illumination of 365 nm wavelength, such as a higher responsivity 24.6 A/W, a higher photoconductive gain 520, and a higher external quantum efficiency 8381%, as compared to those of the control device without a stress liner. Our work provides an effective technical way for improving the performance of MoS2-based photodetectors.

Distributed Brillouin frequency shift extraction via a convolutional neural network

Hao Wu, Yiqing Chang, Can Zhao, li shen, Songnian Fu, and Ming Tang

DOI: 10.1364/PRJ.389970 Received 05 Feb 2020; Accepted 28 Feb 2020; Posted 28 Feb 2020  View: PDF

Abstract: Distributed optical fiber Brillouin sensors detect the temperature and strain along a fiber according to the local Brillouin frequency shift (BFS), which is usually calculated by the measured Brillouin spectrum using Lorentzian curve fitting. In addition, cross-correlation, principal component analysis, and machine learning methods have been proposed for the more efficient extraction of BFS. However, existing methods only process the Brillouin spectrum individually, ignoring the correlation in the time domain, indicating that there is still room for improvement. Here, we propose and experimentally demonstrate a BFS extraction convolutional neural network (BFSCNN) to retrieve the distributed BFS directly from the measured two-dimensional data. Simulated ideal Brillouin spectrums with various parameters are used to train the BFSCNN. Both the simulation and experimental results show that the extraction accuracy of the BFSCNN is better than that of the traditional curve fitting algorithm with a much shorter processing time. The BFSCNN has good universality and robustness and can effectively improve the performances of existing Brillouin sensors.

Fully reversible spectral compression of arbitrary pulsed data signals

Luis Romero Cortes, Reza Maram, and Jose Azana

DOI: 10.1364/PRJ.375990 Received 30 Aug 2019; Accepted 27 Feb 2020; Posted 28 Feb 2020  View: PDF

Abstract: Data signals consisting of arbitrarily modulated sequences of pulses are extensively used for processing and communication applications. The spectral extent of such signals is determined by the bandwidth of the individual pulses in the sequence, and this imposes a fundamental limit to the maximum amount of information that can be transmitted or processed per time period. In this work, we propose and experimentally demonstrate that the frequency spectrum occupied by a data-modulated pulse sequence can be significantly compressed to well-below the individual pulse bandwidth, while still maintaining the temporal duration of the pulses in the sequence, and without losing any of the information carried by the signal. The proposed method involves fully reversible linear transformations of the data signal along the time and frequency domains. We demonstrate successful pulse-shape-preserving spectral compression, and subsequent full waveform and information recovery, of a ~10 Gb/s optical pulse sequence, modulated by an arbitrary data pattern, liberating over 60% of its bandwidth. These findings should prove useful for applications in signal processing, communications, and others.

Continuum Electron Giving Birth to Terahertz Emission

Yizhu Zhang, Kaixuan Zhang, Y. H. JIANG, T.-M. YAN, and Xincheng Wang

DOI: 10.1364/PRJ.377408 Received 10 Sep 2019; Accepted 27 Feb 2020; Posted 28 Feb 2020  View: PDF

Abstract: The origin of terahertz (THz) generation in gas-phase medium is still in controversy although the THz sources have been applied across many disciplines. Herein, the THz generation in a dual-color field is investigated experimentally by precisely controlling the relative phase and polarization of dual-color lasers, where the accompanying third-harmonic generation is employed for in situ determination of the relative phase up to the sub-wavelength accuracy. Joint studies with the strong approximation (SFA) theory reveal that the continuum-continuum (CC) transition within escaped electron wave packet in the single atom gives birth to THz emission, without the necessity of considering the plasma effect. Meanwhile, we develop the analytic form from SFA-based CC description, which is able to reproduce and decompose the classical photocurrent model from the viewpoint of microscopic quantum theory, establishing the quantum-classical correspondence and bringing a novel insight into the mechanism of THz generation. Present studies leave open the possibility for probing the ultrafast dynamics of continuum electron and a new dimension for study of THz-related science and methodology.

Ultra-high-Q Silicon Race-track Resonators

Daoxin Dai, Long Zhang, Lanlan Jie, Ming Zhang, Yi Wang, Yiwei Xie, and Yaocheng Shi

DOI: 10.1364/PRJ.387816 Received 13 Jan 2020; Accepted 27 Feb 2020; Posted 28 Feb 2020  View: PDF

Abstract: An ultra-high-Q silicon race-track resonator is proposed and demonstrated with uniform multimode silicon photonic waveguides. It consists of two multimode straight waveguides connected by two multimode waveguide bends (MWBs). In particular, the MWBs are based on modified-Euler curves, and a bent directional coupler is used for achieving the selective mode-coupling for the fundamental mode and not exciting the higher-order mode in the race-track. In this way, the fundamental-mode is excited and propagates in the multimode race-track resonator with ultra-low loss and low inter-mode coupling. Meanwhile, it helps achieve a compact 180°-bend to make a compact resonator with a maximized free-spectral range (FSR). In this paper, for the chosen 1.6 μm-wide silicon photonic waveguide, the effective radius Reff of the designed 180°-bend is as small as 29 μm. The corresponding free spectral range (FSR) is about 0.9 nm when choosing 260 μm-long straight waveguides in the race-track. The present high-Q resonator is realized with a simple standard single-etching process provided by a multi-project-wafer foundry. The fabricated device, which has a measured intrinsic Q-factor as high as 2.3×10⁶, is the smallest silicon resonators with a >10⁶ Q-factor.

Laser trimming of the operating wavelength of silicon nitride racetrack resonators

Greta De Paoli, Senta Jantzen, Thalía Domínguez Bucio, Ilias Skandalos, Christopher Holmes, Peter Smith, Milan Milosevic, and Frederic Gardes

DOI: 10.1364/PRJ.382529 Received 15 Nov 2019; Accepted 23 Feb 2020; Posted 25 Feb 2020  View: PDF

Abstract: We demonstrate the possibility of post-fabrication trimming of the response of nitrogen-rich silicon nitride racetrack resonators by using an ultraviolet laser. The results revealed the possibility to efficiently tune the operating wavelength of fabricated racetrack resonators to any point within the full free spectral range. This process is much faster than similar, previously presented methods (in the order of seconds, compared to hours) and induces an average material's refractive index change of -2x10^-2. This technique can also be applied to accurately trim the optical performance of any other silicon photonic device based on nitrogen-rich silicon nitride.

Narrow linewidth thermally tuned multi-channel interferece widely tunable semiconductor laser with thermal tuning power below 50 mW

Quanan Chen, Chun Jiang, Wang kuan, Miao Zhang, xiang ma, Ye Liu, Qiaoyin Lu, and Wei-Hua Guo

DOI: 10.1364/PRJ.380002 Received 21 Oct 2019; Accepted 23 Feb 2020; Posted 25 Feb 2020  View: PDF

Abstract: Thermally tuned multi-channel interference (MCI) widely tunable semiconductor laser is designed and demonstrated for the first time, which realizes a tuning range of more than 45 nm, side mode suppression ratios up to 56 dB and Lorentzian linewidth below 160 kHz. AlGaInAs multiple quantum wells (MQWs) were used to reduce linewidth, which have a lower linewidth enhancement factor compared with InGaAs MQWs. To decrease the power consumption of micro-heaters, air gaps were fabricated below the arm phase sections. For a 75-μm-long suspended thermal tuning waveguide, about 6.3 mW micro-heater tuning power is needed for a 2π round trip phase change. Total micro-heater tuning power required is less than 50 mW across the whole tuning range, which is lower than the reported thermally-tuned tunable semiconductor lasers.

Low-efficiency-droop InGaN quantum dot light-emitting diodes operating in the “green gap”

CHUNYU ZHAO, Chak Wah TANG, Billy LAI, Guanghui CHENG, Jiannong Wang, and Kei May Lau

DOI: 10.1364/PRJ.380158 Received 11 Oct 2019; Accepted 22 Feb 2020; Posted 25 Feb 2020  View: PDF

Abstract: Gallium nitride (GaN)-based light-emitting diodes (LEDs) are important to lighting and display applications. In this article, we demonstrate green-emission (512 nm) InGaN quantum dot (QD) LEDs grown on a c-plane sapphire substrate by MOCVD. A radiative lifetime of 707 ps for the uniform InGaN self-assembled QDs is obtained by time resolved photoluminescence measurement at 18 K. The screening of the built-in fields in the QDs effectively improves the performance of QD LEDs. These high quantum efficiency and high temperature stability green QD LEDs are able to operate with negligible efficiency droop and with current density increased up to 106 A/cm². Our results show that InGaN QDs may be a viable option to replace conventional quantum well devices.

High-performance mid wavelength InAs avalanche photodiode using AlAs0.13Sb0.87 as multiplication layer

Jianliang Huang, Chengcheng Zhao, Biyin Nie, Shiyu Xie, Dominic Kwan, xiao meng, Yanhua Zhang, Diana Huffaker, and Wenquan Ma

DOI: 10.1364/PRJ.385177 Received 04 Dec 2019; Accepted 22 Feb 2020; Posted 25 Feb 2020  View: PDF

Abstract: We report on a high-performance mid wavelength infrared avalanche photodetector (APD) with separate absorption and multiplication regions. InAs is used as the absorber material and high bandgap AlAs0.13Sb0.87 is used as the multiplication material. At room temperature, the APD's peak response wavelength is 3.27 μm and the 50% cutoff wavelength is 3.5 μm. The avalanche gain reaches 13.1 and the responsivity is 8.09 A/W at 3.27 μm when the applied reverse bias voltage is 14.6 V. The shot noise limited detectivity D* of the device is 1.42x1010 cm•Hz0.5/W at 3.27 μm.

Individually resolved luminescence from closely stacked GaN/AlN quantum wells

Bowen Sheng, Gordon Schmidt, Frank Bertram, Peter Veit, Yixin Wang, tao wang, Xin Rong, Zhaoying Chen, Ping Wang, juergen blaesing, Hideto Miyake, Hongwei Li, Shiping Guo, zhixin Qin, André Strittmatter, Bo Shen, Jurgen Christen, and Xinqiang Wang

DOI: 10.1364/PRJ.384508 Received 28 Nov 2019; Accepted 17 Feb 2020; Posted 19 Feb 2020  View: PDF

Abstract: Investigating closely stacked GaN/AlN multiple quantum wells (MQWs) by means of cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope we have reached an ultimate spatial resolution of σ_{CL} = 1.8 nm. The pseudomorphically grown MQWs with high interface quality emit in the deep ultra-violet spectral range. Demonstrating the capability of resolving the 10.8 nm separated, ultra-thin quantum wells, a cathodoluminescence profile was taken across individual ones. Applying a diffusion model of excitons generated by a Gaussian-broadened electron probe, the spatial resolution of cathodoluminescence down to the free exciton Bohr radius scale has been determined.

Mid-infrared chalcogenide microfiber knot resonators

Yu Xie, Dawei Cai, Hao Wu, Jing Pan, Ning Zhou, Chenguang Xin, Shaoliang Yu, Pan Wang, Xiaoshun Jiang, Jianrong Qiu, Xin Guo, and Limin Tong

DOI: 10.1364/PRJ.386395 Received 19 Dec 2019; Accepted 15 Feb 2020; Posted 19 Feb 2020  View: PDF

Abstract: A novel type of mid-IR microresonator — chalcogenide glass (ChG) microfiber knot resonator (MKR) is demonstrated with easy fabrication, fiber-compatible feature, resonance tunability, and high robustness. ChG microfibers with typical diameters around 3 µm are taper drawn from As2S3 glass fibers and are assembled into MKRs in liquid without surface damage. The measured Q factor of a typical 824-µm-diameter ChG MKR is about 2.84×10^4 at the wavelength of 4469.14 nm. The free spectral range (FSR) of the MKR can be tuned from 2.0 nm (28.4 GHz) to 9.6 nm (135.9 GHz) by tightening the knot structure in liquid. Benefitting from the high thermal expansion coefficient of As2S3 glass, the MKR exhibits a thermal tuning rate of 110 pm ℃^-1 in resonance peak. When being embedded in PMMA film, a 551-µm-diameter MKR retains a Q factor of 1.1×10^4. The ChG MKRs demonstrated here are highly promising for resonator-based optical technologies and applications in the mid-IR spectral range.

Superconducting nanowire multi-photon detectors enabled by current reservoirs

Kai Zou, Yun Meng, Zhao Wang, and Xiaolong Hu

DOI: 10.1364/PRJ.380764 Received 16 Oct 2019; Accepted 15 Feb 2020; Posted 19 Feb 2020  View: PDF

Abstract: Single-photon detectors are ubiquitous devices in quantum-photonic-based communication, computation, metrology, and sensing. In these applications, N-fold coincidence photon counting is often needed, for example, to characterize entanglement. However, N-fold coincidence photon counting typically requires N individual single-photon detectors and associated bias and readout electronics, and these resources could become prohibitive if N goes large and the detectors need to work at cryogenic temperatures. Here, to break this limit on N, we propose a device architecture based on N cascaded photosensitive superconducting nanowires and one wider nanowire that functions as a current reservoir. We show that by strategically designing the device, the network of these superconducting nanowires can work in a synergic manner as an n-photon detector, where n can be from 1 to N, depending on the bias conditions. We therefore name the devices of this type as superconducting nanowire multi-photon detectors (SNMPDs). In addition to its simple, one-port bias and readout circuitry, the coincidences are counted internally in the detector, eliminating the need of external multi-channel, time-correlated pulse counters. We believe that the SNMPDs proposed in this work would open avenues towards conveniently measuring high-order temporal correlations of light and characterizing multi-photon entanglement.

Experimental study of mode distortion induced by stimulated Raman scattering in high power fiber amplifiers

Chu Chu, Qiang Shu, Zeng Chen, Fengyun Li, Donglin Yan, Chao Guo, Honghuan Lin, Jianjun Wang, Feng Jing, Chuanxiang Tang, and Rumao Tao

DOI: 10.1364/PRJ.383551 Received 25 Nov 2019; Accepted 14 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: Mode distortion induced by stimulated Raman scattering (SRS) in high power fiber amplifier has been experimentally investigated in detail, which includes the evolution of optical spectrums, spatial beam profiles, and time-frequency characteristics, and the temporal-frequency study has been carried out for the first time by using both low speed camera and high speed photodiode traces. It revealed that the temporal-frequency characteristics of the SRS induced mode distortion are different from traditional dynamic mode instability. The experimental results show that the output beam profile remains stable before the mode distortion occurs, and fluctuates obviously after the onset of SRS induced MI but on a time scale of seconds, which is much lower than that of Yb-gain induced MI featuring millisecond-level beam profile fluctuation. It also shows that the mode distortion became measurable companying with the onset of inter-mode four wave mixing (IM-FWM) when the ratio of Raman light reaches to 3%, and the beam quality factor M² degrades gradually from 1.4 to 2.1 as the ratio of Raman light increases. The mode distortion is accompanied by obvious temperature increasing of the output passive fiber, which further confirms the SRS origin of the mode distortion. The cause of the SRS induced mode distortion has been explained in the context of core-pumped SRS effect, and the investigation of accompanying IM-FWM effect indicates that the mainly content of the SRS induced high order mode is LP21 mode.

Towards Simultaneous Observation of Path and Interference of Single Photon in a Modified Mach-Zehnder Interferometer

Fenghua Qi, Zhiyuan Wang, Weiwang Xu, Xuewen Chen, and Zhiyuan Li

DOI: 10.1364/PRJ.386774 Received 27 Dec 2019; Accepted 11 Feb 2020; Posted 13 Feb 2020  View: PDF

Abstract: Classical wisdom of wave-particle duality regulates that a quantum object shows either the particle or wave nature but never the both. Consequently it would be impossible to observe simultaneously the complete wave and particle nature of the quantum object. Mathematically the principle requests that the interference visibility V and which-path distinguishability D satisfy an orthodox limit of V^2+D^2 <= 1. The present work reports a new wave-particle duality test experiment using single photons in a modified Mach-Zehnder interferometer to demonstrate the possibility of breaking the limit. The key element of the interferometer is a weakly-scattering total-internal reflection prism surface, which exhibits a pronounced single-photon interference with a visibility of up to 0.97 and simultaneously provides a path distinguishability of 0.83. Apparently the result of V^2+D^2 ≈1.63 exceeds the orthodox limit set by the classical principle of wave-particle duality for single photons. We expected that more delicate experiments in future should be able to demonstrate the ultimate limit of V^2+D^2 ≈2 and shed new light on the foundations of contemporary quantum mechanics.

Improved performance of UVC-LEDs by combination of high temperature annealing and epitaxially laterally overgrown AlN/sapphire

Norman Susilo, Eviathar Ziffer, Sylvia Hagedorn, Leonardo Cancellara, Carsten Netzel, Neysha Lobo-Ploch, Shaojun Wu, Jens Rass, Sebastian Walde, Luca Sulmoni, Martin Guttmann, Tim Wernicke, Martin Albrecht, Markus Weyers, and M Kneissl

DOI: 10.1364/PRJ.385275 Received 10 Dec 2019; Accepted 01 Feb 2020; Posted 04 Feb 2020  View: PDF

Abstract: We report on the performance of AlGaN-based deep ultraviolet light emitting diodes (UV-LED) emitting at 265 nm grown on stripe patterned high temperature annealed (HTA) epitaxially laterally overgrown (ELO) AlN/sapphire templates. For this purpose, the structural and electro-optical properties of UVC-LEDs on as-grown and on HTA planar AlN/sapphire as well as ELO AlN/sapphire with and without HTA are investigated and compared. Cathodoluminescence measurements reveal dark spot densities of 3.5×10⁹ cm-², 1.1×10⁹ cm-², 1.4×10⁹ cm-² and 0.9×10⁹ cm-² in MQWs samples on as-grown planar AlN/sapphire, HTA planar AlN/sapphire, ELO AlN/sapphire and HTA ELO AlN/sapphire, respectively and are consistent with the threadingdislocation densities (TDDs) determined by TEM and HRXRD. The UVC-LED performance improves with the reduction of the TDDs. The output powers (measured on-wafer in cw operation at 20 mA) of the UV-LEDs emitting at 265 nm were 0.03 mW (planar AlN/sapphire), 0.8 mW (planar HTA AlN/sapphire), 0.9 mW (ELO AlN/sapphire), and 1.1 mW (HTA ELOAlN/sapphire), respectively. Furthermore, Monte Carlo ray-tracing simulations showed a 15% increase in the light extraction efficiency due to the voids formed in the ELO process. These results demonstrate that HTA ELO AlN/sapphire templates provide a viable approach to increase the efficiency of UV-LEDs, improving both the internal quantum efficiency and the light extraction efficiency.

Laser Fabrication of Graphene-Based Supercapacitors

Xiu-Yan Fu, Zhao-Di Chen, Dong-Dong Han, Yonglai Zhang, Hong Xia, and Hong-Bo Sun

DOI: 10.1364/PRJ.382401 Received 15 Nov 2019; Accepted 30 Jan 2020; Posted 31 Jan 2020  View: PDF

Abstract: Supercapacitors have abroad applications in wearable electronics, e-skin, robots, etc. Recently, graphene-based supercapacitors have attracted extensive attentions for their excellent flexibility and electrochemical performance. Laser fabrication of graphene-based supercapacitors exhibits obvious superiority because of the simple procedures and integration compatibility towards future electronics. Here, we comprehensively summarize the state-of-the-art advancements in laser assisted preparation of graphene-based supercapacitors including working mechanism, fabrication procedures and unique characteristics. In the working mechanism section, electric double-layer capacitors and pseudo capacitors are introduced. The latest advancements in this field are comprehensively summarized including laser reduction of graphene oxides, laser treatment of graphene prepared from chemical vapor deposition, and laser induced graphene. Besides, unique characteristics of laser enabled graphene-based supercapacitors, such as structured graphene, graphene hybrids, heteroatom doping graphene-related electrodes, are presented. Subsequently, laser enabled miniaturized, stretchable and integrated graphene-based supercapacitors are also discussed. It is anticipated that laser fabrication of graphene-based supercapacitors holds great promise for developing future energy storage devices.

Versatile on-chip light-coupling and (de)multiplexing from arbitrary polarizations to controlled waveguide modes using integrated dielectric metasurface

Yuan Meng, Zhoutian Liu, Zhenwei Xie, Ride Wang, Tiancheng Qi, Futai Hu, Hyunseok Kim, Qirong Xiao, Xing Fu, Qiang Wu, Sang-Hoon Bae, Mali Gong, and Xiaocong Yuan

DOI: 10.1364/PRJ.384449 Received 27 Nov 2019; Accepted 23 Jan 2020; Posted 23 Jan 2020  View: PDF

Abstract: Metasurface has found broad applicability in free-space optics, while its potential to tailor guided waves remains barely explored. By synergizing the Jones matrix model with generalized Snell’s law under phase-matching condition, we propose a universal design strategy for versatile on-chip mode-selective coupling with polarization-sensitivity, multiple working wavelengths and high efficiency concurrently. The coupling direction, operation frequency and excited mode type can be designed at will for arbitrary incident polarizations, outperforming previous technology that only works for specific polarizations and lacks versatile mode-controllability. Here, using silicon-nanoantennas-patterned silicon-nitride photonic waveguides, we numerically demonstrate a set of chip-scale optical couplers around 1.55 µm, including directional couplers with high coupling efficiency over 57% and directivity about dB. Polarization- and wavelength-demultiplexer scenarios are also proposed with 67% maximum efficiency and an extinction ratio of 20 dB. Moreover, a chip-integrated twisted light generator, coupling free-space linear polarization into optical vortex carrying 1hbar orbital-angular-momentum (OAM), is also reported to validate our mode-control flexibility. This comprehensive method may motivate compact wavelength/polarization (de)multiplexers, multifunctional mode converters, on-chip OAM generators for photonic integrated circuits and high-speed optical telecommunications.


Jason Jerwick, Yongyang Huang, Zhao Dong, Adrienne Slaudades, Alexander Brucker, and Chao Zhou

DOI: 10.1364/PRJ.383034 Received 13 Nov 2019; Accepted 17 Jan 2020; Posted 23 Jan 2020  View: PDF

Abstract: High-speed ophthalmic optical coherence tomography systems are of interest because they allow rapid, motion-free, and wide-field retinal imaging. Space-division multiplexing optical coherence tomography (SDM-OCT) is a high-speed imaging technology which takes advantage of the long coherence length of microelectromechanical vertical cavity surface emitting laser (MEMs VCSEL) sources to multiplex multiple images along a single imaging depth. We demonstrate wide-field retinal OCT imaging, acquired at an effective A-scan rate of 800,000 A-scans/sec with volumetric images covering up to 12.5 mm x 7.4 mm on the retina acquired in less than 1 second. A clinical feasibility study was conducted to compare the ophthalmic SDM-OCT with commercial OCT systems, illustrating the high-speed capability of SDM-OCT in a clinical setting.

Synergistic effects of electric-field-enhancement and charge-transfer based SERS study in Ag₂S quantum dots/plasmonic bow-tie nanoantenna composite system

Subhash Singh, Bin Wang, Chen Zhao, Huanyu Lu, Tingting Zou, ZHI YU, Chaonan Yao, Xin Zheng, Jun Xing, Yuting Zou, Cunzhu Tong, WEILI YU, Bo Zhao, and Chunlei Guo

DOI: 10.1364/PRJ.383612 Received 20 Nov 2019; Accepted 15 Jan 2020; Posted 16 Jan 2020  View: PDF

Abstract: Localized surface plasmonic resonance (LSPR) of nanostructures and the interfacial charge transfer (CT) of semiconductor materials play essential roles in the optical and photo-electronic property study. In this article, a composite substrate of Ag₂S quantum dots (QDs) coated plasmonic Au bow-tie nanoantenna (BNA) arrays with metal-insulator-metal (MIM) configuration was built to study the synergistic effect of LSPR and interfacial CT using surface-enhanced Raman spectroscopy (SERS) in near-infrared (NIR) region. The Au BNA arrays structure with large enhancement of localized electric field (E-field) and broad enhanced spectral region strongly enhanced the Raman signal of adsorbed p-aminothiophenol (PATP) molecules owing to the coupling of LSPR. The as-prepared Au BNA arrays structure facilitated enhancements of the excitation as well as the emission of Raman signal simultaneously, which was proved by finite-different-time-domain (FDTD) simulation. Moreover, Ag₂S semiconductor QDs were introduced into BNA/PATP system to further enhance Raman signals, which benefited from the interfacial CT resonance in BNA/Ag₂S-QDs/PATP system. As a result, the Raman signals of PATP in BNA/Ag₂S-QDs/PATP system were strongly enhanced under 785 nm laser excitation due to the synergetic effect of E-field enhancement and interfacial CT. Furthermore, the SERS polarization dependence effects of BNA/Ag₂S-QDs/PATP system were also investigated. This study establishes a correlation of synergetic effect of interfacial CT and E-field enhancement for SERS applications and provides guidance for the development of SERS study on semiconductor QDs based plasmonic substrates, and can be further extended to other material-nanostructure systems for various optoelectronic and sensing applications.

Enhanced photons communication through Bayesian estimation with SNSPDs array

xiang li, jingrou tan, kaimin zheng, Labao Zhang, Lijian Zhang, Weiji He, Pengwei Huang, haochen li, biao zhang, qi chen, rui ge, tao huang, Xiaoqing Jia, Qingyuan Zhao, Cou Tu, Lin Kang, Jian Chen, Peiheng Wu, and Shuya Guo

DOI: 10.1364/PRJ.377900 Received 01 Oct 2019; Accepted 11 Jan 2020; Posted 26 Feb 2020  View: PDF

Abstract: Laser communication using photons need consider the performance of the single photon detector and the photon-number distribution. Photons communication through superconducting nanowire single photon detector (SNSPD) is a new technology to approach the sensitivity limitations at the level of single photons in deep space communication. However, the bit error rate (BER) of the photons communication is degenerated by the dark noise and the photon number distribution with traditional SNSPDs. In this work, we proposed an enhanced photon communication method based on the photon-number resolution of a SNSPDs array. A theoretical model of BER was introduced to estimate the mean-photon number through resolving photon-number with SNSPDs array and Bayesian estimation. With this propose, the experiment of data transmission indicated that the BER can be reduced by two orders of magnitude compared with traditional methods.

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