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Single-pixel ptychography

Meng Li, Liheng Bian, Guoan Zheng, Andrew Maiden, Yang Liu, Yiming Li, Qionghai Dai, and Jun Zhang

DOI: 10.1364/OL.417039 Received 09 Dec 2020; Accepted 24 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: Ptychography is a predominant non-interferometric technique to image large complex fields, but with quite narrow working spectrum because the diffraction measurements require dense array detection with ultra-high dynamic range. Here we report a single-pixel ptychography technique that realizes non-interferometric and non-scanning complex-field imaging in a wide waveband, where 2D dense detector arrays are not available. A single-pixel detector is placed in the far field to record the DC-only component of the diffracted wavefront scattered from the target field, which is illuminated by a sequence of binary modulation patterns. This decreases the measurements’ dynamic range by several orders of magnitude. We employ an efficient single-pixel phase-retrieval algorithm to jointly recover the field's 2D amplitude and phase maps from the 1D intensity-only measurements. No a priori object information is needed in the recovery process. We validate the technique's quantitative phase imaging nature using both calibrated phase objects and biological samples, and demonstrate its wide working spectrum with both 488-nm visible light and 980-nm near-infrared light.

Large-scale, high-contrast glare suppression with low-transmittance eigenchannels of aperture-target transmission matrices

Hengkang Zhang, Bin Zhang, Kaige Liu, Xing Fu, and Qiang Liu

DOI: 10.1364/OL.418934 Received 06 Jan 2021; Accepted 24 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: Glare suppression with wavefront shaping is a significant technique in terms of actively controlling the speckle light field. A novel glare suppression method based on transmission matrix (TM) measurement is demonstrated in this letter. An aperture-target TM model is proposed, and its low-transmittance eigenchannel is utilized to minimize the speckle intensity inside a given target area. We verified the availability of this method by experimentally realizing high-contrast glare suppression in areas of various sizes and shapes. For a large-scale area containing 100 speckle grains, the average intensity was suppressed to 6.3% of the background intensity. We believe our method has provided an ideal way for glare suppression, and it holds interesting prospects for areas such as speckle optical tweezers and imaging under scattering conditions.

Overcoming the rate–distance limit of device-independent quantum key distribution

Yuan-Mei Xie, Bing-Hong Li, Yu-Shuo Lu, Xiao-Yu Cao, Wen-Bo Liu, Hua-Lei Yin, and Zeng-Bing Chen

DOI: 10.1364/OL.417851 Received 16 Dec 2020; Accepted 24 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: Device-independent quantum key distribution (DIQKD) exploits the violation of a Bell inequality to extract secure key even if the users’ devices are untrusted. Currently, all DIQKD protocols suffer from the secret key capacity bound, i.e., the secret key rate scales linearly with the transmittance of two users. Here we propose a heralded DIQKD scheme based on entangled coherent states to improve entangling rates whereby long-distance entanglement is created by single-photon-type interference. The secret key rate of our scheme can significantly outperform the traditional two-photon-type Bell-state measurement scheme and, importantly, surpass the above capacity bound. Our protocol therefore is an important step towards a realization of DIQKD and can be a promising candidate scheme for entanglement swapping in future quantum internet.

Optical conveyor belt based on plasmonic metasurface with polarization dependent hot-spot arrays

Chi Zhang, min Jiang, Yao Chang, Yang Liu, Guanghui Wang, Fei Xu, and Yanqing Lu

DOI: 10.1364/OL.419201 Received 27 Jan 2021; Accepted 24 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: In this paper, we proposed a novel metasurface convey belt with periodic orientated arrays of gold plasmonic elliptical elements (GPEEs), which can be continuously lighted in a relay way by switching the polarization of excitation beam and can be used to trap, transport and sort particles. The array of hot field can provide larger trapping area and better stiffness. With the incident optical intensity of 0.08 mW/μm^2, the depth of the potential well could be as high as 10kBT. By setting a narrow interval between plasmonic ellipses in principal axes, it can help to further enhance their directional resonant coupling and polarization dependence. Furthermore, based on the considering the Brownian motion of trapped particles in aqueous solution, we analyzed its time response property of particle manipulation with different applied switching frequency from a statistical point of view. As confirmed by numerical analysis, our design offers a novel scheme of particles sorting by using scalable hot-spots array with better performance, which could be used in many on-chip optofluidic applications.

Efficient Type II Second Harmonic Generation in an Indium Gallium Phosphide on insulator wire waveguide aligned with a crystallographic axis

Nicolas Poulvellarie, Carlos Mas Arabi, Charles Ciret, Sylvain Combrié, Alfredo De Rossi, Marc Haelterman, Fabrice Raineri, Bart Kuyken, Simon-Pierre Gorza, and François Leo

DOI: 10.1364/OL.418064 Received 18 Dec 2020; Accepted 23 Feb 2021; Posted 23 Feb 2021  View: PDF

Abstract: We theoretically and experimentally investigate type II second harmonic generation in III-V-on-insulator wire waveguides. We show that the propagation direction plays a crucial role and that longitudinal field components can be leveraged for robust and efficient conversion. We predict that the maximum theoretical conversion is larger than that of type I second harmonic generation for similar waveguide dimensions and reach an experimental conversion efficiency of 12 %/W, limited by the propagation loss.

Asymmetry of the group velocity of light in monoclinic crystals

Pengqian Wang

DOI: 10.1364/OL.420414 Received 20 Jan 2021; Accepted 23 Feb 2021; Posted 23 Feb 2021  View: PDF

Abstract: We have proved that compared to the phase velocity and ray velocity surfaces, the group velocity surface of a monoclinic crystal has a reduced symmetry, due to the loss of the two mirror planes that contain the crystallographic b-axis. We have derived a formula for the calculation of the group velocity of the extraordinary light traveling in a principal plane of a biaxial crystal, which takes into account the rotation of the dielectric frame due to frequency dispersion. The maximum asymmetry of the group velocity of light traveling in the a-c plane is found to be 2.4% at 365 nm in BiB3O6, and 1.4% at 550nm in Sn2P2S6.

Angular spectrum matching for digital holographic microscopy under extremely low light conditions

Myungjin Cho, Kotaro Inoue, and Arun Anand

DOI: 10.1364/OL.416002 Received 25 Nov 2020; Accepted 23 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: Digital holographic microscopy (DHM) is a future three-dimensional (3D) microscopy due to the high resolution and high-precision 3D images. Thus, it is getting attention in bioinformatics, semiconductor defect detection, etc. However, some limitations still exist. Especially, high-speed holographic imaging requires high-power lasers, which are difficult to image on highly absorbent or light-sensitive samples. To overcome these issues, we propose a new digital hologram denoising algorithm called angular spectrum matching (ASM), which achieves hologram imitation to reduce noise for digital holography at low light intensities. The hologram used for the background phase comparison is recorded without objects, removing thelimitation on the power requirement. ASM recoveries dark holograms using this background hologram. We present experimental results showing improved DHM numerical reconstructions and recovered holograms underextremely low light conditions.

5 kW monolithic fiber amplifier employing home-made novel spindle-shaped ytterbium-doped fiber

zeng lingfa, Zhiyong Pan, Xiaoming Xi, Huan Yang, Yun Ye, Liangjin Huang, hanwei zhang, Xiaolin Wang, Zefeng Wang, Pu Zhou, Xiaojun Xu, and Jinbao Chen

DOI: 10.1364/OL.418194 Received 30 Dec 2020; Accepted 22 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: We have demonstrated a 5 kW high power monolithic fiber amplifier employing home-made novel spindle-shaped ytterbium-doped fiber based on main oscillator power amplifier configuration. The ytterbium-doped fiber consists of a spindle-shaped core and cladding along the fiber length, with core/cladding diameter of 27/410 µm at both ends and 39.5/600 µm in the middle. An output power of over 5 kW and beam quality of about 1.9, and an optical-to-optical conversion efficiency of 66.6% were achieved in the amplifier under bidirectional pump scheme. While operating at the maximum power, the laser performance was evaluated and the TMI and SRS effects were well mitigated. To the best of our knowledge, this is the highest power demonstration in continues wave fiber laser employing tapered fiber. Further power scaling is promising by optimizing the structure of the ytterbium-doped fiber.

Non-diffracting and self-accelerating Bessel beams with on-demand tailored intensity profiles along arbitrary trajectories

Wenxiang Yan, Yuan Gao, Zheng Yuan, Zhuang Wang, Zhi-Cheng Ren, Xi-Lin Wang, Jianping Ding, and Hui-Tian Wang

DOI: 10.1364/OL.418928 Received 06 Jan 2021; Accepted 22 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: Owing to their robustness against diffraction, the Bessel beams(BBs) offer special advantages in various applications. To enhance their applicability, we present a method to generate self-accelerating zeroth-order BBs along predefined trajectories with tunable z-direction intensity profiles. The character of tunable z-direction intensity profiles in non-diffracting self-accelerating BBs potentially can attract interest in the regimes of particle manipulation, microfabrication and free-space optical interconnect.

Low loss edge-coupling thin-film lithium niobatemodulator with efficient thermal-optic phase shifter

pan ying, Heyun Tan, Junwei Zhang, mingbo he, Mengyue Xu, Xiaoyue Liu, Renyou Ge, Yuntao Zhu, Xinlun Cai, and chuan liu

DOI: 10.1364/OL.418996 Received 04 Jan 2021; Accepted 22 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: Thin-film lithium niobate on insulator (LNOI) is a very attractiveplatform for optical interconnect and nonlinear optics. It is essential toenable LN photonic integrated circuits with low power consumption.Here, we present an edge-coupling Mach–Zehnder modulator on theplatform with a low fiber-chip coupling loss of 0.5 dB/facet, half-wavevoltage 𝑽𝝅 of 2.36 V, electro-optic (EO) bandwidth of 60 GHz andefficient thermal-optic (TO) phase shifter with half-wave power of6.24mW. Besides, we experimentally demonstrate single-lane 200-Gbit/s data transmission utilizing discrete multi-tone (DMT) signal.The LNOI modulator demonstrated here shows great potential inenergy-efficient large-capacity optical interconnects.

Quantitative spectroscopy of single molecule interaction times

Horst-Holger Boltz, Alexei Sirbu, Nina Stelzer, Martin Lohse, Christof Schütte, and Paolo Annibale

DOI: 10.1364/OL.413030 Received 09 Nov 2020; Accepted 22 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: Single molecule fluorescence tracking provides information at nm-scale and ms-temporal resolution about the dynamics and interaction of individual molecules in a biological environment. While the dynamic behavior of isolated molecules can be characterized well, the quantitative insight is more limited when interactions between two indistinguishable molecules occur. We address here this aspect by developing a solid theoretical foundation for a spectroscopy of interaction times, i.e. the inference of interaction constants from imaging data. The non trivial crossover between a power law to an exponential behavior of the distribution of the interaction times is highlighted here, together with the dependence of the exponential term upon the product of the microscopic reaction rates (affinity). Our approach is validated on simulated as well as experimental datasets.

Selection rules for the orbital angular momentum of optically-produced THz radiation

Alessandro Curcio, Sen Mou, Luigi Palumbo, Stefano Lupi, and massimo petrarca

DOI: 10.1364/OL.416814 Received 08 Dec 2020; Accepted 22 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: In this work we theoretically study the transduction of the Orbital Angular Momentum (OAM) l of infrared pump lasers into the THz domain. For the optical rectification, the transduction of OAM occurs only through a spin-orbit interaction, with the selection rule on the topological charge l=0 valid for any kind of polarization of the pump which means there is no transfer of OAM along the propagation axis. In the difference frequency generation the selection rule for the difference Deltalambda between the topological charge of the pump fields with linear or circular polarizations is l=Deltalambda, whereas the selection rule l in [Deltalambda-2,Deltalambda+2] holds in the case of radial and azimuthal polarizations. Moreover, for THz generation in the latter cases, high diffraction obtained with tightly focused pumps yields l tending to Deltalambda+-2, while l tends to zero for large pump beams.

PMD Estimation and Its Enabled Feedforward Adaptive Equalization Based on Superimposed FrFT Training Sequences

Wang Li, HEXUN JIANG, Huan He, Fengguang Luo, and Ming Tang

DOI: 10.1364/OL.417598 Received 28 Dec 2020; Accepted 22 Feb 2021; Posted 24 Feb 2021  View: PDF

Abstract: Fractional Fourier transform (FrFT) training sequences (TSs) have been proposed to superimpose with signals to achieve time synchronization and polarization mode dispersion (PMD) estimation simultaneously before equalization. The feasibility of the proposed scheme is verified by experiment of 30 GBaud/s PDM-QPSK system. Without sacrificing additional spectrum efficiency, the proposed PMD estimation is robust against to amplifier spontaneous emission (ASE) noise and has steady performance in large PMD ranged from 60ps to 108ps. The root mean square error (RMSE) of PMD is 5.6ps by experimental verification. The RMSE of time offset (TO) is 0.8 sample by simulation. Besides, the PMD estimated before equalization is utilized to adjust the numbers of taps in subsequently tap-varied equalizer, which achieves feedforward adaptive equalization (FFD-AEQ). The number of the AEQ taps is adjusted to an optimal value with the prior-PMD information estimated by FrFT TSs. The experimental results show that the tap-varied AEQ has 52.6% power consumption decrease compared to the tap-fixed AEQ with 13 taps.

Broadband and ultrafine luminescence in diamond facilitated by femtosecond laser driven electron impact and injection of “vacancy-interstitial” pairs

Sergey Kudryashov, Roman KHMELNITSKII, Pavel Danilov, Nikita Smirnov, Alexey Levchenko, Oleg KOVALCHUK, Mayya Uspenskaya, Ekaterina OLEYNICHUK, and Michael Kovalev

DOI: 10.1364/OL.414583 Received 09 Nov 2020; Accepted 21 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: Ultrafast heating of photoionized free electrons by high-NA (0.25-0.65) focused visible-range ultrashort laser pulses provides their resonant impact trapping into intra-gap electronic states of point-defect centers in a natural IaA/B diamond with high concentration of poorly-aggregated nitrogen impurity atoms, exciting their ultrafine-structure, broadband polychromatic luminescence over the entire bandgap. The observed luminescence spectra revealed higher nitrogen aggregation with non-equilibrium intrinsic carbon vacancies, produced simultaneously as Frenkel “vacancy-interstitial” pairs during the same laser exposure.

Spectral filtering in single-mode fibers using resonant coupling with absorbing rods

Svetlana Aleshkina, Tatiana Kashaykina, Mikhail Yashkov, Mikhail Salaganskii, Vladimir Vel'miskin, Mikhail Bubnov, and Mikhail Likhachev

DOI: 10.1364/OL.412429 Received 14 Oct 2020; Accepted 20 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: We investigate (both theoretically and experimentally) a method for fundamental mode spectral filtering in single-mode optical fibers using the resonant mode coupling effect. We demonstrate the possibility of controlling the spectral bandwidth of the fundamental mode suppression band through appropriate choice of fiber parameters and fiber bending. The developed technique can be very useful for the design of fiber-based spectral filters (i.e., active fibers with suppression of laser emission at undesirable wavelengths, suppression of stimulated Raman scattering, etc.).

Silicon photonic on-chip spatial heterodyne Fourier transform spectrometer exploiting the Jacquinot’s advantage

thi-thuy-duong dinh, David Gonzalez-Andrade, Miguel Montesinos Ballester, Lucas Deniel, Bertrand SZELAG, xavier leroux, Eric Cassan, Delphine Marris-Morini, Laurent Vivien, Pavel Cheben, Aitor Velasco, and Carlos Alonso Ramos

DOI: 10.1364/OL.418278 Received 22 Dec 2020; Accepted 20 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: Silicon photonics on-chip spectrometers are finding important applications in medical diagnostics, pollution monitoring and astrophysics. Spatial heterodyneFourier-transform spectrometers (SHFTS) provide a particularly interesting architecture with a powerful passive error correction capability and high spectral resolution. Despite having an intrinsically large optical throughput (étendue, also referred as Jaquinot’s advantage), state-of-the-art silicon SHFTS have not exploited this advantage yet. Here, we propose and experimentallydemonstrate for the first time an SHFTS implementing a wide-area light collection system simultaneously feeding an array of 16 interferometers, with an input aperture as large as 90 µm x 60 µm formed by a two-way- fed grating coupler. We experimentally demonstrate 85 pm spectral resolution, 600 pm bandwidth and 13 dB étendue increase, compared with a device with a conventional grating coupler input. The SHFTS was fabricated using 193-nm deep-UV optical lithography and integrates the large-size input aperture with the interferometer array and monolithic Ge photodetectors, in a 4.5 mm2 footprint.

Label-free plasmonic assisted optical trapping of single DNA molecule

Lei Chen, Wei Liu, Dongyi Shen, Zhihao Zhou, Yuehan Liu, and Wenjie Wan

DOI: 10.1364/OL.420957 Received 28 Jan 2021; Accepted 20 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: DNA molecules are hard to be caught using traditional optical trapping due to the nanometer width of their chains. Here we experimentally demonstrate a label-free optical trapping of a single micrometer λ-DNA in solution by the aid of plasmonic gold nano-particles (GNPs), where a double-laser trap induces strong optical interparticle forces for the tweezer. We examine such sub-resolved interparticle forces by tracking the GNPs’ dynamics in solution. Moreover, surface-enhanced Raman scattering (SERS) signals of trapped λ-DNA has also been measured simultaneously in the same setup. In comparison with prior works, ours benefits from the structureless excitation in a dynamic configuration without fabrication. This technique opens a new avenue to all-optically manipulate biomolecules trapping and sensing for bio-medical applications.

The Use of Machine Learning to Efficiently Predict the Confinement Loss in Anti-Resonant Hollow-Core Fiber

Fanchao Meng, Xiaoting Zhao, Jinmin Ding, Yingli Niu, Xinghua Zhang, Mateusz Smietana, Ryszard Buczynski, Bo Lin, Guangming Tao, Luyun Yang, Xin Wang, shuqin lou, xinzhi sheng, and Sheng Liang

DOI: 10.1364/OL.422511 Received 10 Feb 2021; Accepted 19 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: The fundamental mode confinement loss (CL) of anti-resonant hollow-core fiber (ARF) is efficiently predicted by a classification task of machine learning. The structure-parameters vector is utilized to define the sample space of ARFs. The CL of labeled samples at 1550 nm is numerically calculated via finite element method (FEM). The magnitude of CL is obtained by classification task via decision tree and k-nearest neighbors (k-NN) algorithms with the training and test sets generated by 290700 and 3 00 labelled samples. The test accuracy, confusion matrices and the receiver operating characteristic curves have shown that our proposed method is effective for predicting the magnitude of CL with a short computation runtime compared to FEM simulation. The feasibility of predicting other performance parameters by extension of our method as well as its ability to generalize outside the tested sample space are also discussed. It is likely that the proposed sample definition and the use of a classification approach can be adopted for design application beyond efficient prediction of ARFs CL and inspire artificial intelligence and data driven based research of photonic structures.

314.7W supercontinuum generation ranging from 390nm to 2400nm by tapered photonic crystal fiber

Haoyu Zhang, Fengyun Li, Ruoyu Liao, Kegong Dong, Yue Li, Honghuan Lin, Jianjun Wang, and Feng Jing

DOI: 10.1364/OL.420707 Received 26 Jan 2021; Accepted 19 Feb 2021; Posted 22 Feb 2021  View: PDF

Abstract: To consider both the high-power handling and the blue-extended supercontinuum (SC) generation, a long tapered photonic crystal fiber (PCF) is pumped by a high-power laser source. Supercontinuum ranging from 390nm to 2400nm with 314.7W output power is obtained. And the spectral component below 960nm accounts for 36.1% of total output power, exceeding 113.5W, with a spectral flatness within 16dB. To the best of our knowledge, it's the first time to achieve supercontinuum coverage of all visible wavelengths with more than 300W output power. This result increases the output power of the supercontinuum covering all visible spectrums by two times.

Simultaneous and cascaded frequency conversion under nonlinear stimulated Raman adiabatic passage

Pragati Aashna and Krishna Thyagarajan

DOI: 10.1364/OL.418085 Received 21 Dec 2020; Accepted 19 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: A comprehensive study on two simultaneous three wave mixing processes, a second harmonic generation followed by a difference frequency generation, under nonlinear stimulated Raman adiabatic passage is presented. An input pump is up-converted to its second harmonic which then gets down-converted to a signal and an idler pair with frequencies lying very close to the input pump in such a manner that complete conversion from pump to signal and idler takes place without exciting the second harmonic under the counterintuitive adiabatic passage. This process involves nonlinear STIRAP with a nonlinear dark state similar to atomic population transfers and we bring the analogy from the atomic systems to our nonlinear dynamics to linearise the problem and analytically obtain the adiabaticity condition required for the complete conversion. We also show that the nonlinear STIRAP mechanism results into a large bandwidth of about 380 nm with almost complete conversion of pump to signal and idler.

Origami-based microwave absorber with reconfigurable bandwidth

Xiqiao Chen, Wei Li, Zhuang Wu, zilong zhang, and Yanhong Zou

DOI: 10.1364/OL.419093 Received 05 Jan 2021; Accepted 19 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: Reconfigurability is critical for the research fields in electromagnetic, mechanical, and acoustic, due to the controllability of functionalities. This paper numerically and experimentally demonstrates an origami-based absorber with reconfigurable bandwidth. The proposed structure provides four transformable models: flat sheet, single-arch-folded, double-arches-folded and U-shaped strips filled, respectively, corresponding to the performance of nearly no absorption, one-peak absorption , two-peak absorption, and ultra-broadband absorption (3.4-18 GHz), which clearly demonstrates the bandwidth-enhancement effect. In contrast with the traditional structural absorbers, the transformable flat sheet and U-shaped strips are obtained by three-dimensional (3D) printing, which exhibits an obvious superiority on prototype fabrication. These results provide a feasible strategy for energy dissipation and origami transformation.

Controllable double-helical microstructures by photonic orbital angular momentum for chiroptical response

Jincheng Ni, Yanlei Hu, Shunli Liu, Zhaoxin Lao, Shengyun Ji, Deng Pan, Chenchu Zhang, Bing Xu, Jiawen Li, Dong Wu, and Jiaru Chu

DOI: 10.1364/OL.419798 Received 15 Jan 2021; Accepted 18 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: Three-dimensional helical microstructures are abundant in nature and can also be applied as chiral metamaterials for advanced nanophotonics. Here we report a flexible method to fabricate double-helical microstructures with single exposure by recording the chirality of incident optical vortices. Two coaxial optical vortices can interfere to generate a helical optical field, confirmed by the numerical simulation. The diameters of double-helical microstructures can be tailored by the magnitude of topological charges. Finally, the chirality of double-helical microstructures can be reversibly read by optical vortices, demonstrating a strong chiroptical response. This fast-manufacturing strategy provides the opportunities in efficient yield of helical microstructures.

Experimental demonstration of SHG in high χ² metasurface

Lena Soun, Baptiste Fix, Hasnaa El Ouazzani, sébastien Héron, Nathalie Bardou, Christophe Dupuis, Sophie Derelle, Julien Jaeck, Riad Haidar, and Patrick Bouchon

DOI: 10.1364/OL.415257 Received 17 Nov 2020; Accepted 18 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: Metasurfaces able to concentrate light at various wavelengths are promising for enhancing nonlinear interactions. In this letter, we experimentally demonstrate infrared second harmonic generation by a multi-resonant nanostructure. A 100-GaAs layer embedded in a metal-insulator-metal waveguide is shown to support various localized resonances. One resonance enhances the nonlinear polarization due to the TM-polarized pump wavelength near 3.2 µm, while another is set near the TE-polarized generated wavelength (1.6µm). The measured SHG efficiency is higher than $10^{-9} W^{-1}$ for pump wavelengths ranging from 2.9 to 3.3 µm, which agrees with theoretical computations. This is typically 4 orders of magnitude higher than the equivalent GaAs membrane.

Beyond intensity modulation: new approaches to pump-probe microscopy

Jun Jiang, David Grass, Yue Zhou, Warren Warren, and Martin Fischer

DOI: 10.1364/OL.417905 Received 21 Dec 2020; Accepted 17 Feb 2021; Posted 19 Feb 2021  View: PDF

Abstract: Pump-probe microscopy is an emerging nonlinear imaging technique based on high repetition rate lasers and fast intensity modulation. Here we present new methods for pump-probe microscopy that keep the beam intensity constant and instead modulate the inter-pulse time delay, the relative polarization, or the pulse length. These techniques can improve image quality for samples that have poor heat dissipation or long-lived radiative states, and can selectively address nonlinear interactions in the sample. We experimentally demonstrate this approach and point out the advantages over conventional intensity modulation.

Coherent laser detection of the fW-level frequency-shifted optical feedback based on DFB fiber laser

Yuanyang Zhao, Desheng Zhu, Yourui Tu, Lingling Pi, Hongtao Li, Lin Xu, Zhijia Hu, Yuecheng Shen, Benli Yu, and Liang Lu

DOI: 10.1364/OL.418639 Received 30 Dec 2020; Accepted 17 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: The theoretical basis and experimental realization of an all-fiber self-mixing laser Doppler velocimetry based on frequency-shifted feedback in a distributed feedback fiber laser are presented, which employs a pair of fiber-coupled acousto-optic modulators to adjust the modulation intensity and frequency of the laser self-mixing effect. Moreover, the minimum optical feedback intensity for the velocity signal successfully measured by the interferometer is 5.12fW, corresponding to 0.16 photons per Doppler cycle. The results demonstrate that the proposed scheme can adapt to the non-contact measurement requirements of the wide-range speed and weak feedback level in the complex environment.

800-Gb/s/200-m FSO link with WDM-PAM4 scheme and SLM-based beam tracking technology

Hai-Han Lu, Xu-Hong Huang, Wen-Shing Tsai, Chao-Yu Feng, Cing-Ru Chou, Yi-Hao Chen, Yu-Ting Huang, and Agustina Nainggolan

DOI: 10.1364/OL.418822 Received 30 Dec 2020; Accepted 17 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: An 800-Gb/s/200-m free-space optical (FSO) link with wavelength-division multiplexing (WDM)-four-level pulse amplitude modulation (PAM4) scheme and spatial light modulator (SLM)-based beam tracking technology is constructed. It is the first one that adopts a WDM-PAM4 scheme and an SLM-based beam tracking technology to simultaneously afford a high transmission capacity of 800 Gb/s and resolve the laser beam misalignment problem due to target device movement. By adopting a 16-wavelength WDM-PAM4 scheme, the transmission capacity of FSO link is considerably enhanced with an 800-Gb/s (50 Gb/s PAM4/λ x 16 λ) total capacity. By deploying an SLM-based beam tracking technology, the laser beam misalignment problem is practically resolved for providing an FSO link with high link accessibility. Sufficiently low BER and qualified PAM4 eye diagrams are acquired at an 800-Gb/s/200-m operation. This demonstrated WDM-PAM4 FSO link fully meets the requirements of high-speed, long-reach, and high reliability transmissions.

Vanadium-dioxide microstructures with designable temperature-dependent thermal emission

Romil Audhkhasi and Michelle Povinelli

DOI: 10.1364/OL.414705 Received 13 Nov 2020; Accepted 17 Feb 2021; Posted 18 Feb 2021  View: PDF

Abstract: We propose gold-vanadium dioxide microstructures for which the difference in thermally radiated power between low and high temperature states can be tuned via structural design. We start by incorporating VO2 in a gold-dielectric-gold waveguide to achieve a temperature-dependent mode effective index. We show that a cavity formed in this waveguide structure has a fundamental resonance wavelength that shifts with temperature. We calculate the thermal radiated power from the cavity at temperatures above and below the phase transition of VO2 for wavelengths between 8 – 14 μm. We show that the difference in radiated power can be made positive, negative or zero simply by adjusting the cavity length. Finally, we use our cavity to design thermally emissive metasurfaces with spatial emission patterns that can be inverted with temperature. Our emitters could serve as building blocks in the realization of metasurfaces enabling complex thermal radiation control.

Lithographic pattern quality enhancement of DMD lithography with spatiotemporal modulated technology

Shuping Guo, Lu Feng, Zheng Xiong, Long Huang, Liu Hua, and Jinhuan Li

DOI: 10.1364/OL.415788 Received 03 Dec 2020; Accepted 17 Feb 2021; Posted 18 Feb 2021  View: PDF

Abstract: In this paper, we propose the spatiotemporal modulation projection lithography (STPL) technology, which is a spatiotemporal modulation technology applied to the conventional digital micromirror device (DMD) projection lithography system. Through coordinating the micro-movement of the piezoelectric stage, the flexible pattern generation of DMD as well as the exposure time, the proposed STPL enables fabricate a microstructure with smooth edges, accurate linewidth and accurate line position. Further application on fabricating diffraction lens has been implemented. The edge sawtooth of the Fresnel zone plate fabricated by using the STPL is reduced to 0.3 μm, the error between the actual measured linewidth and the ideal linewidth is only within ±0.1 μm, and the focal length is 15 mm, which is basically consistent with the designed focal length. These results indicated that STPL can serve as a significant role in the micromanufacturing field for achieving high fidelity microdevices. length is 15 mm, which is basically consistent with the designed focal length. These results indicated that STPL can serve as a significant role in the micromanufacturing field for achieving high fidelity microdevices.

Voltage tunable mid-wave infrared reflective varifocal metalens via an optomechanic cavity

Yikun Chen, Shiliang Pu, Chongzhi Wang, and Fei Yi

DOI: 10.1364/OL.417224 Received 23 Dec 2020; Accepted 16 Feb 2021; Posted 18 Feb 2021  View: PDF

Abstract: Metalenses enable the multifunctional control of light beam with an optically thin layer of nanonantennas. Efficient on-chip voltage tuning of the focal length is the crucial step towards the integration of metalenses into dynamically tunable optical systems. We propose and numerically investigate the on-chip electrical tuning of a reflective metalens via an optomechanic cavity. Light is focused by an array of silicon nanopillar antennas separated from a deformable metallic reflector by a small air gap. A transparent electrode is inserted into the optomechanic cavity to electrostatically bend the reflector and rearrange the reflection phase profile, resulting in a shift in the focal point. Two modes of voltage tuning via the relative curvature change of the reflector are analyzed. In mode 1, the size of the air gap is modified through the nearly parallel shift of the reflector, while in mode 2 the distribution of the air gap size is tailored by the curvature change of the reflector. With the designed working wavelength of 3.8 μm and the initial focal length of 80.35 μm, the focal length is shifted by 20.3 μm in mode 1 and 7.25 μm in mode 2. Such device can be used as a free space coupler between quantum cascade lasers and mid-infrared fibers with variable coupling efficiency.

MEMS-tuned resonant filters spanning the 8-12 μm band

Yeong Ko, Kyu Lee, Subrata Das, Neelam Gupta, and Robert Magnusson

DOI: 10.1364/OL.418545 Received 28 Dec 2020; Accepted 16 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: The spectral band covering ~8 to 12 μm is atmospherically transparent and therefore important for terrestrial imaging, day/night situational awareness systems and spectroscopic applications. There is a dearth of tunable filters spanning the band. Here, we propose and demonstrate a new tunable-filter method engaging the fundamental physics of the guided-mode resonance (GMR) effect realized with a nonperiodic lattice. The filter is fashioned with the Ge/ZnSe materials system and interrogated with a ~1.5 mm Gaussian beam to show clear transmittance nulls. To expand the tuning range, the device parameters are optimized for sequential operation in TM and TE polarization states. The theoretical model exhibits a tunable range exceeding 4 μm thus covering the band fully. In experiment, a prototype device exhibits a spectral range of 8.6-10.0 µm in TM and 9.9-11.7 µm in TE polarization or >3 μm total. With additional efforts in fabrication, we expect to achieve the full range.

Multimode fiber enables control of spatial coherence in Fourier-domain full-field optical coherence tomography

Egidijus Auksorius, Dawid Borycki, and Maciej Wojtkowski

DOI: 10.1364/OL.417178 Received 09 Dec 2020; Accepted 15 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: Fourier-domain full-field optical coherence tomography (FD-FF-OCT) has recently emerged as a fast alternative to point-scanning confocal OCT in eye imaging. However, when imaging the cornea with FD-FF-OCT, a spatially coherent laser can focus down on the retina to a spot that exceeds the maximum permissible exposure level. Here we demonstrate that a long multimode fiber with a small core can be used to reduce the spatial coherence of the laser, and thus, enable ultrafast in vivo volumetric imaging of the human cornea without causing risk to the retina

Accelerating ptychographic reconstructions using spectral initializations

Lorenzo Valzania, Jonathan Dong, and Sylvain Gigan

DOI: 10.1364/OL.406156 Received 24 Aug 2020; Accepted 15 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: Ptychography is a promising phase retrieval technique for label-free quantitative phase imaging. Recent advances in phase retrieval algorithms witnessed the development of spectral methods, in order to accelerate gradient descent algorithms. Using spectral initializations on experimental data, for the first time we report three times faster ptychographic reconstructions than with a standard gradient descent algorithm and improved resilience to noise. Coming at no additional computational cost compared to gradient-descent-based algorithms, spectral methods have the potential to be implemented in large-scale iterative ptychographic algorithms.

Carving beams of light

Michel Zamboni-Rached

DOI: 10.1364/OL.419516 Received 18 Jan 2021; Accepted 14 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: Some years after the appearance of the so-called non-diffracting beams, there was the development of methods capable of structuring them spatially, being the so called Frozen Waves method the first and, perhaps, the most efficient one. That method allowed modelling the longitudinal intensity pattern of non-diffracting beams, being, however, little efficient in controlling their transverse spatial pattern, granting only the possibility of choosing their transverse dimensions, which remain invariant throughout the propagation. In this work, we have extended the Frozen Wave method in such a way to control, in addition to the longitudinal pattern, the transverse beam structure along the propagation.The new transversally and longitudinally structured beams can have potential applications in areas such as photonics, optical manipulation, optical atom guidance, lithography, etc..

Simultaneous refractive index and temperature sensing based on fiber surface waveguide and fiber Bragg gratings

Dongning Wang, QI CHEN, and Gao Feng

DOI: 10.1364/OL.419636 Received 12 Jan 2021; Accepted 14 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: An optical fiber sensor based on fiber surface waveguide and Bragg grating is proposed for simultaneous refractive index and temperature sensing. The device consists of two fiber Bragg gratings fabricated by femtosecond laser, of which one is situated in the fiber core for temperature sensing, and the other is located in the fiber surface waveguide for both temperature and refractive index measurement. The refractive index and temperature sensitivities achieved are 10.3 nm/RIU and 9.94 pm/℃, respectively. The device is featured with compact structure, high robustness and convenient operation.

Plasmonic hollow fibers with distributed inner-wall hotspots for direct SERS detection of flowing liquids

Yunyun Mu, Miao Liu, Jiajun Li, and Xinping Zhang

DOI: 10.1364/OL.415733 Received 23 Nov 2020; Accepted 14 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: Plasmonic hollow fibers are fabricated by coating silver/gold alloyed nanoparticles (Ag-Au-ANPs) onto the inner walls of hollow fibers. The Ag-Au-ANPs were synthesized chemically and dissolved in acetone to prepare a colloidal solution, which was flowed through the hollow fiber multiple times, so that a thin layer of colloidal Ag-Au-ANPs was produced on the inner wall. Annealing at 400 ℃ enables melting/aggregation of the metallic nanoparticles and consequent formation of closely arranged plasmonic nanostructures fixed solidly on the inner wall. Surface enhanced Raman scattering (SERS) mechanism is thus established for the liquids flowing through the hollows. SERS measurements show an enhancement factor larger than 104 for such plasmonic hollow fibers in the direct detection of R6G/ethanol solutions. Confinement of the excitation laser energy inside the hollow space is an additional contribution to the enhancement mechanism. This is a promising design for the direct on-site SERS detection of molecules in flowing liquids with low concentrations.

TEMPORAL CONTRAST ENHANCEMENT AND COMPRESSION OF OUTPUT PULSES OF ULTRA-HIGH POWER LASERS

Sergey Mironov, Efim Khazanov, and Mikhail STARODUBTSEV

DOI: 10.1364/OL.415430 Received 20 Nov 2020; Accepted 13 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: We propose a technique for a significant contrast enhancement at femtosecond laser pulse compression by broadening their spectrum due to self-phase modulation and subsequent reflection from chirping mirrors with a dip in the reflection coefficient in the central region of the broadened spectrum. This dip provides almost zero reflection of the pulse pedestal, only slightly distorting the pulse itself.

Strong light emission from defective hexagonal boron nitride monolayer coupled to near-touching random plasmonic nanounits

Zeinab Eftekhari, Amir Ghobadi, Mahmut Soydan, Deniz Yildirim, neval cinel, and Ekmel Ozbay

DOI: 10.1364/OL.415475 Received 19 Nov 2020; Accepted 13 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: In this paper, we demonstrate strong light emission from defective hexagonal boron nitride (hBN) defect centers upon their coupling with disorder near-touching plasmonic units. Based on numerical simulations and characterization results, the plasmonic design at thin layer thicknesses of 20 nm can provide above two orders of magnitude enhancement in photoluminescence (PL) spectra. Moreover, this plasmonic platform shortens the luminescence lifetime of the emitters. The proposed design can be easily extended to other plasmonic-emitter combinations where strong light-matter interaction can be achieved using large scale compatible routes.

Direct generation of relativistic isolated attosecond pulses in transmission from laser-driven plasmas

Yan Jiang, Zi-Yu chen, Zhanjun Liu, li cao, C. Y. Zheng, Rui Xie, Yue Chao, and Xiantu He

DOI: 10.1364/OL.418144 Received 21 Dec 2020; Accepted 13 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: Isolated attosecond pulses are useful to perform pump-probe experiments at an unpresented time resolution, and provide a new tool for ultrafast metrology. However, it is still a challenging task to generate such pulses of high intensity, even for a few-cycle laser. Through particle-in-cell simulations, we show that it is possible to directly generate a giant isolated attosecond pulse in the transmission direction from a relativistic laser driven plasmas. Compared to attosecond pulse generation in the reflection direction, no further spectral filtering is needed. The underlying radiation mechanism is coherent synchrotron emission, and the transmitted isolated attosecond pulse can reach relativistic intensity. This provides a promising alternative to generate intense isolated attosecond pulses for ultrafast studies.

Optimum design of NOLM-driven mode-locked fiber lasers

Alix Malfondet, Alexandre Parriaux, Katarzyna Krupa, Patrice Tchofo Dinda, and Guy Millot

DOI: 10.1364/OL.418281 Received 23 Dec 2020; Accepted 12 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: Most of the saturable absorbers commonly used to perform mode locking in laser cavities affect the trigger conditions of the laser oscillation, which requires manually forcing the laser start-up by various means such as polarization controllers. We present a procedure for designing a laser cavity driven by a nonlinear optical loop mirror (NOLM), which allows the laser to operate optimally without interfering with the oscillation triggering conditions, thus opening up possibilities for integration of this type of laser.

Single shot interferometric measurement of cavitation bubble dynamics

Bryce Wilson, FAN Zhenkun, Rahul Sreedasyam, Elliot Botvinick, and Vasan Venugopalan

DOI: 10.1364/OL.416923 Received 08 Dec 2020; Accepted 12 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: We demonstrate an interferometric method to provide direct, single shot measurements of cavitation bubble dynamics with nanoscale spatial and temporal resolution with results that closely match theoretical predictions. Implementation of this method reduces the need for expensive and complex ultra-high speed camera systems for the measurement of single cavitation events. This method can capture dynamics over large time intervals with sub-nanosecond temporal resolution and spatial precision surpassing the optical diffraction limit. This method provides an accurate approach for precise measurement of cavitation bubble dynamics suitable for metrology applications such as optorheological materials characterization.

Design of opposite thermal behaviors in Tm3+/Eu3+ co-doped YVO4 for high-sensitive optical thermometry

Shaoshuai Zhou, XiaoMan Li, and Shoubao Zhang

DOI: 10.1364/OL.419557 Received 11 Jan 2021; Accepted 12 Feb 2021; Posted 12 Feb 2021  View: PDF

Abstract: Temperature-induced red-shift of V-O charge transfer band edge and temperature quenching effect were combined for designing ratiometric optical thermometry. Following this approach, opposite thermal behaviors of Tm3+ and Eu3+ emissions were realized in the range of 300 to 380 K in Tm3+/Eu3+ co-doped YVO4. Applying the temperature dependent fluorescence intensity ratio of Eu3+ to Tm3+ as temperature readout, the maximal relative sensitivity reaches up to 4.6% K-1 around 330 K. This result makes the as-proposed strategy an excellent candidate for developing high-sensitive optical thermometry.

Complete polarization conversion using anisotropic temporal slabs

Jingwei Xu, Wending Mai, and Douglas Werner

DOI: 10.1364/OL.415757 Received 01 Dec 2020; Accepted 11 Feb 2021; Posted 12 Feb 2021  View: PDF

Abstract: It is well known that control over the polarization of electromagnetic waves can be achieved by utilizing artificial anisotropic media such as metamaterials. However, most of the related research has been focused on time-invariant systems. Inspired by the concept of temporal boundaries, we propose a method to realize polarization conversion in real time by employing time-variant materials, whose permittivity or permeability switches between isotropic and anisotropic values. The criteria for complete polarization conversion are studied for several polarization angles, both analytically and numerically.

Tunable optical second-order Volterra nonlinear filter using wave mixing and delays to equalize a 10-/20-Gbaud 4-APSK channel

Kaiheng Zou, Peicheng Liao, Huibin Zhou, Ahmad Fallahpour, Amir Minoofar, Ahmed Almaiman, Fatemeh Alishahi, Moshe Tur, and Alan Willner

DOI: 10.1364/OL.418088 Received 21 Dec 2020; Accepted 11 Feb 2021; Posted 12 Feb 2021  View: PDF

Abstract: We experimentally demonstrate a tunable optical second-order Volterra filter using wave mixing and delays. Wave mixing is performed in a periodically poled lithium niobate waveguide with the cascaded sum-frequency generation and difference-frequency generation processes. Compared to conventional optical tapped delay line structures, second-order taps are added through the wave mixing of two signal copies. We measure the frequency response of the filter by sending a frequency-swept sinusoidal wave as the input. The tap weights are tuned with a liquid-crystal-on-silicon waveshaper for different filter configurations. With the additional second-order taps, the filter is able to perform a nonlinear function. As an example, we demonstrate the compensation of a nonlinearly distorted 10-/20-Gbaud 4-amplitude-phase-shift-keying signal.

Simultaneous control of intensity, phase, and polarization in real-time under weak oscillation theory

Runzhang Xie, Peng Wang, Fang Wang, Wei-Da Hu, Xiaoshuang Chen, and Wei Lu

DOI: 10.1364/OL.412851 Received 21 Oct 2020; Accepted 11 Feb 2021; Posted 11 Feb 2021  View: PDF

Abstract: Metasurfaces are planar structures that modify the polarization, phase, and amplitude of light in reflection or transmission. Manipulating polarization, phase, and amplitude simultaneously in real-time is an ultimate pursuit of controlling light. Several types of controllable metasurfaces have been realized, but with either low transmission efficiencies or limited control over amplitude, polarization, and phase in real-time. Here, in contrast to previous designs, which use single-layered or few-layered artificial elements changing the total field strongly, we present a weak oscillation theory dealing with a new type of optical systems consist of many layers of artificial oscillators, each layer of which is weakly interacted with the external field. Rather than a redesign, higher conversion efficiency is achieved by simply appending more layers to the structure, which is vital in technology standardization and commercialization. As an application of our theory, we demonstrate and simulate a graphene-based metasurface structure to show that the oscillator system could change the focal length by changing the bias voltages. The polarization state to focus can also be selected by the bias voltage. The weak oscillation theory presents both a new perspective and a paradigm shift on optics.

Magnetic wire: transverse magnetism in one-dimensional plasmonic system

Subhajit Karmakar, Deepak Kumar, Bishnu Pal, Ravi Varshney, and Dibakar Roy Chowdhury

DOI: 10.1364/OL.414005 Received 02 Nov 2020; Accepted 11 Feb 2021; Posted 11 Feb 2021  View: PDF

Abstract: We experimentally demonstrate ‘magnetic wire’ in coupled cut-wire pair based metasurfaces operating at terahertz (THz) frequency domain. Here, dominant transverse magnetic dipole mode (non-axial circulating conduction current) is excited in one of the plasmonic wires in the coupled system (unit cell); while the other wire remains ‘electric’ in nature. Despite having large asymmetry-induced strong radiation channels in such metasurface, non-radiative current distributions are obtained as a direct consequence of interaction between a electric and the realized magnetic wire(s). Our findings demonstrate a versatile platform to transform an electric to a magnetic wire and vice-versa through asymmetry induced polymorphic hybridization with potential applications in photonic or electrical integrated circuits.

Dynamically reversible and strong circular dichroism based on Babinet-invertible chiral metasurfaces

Xiaoqing Luo, Fangrong HU, and Guangyuan Li

DOI: 10.1364/OL.421016 Received 28 Jan 2021; Accepted 11 Feb 2021; Posted 11 Feb 2021  View: PDF

Abstract: We propose a Babinet-invertible chiral metasurface for achieving dynamically reversible and strong circular dichroism (CD). The proposed metasurface is composed of VO₂-metal hybrid structure, and when VO₂ transits between the dielectric state and the metallic state, the metasurface unit cell switches between complementary structures that are designed according to the Babinet principle. This leads to a large and reversible CD tuning range between ±0.5 at 0.97 THz, which is larger than the literature. We attribute the CD effect to extrinsic chirality of the proposed metasurface. We envision that the Babinet-invertible chiral metasurface proposed here will advance the engineering of active and tunable chiro-optical devices and promote their applications.

High-performance all-silicon polarizer with 415 nm bandwidth

Yaocheng Shi, weixi liu, and Daoxin Dai

DOI: 10.1364/OL.416929 Received 06 Dec 2020; Accepted 11 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: On-chip silicon polarizer have been widely used in polarization controller. However, there are few researches on all-silicon polarizer covering the whole optical communication band due to the strong waveguide dispersion for silicon waveguides. In this paper, we demonstrated an all-silicon TE polarizer with high polarization extinction ratio and low insertion loss, working for the whole optical communication band. The device is based on shallow-etched waveguide realized on silicon-on-insulator (SOI) platform. The optical field of TE polarization is designed to be tightly confined in the shallow-etched silicon waveguide while that of TM polarization is weakly confined. As a result, TE polarization propagates through the waveguide with low loss while TM polarization leaks into the substrate and decays finally. The measurements show that a maximum insertion loss < 0.25 dB and polarization extinction ratio (PER) > 20 dB over an ultra-broad operation band from 1260 nm~1675 nm have been achieved for the proposed polarizer.

On-chip optical true time delay lines based on subwavelength grating waveguides

Yue Wang, hao sun, Mostafa Khalil, Wei Dong, Ivana Gasulla Mestre, Jose Capmany, and Lawrence Chen

DOI: 10.1364/OL.414477 Received 10 Nov 2020; Accepted 11 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: An optical true time delay line (OTTDL) is a fundamental building block for signal processing applications in microwave photonics (MWP) and optical communications. Here, we experimentally demonstrate an index-variable OTTDL based on an array of forty subwavelength grating (SWG) waveguides on silicon-on-insulator (SOI). Each SWG waveguide in the array is 34 mm long and arranged in a serpentine manner; the average incremental delay between waveguides is about 4.7 ps and the total delay between the first and last waveguides is approximately 181.9 ps. The waveguide array occupies a chip area of ~ 6.5 mm × 8.7 mm = 56.55 mm². The proposed OTTDLs bring potential advantages in terms of compactness as well as operation versatility to a variety of microwave signal processing applications.

Quasi-coherent noise-like pulses in a mode-locked fiber laser with 3D rotatable polarization beam splitter

renlai zhou, Qian Li, Hongyan Fu, and Nakkeeran Kaliyaperumal

DOI: 10.1364/OL.420832 Received 26 Jan 2021; Accepted 10 Feb 2021; Posted 18 Feb 2021  View: PDF

Abstract: For the first-time, we experimentally observed a novel type quasi-coherent noise-like pulses in a simplified nonlinear polarization evolution mode-locking fiber laser when appropriate polarization was maintained for the lasing light through a three-dimensionally rotatable polarization beam splitter inside the cavity. The degree of first-order coherence was evaluated after an interferogram measurement. Evolution of the measured shot-to-shot spectrum revealed that the noise-like pulses possess quasi-coherence. Self-started ultrafast soliton pulses switching to quasi-coherent noise-like pulses at higher pump power levels were due to the preservation of the soliton features, mainly the Kelly sidebands in the spectrum. Quasi-coherent noise-like pulses with average power of 56.58 mW and 10.4% slope efficiency were achieved with single pulse energy of 3.22 nJ.

Ptychographic optical coherence tomography

Mengqi Du, Lars Loetgering, Kjeld Eikema, and Stefan Witte

DOI: 10.1364/OL.416144 Received 30 Nov 2020; Accepted 10 Feb 2021; Posted 12 Feb 2021  View: PDF

Abstract: Ptychography is a robust computational imaging technique that can reconstruct complex light fields beyond conventional hardware limits. However, for many wide-field computational imaging techniques, including ptychography, depth sectioning remains a challenge. Here we demonstrate a high-resolution 3D computational imaging approach, which combines ptychography with spectral-domain imaging, inspired by optical coherence tomography (OCT). This results in a flexible imaging system with the main advantages of OCT, such as depth-sectioning without sample rotation, decoupling of transverse and axial resolution, and a high axial resolution only determined by the source bandwidth. The interferometric reference needed in OCT is replaced by computational methods, simplifying hardware requirements. As ptychography is capable of deconvolving the illumination contributions in the observed signal, speckle-free images are obtained. We demonstrate the capabilities of ptychographic optical coherence tomography (POCT) by imaging an axially discrete lithographic structure and an axially continuous mouse brain sample.

MCVD based GRIN-axicon for generation of scalable Bessel-Gauss beams

Mireille Quemener, Nicolas Grégoire, Steeve Morency, Daniel Cote, and Simon Thibault

DOI: 10.1364/OL.415773 Received 24 Nov 2020; Accepted 10 Feb 2021; Posted 10 Feb 2021  View: PDF

Abstract: We introduce the GRIN-axicon, a versatile component that generates high-quality scalable Bessel-Gauss beams. To our knowledge, the GRIN-axicon is the only optical component that can be introduced in both larger-scale laboratory setups and miniaturized all-fiber optical setups while having an easy control of the dimensioning of the generated focal line. We show that a GRIN with hyperbolic secant refractive index profile with sharp central dip and no ripples generates a Bessel-Gauss beam having high-intensity central lobe when coupled to a simple lens. Those fabrication characteristics suit well for MCVD process and enable easy manufacturing of an adaptable component that can fit in any optical setup.

Shear deformation response of a holographic sensor based on elastic poly(MMA-co-LMA) photopolymer

Hongpeng Liu, wei mingzhao, Li li, wang baohua, Yu Dan, and Wang weibo

DOI: 10.1364/OL.413162 Received 04 Nov 2020; Accepted 10 Feb 2021; Posted 10 Feb 2021  View: PDF

Abstract: A holographic sensor based on camphorquinon (CQ) doped poly(methyl methacrylate-co-lauryl methacrylate) (poly(MMA-co-LMA)) elastic photopolymer is developed for characterizing the shear deformation of material. Shear angle and its transverse displacement, which induced by a couple of shear stress, are analyzed using a diffraction spectrum of transmission holographic sensor. The dependence of peak wavelength shift on the shear deformation presents a good linear relationship which provides a quantitative characterization means. The detectable maximum of shear angel exceeds 26.1 degrees, and the peak wavelength shift closes to 4.0 nm. The available sensitivity is better than 3.33 degree/0.5 nm (shear angle/ wavelength shift) using commercial spectrometer with 0.5 nm of resolution. Finally, the reversibility response of shear deformation further confirmed the practical applicability of the elastic polymer based shear deformation sensor. The spectrum measurement of shear deformation supplies a novel measurement method for mechanical deformation of materials and expands the application of holographic sensor.

Topological rainbow based on graded topological photonic crystals

Hongyu Zhang, Long Qian, Chenyang Wang, Chang-Yin Ji, Yaotian Liu, Jiali Chen, and Cuicui Lu

DOI: 10.1364/OL.419271 Received 08 Jan 2021; Accepted 10 Feb 2021; Posted 10 Feb 2021  View: PDF

Abstract: Topological photonic crystal provides a robust platform for nanophotonic devices. However, few reports have been found to realize multiple frequencies routing based on topological photonic states, which have restricted further applications in the field of nanophotonic devices. Here, for the first time, we propose an efficient method to realize topological rainbow based on graded dielectric topological photonic crystals, which are constructed by changing the degree of lattice contraction and expansion. The topological edge states of different frequencies are separated and trapped at different positions. The all-dielectric planar nanostructures of graded topological photonic crystals are low-loss, robust, and easy for integration. This work plays a key role in robust nanophotonic wavelength routers, optical storage, and optical buffers.

Broadband Fiber-based Entangled Photon-Pair Source at Telecom O-band

Changjia Chen, Calvin Xu, Arash Riazi, Eric Zhu, Alexey Gladyshev, Peter Kazansky, and Li Qian

DOI: 10.1364/OL.415409 Received 25 Nov 2020; Accepted 09 Feb 2021; Posted 10 Feb 2021  View: PDF

Abstract: In this letter, we report a polarization-entangled photon-pair source based on type-II spontaneous parametric down conversion at telecom O-band in periodically poled silica fiber (PPSF). The photon-pair source exhibits more than 130 nm ($\sim$24 THz) emission bandwidth centered at 1306.6 nm. The broad emission spectrum results in a short biphoton correlation time and we experimentally demonstrate a Hong-Ou-Mandel interference dip with a full width of 26.6 fs at half maximum. Owing to the low birefringence of the PPSF, the biphotons generated from type-II SPDC are polarization-entangled over the entire emission bandwidth, with a measured fidelity to a maximally entangled state greater than 95.4$\%$. The biphoton source provides the broadest bandwidth entangled biphotons at O-band to our knowledge.

All-passive multiple-place optical phase noise cancellation

Liang Hu, Ruimin xue, Xueyang Tian, Guiling Wu, and Jianping Chen

DOI: 10.1364/OL.415930 Received 10 Dec 2020; Accepted 09 Feb 2021; Posted 10 Feb 2021  View: PDF

Abstract: We report on the realization of delivering coherent optical frequency to multiple places based on passive phase noise cancellation. This technique mitigates any active servo controller on the main fiber link and at arbitrary access places as opposed to the conventional technique in which an active phase compensation circuit has to be adopted to stabilize the main fiber link. Although the residual fiber phase noise power spectral density (PSD) in the proposed technique turns out to be a factor of 7 higher than that of in the conventional multiple-access technique, it could largely suppress the phase noise introduced by the servo bumps, improve the response speed and phase recovery time and minimize hardware overhead in systems with many stations and connections without the need of the active servo circuits including phase discriminators and active compensators. The proposed technique could considerably simplify future efforts to make precise frequency signals available to many users, as demanded by some large-scale science experiments.

Bifunctional Luneburg-Fisheye Lens Based on the Manipulation of Spoof Surface Plasmon

jin zhao, Yi-Dong Wang, Li-Zheng Yin, Feng-Yuan Han, Tie-Jun Huang, and Pu-Kun Liu

DOI: 10.1364/OL.417116 Received 11 Dec 2020; Accepted 09 Feb 2021; Posted 10 Feb 2021  View: PDF

Abstract: Manipulation of spoof surface plasmons (SSPs) has recently intrigued enormous interest due to the capability of guiding waves with subwavelength footsteps. However, most of the previous studies, manifested for a single functionality, are not suitable for multifunctional integrated devices. Herein, a bifunctional Luneburg-fisheye lens is proposed based on a two-dimension metal pillar array. Firstly, by tuning the dimension of the metal pillars in the array, its ability to precisely manipulate the SSPs along one direction is confirmed, achieving subwavelength focusing and imaging with the resolution up to 0.14λ. Then, separately controlling the propagation of the SSPs along the orthotropic directions is further implemented, and the bifunctional Luneburg-fisheye lens is realized. It is experimentally characterized as a Luneburg lens along the x-axis, whereas in the y-axis, it presents the properties of a Maxwell fisheye lens. This bifunctional lens can reduce the system complexity and exert flexibility in multifunctional applications, while the proposed metal pillar-based design method broadens the application range of gradient refractive-index lens in the microwaves, terahertz, and even optical ranges.

Optical coupling of overlapping nanopillars

Alexey Dmitriev and M Rybin

DOI: 10.1364/OL.415334 Received 26 Nov 2020; Accepted 09 Feb 2021; Posted 09 Feb 2021  View: PDF

Abstract: Engineering of nanophotonic devices for controlling light requires deep understanding of interaction between their subwavelength structure elements. Theoretical approaches based on the multiple scattering theory provide simple analytics valuable for design. However they consider different elements separated by the surrounding medium. Here we develop an approach to study wave coupling in the case of overlapping particles. We consider the simplest system that is a dimer of nanopillars and find out that it can be described by a three oscillator model. Two modes correspond to the multipole response of isolated particles that interact through the radiating and evanescent waves in accordance to the conventional multiple scattering theory, but there exists a third effective non-resonant oscillator supporting a direct mode coupling via the intersecting part. Our simple model yields results with a reliable agreement to the numerical simulations and allows insight into the physical processes underlying the collective response of a cluster of overlapped subwavelength particles.

Air-backed photoacousitc transmitter for significantly improving negative acoustic pressure output

Yujie Chen, Qi Li, Haobo Zhu, Yan Wang, Xinyue Zhang, and Hongbin Yu

DOI: 10.1364/OL.415850 Received 25 Nov 2020; Accepted 09 Feb 2021; Posted 09 Feb 2021  View: PDF

Abstract: Aiming to pursue ultrasound signal with significantly improved negative acoustic pressure level, which is one of the critical characteristics for exciting ultrasound cavitation effect, a real applicable air-backed photoacoustic transmitter is presented. Different from conventional solution relying on complicated focusing structure design, it works based on acoustic signal phase reversal and amplitude superposition strategy, which converts the initial backward transmission acoustic wave with positive pressure into forward transmission one possessing negative pressure efficiently through using an innovative sandwich-like suspending photoacoustic layer with optimized thickness. For proof of concept demonstration, photoacoustic transmitter prototypes adopting polydimethylsiloxane(PDMS)-candle soot nanoparticles(CSNPs)/PDMS-PDMS composite as photoacoustic conversion layer are fabricated and characterized. From experiment results, acoustic signal with remarkable ratio of negative pressure level to positive one of 1.3 has been successfully realized, which is the largest value ever being reported, to the best of our knowledge. Moreover, when compared to the commonly used glass and PDMS-backing conditions in photoacoustic area, nearly 200% and 400% enhancement in negative pressure output can be achieved, respectively. Based on the same strategy, various photoacoustic materials can also be easily involved into the transmitter structure for further performance improvement.

Passive polarimetric imaging of millimeter and terahertz waves for personnel security screening

Yayun Cheng, Lingbo Qiao, Dong Zhu, Yingxin Wang, and Ziran Zhao

DOI: 10.1364/OL.418497 Received 30 Dec 2020; Accepted 09 Feb 2021; Posted 09 Feb 2021  View: PDF

Abstract: Passive millimeter and terahertz wave imaging is a powerful way for personnel security inspection and scene monitoring. The existing systems usually have a single polarization mode. To obtain more information, polarimetric imaging has been preliminarily explored recently. However, there is no work exhibiting high performance polarimetric imaging to analyze and interpret polarization characteristics. In this paper, we reports on the development of a W-band passive polarimetric imaging system for human body screening, and first present the polarization characteristics analysis of several typical scenarios. The experimental system has the spatial resolution of better than 1cm at 2.5m distance, and has the thermal sensitivity of better than 0.3K. The system can display polarization properties of human bodies and concealed objects. The experimental results demonstrate that passive polarimetric imaging has a great potential for concealed object enhancement, detection, segmentation and recognition.

Broadband absorption mechanism based on quantum dots glass for filtering field

Panpan Li, Yongmin Duan, Yang Lu, Shiqing Xu, and Junjie Zhang

DOI: 10.1364/OL.420229 Received 19 Jan 2021; Accepted 09 Feb 2021; Posted 09 Feb 2021  View: PDF

Abstract: Traditional filters are faced with the problems of high price and complex preparation process. It is necessary to find a new method or material to make up for these shortcomings. In this paper, CsPbBr3 nanocrystals with controllable size were prepared in tellurite glass matrix by a simple and effective heat-treatment process. The absorption wavelength of CsPbBr3 quantum dots gradually shifted to red due to the quantum confinement effect. In addition, the intrinsic photoluminescence intensity of CsPbBr3 quantum dots was reduced by Ce4+ doped quantum dots CsPbBr3 glass, which could shield a short wavelength of 200 nm-530 nm. The uniformly distributed quantum dots ensure high transmittance (> 80%) of a long wavelength (560 nm-800 nm), demonstrating that the quantum dot glass material has a broad application prospect as a shortwave shielding material.

Resolution improvement of dark-field microscopy via micro-particle near-field illumination

Xiao-rui Wang, Jinzhong Ling, Yucheng Wang, and xin liu

DOI: 10.1364/OL.418159 Received 28 Dec 2020; Accepted 09 Feb 2021; Posted 17 Feb 2021  View: PDF

Abstract: In this article, a novel approach to improve the imaging resolution of dark-field microscopy is proposed and demonstrated. Inspired by existing super-resolution imaging method based on near-filed illumination using a prism or micro-fiber, a micro-particle generated full directions evanescent field for sample illumination was demonstrated to realize multi-orientation near-field illumination in one snap shot and to obtain super-resolution image by spatial frequency shifting. The ultimate resolution and the additional magnification factor of this method were analyzed theoretically. Imaging experiments were carried on a standard microscope calibration target MetroChip and a Blu-ray disc characterized with sub-wavelength microstructures. High imaging resolution was demonstrated experimentally, and two novel illumination modes were proposed to overcome imaging direction selectivity. Our work opened up a new perspective of super-resolution imaging with near-field illumination.

Transformation of spin and orbital angular momentum in second harmonic generation process at oblique incidence of light from the surface of an isotropic medium with spatial dispersion of quadratic nonlinearity

Kirill Grigoriev, Vladislav Diukov, and Vladimir Makarov

DOI: 10.1364/OL.419855 Received 14 Jan 2021; Accepted 08 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: We discuss key features of the conversion of spin and orbital angular momentum of electromagnetic waves in the process of second harmonic generation from the surface of the isotropic medium at oblique incidence. The conservation of the projections of spin and orbital parts of angular momentum of interacting waves onto the normal to the surface is shown for arbitrary case of polarization and mode structure of the incident light beam.

Enhancing the Efficiency of Solution-Processed Inverted Quantum Dot Light-Emitting Diodes via Ligand Modification with 6-mercaptohexanol

Heeyeop Chae, Woosuk Lee, Changmin Lee, BO KIM, and Yonghyeok Choi

DOI: 10.1364/OL.414574 Received 09 Nov 2020; Accepted 08 Feb 2021; Posted 18 Feb 2021  View: PDF

Abstract: In this study, the surface hydrophilicity of the quantum dot (QD) emitting layer (EML) was modified via ligand exchange to prevent QD EML damage upon hole transport layer (HTL) deposition for all-solution-processed inverted QD-light-emitting diodes (QLEDs). The conventional hydrophobic oleic acid ligand (OA-QDs) was partially replaced with a hydrophilic 6-mercaptohexanol (OH-QDs) through one-pot ligand exchange. Owing to this replacement, the contact angle of a water droplet on the OH-QD films was reduced to 71.7° from 89.5° on the OA-QD films, indicating the conversion to hydrophilic hydroxyl ligands. The OH-QD EML maintained its integrity without any noticeable damage even after HTL deposition, enabling all-solution processing for inverted QLEDs with well-organized multilayers. Inverted QLEDs with the OH-QD EMLs were compared with those with OA-QD EMLs; the maximum current efficiency of the device with the OH-QD EML significantly improved to 39.0 cd A-1 from 5.3 cd A-1, and the peak external quantum efficiency improved to 9.3% from 1.2%, which is a 7-fold increase over the OA-QD device. This approach is believed to be effective for forming solid QD films with resistance to chlorobenzene, a representative HTL solvent, and consequently contributes to high-efficiency all-solution-processed inverted QLEDs.

Full Three-Dimensional Wavelength-Scale Plasmomechanical Resonator

Min-Kyo Seo and Shinho Lee

DOI: 10.1364/OL.416695 Received 04 Dec 2020; Accepted 08 Feb 2021; Posted 12 Feb 2021  View: PDF

Abstract: Plasmomechanical systems have received considerable interest in mediating the strong interaction between the optical field and mechanical motion. However, typical plasmomechanical systems, based on mechanical oscillators significantly larger than the wavelength of light, do not take full advantage of the optical field concentration beyond the optical diffraction limit of the employed plasmonic resonators. Here, we present a full three-dimensional wavelength-scale plasmomechanical resonator consisting of a plasmonic nano-antenna and a hydrogen silsesquioxane nano-wall. The experimental results demonstrated the precise detection of longitudinal mechanical oscillation on a picometer scale, and we investigated the tunability and thermoelastic effect of the mechanical resonance.

Dynamic manipulation of nonlinear Talbot effect with structured light

Lin Li, Haigang Liu, and Xianfeng Chen

DOI: 10.1364/OL.416988 Received 08 Dec 2020; Accepted 08 Feb 2021; Posted 12 Feb 2021  View: PDF

Abstract: Nonlinear Talbot effect has sparked considerable interest of the researchers since it was proposed in recent years, because it has many advantages compared with the Talbot effect in linear optics. In previous researches, such nonlinear Talbot effect is only observed in nonlinear photonic crystals, which cannot dynamically manipulate in real time. Here, we report and experimentally demonstrate the high efficiency and dynamic manipulation of such nonlinear Talbot effect with structured light. Different with previous scheme, the nonlinear self-imaging effect observed in our experiment originates from the spatial phase structure of the incident fundamental frequency light. In our experiments, integer and fractional second-harmonic Talbot self-imaging are observed. Our results not only extend a novel technique for dynamic manipulation of the nonlinear Talbot effects, but also may have the potential applications in parallel optical lithography, optical imaging, and optical computing.

Narrow-linewidth self-injection locked diode laser with high-Q fiber Fabry-Pérot resonator

Liyun Hao, Xiaohan Wang, Dongjie Guo, Kunpeng Jia, Pengfei Fan, Jian Guo, Xin Ni, Gang Zhao, Zhenda Xie, and Shining Zhu

DOI: 10.1364/OL.415859 Received 08 Jan 2021; Accepted 08 Feb 2021; Posted 16 Feb 2021  View: PDF

Abstract: Narrow linewidth lasers are essential for various applications, but are limited by their size, weight, power and cost requirements. Here we demonstrate a self-injection locked diode laser fabricated with a high-Q fiber Fabry-Pérot resonator, with 145 Hz free-running linewidth. The locking scheme is all-fiber for plug-and-play operation. White frequency noise of 50Hz2/Hz is measured with over 42 dB reduction from the low cost TO-can distributed feedback laser diode, and shows its wide applications in a compact and cost-effective way.

Vector vortex state preservation in the Fresnel cylindrical diffraction

Yanwen Hu, Guangcui Mo, Zixian Ma, Shenhe Fu, Si-Qi Zhu, Hao Yin, Zhen Li, and Zhenqiang Chen

DOI: 10.1364/OL.416414 Received 01 Dec 2020; Accepted 08 Feb 2021; Posted 09 Feb 2021  View: PDF

Abstract: Vector vortex light beam, which exhibits a space-variant polarization state and is coupled with orbital angular momentum of light, has been drawing much attention, due to its fundamental interest and potential applications in a wide range. Here we reveal both theoretically and experimentally that a diffractive structure having cylindrical symmetry is shown to be transparent for the vector vortex state of light with arbitrary topology. We demonstrate such an intriguing phenomenon in the Fresnel diffraction condition, where vector Helmholtz wave equation could be utilized in the paraxial regime. Our demonstration has implications in control and manipulation of the vector vortex light beams in diffractive optics and hence it may find potential applications.

Third optical harmonic generation reveals circular anisotropy in tilted silicon nanowire array

Alexey Ustinov, Liubov Osminkina, Denis Presnov, and Leonid Golovan

DOI: 10.1364/OL.417684 Received 23 Dec 2020; Accepted 08 Feb 2021; Posted 09 Feb 2021  View: PDF

Abstract: In this paper, we report on the circular anisotropy of the third-harmonic (TH) generation in an array of silicon nanowires (SiNW) of approximately 100 nm in diameter. SiNW arrays were obtained by metal-assisted chemical etching of (110) crystalline silicon (c-Si) wafer, with the SiNWs tilted to the c-Si substrate at an angle of 45°. Numerical simulations of the linear scattering of circularly polarized light by the SiNWs indicate asymmetric scattering diagrams off a single SiNW as well as an ansatz-structure composed of thirteen nanowires used as a geometrical approximation of the real SiNW array, which is a manifestation of the photonic spin Hall effect mediated by the synthetic gauge field arising due to the special guided-like mode structure in each and every SiNW in an array. Furthermore, despite strong light scattering in a SiNW array, the experimentally measured TH signal generated in it demonstrated to be significantly dependent on the polarization state of incident radiation and a SiNW array spacial orientation in regard to its wave vector direction.

Rayleigh speckle-based wavemeter with high dynamic range and fast reference speckles establishment process assisted by optical frequency combs

Xinyu Fan, Yangyang Wan, Wang Shuai, Zhaopeng Zhang, Bingxin Xu, and Zuyuan He

DOI: 10.1364/OL.419539 Received 11 Jan 2021; Accepted 08 Feb 2021; Posted 10 Feb 2021  View: PDF

Abstract: The compact speckle-based spectrometers which acquire the lightwave wavelength from the speckle generated by the turbid medium are promising for high-resolution spectral analysis. For these devices, the reference establishment process is time-consuming and it is very difficult to obtain the reference speckles covering a wide bandwidth with high-resolution, which restricts the dynamic range in frequency (the ratio of bandwidth to resolution). In this Letter, we introduce optical frequency combs (OFCs) to overcome these problems existed in the wavemeter based on Rayleigh speckle obtained from a single-mode fiber. In the experiment, the proposed wavemeter has a 1.5 nm bandwidth with 60 am resolution, covering a dynamic range in frequency of 2×10^7, and a fast reference speckles establishment process which only takes 0.9 ms. The proposed method assisted by OFCs shows a good prospect for a more practical speckle-based wavemeter with higher dynamic ranges.

Light scattering from stationary PT-symmetric collections of particles

Olga Korotkova and Paulo Brandão

DOI: 10.1364/OL.418537 Received 29 Dec 2020; Accepted 08 Feb 2021; Posted 08 Feb 2021  View: PDF

Abstract: A potential scattering theory from deterministic and random PT collections of particles with gain and loss is introduced and the forms of their structure and pair-structure factors are elucidated. An example relating to light scattering from a random distribution of a pair of particles with gain and loss is considered.

Diffraction-limited broadband optical meta-power-limiter

Abdoulaye Ndao and LiYi Hsu

DOI: 10.1364/OL.418745 Received 30 Dec 2020; Accepted 07 Feb 2021; Posted 10 Feb 2021  View: PDF

Abstract: In recent years, the need for high-power laser is of great interest for different applications ranging from direct-laser processing, light detection, medicine, and lighting. However, high-power lasers with high intensities give rise to fundamental problems for optical detectors and imaging systems with low threshold damage, which still need reliable solutions. Here, we report and numerically demonstrate a hybrid system that synergically combines a broadband optical power limiter with a transmittance difference between on-state (70℃) and off-state (25℃) about 62.5%, and a diffraction-limited broadband metalens from 1534 nm to 1664 nm. Such a metalens power limiter could be used in any system requiring an intermediate focal plane in the optical path to the detector from damage by exposure to high-intensity lasers.

A compact long range single-photon imager with dynamic imaging capability

Feihu Xu, Peng-Yu Jiang, and Zheng-Ping Li

DOI: 10.1364/OL.416327 Received 23 Dec 2020; Accepted 07 Feb 2021; Posted 08 Feb 2021  View: PDF

Abstract: Single-photon light detection and ranging (LiDAR) has emerged as a strong candidate technology for active imaging applications. Benefited from the single-photon sensitivity in detection, long-range active imaging can be realized with a low-power laser and a small-aperture transceiver. Here we present a compact co-axial single-photon LiDAR system for kilometer-range 3D imaging. A fiber-based transceiver with 2.5 cm effective aperture was employed to realize a robust and compact architecture while a tailored temporal filtering approach guaranteed the high signal-to-noise level. Moreover, a MEMS (Micro-Electro-Mechanical System) scanning mirror was adopted to achieve fast beam scanning. In experiment, high-resolution three-dimensional images of different targets up to 12.8 km were acquired to demonstrate the long-range imaging capability. Furthermore, it exhibits the ability to achieve real-time imaging at 5 frames per second over a distance of 1 km. The results indicate potential in varieties of applications such as autonomous systems and long-range target detection.

Diffuse reflectance from turbid media of strong absorption well beyond the leveling zone of the Kubelka-Munk formulation

Daqing Daching Piao and Tengfei Sun

DOI: 10.1364/OL.415650 Received 20 Nov 2020; Accepted 07 Feb 2021; Posted 09 Feb 2021  View: PDF

Abstract: Simple and robust expression of diffuse reflectance from turbid media is important to many quality-control processes. The well-known Kubelka-Munk (KM) model incorrectly “levels” the diffuse reflectance at strong absorption. We demonstrate that diffuse reflectance from a medium of strong absorption well beyond the leveling zone of the KM formulation follows a simple semi-empirical dependence upon absorption/scattering ratio of the medium, when assessed over a large area centered at the illumination point. The semi-empirical dependence gives ~11% mean errors against Monte Carlo simulations for the medium to have an absorption coefficient covering 0.001 to up to 1000 times stronger than the reduced scattering coefficient for the scattering anisotropy within [0.5, 0.9]. A slight modification of the KM coefficient also improves noticeably the KM-model prediction of diffuse reflectance for strong absorption. The new simple model and the modified KM formulae were compared against measurements from a turbid medium of which the absorption coefficient was changed over four orders of magnitude.

Phase noise of electro-optic dual frequency combs

Callum Deakin, Zichuan Zhou, and Zhixin Liu

DOI: 10.1364/OL.418543 Received 28 Dec 2020; Accepted 06 Feb 2021; Posted 08 Feb 2021  View: PDF

Abstract: Dual frequency combs are emerging as new tools for spectroscopy and signal processing. The relative phase noise of the tone pairs determines the performance (e.g. signal to noise ratio) of the detected spectral components. Although previous research has shown that the signal quality generally degrades with the increase of frequency difference between tone pairs, the scaling of the relative phase noise of dual frequency comb systems has not been fully characterized. In this paper, we model and characterize the phase noise of a coherent electro-optic dual frequency comb system. Our results show that at high offset frequencies, the phase noise is an incoherent sum of the timing phase noise of the two combs, multiplied by line number. At low offset frequencies, however, the phase noise scales more slowly due to the coherence of the common frequency reference.

Multispectral compressive fluorescence lifetime imaging microscopy with a SPAD array detector

Alberto Ghezzi, Andrea Farina, Andrea Bassi, Gianluca Valentini, Ivan Labanca, Giulia Acconcia, Ivan Rech, and Cosimo D'Andrea

DOI: 10.1364/OL.419381 Received 08 Jan 2021; Accepted 06 Feb 2021; Posted 08 Feb 2021  View: PDF

Abstract: Multispectral Fluorescence Lifetime Imaging Microscopy (λFLIM) is a promising tool for studying functional and structural biological processes. The rich information content provided by a multidimensional dataset is often in contrast with the acquisition speed. In this work, we develop and experimentally demonstrate a wide-field λFLIM setup, based on a novel time-resolved 32x1 Single Photon Avalanche Diodes (SPAD) array detector working in a single pixel camera scheme, which parallelizes the spectral detection reducing the measurement time. The proposed system, which implements a single-pixel camera with compressive sensing scheme, represents an optimal microscopy framework towards the design of real-time λFLIM intravital microscopy setups.

Information transfer from optical phase to a plasmonic digital encoder composed of a gold nanorod pair

Donghai Li, Shufeng Wang, Saisai Chu, yingbo he, Guowei Lu, Hongbing Jiang, and Qihuang Gong

DOI: 10.1364/OL.417300 Received 18 Dec 2020; Accepted 06 Feb 2021; Posted 11 Feb 2021  View: PDF

Abstract: Small all-optical devices are central to the optical computing. Plasmonic digital encoders (PDEs) with featured dimension of ~1 µm, holds the key for transferring information from far field to photonic processing systems. Here we propose a design of PDE composed of two gold nanorods (AuNRs), whose pattern represent a 2-bit digital information, instead of one nanorod for each bit. We implanted information into the spectral phase of a femtosecond pulse by pulse shaping and controlled the two-photon photoluminescence pattern of a AuNR pair. The high contrast ratios were achieved with 13.01 and 6.02 dB for binary codes “1-0” and “0-1”, respectively.

Estimating the effective pressure from nanosecond laser-induced breakdown in water

fabing Li, Ying Wang, haiying zhao, Xin Xu, Liu Cunming, zhiwei men, and chenglin sun

DOI: 10.1364/OL.420399 Received 20 Jan 2021; Accepted 06 Feb 2021; Posted 11 Feb 2021  View: PDF

Abstract: Nanosecond laser-induced breakdown (LIB) in liquids (e.g. water) can produce the dynamic high pressure and high temperature. However, since the high pressure needs to negate the effect of high temperature in some degree, it is only partially effective. As a result, it is difficult to directly measure the effective pressure due to the transient and complex LIB process. Here, we presented a simple method based on the Raman spectroscopy to indirectly determine the effective pressure caused by LIB in the liquid pure H2O and low concentration H2O-H2O2 mixtures. By comparing the Raman shifts of ice-VII mode for pure H2O and H2O-H2O2 mixtures under laser pumping and static high pressure, the LIB effective pressure can be first estimated. Then, with the relationship between the LIB effective pressure and ice-VII-point Stimulated Raman Scattering (SRS) thresholds for pure and mixture water solution, the obtained empirical equation can be used to estimate the LIB effective pressures for different water solutions, with the uncertainty of 0.14 – 0.25 Gpa. Hopefully, our study here would advance the measurements of effective pressure in the LIB process.

Modulation of MXene Nb2CTx saturable absorber for passively Q-switched 2.85 μm Er:Lu2O3 Laser

Chenyang Feng, Wenchao Qiao, Yizhou Liu, jia Huang, Yangyang Liang, Yuefeng zhao, Yuzhi Song, and Tao Li

DOI: 10.1364/OL.416644 Received 06 Dec 2020; Accepted 06 Feb 2021; Posted 18 Feb 2021  View: PDF

Abstract: We proposed a passively stabilized Q-switched Er:Lu2O3 laser at 2845 nm applying MXene Nb2CTx nanosheets saturable absorber prepared by liquid-phase exfoliation method. The surface morphology and nolinear properties of this nanosheet were systematically characterized. 542 mW average output power of the Q-switched laser was obtained under 7.26 W absorbed pump power. Meanwhile, the Q-switched pulse duration was measured to be 2 .7 ns with 142.9 kHz repetition rate corresponding to a peak power of 16.96 W.

Simultaneous gain profile design and noise figure prediction for Raman amplifiers using machine learning

Uiara de Moura, Ann Margareth Rosa Brusin, Andrea Carena, Darko Zibar, and Francesco Da Ros

DOI: 10.1364/OL.417243 Received 11 Dec 2020; Accepted 05 Feb 2021; Posted 08 Feb 2021  View: PDF

Abstract: A machine learning framework predicting pump powers and noise figure profile for a target distributed Raman amplifier gain profile is experimentally demonstrated. We employ a single-layer neural network to learn the mapping from the gain profiles to the pump powers and noise figures. The obtained results show highly-accurate gain profile designs and noise figure predictions, with a maximum error on average of ~0.3 dB. This framework provides the comprehensive characterization of the Raman amplifier and thus is a valuable tool for predicting the performance of the next-generation optical communication systems, expected to employ Raman amplification.

Continuous-wave cascaded second Stokes diamond Raman laser at 1477\,nm

Matthias Heinzig, Gonzalo Palma Vega, Benjamin Yildiz, Till Walbaum, Andreas Tünnermann, and Thomas Schreiber

DOI: 10.1364/OL.416549 Received 07 Dec 2020; Accepted 03 Feb 2021; Posted 03 Feb 2021  View: PDF

Abstract: We report a high brightness cascaded Stokes diamond Raman laser with a diffraction limited beam Quality pumped by an Yd-doped fiber laser. The Raman laser operate at 1477\,nm and reached a output power of 63\,W with 214\,W pump power in continuous wave mode. Conversion efficiency over 30\% is achieved using a single pump pass concentric cavity that is highly resonant at the first Stokes and has high outcoupling at the second Stokes (45\%). Thermal limitations were investigated as well as the temporally behavior of the first and second Stokes intra cavity power.

Maskless fabrication of plasmonic metasurfaces in polymer film using spatial light modulator

Mohammad Bitarafan, Shambhavee Annurakshita, Juha Toivonen, and Godofredo Bautista

DOI: 10.1364/OL.418459 Received 28 Dec 2020; Accepted 03 Feb 2021; Posted 04 Feb 2021  View: PDF

Abstract: We demonstrate a high-speed optical technique to fabricate plasmonic metasurfaces in a polymer film. The technique is based on a programmable spatial light modulator, which is used to spatially-control the photoreduction sites of gold ions in a polyvinyl alcohol film doped with a gold precursor. After irradiation, annealing was used to induce the growth of nanoparticles, producing plasmonic microstructures. Using 473 nm excitation wavelength, microscopic plasmonic gratings and meta-atom arrays with arbitrary orientations, an effective nanostructure size of ~700 nm and constituent nanoparticles with average size of ~37 nm were created. The technique enables a cost-effective and straightforward light-based approach to fabricate plasmonic metasurfaces with tunable properties.

10-W-scale Kerr-lens mode-locked Yb:CALYO laser with sub-100-fs pulses

Dacheng Zhang, Tian Wenlong, Rui Xu, Li Zheng, Xuan Tian, Xiaodong Xu, Jiangfeng Zhu, Jun Xu, and Zhiyi Wei

DOI: 10.1364/OL.419370 Received 08 Jan 2021; Accepted 03 Feb 2021; Posted 04 Feb 2021  View: PDF

Abstract: We reported a high power pure Kerr-lens mode-locked Yb:CALYO laser based on the dual-confocal cavity delivering sub-100 fs pulses. The output pulses at 81 MHz have an average power of 10.4 W and the pulse duration of 98 fs, corresponding to the peak power of 1.14 MW. This is, to the best of our knowledge, the highest average power ever reported for a Kerr-lens mode-locked Yb-bulk oscillator. The analysis of the dual-confocal cavity was also conducted which indicates a way to achieve higher average power. We believe the result described in this paper may pave a way to develop the Kerr-lens mode-locked bulk lasers with much higher average power.

Supercontinuum generation from Zinc Borate glasses: Bandgap vs Rare-earth doping

M M NEETHISH, V.V. Ravi Kanth Kumar, Samuel Nalam, Sree Srikantaiah, and Prem Kiran Paturi

DOI: 10.1364/OL.418594 Received 29 Dec 2020; Accepted 03 Feb 2021; Posted 05 Feb 2021  View: PDF

Abstract: We report an enhanced supercontinuum emission (SCE) from a rare-earth (Pr3+) doped low bandgap Zinc Borate glass when excited with 60 fs pulses from a Ti-Sapphire laser. The emission associated with the absorption bands due to Pr3+ doping (around 420 – 500 nm and 580 – 600 nm) is observed to assist the enhanced SCE.

200 μJ, 13 ns Er:ZBLAN mid-infrared fiber laser actively Q-switched by an electro-optic modulator

Yanlong Shen, Yousheng Wang, Feng Zhu, Lianying Ma, Liu Zhao, Zhengge Chen, Hao Wang, Chao Huang, Ke Huang, and Guobin Feng

DOI: 10.1364/OL.418950 Received 07 Jan 2021; Accepted 02 Feb 2021; Posted 05 Feb 2021  View: PDF

Abstract: We report, as far as we know for the first time, on a pulsed 2.7 μm Er:ZBLAN fiber laser Q-switched by an electro-optic modulator. Q-switched operation was achieved with a repetition rate range of 100 Hz~50 kHz. Pulse energy of 205.7 μJ and pulse width down to 13.1 ns, yielding a peak power of 15.7 kW, were obtained at a repetition rate of 100 Hz. The linewidth of the output spectrum was as narrow as 0.4 nm. The pulse width and the pulse peak power, to the best of our knowledge, are currently the shortest and the highest in the 3-μm band Q-switched fiber lasers, respectively.

Brightness enhancement in random Raman fiber laser based on a graded-index fiber with high-power multimode pumping

Yizhu Chen, Chenchen Fan, Tianfu Yao, Hu Xiao, Jinyong Leng, Pu Zhou, Ilya Nemov, Alexey Kuznetsov, and Sergey Babin

DOI: 10.1364/OL.416740 Received 14 Dec 2020; Accepted 02 Feb 2021; Posted 05 Feb 2021  View: PDF

Abstract: A brightness-enhanced random Raman fiber laser (RRFL) with maximum power of 306 W at 1120 nm is demonstrated. A half-open cavity is built based on a graded-index (GRIN) passive fiber and single high-reflective fiber Bragg grating written in it directly. Thanks to the beam cleanup effect in the GRIN fiber enhanced in the half-open RRFL cavity, the output beam quality factor M² is improved from 9.15 (pump) to 1.76-2.35 (Stokes) depending on power, while the pump-Stokes brightness enhancement (BE) factor increases proportionally to output power reaching 6.1 in maximum. To the best of our knowledge, this is the highest power GRIN RRFL with BE.

Ultrasound liquid crystal lens with enlarged aperture using traveling waves

Jessica Onaka, Takahiro Iwase, Marina Fukui, Daisuke Koyama, and Mami Matsukawa

DOI: 10.1364/OL.414295 Received 06 Nov 2020; Accepted 01 Feb 2021; Posted 01 Feb 2021  View: PDF

Abstract: A new type of ultrasonically controlled concave liquid crystal lens based on traveling waves (TWs) with a divided electrode structure and an appropriate driving scheme is proposed. The lens uses an annular piezoelectric ceramic that is divided into four parts for four-phase driving and consists of a liquid crystal layer in a sandwich structure between two circular glass substrates. The lens configuration was simulated by finite element analysis using Ansys software. In this work, we discuss the use of TWs to expand the lens aperture and clarify the lens’ optical characteristics using a Shack–Hartmann wavefront sensor.

Microwave-photonic Low-coherence Interferometry for Dark Zone Free Distributed Optical Fiber Sensing

Xuran Zhu, Liwei Hua, Jincheng Lei, Jianan Tang, Lawrence Murdoch, and Hai Xiao

DOI: 10.1364/OL.414699 Received 10 Nov 2020; Accepted 31 Jan 2021; Posted 01 Feb 2021  View: PDF

Abstract: A microwave-photonic low-coherence interferometry (MPLCI) system is proposed for fully distributed optical fiber sensing. Assisted by an unbalanced Michelson interferometer (MI), a low-coherence laser source is used to interrogate cascaded Fabry–Pérot interferometers (FPIs) along with an optical fiber for a dark zone free (or spatially continuous) distributed measurement. By combining the advantages of microwaves and photonics, the MPLCI system can synergistically achieve high sensitivity and high spatial resolution. Our tests have confirmed a strain resolution of 95 nε at the spatial resolution of 10 cm.

Singlet oxygen phosphorescence imaging by superconducting single photon detector and time-correlated single photon counting

Pavel Morozov, Maria Lukina, Marina Shirmanova, Alexander Divochiy, Gregory Goltsman, Wolfgang Becker, and Vladislav Shcheslavskiy

DOI: 10.1364/OL.415229 Received 18 Nov 2020; Accepted 31 Jan 2021; Posted 01 Feb 2021  View: PDF

Abstract: This paper presents a novel optical configuration for direct time-resolved measurements of luminescence from singlet oxygen both in solutions and from cultured cells upon photodynamic therapy. The system is based on the superconducting single photon detector coupled to the confocal scanner that is modified for the near-infrared measurements. The recording of a phosphorescence signal from singlet oxygen at 1270 nm have been done by time-correlated single photon counting. The performance of the system is verified by measurement of phosphorescence from singlet oxygen generated by the photosensitizers that are commonly used in photodynamic therapy, methylene blue and chlorine e6. The described system can be easily upgraded to the configuration when both phosphorescence from singlet oxygen and fluorescence from the cells can be detected in the imaging mode. Thus, co-localization of the signal from singlet oxygen with the areas inside the cells can be done.

Efficient erbium-doped thin-film lithium niobate waveguide amplifiers

Zhaoxi CHEN, Qing Xu, Ke ZHANG, Wing-Han Wong, De-Long Zhang, Edwin Pun, and Cheng Wang

DOI: 10.1364/OL.420250 Received 20 Jan 2021; Accepted 31 Jan 2021; Posted 10 Feb 2021  View: PDF

Abstract: Lithium niobate on insulator (LNOI) is an emerging photonic platform with great promises for future optical communications, nonlinear optics and microwave photonics. An important integrated photonic building block, active waveguide amplifiers, however, is still missing in the LNOI platform. Here we report an efficient and compact waveguide amplifier based on erbium-doped LNOI waveguides, realized by a sequence of erbium-doped crystal growth, ion slicing and lithography-based waveguide fabrication. Using a compact 5-mm-long waveguide, we demonstrate on-chip net gain of > 5 dB for 1530-nm signal light with a relatively low pump power of 20.9 mW at 980 nm. The efficient LNOI waveguide amplifiers could become an important fundamental element in future lithium niobate photonic integrated circuits.

Silicon reconfigurable mode-selective modulation for on-chip mode-multiplexed photonic systems

Gangqiang Zhou, Yuyao Guo, Liangjun Lu, Jianping Chen, and Linjie Zhou

DOI: 10.1364/OL.413865 Received 30 Oct 2020; Accepted 29 Jan 2021; Posted 03 Feb 2021  View: PDF

Abstract: Recently, optical mode-division multiplexing has drawn a lot of attention due to its ability to increase the optical communication capacity in one physical channel with a single wavelength carrier. In this paper, we demonstrate reconfigurable mode-selective modulation potentially useful for on-chip mode-multiplexed photonic systems. The device consists of two mode-exchangers and one TE₁ mode modulator. The mode-exchanger is based on a Mach-Zehnder interferometer (MZI) that performs mode exchange between TE₀ and TE₁ modes. The TE₁ mode modulator consists of a pair of 1×3/3×1 MMIs acting as mode (de)multiplexer. It only selectively modulates the TE₁ mode while bypassing the TE₀ mode. 32 Gb/s OOK modulation is successfully demonstrated for both input TE₀ and TE₁ modes. This device can be used as a building block for on-chip multimode interconnect networks.

Open-aperture Z-scan Study for Absorption Saturation: Accurate Measurement of Saturation Intensity in YLF:Yb for Optical Refrigeration

Azzurra Volpi, Jackson Kock, Alexander Albrecht, Markus Hehlen, Richard Epstein, and Mansoor Sheik-Bahae

DOI: 10.1364/OL.419551 Received 11 Jan 2021; Accepted 28 Jan 2021; Posted 24 Feb 2021  View: PDF

Abstract: Knowledge of saturation intensity of gain or absorption plays a fundamental role in a variety of applications ranging from lasers to many nonlinear optical processes. Here we present an analytical expression for the open-aperture Z-scan transmission for accurately measuring the saturation intensity in the low absorbance samples but at arbitrary pump intensities. We exploit this formalism to investigate the absorption saturation of LiYF4:Yb3+ (YLF:Yb) in the anti-Stokes excitation region for optical refrigeration at high pump intensities . An absorption saturation intensity of 14.5±1 kW/cm^2 was measured in YLF:Yb at 1020 nm (E||c) at room temperature .

Direct generation of sub-ps pulse via multi-section gain switching

Takahiro Nakamura, Takashi Ito, HIDEKAZU NAKAMAE, CHANGSU KIM, Yuji Hazama, Yohei Kobayashi, R Kuroda, and Hidefumi Akiyama

DOI: 10.1364/OL.409822 Received 13 Oct 2020; Accepted 27 Jan 2021; Posted 12 Feb 2021  View: PDF

Abstract: We have directly generated optical pulses having a duration of 0.56 ps with a peak power of 25 W by gain switching of multi-section semiconductor lasers in which the optimized lengths of the absorption and gain regions were 50 and 200 μm, respectively. Even though the experiment was conducted via impulsive optical pumping at a low temperature, we observed that the multi-section gain switching suppresses the low-energy tail and chirping inherent to conventional gain switching in single-section lasers and is useful in direct short-pulse generation.

Sensing Single Photon in a Cat State

Prasanta Panigrahi, Gargi Tyagi, and ARMAN .

DOI: 10.1364/OL.415713 Received 24 Nov 2020; Accepted 26 Jan 2021; Posted 27 Jan 2021  View: PDF

Abstract: The cat state is shown to `store' a single photon through the superposition of its orthogonal counterpart with itself, and an excited oscillator state. Photon addition leads to a $\pi $ phase shift at origin in the observed phase space interference of the Wigner function, which also displays negativity, controlled by the average photon number ($|\alpha|^2$) of coherent states comprising the cat state. The maxima and minima of the sub-Planck tiles in the phase space of the kitten state are interchanged after photon addition, leading to their orthogonality. Interestingly, photon addition to Yurke-Stoler state characterized by Poissonian statistics leads to a sub-Poissonian distribution.

Threshold gain of coherently-coupled aperiodic lattice Y lasers

Wei Jiang and Subhasish Chakraborty

DOI: 10.1364/OL.417552 Received 18 Dec 2020; Accepted 24 Jan 2021; Posted 25 Jan 2021  View: PDF

Abstract: By incorporating a holographically designed aperiodic photonic lattice within one of the arms of a Y-coupled Fabry-Perot quantum cascade laser architecture, it has been demonstrated that the multiband mode control exerted by the photonic lattice on emission spectra can, owing to the mutual optical coupling between the arms, be transferred to the second unpatterned arm. However, the underlying theoretical mechanism on how the lattice influences the threshold gain spectral properties of the Y architecture has, until now, remained unstudied. Here, we use the transfer matrix formalism, originally developed for studying aperiodic lattice lasers, to investigate this. A detailed threshold gain spectral study revealed that although the effects of facet feedback of the Y-coupled laser chip are present, due to the enhanced photonic density-of-states at user-specified frequencies, the aperiodic lattice has remarkable control over the Y architecture laser spectra, under the mutual optical coupling between the arms. Finally, indicated by the fringe patterns akin to double-slit interference, of the measured far-field beam profiles, phase-locked terahertz emissions from the Y architecture are demonstrated.

Ultra-sensitive slot-waveguide-enhanced Raman spectroscopy for aqueous solutions of non-polar compounds using a functionalized silicon nitride photonic integrated circuit

Zuyang Liu, Haolan Zhao, Bettina Baumgartner, Bernhard Lendl, Andre Skirtach, Nicolas Thomas, Roel G. Baets, and Andim Stassen

DOI: 10.1364/OL.416464 Received 02 Dec 2020; Accepted 22 Jan 2021; Posted 01 Feb 2021  View: PDF

Abstract: We demonstrate an ultra-sensitive waveguide enhanced Raman sensor for low concentration organic compounds dissolved in water.The spectra are obtained using silicon nitride slot waveguides coated with a thin film of hexamethyldisilazane (HMDS) modified mesoporous silica.Enriched locally by at least 100-fold within the coating, µM level of cyclohexanone is probed.The sensor is also capable of simultaneous quantification of multiple analytes, and the adsorbed analytes can be completely released from the coating.These properties make this on-chip Raman sensor promising for diverse applications, especially for the monitoring of non-polar organics and biomolecules in aqueous environments.

Time-resolved study of laser emission in nitrogen gas pumped by two near IR femtosecond laser pulses

Rostyslav Danylo, guillaume lambert, Yi Liu, Vladimir Tikhonchuk, Aurelien Houard, and Andre Mysyrowicz

DOI: 10.1364/OL.414863 Received 16 Nov 2020; Accepted 18 Jan 2021; Posted 19 Jan 2021  View: PDF

Abstract: The time profile of the lasing signal at 391.4 nm emitted by a weakly ionized gas of nitrogen molecules at low pressure is measured under double excitation with intense femtosecond laser pulses at 800 nm. An abrupt decrease of the emission occurs at the time of arrival of the second pulse. It is explained by a transfer of population from ground to first excited ionic level and disruption of the coherence, terminating the conditions for lasing in a V-scheme without population inversion.

Optical frequency generation using fiber Bragg grating filters for applications in portable quantum sensing

Calum Macrae, Michael Holynski, and Kai Bongs

DOI: 10.1364/OL.415963 Received 01 Dec 2020; Accepted 14 Jan 2021; Posted 14 Jan 2021  View: PDF

Abstract: A method for the agile generation of the optical frequencies required for laser cooling and atom interferometry of rubidium is demonstrated. It relies on fiber Bragg grating technology to filter the output of an electro-optic modulator and was demonstrated in a robust, alignment-free, single-seed, frequency-doubled, telecom fiber laser system. The system was capable of 50 ns frequency switching over a ~40 GHz range, ~0.5 W output power and amplitude modulation with a ~15 ns rise/fall time and an extinction ratio of 120 ± 2 dB. The technology is ideal for enabling high-bandwidth, mobile industrial and space applications of quantum technologies.

Constructing achromatic polarization-dependent bifocal metalens with height-gradient metastructures

Xiang Xiong, xi wang, Zheng-Han Wang, Yajun Gao, Ruwen Peng, and Mu Wang

DOI: 10.1364/OL.414668 Received 12 Nov 2020; Accepted 07 Jan 2021; Posted 08 Feb 2021  View: PDF

Abstract: Metalenses processes extraordinary capability in tailoring the wavefront of light with compact metastructures. However, it remains challenging to eliminate chromatic aberration and realize multifunction. Here we report an achromatic bifocal metalens (ABM) made of three-dimensional standing nanoblocks (SNBs). By introducing a height gradient to SNBs, the ABM can achieve achromatic focusing in the wavelength range of 760-1550 nm, with two different focal lengths by merely orthogonally switching the linear polarization of the incident beam. Such an achromatic multi-functional element may have applications in polarization sensing/imaging and shared-aperture optics design among many others.

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