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Accepted papers to appear in an upcoming issue

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The Talbot effect in subwavelength multi-lobe superoscillation

Zhigui Deng, Niv Shapira, Roei Remez, Yongyao Li, and Ady Arie

DOI: 10.1364/OL.388263 Received 15 Jan 2020; Accepted 27 Mar 2020; Posted 27 Mar 2020  View: PDF

Abstract: The self-imaging of periodic light patterns, also known as the Talbot effect, is usually limited to periods that are larger than the wavelength. Here we present, theoretically and experimentally, a method to overcome this limitation by using superoscillating light patterns. The input intensity distribution is a periodic band-limited function with relatively large periods, but it contains regions of multi-lobe periodic oscillations with periods that are smaller than half of the wavelength. We observe the revival of the input pattern, including the subwavelength superoscillating regions, at large distances of more than 40 times the optical wavelength. Moreover, at fractional Talbot distances, we observe even faster local oscillations, with periods of approximately one third of the optical wavelength.

Landau broadening of plasmonic resonances in Mie theory

Renat Ikhsanov, Igor Protsenko, Igor Smetanin, and Alexander Uskov

DOI: 10.1364/OL.389329 Received 11 Feb 2020; Accepted 27 Mar 2020; Posted 27 Mar 2020  View: PDF

Abstract: For the first time, Landau damping in the metal nanosphere is considered out of the frames of the quasi-static approximation but with using instead the exact Mie theory. The refusal of the quasistatic approximation leads to the possibility to excite by a plane wave not only the dipole mode but also the modes of a higher order. In resonance approximation, when one considers excitation of a single-mode, the analytical formula for the Landau damping coefficient for various modes have been derived. It was demonstrated that the simultaneous excitation of several eigenmodes, which are overlapped in the frequency domain, can lead to substantial correction of the Landau damping coefficients for the modes.

Fiber-optic photoacoustic gas sensor with temperature self-compensation

Ke Chen, Beilei Yang, Min Guo, Hong Deng, Bo Zhang, Shuai Liu, Chenyang Li, Ran An, Wei Peng, and Qingxu Yu

DOI: 10.1364/OL.390898 Received 19 Feb 2020; Accepted 27 Mar 2020; Posted 27 Mar 2020  View: PDF

Abstract: A high-precision fiber-optic photoacoustic (PA) gas sensor with temperature self-compensation has been presented. The target gas diffused into the micro-chamber absorbs the laser energy to generate a PA signal, which is detected by a Fabry-Perot (F-P) interferometric cantilever. The temperature affects not only the acoustic sensitivity of the cantilever but also the PA conversion efficiency. The test result of the PA frequency response demonstrates that there is a temperature insensitive operating frequency of 1207.4 Hz in the range of 0-80 °C. The temperature self-compensation measurement is realized by setting the laser modulation frequency to 603.7 Hz and using the second-harmonic detection technique.

Phase stabilization of a coherent fibre network by single-photon counting

Salih Yanikgonul, Ruixiang Guo, ANGELOS XOMALIS, Anton Vetlugin, Giorgio Adamo, Cesare Soci, and Nikolay Zheludev

DOI: 10.1364/OL.381388 Received 24 Oct 2019; Accepted 26 Mar 2020; Posted 26 Mar 2020  View: PDF

Abstract: Coherent optical fibre networks are extremely sensitive to thermal, mechanical and acoustic noise, which requires elaborate schemes of phase stabilization with dedicated auxiliary lasers, multiplexers and photodetectors. This is particularly demanding in quantum networks operating at the single-photon level. Here we propose a simple method of phase stabilization based on single-photon counting and apply it to quantum fibre networks implementing single-photon interference on a lossless beamsplitter and coherent perfect absorption on a metamaterial absorber. As a proof of principle, we show dissipative single-photon switching with visibility close to 80%. This method can be employed in quantum networks of greater complexity without classical stabilization rigs, potentially increasing efficiency of the quantum channels.

Experimental optical trapping with Frozen Waves

RAFAEL BONILLA, Leonardo Ambrosio, Antonio Neves, Michel Zamboni-Rached, and Marcos Gesualdi

DOI: 10.1364/OL.390909 Received 18 Feb 2020; Accepted 25 Mar 2020; Posted 26 Mar 2020  View: PDF

Abstract: We report, to the best of our knowledge, the first optical trapping experimental demonstration of micro-particles with Frozen Waves. Frozen Waves are an efficient method to model longitudinally the intensity of non-diffracting beams obtained by superposing co-propagating Bessel beams with the same frequency and order. Based on this, we investigate the optical force distribution acting on micro-particles of two types of Frozen Waves. The experimental setup of a holographic optical tweezers using spatial light modulators has been assembled and optimized. The results show that it is possible to obtain greater stability for optical trapping using Frozen waves. The significant enhancement in trapping geometry from this approach shows promising applications for optical tweezers, micro-manipulations over a broad range.

Demonstration of carpet cloaking by anisotropic zero refractive index medium

Emre Bor, Utku Yasa, Hamza Kurt, and Mirbek Turduev

DOI: 10.1364/OL.388432 Received 17 Jan 2020; Accepted 25 Mar 2020; Posted 27 Mar 2020  View: PDF

Abstract: In this Letter, we numerically and experimentally demonstrated the carpet cloaking effect by a rectangular lattice two-dimensional photonic crystal (PC) exhibiting semi-Dirac cone (SDC) dispersion phenomenon. The proposed SDC PC of anisotropic zero refractive index medium operates as an optical carpet cloak for perfect electric conductor surface bump. The experimental verification of the cloak is realized at microwave frequencies around 12.10 GHz via dielectric rods. A good agreement between experimental measurements and numerical calculations are observed. Finally, features such as rendering larger objects invisible are possible with the proposed idea.

Laser-induced damage thresholds and mechanism of silica glass induced by ultra-short soft X-ray laser pulse irradiation

Katsuhiro Mikami, Masahiko Ishino, Thanh-Hung Dinh, Shinji Motokoshi, Noboru Hasegawa, Akira Kon, Yuichi Inubushi, Shigeki Owada, Hiroo Kinoshita, and Masaharu Nishikino

DOI: 10.1364/OL.389288 Received 30 Jan 2020; Accepted 25 Mar 2020; Posted 27 Mar 2020  View: PDF

Abstract: Laser-induced damage thresholds (LIDTs) of silica glasses obtained by the femtosecond soft X-ray free-electron laser (SXFEL, 13.5 nm, 70 fs) and the picosecond soft X-ray laser (SXRL, 13.9 nm, 7 ps) are evaluated. The volume of the hydroxyl group in the silica glasses influenced its LIDTs. LIDTs obtained in this research by the femtosecond SXFEL and the picosecond SXRL were nearly identical but were different from that by the nanosecond soft X-ray pulse. The photoionization processes of silica glass in the laser-induced damage mechanism (LIDM) are also discussed. In the ultra-short soft X-ray pulse irradiation regime, the LIDM can be speculated to include the spallation process with a scission of bondings.

Three-dimensional interferometric scattering microscopy via remote focusing technique

il-buem lee, hyeon-min moon, Jin-Sung Park, katerina zambochova, Seok-Cheol Hong, and Minhaeng Cho

DOI: 10.1364/OL.386172 Received 19 Dec 2019; Accepted 24 Mar 2020; Posted 25 Mar 2020  View: PDF

Abstract: The interferometric scattering (iSCAT) microscopy enables us to track nm-sized objects with high spatial and temporal resolutions and permits label-free imaging of biomolecules. Its superb sensitivity, however, comes at a cost by several downsides such as slow three-dimensional (3-D) imaging and limited vertical tracking. Here, we propose a new method, Remote Focusing-iSCAT (RF-iSCAT) microscopy, to visualize a volume specimen by imaging sections at different depths without a translation of either objective lens or sample stage. We demonstrate the principle of RF-iSCAT by determining the z-position of submicron beads by translating the reference mirror instead. The images thereby are free from artifactual fringe variation. RF-iSCAT features an unprecedentedly long range of vertical tracking and permits fast but vibration-free vertical scanning. We anticipate that RF-iSCAT would enhance the utility of iSCAT for dynamics study.

Single Mode Bragg Ring Laser Diodes

Bilal Janjua, Meng Lon Iu, Paul Charles, Eric Chen, and Amr Helmy

DOI: 10.1364/OL.387448 Received 13 Jan 2020; Accepted 24 Mar 2020; Posted 24 Mar 2020  View: PDF

Abstract: We have designed and fabricated a monolithic semiconductor ring laser based on a Bragg waveguide structure. Through careful control of the waveguiding we have overcome the inherent 'leaky' nature of this waveguide mode and demonstrated a ring laser lasing in the Bragg mode. Best behavior was obtained from lasers with a diameter of 400 μm, where they exhibited output power >1 mW, in continuous wave (CW) operation. A tangent waveguide provided access to the ring cavity using two ports through evanescent coupling. To meet the stringent waveguiding requirements imposed by the Bragg structure, a 2-step etching process, consisting of a shallow etched coupler and a deep-etched bend section of the ring was developed in order to reduce the bend and scattering losses. The laser showed a threshold current density of ~2.2 kA/cm2 in CW operation with single longitudinal mode operation with a signal to noise ratio of 30 dBm obtained at 1.5 Ith. Broadband phase-matching of χ(2) nonlinearity is observed, offering self-pumped parametric C-band conversion > 40 nm with efficiency of 142% W-1cm-2.

Phase-modulated axilenses for infrared multi-bandspectroscopy

Yuyao Chen, Wesley Britton, and Luca Dal Negro

DOI: 10.1364/OL.388704 Received 24 Jan 2020; Accepted 23 Mar 2020; Posted 24 Mar 2020  View: PDF

Abstract: We design and characterize compact phase-modulatedaxilens devices that combine efficient point focusingand grating selectivity within 4-level phase mask configurations.Specifically, we select and characterizein detail two device configurations designed for long wavelengthinfrared (LWIR) operation in the 6μm-12μmwavelength range. These devices are ideally suitedfor monolithic integration atop the substrate layersof infrared focal plane arrays (IR-FPAs) for use inmulti-band LWIR photo-detection. We systematicallystudy their focusing efficiency, spectral response, andcross talk ratio, and we demonstrate a single componentmicrospectrometer. Our design method leveragesRayleigh-Sommerfeld (RS) diffraction theory that isvalidated numerically using the Finite Element Method(FEM). The proposed devices are broadband and polarizationinsensitive and add fundamental spectroscopiccapabilities to miniaturized optical components for anumber of applications in LWIR detection and spectroscopy.

Dual Polarization Dirac Cones in a Simple 2D Square Lattice Photonic Crystal

Jesse Rodriguez, Benjamin Wang, and Mark Cappelli

DOI: 10.1364/OL.389163 Received 27 Jan 2020; Accepted 23 Mar 2020; Posted 24 Mar 2020  View: PDF

Abstract: We report on dual-polarization Dirac cones in a simple square lattice two-dimensional photonic crystal (PC) based on transmission at accidental degeneracies centered at the k = 0 symmetry (Γ) point. Finite difference time domain simulations are used to identify the material and geometric parameters for Dirac-like dispersion. A configuration that produces a Dirac-like point for both transverse electric and transverse magnetic polarizations at the same frequency is presented. The PC dispersion shows the expected three-fold degenerate linear branch crossings at the Dirac-like point. Full-field electromagnetic wave simulations exhibit some common behaviors of devices based on Dirac-like dispersion, such as cloaking and waveguiding. The configuration works for a considerable range of the parameter space, and thus is experimentally realizable with a wide range of materials.

Full path single-shot imaging of femtosecond pulse collapse in air turbulence

Ilia Larkin, Jesse Griff-McMahon, aaron SCHWEINSBERG, Andrew Goffin, Anthony Valenzuela, and Howard Milchberg

DOI: 10.1364/OL.389495 Received 30 Jan 2020; Accepted 23 Mar 2020; Posted 24 Mar 2020  View: PDF

Abstract: In a single shot, we measure the full propagation path, including the evolution to pulse collapse, of a high power femtosecond laser pulse propagating in air. This technique enables single-shot examination of the effect of parameters that fluctuate on a shot-to-shot basis, such as pulse energy, pulse duration, and air turbulence-induced refractive index perturbations. We find that even in lab air over relatively short propagation distances, turbulence plays a significant role in determining the location of pulse collapse.

Graphene hybrid waveguide stimulation using photoconductive terahertz generator

Bahareh Hosseini Fakhar, Mohammad Ghazialsharif, and Mohammad Sadegh Abrishamian

DOI: 10.1364/OL.391352 Received 24 Feb 2020; Accepted 23 Mar 2020; Posted 24 Mar 2020  View: PDF

Abstract: Typically, terahertz (THz) surface plasmon polaritons (SPP) excitation involves phase-matching engineering and THz plane-wave generation. This requires antennas, lenses, and other optical phase-matching devices. Herein, we demonstrate a novel method to excite THz SPPs in graphene directly by using an 800 nm optical pump and a photoconductive source. We miniaturize the SPP excitation setup by eliminating the plane-wave generator and the need for mode matching between the plane-wave and the waveguide, thereby improving the power efficiency of THz SPP excitation; an average SPP power of 0.6 mW is obtained for an optical pump power of 25 mW.

Nonlinear refraction in CH₃NH₃PbBr₃ single crystals

Christian Kriso, Markus Stein, Tobias Haeger, Neda Pourdavoud, Marina Gerhard, Arash Rahimi-Iman, Thomas Riedl, and Martin Koch

DOI: 10.1364/OL.383917 Received 20 Nov 2019; Accepted 23 Mar 2020; Posted 24 Mar 2020  View: PDF

Abstract: We measure both nonlinear absorption and nonlinear refraction in a CH₃NH₃PbBr₃ single crystal using the Z-scan technique with femtosecond laser pulses. At 1000 nm, we obtain values of 5.2 cm/GW and 9.5∙10-14 cm²/W for nonlinear absorption and nonlinear refraction, respectively. Sign and magnitude of the observed refractive nonlinearity are reproduced well by the two-band model. Our results suggest that the large nonlinear refractive index measured in perovskite nanostructures cannot be explained by an intrinsically high nonlinearity in this emerging material class but must be caused by carrier and confinement effects.

High-power cladding pumped Raman fiber amplifier with record beam quality

Yizhu Chen, Tianfu Yao, Hu Xiao, Jinyong Leng, and Pu Zhou

DOI: 10.1364/OL.388297 Received 17 Jan 2020; Accepted 23 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: In this paper, a high power all-fiberized Raman amplifier based on cladding-pumping scheme with brightness enhancement is presented. The multi-cladding passive fiber is employed as wavelength shifting and the only gain media in the laser system, and the maximum output power is 762.6 W emitting at 1130 nm. Through cladding-pumping process, the beam quality parameter M² improves from 6.12 of seed laser to 2.24 at maximum amplified power, with 1.9 at best. To the best of our knowledge, this is the highest power and highest beam quality at this power level in the fields of cladding-pumped Raman amplifiers, which is completely irrelevant with rare-earth-doped gain scheme and obtains brightness enhancement at the same time.

Deep neural network inversion for 3D laser absorption imaging of methane in reacting flows

Chuyu Wei, Kevin Schwarm, Daniel Pineda, and Mitchell Spearrin

DOI: 10.1364/OL.391834 Received 02 Mar 2020; Accepted 23 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: Mid-infrared laser absorption imaging of methane in flames is performed with a learning-based approach to the limited view-angle inversion problem. A deep neural network is trained with a spectral absorbance model and Radon transform based on superimposed Gaussian field distributions. Prediction capability of the neural network is compared to linear tomography methods at varying number of view angles for simulated fields representative of a flame pair. Experimental 3D imaging is demonstrated on a methane-oxygen laminar flame doublet (Less than 1 cm) backlit with tunable radiation from an interband cascade laser near 3.16 μm. Spectrally-resolved data at each pixel provides for species-specific projected absorbance. 2D images were collected at 6 different projection angles on a high-speed infrared camera, yielding an aggregate of 27,648 unique lines of sight capturing the scene with a pixel resolution of approximately 70 μm. Mole fraction measurements are inferred from the predicted absorption coefficient images using an estimated temperature field, showing consistency with expected values from reactant flow rates. To the authors' knowledge, this work represents the first 3D imaging of methane in a reacting flow.

Electro- and photon-induced cooling in BNT-BT-SBET relaxors with in situ optical temperature sensing

Lejian Wang, Jingji Zhang, Jiangying Wang, Yaxuan Yao, Lingling Ren, Xue Chen, Martin Birkett, Laurent Dala, and Ben Xu

DOI: 10.1364/OL.391422 Received 27 Feb 2020; Accepted 22 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: A novel lead-free luminescent ferroelectric ceramic, Bi0.5Na0.5TiO3-0.06BaTiO3-0.055Sr0.7Bi0.18Er0.02□0.1TiO3, is developed with an adiabatic temperature change of 0.7 K under an electric-field of 60 kV/cm at room temperature, an anti-stokes fluorescence cooling and a maximum optical temperature sensitivity of 0.0055 K-1 at 522 K. Interestingly, the electrocaloric response reaches a saturation at permittivity-shoulder temperature of 100 oC, meanwhile the maximized emission intensity of 2H11/2→4I15/2 occurs. Temperature- and poling-field-tunable enhancement of 2H11/2→4I15/2 emission intensity is due to the population inversion from the 4S3/2 to 2H11/2 states caused by an incoherent regime consisting of ferroelectric phase and polar nanoregions in a relaxor matrix.

Ultra-compact optical switch using single semi-symmetric Fano nanobeam cavity

Ziwei Cheng, Jianji Dong, and Xinliang Zhang

DOI: 10.1364/OL.383250 Received 15 Nov 2019; Accepted 22 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: We propose and theoretically analyze a generalized 4-port waveguide-cavity system with a novel semi-symmetric Fano structure. By applying the system to a silicon based photonic crystal nanobeam cavity (PCNC), we experimentally demonstrate an ultra-compact crossbar optical switch with high drop port transmission. A low insertion loss of 1.5 dB in drop port at the cross state is achieved comparing to the 8.5 dB insertion loss in a traditional non-Fano structure. The device is also benefited by the high quality factor and small mode volume for efficient switching and the ultra-compact footprint of 14 μm² in the core structure. The proposed PCNC switch shows great potential in various optical systems on chip to increase the integration and reduce energy consumption.

Fully controllable three-dimensional perpendicular-magnetic recording using a single objective lens

Zhang Xiaoqiang, Guanghao Rui, Yong Xu, Fan Zhang, Yinchang Du, xiaoyang lin, Anting Wang, and weisheng zhao

DOI: 10.1364/OL.389376 Received 28 Jan 2020; Accepted 22 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: With features of fast and energy-efficient data writing, all-optical helicity-dependent switching (AO-HDS) has emerged as a competitive technology to satisfy the demand for the next-generation volume data storage. Unfortunately, to recording the different bits of information in different positions of the magnetic recording film, the laser beam, the objective lens, or the magnetic recording film should be moved, limiting the advantage of AO-HDS in the fast magnetic recording. To achieve on-the-fly magnetic recording, the induced magnetization should be fully controllable. In this paper, by focusing an azimuthally polarized vortex beam (APVB) with different orbital angular momentum (OAM) and introducing an additional phase, a feasible strategy constructing diffraction-limited light-induced pure longitudinal multi-magnetization spots capable of dynamically controlling the position of each spot using a single objective lens is proposed. The distributions of the focused APVBs with different OAM and the induced magnetizations are surveyed as well. We believe that this is a practical and flexible three-dimensional magnetic recording technique with dynamically controllable the recording position.

High-performance and Ultra-flexible PEDOT/Silver Nanowires/Graphene Films for Electrochromic Applications

Hongchao Peng, Hao Jiang, Shijian Tu, Sihang Zhang, Erhui Ren, Bin Yan, Qin Yang, and Sheng Chen

DOI: 10.1364/OL.388769 Received 21 Jan 2020; Accepted 22 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: We fabricate a kind of flexible electrochromic (EC) film by spraying the mixed dispersion of silver nano-wires (AgNWs) and poly (3, 4-ethylenedioxythiophene) (PEDOT) on the graphene (GR) electrode. AgNWs are embedded in the PEDOT nanoparticles, forming an interlaced conductive network and double electron transport channels, therefore the disadvantages of poor conductive graphene electrode have been remedied effectively. The subsequent graphene-based PEDOT /AgNWs composite films have revealed remarkable optical contrast (63%), high coloration efficiency (182.8 cm-2 C-1) and good cycle stability (keep the optical contrast about 60% after switching cycles of 16000 s). In addition, the graphene-based composite films can keep good EC performances after 2000 cycles of bending tests, while those of the ITO/PET-based composite films decrease dramatically. The ultra-flexible graphene-based PEDOT/AgNWs composite films with excellent electrochromic properties present an opportunity for fabricating large-area flexible EC devices. © 2019 Optical Society of America

Investigation on the reflective MoOx/Al p-contact layer for AlGaN-based DUV-LEDs

Liang Li, Mei Cui, Hua Shao, Yijun Dai, Li Chen, Zi Hui Zhang, Jason Hoo, Shiping Guo, Wen'an Lan, Lili Cao, Hui Xu, Wei Guo, and Jichun Ye

DOI: 10.1364/OL.387275 Received 06 Jan 2020; Accepted 22 Mar 2020; Posted 24 Mar 2020  View: PDF

Abstract: We report a high reflective Ohmic contact for flip-chip AlGaN deep ultraviolet light emitting diode (DUV-LED) using a MoOx/Al structure. The influences of MoOx thickness and annealing condition on the electrical and optical behaviors of the MoOx/Al structure were investigated. Surface morphology of MoOx with different thicknesses reveals a 3D growth mode. Interface stoichiometry of p-GaN/MoOx/Al was analysed to understand the underlying reason for the Ohmic contact formation between MoOx and Al. Energy band diagram at the interface was further proposed to demonstrate the efficient hole injection through MoOx/Al electrode and facilitate the development of next-generation DUV emitters.

Highly-elliptical-core fiber with stress-induced birefringence for mode multiplexing

Alessandro Corsi, Jun Ho Chang, Ruohui Wang, Lixian Wang, Leslie Rusch, and Sophie LaRochelle

DOI: 10.1364/OL.387751 Received 13 Jan 2020; Accepted 22 Mar 2020; Posted 24 Mar 2020  View: PDF

Abstract: We report the polarization–maintaining properties of a highly–elliptical–core fiber surrounded by a trench that was designed to optimize the modal effective indices and bending loss for a total of five spatial modes with twofold polarization degeneracy (ten channels). In addition to the asymmetric core structure, the birefringence of the fiber is increased by the thermal stress introduced during the fabrication. We examine this effect and compare the calculated modal effective index differences to experimentally measured values. The results show a modal birefringence larger than 10-4 for all guided spatial modes. The fiber has a propagation loss, averaged over all mode groups, of 0.45 dB/km. The mode stability to bending is evaluated by selectively exciting/detecting each spatial mode while perturbing the fiber. This few–mode polarization–maintaining fiber is of interest for MIMO–free mode division multiplexing transmission systems.

Hilbert transform based crosstalk compensation for color fringe projection profilometry

Yuwei Wang, Lu Liu, Jun Wu, xiangcheng chen, and Yajun Wang

DOI: 10.1364/OL.392061 Received 05 Mar 2020; Accepted 21 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: This letter presents an effective crosstalk compensation method for color fringe projection profilometry. The crosstalk induced phase error has been studied, and we find that the phase error doubles the frequency of the color fringe. Therefore, Hilbert transform is applied on the color fringe to shift the carried phase by π/2. Two phase maps are recovered from the normalized and the transformed patterns, and their average phase is used for three-dimensional (3D) reconstruction. Simulations and experiments confirm that the proposed method can effectively suppress the crosstalk induced phase error to improve the measurement accuracy.

Entanglement swapping with autonomous polarization-entangled photon pairs from warm atomic ensemble

Jiho Park, Han Seb Moon, and Heonoh Kim

DOI: 10.1364/OL.388613 Received 17 Jan 2020; Accepted 20 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: Entanglement swapping forms a key concept in the realization of scalable quantum networks and large-scale quantum communication. For the practical implementation of entanglement swapping, completely autonomous entanglement sources and a joint Bell-state measurement (BSM) between two independent photons are essential. Here, we experimentally demonstrate entanglement swapping between two independent polarization-entangled photon-pair sources obtained via spontaneous four-wave mixing (SFWM) in a Doppler-broadened atomic ensemble of 87Rb atoms. From the joint BSM, we estimate the S parameter in the Clauser–Horne–Shimony–Holt (CHSH) form of Bell’s inequality and confirm the violation of the Bell–CHSH inequality by S = 2.32 ± 0.07 with 4.5 standard deviations. We believe that this work is an important step towards realizing practical scalable quantum networks.

High-order rogue waves excited from multi-Gaussian perturbations on continuous wave

Peng Gao, Lichen Zhao, Zhan-Ying Yang, Xiaohui Li, and Wen-Li Yang

DOI: 10.1364/OL.389012 Received 04 Feb 2020; Accepted 20 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: Peregrine rogue wave excitation has applications on gaining high-intensity pulse etc, and high-order rogue wave exhibits higher intensity. Exact solution and collision between breathers are two existing ways to excite high-order ones. Here we numerically report a new possible way for it, which is from multi-Gaussian perturbationson continuous wave. Theorder and maximal intensity of rogue waves can be adjusted by the number of perturbations. The maximal intensity approaches 63.8 times than the power of initial background wave, and it keeps a large value under the influence of fiber loss and noise. Our results provide the guidance for gaining high-intensity pulses in experiment and understanding the universality of rogue wave generation.

Hidden linear optical response reveals crystalline symmetry

Xiangdong Zhu and Hang Zhang

DOI: 10.1364/OL.390183 Received 10 Feb 2020; Accepted 20 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: We show that a linear quadrupole response from a crystalline material to externally applied optical fields exists and has gone unnoticed, even though its non-linear optical cousin has been studied and explored extensively for decades. Such a linear quadrupole response reveals the symmetry of the crystal, just as its nonlinear optical counterpart has done and can be used to investigate phase transitions non-intrusively with high spatial and temporal resolution.

Poisson distribution of extreme events in radiation of random distributed feedback fiber laser

Oleg Gorbunov, Srikanth Sugavanam, Ilya Vatnik, and Dmitry Churkin

DOI: 10.1364/OL.390492 Received 13 Feb 2020; Accepted 20 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: In the present paper we experimentally investigate extreme events in the time dynamics of the random distributed feedback fiber laser. We found that the probability of extreme events depends on the wavelength of the generated light. On spectrum tails we registered extreme events with intensity up to 50 times higher than the average generation power. Analysis of return times between successive rogue waves reveals their exponential distribution. Further investigation proves that appearance of extreme waves in laser radiation obeys Poisson law. Characteristic radiation time varies from nanoseconds to tens of microseconds for most intense waves.

On-chip biochemical sensor using wide Gaussian beams in silicon waveguide-integrated plasmonic crystal

Xu Han, Guanghui Ren, Thach Nguyen, Huifu Xiao, Yonghui Tian, and Arnan Mitchell

DOI: 10.1364/OL.391067 Received 20 Feb 2020; Accepted 20 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: On-chip biochemical sensor based on two-dimensional waveguide-integrated plasmonic crystal formed by nanogap tiles (NGT) array is realized. By using on-chip optical lenses, an ultra-wide collimated Gaussian beam is launched, coupled with surface plasmonic crystals and collected with relatively low additional insertion loss, allowing large sensing area. The optical field enhancement and stop-band shift of the NGTs device for biochemical sensing applications are numerically analyzed and experimentally demonstrated with sensitivity reaching up to ~260 nm/RIU. Our sensor is demonstrated with monolayer thiol molecules illustrating that it can be functionalized with this class of molecule which is commonly used with bulk surface plasmon resonance sensors.

Enhanced laser speckle optical sensor for in-situ strain measurement and structural health monitoring

YONG PANG, Bernard Chen, Siu Yu, and Siva Lingamanaik

DOI: 10.1364/OL.391582 Received 03 Mar 2020; Accepted 20 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: Laser speckling, as a light phenomenon, records unique surface information can be employed as an advanced speckle fabrication technique for strain measurement. In this letter, an enhanced laser speckle optical sensor (LSOS) for non-destructive, non-contact and high-accuracy strain measurement has been developed. Sub-systems of laser beam shaping and telecentric imaging were incorporated into the LSOS to achieve optimized speckle patterns and a field of view (FOV) separation was introduced to extend sensor gauge length. Validation tests confirmed that the LSOS achieved consistent results with resistive strain gauges. Sensing robustness and practicality were demonstrated in field tests. The results showed that the LSOS is capable of achieving accurate strain measurements in an external environment with maximum root mean squared (RMS) error of 13.34με.

Graphene electrodes for electric poling of electro- optic polymer films

Wen Wang, JieYun Wu, Kaixin Chen, Quandong Huang, Jingdong Luo, and Kin Chiang

DOI: 10.1364/OL.390656 Received 17 Feb 2020; Accepted 20 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: We propose the use of graphene electrodes for electric poling of electro-optic (EO) polymer films. The use of graphene electrodes can waive the use of buffer layers and minimize the poling voltage. To demonstrate the idea, we prepared EO polymer thin-film waveguides for poling with traditional Au/ITO electrodes and graphene electrodes, where the EO polymer is a guest-host system formed by doping 15 wt% of dipolar polyene chromophore AJLZ53 into the random copolymer P(S-co-MMA). Our experiments confirm that the use of graphene electrodes can significantly reduce the poling voltage. For a 3.8-µm thick EO polymer film, we achieve high EO coefficients of 82 pm/V at 1541 nm and 110 pm/V at 1300 nm with a poling voltage of 420 V. In addition, the use of graphene electrodes allows more flexible waveguide designs and can potentially simplify the fabrication of devices based on EO polymer.

High-field mid-infrared pulses derived from frequency domain optical parametric amplification

adrien leblanc, gille dalla-barba, Philippe Lassonde, Antoine Laramée, Bruno Schmidt, Eric Cormier, Heide Ibrahim, and François Légaré

DOI: 10.1364/OL.389804 Received 05 Feb 2020; Accepted 20 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: We present a novel approach for scaling the peak power of mid-infrared laser pulses with few-cycle duration and carrier-to-envelope phase stabilization. Using frequency domain optical parametric amplification (FOPA), selective amplification is performed on two spectral slices of broadband pulses centered at 1.8µm wavelength. In addition to amplification, the Fourier plane is used for specific pulse shaping to control both the relative polarization and the phase/delay between the two spectral slices of the input pulses. At the output of the FOPA, intrapulse difference frequency generation provides carrier-envelope phase stabilized two-cycle pulses centered at 9.5µm wavelength with 25.5µJ pulse energy. The control of the carrier-envelope phase is demonstrated through the dependence of high harmonic generation in solids. This architecture is perfectly adapted to be scaled in the future to high average and high peak powers using picosecond Ytterbium laser technologies.

Dynamics of On-Chip Asymmetrically Coupled Semiconductor Lasers

Benjamin Lingnau, Alison Perrott, Mohamad Dernaika, ludovic Caro, Frank Peters, and Bryan Kelleher

DOI: 10.1364/OL.390401 Received 17 Feb 2020; Accepted 20 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: We investigate the dynamics of asymmetrically coupled semiconductor lasers on photonic integrated circuits in experiment and theory. The experimental observations are explained using a rate-equation model for coupled lasers incorporating a saturable coupling waveguide. We perform a bifurcation analysis of the coupled laser dynamics, focusing on the effects of the coupling phase and the dynamical difference between passive and saturable coupling waveguides. For a passive waveguide, we find a bifurcation scenario closely resembling the well-known optical injection setup, which is largely insensitive to the coupling phase. When the coupling waveguide is saturable, the dynamics become increasingly complex and unpredictable, with a strong phase-dependence. Our results show the possibility of a simple layout for reproducible laser dynamics on a chip.

Mode-Locked Chirped-pulse Generation from Optical Parametric Oscillators with an Aperiodic Quasi-Phase-Matching Crystal

Pei Liu, Jiaxing Heng, and Zhaowei Zhang

DOI: 10.1364/OL.391175 Received 20 Feb 2020; Accepted 20 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: We describe stable mode-locking and chirped-pulse generation from a laser oscillator using an aperiodic quasi-phase-matching crystal as the gain medium. We examine the mode-locking dynamics, and reveal that the excitation of chirped-pulse oscillation is assisted by the position-dependent conversion-wavelength and the pump-to-signal temporal walk-off along the gain medium. Stable mode-locking is confirmed by numerical simulations and experimental demonstrations. In a preliminary experiment, we obtained a signal wave covering 1293–1734 nm and an idler wave covering 2.9-5.0 μm from an oscillator based on a 2 mm-long aperiodically-poled LiNbO3 crystal. These results represent a new regime of laser mode-locking, which is promising for generating light pulses with very wide instantaneous-bandwidth and at arbitrary wavelengths.

Long-wave infrared ZnGeP₂ optical parametric oscillator with exceeding 3 W at around 10 μm

gaoyou liu, Yi Chen, baoquan yao, Ruixue Wang, ke Yang, Chao Yang, Shuyi Mi, Tongyu Dai, and Xiaoming Duan

DOI: 10.1364/OL.389603 Received 03 Feb 2020; Accepted 20 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: Pumped by a 90 W Q-switched Ho:YAG laser at 2091 nm with pulse repetition frequency of 10 kHz, a long-wave infrared ZnGeP₂ (ZGP) optical parametric oscillator at 9.8 μm based on type-I phase matching ZGP crystal was demonstrated. The maximum average output power of 3.51 W at 9.8 μm was achieved, corresponding to slope efficiency of 4.81% and overall optical conversion efficiency of 3.9% from Ho to long-wave infrared. The beam quality factor M² was measured to be 2. with pulse width of 19.6 ns and linewidth (FWHM) of 142 nm at 3.51 W. Meanwhile, using a type-II ZGP, a wavelength tuning range of 9.2~11.1 μm was realized and the maximum average output powers of idler beam were 3.11 W, 2.97 W, 2.2 W and 1.27 W, corresponding to the peak wavelength of 9.6 μm, 9.8 μm, 10.1 μm and 10.6 μm with full incident pump power.

55 W kHz-linewidth core- and in-band-pumped linearly-polarized single-frequency fiber laser at 1950 nm

Changsheng Yang, Xianchao Guan, Quan Gu, weiwei wang, Tianyi Tan, Qilai Zhao, Wei Lin, Xiaoming Wei, Zhongmin Yang, and Shanhui Xu

DOI: 10.1364/OL.388826 Received 21 Jan 2020; Accepted 19 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: Based on core- and in-band-pumped polarization-maintaining Tm3+-doped single-cladding fiber (PM-TSF, the core diameter is 9 μm) by a 1610 nm fiber laser and a distributed Bragg reflector seed laser, a linearly-polarized single-frequency fiber laser (LP-SFFL) at 1950 nm with an output power of 55.3 W and a laser linewidth of 6.95 kHz is demonstrated. The output beam quality of Mx2 and My2 are measured to be 1.01 and 1.03, respectively. The slope efficiency with respect to the launched pump power is 71.0%, in comparison with a theoretical quantum efficiency of 82.6%. A polarization-extinction ratio of 19 dB and an optical signal-to-noise ratio of 58 dB are obtained from the 1950 nm LP-SFFL. To the best of our knowledge, this is the highest power of 2.0-μm SFFL output directly from a strict single-mode active fiber to date. Our experiment offers a promising solution to the current limitations of the high-performance fiber lasers at 2.0 μm, which is particularly essential for coherent detection.

High-Speed Integrated Micro-LEDs Array for Visible Light Communication

Hao-Yu Lan, I-Chen Tseng, Yun-Hsiang Lin, Gong-Ru Lin, Ding-Wei Huang, and Chao-Hsin Wu

DOI: 10.1364/OL.391566 Received 27 Feb 2020; Accepted 18 Mar 2020; Posted 19 Mar 2020  View: PDF

Abstract: In this work, we report high-speed integrated 14 μm-in-diameter micro-LED (μLED) arrays with the parallel configuration, including 2 × 2, 2 × 3, 2 × 4, and 2 × 5 arrays. The small junction area of μLED (~ 191 μm²) in each element facilitates the operation of higher injection current density up to 13 kA/cm², leading to the highest modulation bandwidth of 615 MHz. The optical power of 2 × 5 array monotonically increases (~ 10 times higher) as the numbers of array increases (1 to 10) while retaining the fast modulation bandwidth. A clear eye diagram up to 1 Gbps without any equalizer further shows the capability of this high-speed transmitter for VLC. These results mean that tailoring the optical power of μLEDs in a parallel-biased integrated array can further enhance the data transmission rate without degradation of modulation bandwidth.

Poynting singularities in the transverse flow-field of random vector waves

Matthijs van Gogh, Thomas Bauer, Lorenzo De Angelis, and Laurens Kuipers

DOI: 10.1364/OL.389301 Received 29 Jan 2020; Accepted 18 Mar 2020; Posted 19 Mar 2020  View: PDF

Abstract: In order to utilize the full potential of tailored flows of electromagnetic energy at the nanoscale, we need to understand its general behaviour given by its generic representation of interfering random waves. For applications in on-chip photonics as well as particle trapping, it is important to discern the topological features in the flow field between the commonly investigated cases of fully vectorial light fields and their 2D equivalents. We demonstrate the distinct difference between these cases in both the allowed topology of the flow-field and the spatial distribution of its singularities, given by their pair correlation function g(r). Specifically, we show that a random field confined to a 2D plane has a divergence-free flow-field and exhibits a liquid-like correlation, whereas its freely propagating counterpart has no clear correlation and features a transverse flow-field with the full range of possible 2D topologies around its singularities.

Multimode fiber based single-shot full-field measurement of optical pulses

Wen Xiong, Shai Gertler, Hasan Yilmaz, and Hui Cao

DOI: 10.1364/OL.388616 Received 24 Jan 2020; Accepted 17 Mar 2020; Posted 17 Mar 2020  View: PDF

Abstract: Multimode fibers are widely explored for optical communication, spectroscopy, imaging and sensing applications. Here we demonstrate a single-shot full-field temporal measurement technique based on a multimode fiber. The complex spatio-temporal speckle field is created by a reference pulse propagating through the fiber, and it interferes with a signal pulse. From the time-integrated interference pattern, both the amplitude and the phase of the signal are retrieved. The simplicity and high sensitivity of our scheme illustrate the potential of multimode fibers as versatile and multi-functional sensors.

Generation of resonant geometric modes from off-axis pumped degenerate cavities Nd:YVO4 lasers with external mode converters

Hsing-Chih Liang and Han-Yu Lin

DOI: 10.1364/OL.390278 Received 10 Feb 2020; Accepted 17 Mar 2020; Posted 18 Mar 2020  View: PDF

Abstract: We propose a theoretical model to generate the resonant geometric modes localized on ray periodic trajectories with orbital angular momentum. Base on the numerical analysis, we realize resonant geometric modes in the off-axis pumped degenerate cavities Nd:YVO4 lasers with external mode converter. Experimental results reveal that laser output modes display the planar geometric modes when the off-axis displacement is sufficiently large and the cavity length is set to satisfy the degenerate conditions. To generate the vortex beams, the planar geometric modes are transformed into circular geometric modes. Finally, the resonant geometric modes with large OAM can be generated from converting the circular geometric modes with axicon lens. Experimental results are performed to approve the theoretical analyses. We believe that our present method is scientific interesting and can be applied for further application.

Correlation Signatures For Coherent Three-photon Scattering in Waveguide QED

Zihao Chen, Yao Zhou, and Jung-Tsung Shen

DOI: 10.1364/OL.391756 Received 02 Mar 2020; Accepted 17 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: We study correlation signatures of a coherent three-photon scattering process by computationally investigating scattering photon wavefunctions, correlation functions, and nonlinear conditional phases. It is shown that both three-photon bound states (photonic trimers) and hybrid states, which consist of two-photon bound states (dimers) and one unbound photon, contribute to all correlation signatures. All computational correlation signatures are in good agreement with a recent experiment wherein a weak coherent photonic state propagates through a quantum nonlinear Rydberg medium (Liang et.~al., Science 359, 783 (2018)).

Observation of a comb of squeezed states with strong squeezing factor by a bichromatic local oscillator

Yaohui Zheng, Shaoping Shi, Yajun Wang, Long Tian, Jinrong Wang, and Xiaocong Sun

DOI: 10.1364/OL.385912 Received 16 Dec 2019; Accepted 17 Mar 2020; Posted 17 Mar 2020  View: PDF

Abstract: We demonstrate the experimental detection of an optical squeezing covering several higher resonances of the optical parametric amplifier by adopting a bichromatic local oscillator (BLO). The BLO is generated froma waveguide electro-optic phase modulator (WGM) and subsequent optical mode cleaner (OMC), without the need of additional power balance and phase control. The WGM is for the generation of the frequency-shiftedsideband beams with equal power and certain phase difference, the OMC is for filtering unwanted optical modes. The squeezing factors over a measurement frequency range from 9 kHz to 16.64 GHz are superior to 10 dB below the shot noise limit.

Hidden Phase-Retrieved Tomography

Daniele Ancora, Diego Di Battista, Asier Marcos Vidal, Stella Avtzi, Giannis Zacharakis, and Andrea Bassi

DOI: 10.1364/OL.385970 Received 13 Dec 2019; Accepted 16 Mar 2020; Posted 17 Mar 2020  View: PDF

Abstract: Tomography is a well established methodology able to provide structural and functional information on the measured object. At optical wavelengths, the unpredictable scattering of light is often considered a problem to overcome, rather than a feature to be exploited.Advances in disordered photonics, instead, have shed new light on possibilities offered by opaque materials, treating them as auto-correlation lenses able to create images and focus light.In this letter we propose tomography through disorder, introducing a modified Fourier-slice theorem, cornerstone of the computed tomography, aiming to reconstruct a three-dimensional fluorescent sample secreted behind an opaque curtain.

Controllable formation of laser-induced periodic surface structures on ZnO film by temporally shaped femtosecond laser scanning

Shaojun Wang, Lan Jiang, Weina Han, Wei Liu, Jie Hu, suocheng wang, and Yongfeng Lu

DOI: 10.1364/OL.388770 Received 22 Jan 2020; Accepted 16 Mar 2020; Posted 16 Mar 2020  View: PDF

Abstract: We achieved the controllable formation of laser-induced periodic surface structures (LIPSS) on ZnO films deposited on fused silica induced by modulated temporally shaped femtosecond laser pulses (800 nm, 50 fs, 1 kHz) through the laser scanning technique. Two-dimensional (2D) high spatial frequency LIPSS (HSFL) with period from 100 nm to 200 nm could be flexibly modulated based on the pre-processed nanostructures with appropriate fs laser irradiation conditions (fluence, scanning speed and pulse delay). The finite-difference time-domain (FDTD) method combined with the Drude model was employed to calculate the redistributions of electric fields, which suggested the origin of HSFL formation.

High power DUV picosecond pulse laser with a gain-switched-LD seeded MOPA and a large CLBO crystal

Kenta Kohno, Yosuke Orii, HISASHI SAWADA, DAISUKE OKUYAMA, KIMIHIKO SHIBUYA, Seiji Shimizu, George Okada, Junichi Nishimae, Masashi Yoshimura, and Yusuke Mori

DOI: 10.1364/OL.389017 Received 12 Feb 2020; Accepted 16 Mar 2020; Posted 16 Mar 2020  View: PDF

Abstract: We report 14-W average output power of 266-nm deep ultraviolet (DUV) picosecond pulse by the fourth harmonic generation from two consecutive frequency-doubling stages of 1064-nm pulse based on a gain switched laser diode seeded hybrid fiber/solid-state master oscillator and power amplifier (MOPA) system. Employing gain-switched operation of a narrow spectral linewidth distributed feedback laser diode (DFB-LD) and Yb-doped fiber and a two-stage Nd:YVO4 solid-state amplifier, we obtained 46.5 W average-power near Fourier-transform-limit 13 ps pulse at 200 kHz repetition rate. The narrow linewidth pulse characteristics enabled highly efficient frequency conversion, and the conversion efficiency from 532 nm to 266 nm was 56%, and 1064 nm to 266 nm was 31%. The beam quality factor of the generated DUV pulse was M2 < 1.2. The highly efficient FHG process resulted in appeasing thermal stress caused by nonlinear absorption in the crystal, and more than 5000 hours of continuous operation was achieved without any power down or beam profile degradation.

Study of UV repetition laser-induced absorption on fused silica surface using a surface thermal lensing technique

Kai Ke, Jian Chen, Gao Fan, Xiang Zhang, and Xiao Yuan

DOI: 10.1364/OL.391833 Received 02 Mar 2020; Accepted 16 Mar 2020; Posted 18 Mar 2020  View: PDF

Abstract: A surface thermal lensing (STL) technique is used to study the time evolution of the absorption of fused silica surface when exposed to UV pulsed laser irradiation in the nanosecond regime. The time evolution of the absorption is characterized as a saturated exponential increase and the disruptive change indicates that the macro-damage occurs. A time-dependent absorption defect model is used to fit the experimental data and a one-photon process is proposed to explain the results. This method can be used to estimate the operation lifetime of optical components.

Snell-like and Fresnel-like formulas of dual-phase-gradient metasurface

Bing Hu, Jianfeng Xie, Shengnan Tian, Hanming Guo, and Songlin Zhuang

DOI: 10.1364/OL.389067 Received 24 Jan 2020; Accepted 16 Mar 2020; Posted 17 Mar 2020  View: PDF

Abstract: By patterning metasurface with two phase gradients that are both space- and polarization-orthogonal, we derived the three dimensional (3D) Snell-like formula and Fresnel-like formula of the proposed metasurface. Theoretically, the dual-phase-gradient metasurface resembles biaxial-like birefringence, i.e. decomposing any homogeneously polarized incident beam into two anomalously refracted beams whose polarizations vary as the incident beam. According to the Fresnel-like formula, the relative intensity between the two anomalously refracted beams not only depend on the incidence angle and polarization ellipticity of incident beam similar as biaxial crystals, but also depend on the polarization ellipticity orientation even for a given incident polarization, which is an unique property absent in biaxial crystals. All the theoretical analyses were numerically demonstrated. The 3D Snell-like and Fresnel-like formulas will make the design of functional devices based on dual-phase-gradient metasurface much easier.

Enhancing THz molecular fingerprint detection by a dielectric metagrating

Jinfeng Zhu, Shan Jiang, Yinong Xie, Fajun Li, Liang-Hui Du, Kun Meng, Li-Guo Zhu, and Jun Zhou

DOI: 10.1364/OL.389045 Received 24 Jan 2020; Accepted 16 Mar 2020; Posted 19 Mar 2020  View: PDF

Abstract: THz sensing of molecular fingerprint enables wide applications on biomedicine and security detection. Conventional detection approaches face big barriers on trace analysis of analyte due to the difficulties of enhancing the broadband molecular absorption. In order to achieve strong broadband wave-matter interaction for the analyte, we propose a method based on THz wave angular scanning on a dielectric metagrating. In virtue of the guided-mode resonance, one can strengthen the local electric field in various trace-amount analytes by tuning the polarization and incident angle, which leads to significant enhancement on the broadband signal of molecular fingerprint. The overall enhancement factors can be higher than 13.9 dB in a broad THz band. The study paves the way for more applications on THz trace-amount detection.

Urbach-edge-assisted electro-absorption for enhanced free-space optical modulation

Ilija Hristovski, Nikolai Lesack, Luke Herman, and Jonathan Holzman

DOI: 10.1364/OL.388915 Received 22 Jan 2020; Accepted 16 Mar 2020; Posted 20 Mar 2020  View: PDF

Abstract: In this work, we introduce an electro-absorption- (EA-) based retro-modulator for the realization of effective passive downlinks in free-space optical communication systems. The fundamental demands for deep modulation and broad directionality in such systems are met by the retro-modulator's corner-cube assembly of EA-modulators. The EA-modulators incorporate semi-insulating InP as its band-edge absorption exhibits an Urbach tail near the 980-nm wavelength of the laser light. This enables an Urbach-edge-assisted EA process, which allows the field-induced absorption to be maximized via temperature. Theoretical results, from a uniting of the Einstein model and Franz-Keldysh effect, and experimental results, from a prototype, show good agreement with deep modulation depths, beyond 15%. Such functionality can meet the core demands of passive downlinks in emerging free-space optical communication systems.

Mode-selective few-mode Brillouin fiber lasers based on intra- and inter-modal SBS

Ning Wang, Juan Carlos Alvarado Zacarias, Md Selim Habib, He Wen, Jose Antonio-Lopez, Pierre Sillard, Adrian Amezcua, Axel Schülzgen, Rodrigo Amezcua Correa, and Guifang Li

DOI: 10.1364/OL.385444 Received 17 Dec 2019; Accepted 15 Mar 2020; Posted 18 Mar 2020  View: PDF

Abstract: Mode-selective fiber lasers have advantages in a number of applications. Here we propose and experimentally demonstrate a transverse mode-selective few-mode Brillouin fiber laser using the mode-selective photonic lantern. We generated the lowest three orders LP modes based on both intra- and inter-modal SBS. Their slope efficiencies, optical spectra, mode profiles, and linewidths were measured.

Temporally interleaved optical time-stretch imaging

Yueyun Weng, Gai Wu, Liye Mei, Qijun Wang, Keisuke Goda, Sheng Liu, and Cheng Lei

DOI: 10.1364/OL.381006 Received 22 Oct 2019; Accepted 15 Mar 2020; Posted 16 Mar 2020  View: PDF

Abstract: Optical time-stretch imaging has shown potential in diverse fields for its capability of acquiring images at high speed and high resolution. However, its wide application is hindered by the stringent requirement on the instrumentation hardware caused by the high-speed serial data stream. Here we demonstrate temporally interleaved optical time-stretch imaging that lowers the requirement without sacrificing the frame rate or spatial resolution by interleaving the high-speed data stream into multiple channels in the time domain. Its performance is validated with both a USAF-1951 resolution chart and a single-crystal diamond film. We achieve a 101-Mfps 1D scanning rate and 3-μm spatial resolution with only 2.5-GS/s sampling rate by using a two-channel-interleaved system.

Amplitude-phase Optimized Long Depth of Focus Femtosecond Axilens Beam for Single-exposure Fabrication of High-aspect-ratio Microstructures

Deng Pan, Bing Xu, Shunli Liu, Jiawen Li, Yanlei Hu, Dong Wu, and Jiaru Chu

DOI: 10.1364/OL.389946 Received 06 Feb 2020; Accepted 15 Mar 2020; Posted 16 Mar 2020  View: PDF

Abstract: Fabrication of High-aspect-ratio (HAR) micro/nano-structures by two-photon polymerization (TPP) has become a hot topic because of the advantages of ultra-high resolution and true 3D printing ability. However, the low efficiency caused by point-by-point scanning strategy limits its application. In this letter, we propose a strategy for the rapid fabrication of HAR microstructures by combining TPP with amplitude-phase optimized long depth of focus laser beam (LDFB). The optimization of LDFB is implemented by modulating the amplitude and phase on a phase-only spatial light modulator (SLM), which can suppress the side lobe and smooth energy oscillations effectively. The LDFB is used for one-step exposure (10 ms) of HAR micropillars and various microstructures, which greatly increase the fabrication efficiency. As a demonstration, several typical HAR microstructures such as assembies, microchannels, microtubes and cell scaffolds are prepared. Moreover, the micro-capture arrays are rapidly fabricated for the capture of microspheres and the formation of microlens arrays which show focusing and imaging ability.

Multiwavelength three-dimensional microscopy with white light based on computational coherent superposition

Tatsuki Tahara, Tomoyoshi Ito, Yasuyuki Ichihashi, and Ryutaro Oi

DOI: 10.1364/OL.386264 Received 19 Dec 2019; Accepted 14 Mar 2020; Posted 16 Mar 2020  View: PDF

Abstract: We propose multiwavelength three-dimensional microscopy that exploits the holographic multiplexing of spatially incoherent light at multiple wavelengths. The proposed microscopy employs interference of spatially and temporally incoherent light to record three-dimensional space and multiple wavelengths simultaneously as wavelength-multiplexed holograms. Extraction of wavelength information from the holograms is realized by the computational coherent superposition (CCS) scheme. We constructed a fully mechanical-motion-free holographic multiwavelength three-dimensional microscopy system with spatially and temporally incoherent light and conducted experiments with white light for the experimental demonstration of the technique.

High-Speed All-Optical Thresholding via Carrier Lifetime Tunability

Aashu Jha, Chaoran Huang, Thomas Ferreira de Lima, and Paul Prucnal

DOI: 10.1364/OL.387497 Received 06 Jan 2020; Accepted 14 Mar 2020; Posted 16 Mar 2020  View: PDF

Abstract: We theoretically study the effect of free-carrier lifetime on processing speed and strength of nonlinearity, pertaining to our all-optical thresholder. We find that optimal device performance necessitates tuning lifetime while optimizing for both speed and nonlinearity. We also experimentally demonstrate device processing speed improvement from 400 Mbps to 2.5 Gbps by incorporating PN-junction mediated free-carrier lifetime tuning mechanism. Our study on the significance of free-carrier lifetime is universally applicable to any optical signal processing system reliant on silicon photonic nonlinearities.

Bragg solitons in topological Floquet insulators

Sergey Ivanov, Yaroslav Kartashov, Lukas Maczewsky, Alexander Szameit, and Vladimir Konotop

DOI: 10.1364/OL.390694 Received 17 Feb 2020; Accepted 13 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: We consider a topological Floquet insulator consisting of two honeycomb arrays of identical helical waveguides. The arrays differ by the directions of rotations of the waveguides. The interface between two such arrays supports two distinct topological edge states, which can be resonantly coupled by additional weak longitudinal refractive index modulation with a period larger than the helix period. In the presence of Kerr nonlinearity, such coupled edge states allow to construct a new type of topological Bragg soliton. Theory and examples of such solitons are presented.

All-Digital Stokes Polarimetry with a Digital Micro-mirror Device

Angela Dudley, Isaac Nape, Najmeh TabeBordbar, Carmelo Rosales, Shanti Bhattacharya, Andrew Forbes, and Amogh Manthalkar

DOI: 10.1364/OL.389346 Received 28 Jan 2020; Accepted 13 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: Stokes polarimetry is widely used to extract the polarisation structure of optical fields, typically from six measurements, although it can be extracted from only four. To measure the required intensities, most approaches are based on optical polarisation components. In this work, we present an all-digital approach that enables a rapid measure of all four intensities without any moving components. Our method employs a Polarisation Grating (PG) to simultaneously project the incoming mode into left- and right-circular polarised states, followed by a polarisation-insensitive Digital Micro-mirror Device (DMD), which digitally introduces a phase retardance for the acquisition of the remaining two polarisation states. We demonstrate how this technique can be applied to measuring the SoP, vectorness and intra-modal phase of optical fields, without any moving components and shows excellent agreement with theory, illustrating fast, real-time polarimetry.

Photon-counting 3D integral imaging with less than a single photon per pixel on average using a statistical model of the EM-CCD camera

HISAYA HOTAKA, Timothy O'Connor, Shinji Ohsuka, and Bahram Javidi

DOI: 10.1364/OL.389776 Received 04 Feb 2020; Accepted 13 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: We investigate photon-counting 3D integral imaging (PCII) with an electron multiplying charged-coupled device (EM-CCD) camera using dedicated statistical models. Using conventional integral imaging reconstruction methods with this camera in photon counting conditions may result in degraded reconstructed image quality if multiple photons are detected simultaneously in a given pixel. We propose an estimation method derived from the photon detection statistical model of the EM-CCD to address the problems caused by multiple photons detected at the same pixel and provide improved 3D reconstructions. We also present a simplified version of this statistical method which can be used under the correct conditions. The imaging performance of these methods are evaluated on experimental data by the peak signal-to-noise ratio (PSNR) and the structural similarity index measure (SSIM). The experiments demonstrate 3D integral imaging substantially outperforms 2D imaging in degraded conditions. Furthermore, using the proposed methods, we achieve imaging in photon-counting conditions where, on average, less than a single photon per pixel was detected by the camera. To the best of our knowledge, this is the first report of PCII with the EM-CCD camera employing its statistical model in 3D reconstruction of PCII.

Simultaneous fluorescence lifetime and Raman fiber-based mapping of tissues

Joao Lagarto, Vladislav Shcheslavskiy, Francesco Pavone, and Riccardo Cicchi

DOI: 10.1364/OL.389300 Received 28 Jan 2020; Accepted 13 Mar 2020; Posted 16 Mar 2020  View: PDF

Abstract: We report the development of a novel fiber-based system to realize co-registered simultaneous acquisition of fluorescence lifetime data and Raman spectra from the same area. Fluorescence lifetime measurements by means of time-correlated single photon counting are realized with periodic out-of-phase external illumination of the field of view, enabling acquisition of data under bright illumination of the specimen. Raman measurements in the near-infrared are realized asynchronously. We present a detailed characterization of this technique and validate its potential to report intrinsic contrast. Fiber-based fluorescence lifetime and Raman maps report complementary structural, compositional and molecular contrast in biological tissues with diverse compositional features.

Reduction in mesoscopic conductance with increasinggain in amplifying Anderson-localized systems

Sushil Mujumdar, krishna joshi, Sandip Mondal, and Randhir Kumar

DOI: 10.1364/OL.387162 Received 17 Jan 2020; Accepted 13 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: The consequences of a nonconservative environment on the transport of photons under conditions of Anderson localization in a disordered system are a topic of great interest. In this work, we experimentally demonstrate the systematic decrease in the localization length of a quasi-one-dimensional localizing system when gain is added to it. We quantify the generalized conductance of the system using the variance of the fluctuations in the localized eigenfunctions, and show a decrease in conductance with gain. We theoretically model this system using a combination of transfer matrix calculations and rate equations for a two-level lasing system, and find very good qualitative agreement with the experimental results. We show that higher disorder can be emulated in a system with weak disorder using appropriate gain. The decreasing conductance is explained using the reduced probability of outcoupling of photons relative to their peak position within the system.

Off-Axis Spiral Phase Mirrors for Generating High Intensity Optical Vortices

Andrew Longman, Carlos Salgado, Ghassan Zeraouli, Jon Apinaniz, José Antonio Pérez Hernández, Mohammed Eltahlawy, Luca Volpe, and Robert Fedosejevs

DOI: 10.1364/OL.387363 Received 23 Jan 2020; Accepted 12 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: In this work, we present a novel and practical method for generating optical vortices in high-power laser systems. Off-axis spiral phase mirrors are used at oblique angles of incidence in the beam path after amplification and compression allowing for the generation of high-power optical vortices in almost any laser system. An off-axis configuration is possible via modification of the azimuthal gradient of the spiral phase helix and is demonstrated with a simple model using a discrete spiral staircase. This work presents the design, fabrication, and implementation of off-axis spiral phase mirrors in both low and high-power laser systems.

Accurate quantification of photothermal heat originated from a plasmonic metasurface

Hossein MEHRZAD, Fatemeh Habibimaghaddam, Ezeddin Mohajerani, and Mohammad Mohammadimasoudi

DOI: 10.1364/OL.387789 Received 13 Jan 2020; Accepted 12 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: Photothermal effect in plasmonic nanostructures (thermo-plasmonic), as a nanoscale heater, has been widely used in biomedical technology and optoelectronic devices. However, the big challenge in this effect is the quantitative characterization of the delivered heat to the surrounding environment. In this work, a plasmonic metasurface (as a nano-heater), and a Fabry Perot (FP) cavity including liquid crystal (as a thermometer element) are integrated. Metasurface is manufactured through bottom-up deposition method. This metasurface has near perfect absorption, so that in the photothermal experiment, under the low intensity (<0.25W/〖cm〗^2) of irradiating laser, undergoes efficient temperature rising. Generated heat from metasurface dissipated to the liquid crystal (LC) layer and makes spectral shift of FP modes. Based on consistency of anisotropic thermotropic data of used LC and spectral shift of FP modes, more than 50℃ temperature elevation with accuracy of 1.3℃ has been measured. The calculated figure of merit (FoM) of constructed device -indicating of temperature sensitivity- is 22, which is four times more than other reported thermometry devices with broad spectral width. In addition, this integrated device can be also used as an all optical device to control the plasmonic resonance spectrum.

Conical vs Gaussian terahertz emission from two-color laser-induced air plasma filaments

Emmanuel Abraham, Christian Sørensen, léo Guiramand, Jérôme Degert, Marc Tondusson, Esben Skovsen, and Eric FREYSZ

DOI: 10.1364/OL.390112 Received 07 Feb 2020; Accepted 12 Mar 2020; Posted 12 Mar 2020  View: PDF

Abstract: We demonstrate that the far field terahertz beam generated from a Ti:Sapphire two-color laser-induced filament can exhibit a conical or Gaussian distribution depending on the filtering experimental conditions. Using both an incoherent Golay cell detector and a two-dimensional coherent electro-optic detection covering the 0.2-2.6 THz spectral range, in our experimental conditions, we provide evidence that the conical emission is due to photo-induced carriers in the silicon filter typically used to block the remaining pump laser light. Moreover, the low frequency terahertz beam retrieves an almost TEM00 Gaussian spatial distribution when the silicon filter is preceded by a large bandgap ceramic filter, which stops the pump beam, preventing carrier generation in the silicon filter.

Transient grating in a thin gas target for characterization of extremely short optical pulses

Yuichiro Kida

DOI: 10.1364/OL.390135 Received 10 Feb 2020; Accepted 12 Mar 2020; Posted 12 Mar 2020  View: PDF

Abstract: Transient-grating cross-correlation frequency-resolved optical gating (TG XFROG) with a thin gas target is discussed towards characterization of subfemtosecond optical pulses. For evaluation of the reliability, sub-10-fs near-infrared pulses are characterized, results of which are compared with those given by the sum-frequency-generation XFROG. The TG XFROG covers the nanojoule energy range or that for the advanced few-cycle UV pulses reported recently. It is also shown that the TG XFROG fails to characterize and heavily underestimates the durations of intense test pulses. The FROG technique sensitively detects the onset of this anomalous behavior that represents a serious issue for pulse characterizations.

Single-cycle, MHz-repetition rate THz source with 66 mW of average power

Frank Meyer, Tim Vogel, Shahwar Ahmed, and Clara Saraceno

DOI: 10.1364/OL.386305 Received 19 Dec 2019; Accepted 12 Mar 2020; Posted 12 Mar 2020  View: PDF

Abstract: We demonstrate THz generation using the tilted pulse front method in Lithium Niobate, driven at unprecedented high average power of more than 100 W and at 13.3 MHz repetition rate, provided by a compact amplifier-free modelocked thin-disk oscillator. The conversion efficiency was optimized with respect to pump spot size and pump pulse duration, enabling us to generate a maximum THz average power of 66 mW, which is the highest reported to date from a laser-driven, few-cycle THz source. Furthermore, we identify beam walk-off as the main obstacle that currently limits the conversion efficiency in this excitation regime (with moderate pulse energies and small spot sizes). Further upscaling to the watt level and beyond is within reach, paving the way for linear and nonlinear high-average power THz spectroscopy experiments with exceptional signal-to-noise ratio at MHz repetition rates.

Penetration capability of near infrared Laguerre-Gaussian vortex beams through highly scattering media

Francesco di Bartolo, Muhammad Malik, Mirco Scaffardi, Antonella Bogoni, Simona Celi, Paolo Ghelfi, and Antonio Malacarne

DOI: 10.1364/OL.387998 Received 14 Jan 2020; Accepted 12 Mar 2020; Posted 16 Mar 2020  View: PDF

Abstract: The higher capability of optical vortex beams of penetrating turbid media (e.g. biological fluids) with respect to the conventional Gaussian beams is, for the first time, demonstrated in the 1.3 µm wavelength range, that is the one conventionally used for optical coherence tomography procedures in endoscopic scenarios such as intravascular ones. The effect has been demonstrated by performing transmittance measurements through suspensions of polystyrene microspheres in water with different particulate concentrations and, in reflection, by using samples of human blood with different thicknesses. The reduced backscattering / increased transmittance into such highly scattering media of Laguerre-Gaussian beams with respect to Gaussian ones, in the near infrared wavelength region, could be potentially exploited in clinical applications, leading to novel biomedical diagnoses and/or procedures.

High Verdet constant of Te₂₀As₃₀Se₅₀ glass in the mid-infrared

Masoud Mollaee, Pierre Lucas, Julien Ari, Xiushan Zhu, Michal Lukowski, Tariq Manzur, and Nasser Peyghambarian

DOI: 10.1364/OL.390236 Received 11 Feb 2020; Accepted 12 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: Magneto optical properties of tellurium-arsenic-selenium glass (Te₂₀As₃₀Se₅₀) were measured and analyzed. A Verdet constant of 870 deg/T/m at 1950 nm with the figure of merit of more than 500 deg/T, which is the highest value reported in glass materials at this wavelength, was measured. Compared to other chalcogenide glasses, such as Ge₁₀Se₉₀ and Ge₂₅As₁₅S₆₀, Te₂₀As₃₀Se₅₀ glass exhibits higher Verdet constants, broader mid-infrared transparency window, and longer infrared absorption edge, making it a very promising material to fabricate magneto-optical devices for mid-infrared applications.

High-speed single-pixel imaging by frequency-time-division multiplexing

Hiroshi Kanno, Hideharu Mikami, and Keisuke Goda

DOI: 10.1364/OL.390345 Received 13 Feb 2020; Accepted 12 Mar 2020; Posted 23 Mar 2020  View: PDF

Abstract: We propose and experimentally demonstrate high-speed single-pixel imaging by integrating frequency-division multiplexing and time-division multiplexing (techniques widely used in telecommunications) and applying the combined technique, namely frequency-time-division multiplexing (FTDM), to optical imaging. Specifically, FTDM single-pixel imaging uses an array of broadband, spatially distributed, dual frequency combs (i.e., a spatial dual combs) for multidimensional illumination and detects an image-encoded time-domain signal with a single-pixel photodetector in a FTDM manner. As a proof-of-principle demonstration, we use the method to show ultrafast two-color (bright-field and fluorescence) single-pixel microscopy of breast cancer cells at a high frame rate of 32,000 fps and ultrafast image velocimetry of fluorescent particles flowing at a high speed of >2 m/s.

Wide-angle speckleless DMD holographic display using structured illumination with temporal multiplexing

Byoungho Lee, Dongheon Yoo, Jinsoo Jeong, Byounghyo Lee, and Seungjae Lee

DOI: 10.1364/OL.390552 Received 14 Feb 2020; Accepted 11 Mar 2020; Posted 12 Mar 2020  View: PDF

Abstract: We propose a digital micromirror device (DMD) holographic display, where speckleless holograms can be observed in the expanded viewing zone. Structured illumination (SI) is applied to expand the small diffraction angle of the DMD using a laser diode (LD) array. To eliminate diffraction noise from SI, we utilize an active filter array for the Fourier filter and synchronize it with the LD array. The speckle noise is reduced via temporal multiplexing, where the proposed system supports a dynamic video of 60 Hz using the DMD’s fast operation property. The proposed system is verified and evaluated with experimental results.

Plasmonic transmitted optical differentiator based on subwavelength gold gratings

Weiji Yang, Xiaoxu Deng, Xuanyi Yu, and Jialin Zhang

DOI: 10.1364/OL.390566 Received 21 Feb 2020; Accepted 11 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: A nanoscale plasmonic optical differentiator based on subwavelength gold gratings is investigated theoretically and experimentally without Fourier-transform lens and prisms. In the vicinity of surface plasmon resonance (SPR), the transfer function of subwavelength gold gratings is derived by optical scattering matrix theory. Simulated by FDTD solutions, the wavelengths of optical spatial differentiation performed by subwavelength gold gratings are tuned by gratings period and duty cycle, while the throughput of edge extraction is mainly adjusted by the gratings thickness. Without Fourier transformation, the fabricated plasmonic optical differentiator experimentally achieves real-time optical spatial differentiation in transmission and implements SPR enhanced high-throughput edge extraction of a micro-scale image with resolution of 10μm at 650nm, which has potential applications in areas of optical analog computing, optical imaging and optical information processing.

High-efficient stimulated Raman scattering of sub-picosecond laser pulses in BaWO4 for 10.6 μm difference frequency generation

Igor Kinyaevskiy, Valeri Kovalev, Pavel Danilov, Nikita Smirnov, Sergey Kudryashov, Leonid Seleznev, Elizaveta Dunaeva, and Andrey Ionin

DOI: 10.1364/OL.391550 Received 26 Feb 2020; Accepted 11 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: Transient stimulated Raman scattering of 0.3-ps 515-nm laser pulses in BaWO4 crystal was experimentally demonstrated with efficiency up to ~20% for the Stokes component with wavenumber of ~925 cm-1 in a simple single-pass geometry. This anomalously high efficiency was obtained due to the laser pulses self-phase modulation resulting in spectral broadening and seeding the stimulated Raman scattering. Applicability of seed pulse production for high-pressure sub-picosecond CO2 laser amplifier via difference frequency generation in LiGaS2 crystal was numerically verified.

Coherent Anti-Stokes Raman Scattering Imaging using Silicon Photomultipliers

Christian Allen, Ben Hansson, Olivia Raiche-Tanner, and Sangeeta Murugkar

DOI: 10.1364/OL.390050 Received 14 Feb 2020; Accepted 10 Mar 2020; Posted 12 Mar 2020  View: PDF

Abstract: Silicon photomultipliers (SiPMs) are an emerging solid-state alternative to photomultiplier tubes (PMTs) for low light detection, with similar gain but lower cost and lower operating voltage. We demonstrate Coherent anti-Stokes Raman scattering (CARS) imaging in a side-by-side comparison of an uncooled SiPM with an uncooled multialkali PMT as well as a state-of-the-art cooled GaAsP PMT. We determine the optimum reverse bias voltage for acquiring the best signal to noise ratio (SNR) for CARS imaging of lipids at 2850 cm-1. We find that despite the higher dark counts, the SNR of CARS images acquired with the uncooled SiPM biased at an optimum voltage is better than that of the multialkali PMT and close to that of the cooled GaAsP PMT (1.5 and 0.8 times respectively). This is due to the higher gain and lower excess noise factor related to the pulse height variability in the SiPM.

Dynamic photonic crystal in colloidal quantum dots solution: formation, structure analysis and dimensionality switching

Alexander Smirnov, Kseniia Ezhova, Vladimir Mantsevich, and Vladimir Dneprovskii

DOI: 10.1364/OL.389127 Received 24 Jan 2020; Accepted 10 Mar 2020; Posted 11 Mar 2020  View: PDF

Abstract: We demonstrated, for the first time, a simple method to create three-dimensional (3D) dynamic photonic crystal (PhC) with controllable lattice symmetry by the interference of four non-coplanar laser beams in non-linear optical medium [colloidal solution of CdSe/ZnS quantum dots (QDs)]. 3D dynamic PhC was formed due to the periodical changing of refraction and absorption of resonantly excited excitons in the colloidal solution of QDs. The formation of dynamic PhC was confirmed by the observed self-diffraction of the laser beams on the dynamic structure which they have created. Tuning of the PhC dimensionality to the two-dimensional (2D) and one-dimensional (1D) was done by reduction of the interfering beams number to three and two, respectively, and by the control under the interacting beams polarization. Physical processes responsible for the observed self-action effects that arise in CdSe/ZnS QDs are discussed in details.

Ultra-broadband Polarization Beam Splitter with Silicon Subwavelength-Grating Waveguides

Daoxin Dai, Chenlei Li, Ming Zhang, and John Bowers

DOI: 10.1364/OL.389207 Received 28 Jan 2020; Accepted 10 Mar 2020; Posted 11 Mar 2020  View: PDF

Abstract: An ultra-broadband polarization-beam splitter (PBS) with low excess losses (ELs) and high extinction ratios (ERs) is proposed and demonstrated for the case with 340 nm-thick silicon-on-insulator (SOI) waveguides. Here the PBS is realized by using cascaded adiabatic dual-core tapers, which consists of a strip-core and a subwavelength-grating (SWG)-core. For the designed PBS, which has a 33.6 μm-long mode-evolution region, the ELs are <0.3 dB and the ERs are >20 dB for both TE- and TM-polarizations in an ultra-broad bandwidth of >300 nm (1400-1700 nm) in theory. For the fabricated PBS, the measured bandwidths for achieving ERs of ~20 dB and ~25 dB are about 240 nm and 220 nm, while the 1 dB-bandwidth is as large as 270 nm, which are the largest one reported until now. © 2020 Optical Society of America. http://dx.doi.org/10.1364/AO.99.099999

Scalable Hermite-Gaussian mode-demultiplexing hybrids

He Wen, Huiyuan Liu, Yuanhang Zhang, Rachel Sampson, Shengli Fan, and Guifang Li

DOI: 10.1364/OL.387460 Received 15 Jan 2020; Accepted 10 Mar 2020; Posted 10 Mar 2020  View: PDF

Abstract: We propose a Hermite-Gaussian (HG) mode-demultiplexing hybrid (MDH) for coherent detection of mode-division multiplexed (MDM) signals. The MDH, which performs multiple functionalities including demultiplexing, local oscillator (LO) splitting, and optical 90-degree mixing, is realized based on the multi-plane light conversion (MPLC) technique. An isosceles right triangle output layout is employed to reduce the number of phase masks to fewer than the number of modes, significantly simplifying the construction of the MDH. A ten-HG mode MDH with only five phase masks is demonstrated by numerical simulation, achieving an insertion loss (IL) and mode dependent loss (MDL) as low as -2.3 dB and 1.7 dB, respectively. The IL was further reduced to -1.5 dB through optimization of MDH parameters, such as the beam waists of the input HG modes and the output spots.

Enhanced nanoplasmonic heating in standoff sensing of explosive residues with infrared reflection-absorption spectroscopy

Nicholas Simin, Yangkyu Park, Dongkyu Lee, Thomas Thundat, and Seonghwan Kim

DOI: 10.1364/OL.387653 Received 20 Jan 2020; Accepted 10 Mar 2020; Posted 10 Mar 2020  View: PDF

Abstract: Various standoff sensing techniques employing optical spectroscopy have been developed to address challenges in safely identifying trace amounts of explosives at a distance. A flexible anodic aluminum oxide (AAO) microcantilever and a high-power quantum cascade laser utilized as the infrared (IR) source are used for standoff IR reflection-absorption spectroscopy to detect explosive residues on metal surface. Standoff sensing of trinitrotoluene (TNT) is demonstrated by exploiting the high thermomechanical sensitivity of a bimetallic AAO microcantilever. Moreover, sputtering gold onto the fabricated AAO nanowells generates a strong scattering and absorption of IR light in the wavelength range of 5.18 μm to 5.85 μm resulting in enhanced nanoplasmonic heating. Utilizing the IR absorption enhancement in this wavelength range, the plasmonic AAO cantilever could detect TNT molecules 7 times better than the bimetallic AAO cantilever.

Compact Ho:YLF-pumped ZnGeP$_2$-based optical parametric amplifiers tunable in the molecular fingerprint regime

Siqi Cheng, Gourab Chatterjee, Friedjof Tellkamp, Tino Lang, Axel Ruehl, Ingmar Hartl, and R. J. Dwayne Miller

DOI: 10.1364/OL.389535 Received 03 Feb 2020; Accepted 09 Mar 2020; Posted 10 Mar 2020  View: PDF

Abstract: We report on a compact mid-infrared laser architecture, comprising a chain of ZnGeP$_2$-based optical parametric amplifiers (OPAs), which afford a higher energy yield ($\lesssim 60$ $\mu$J at 1 kHz) compared to most conventional OPA gain media transparent in the $2-8$ $\mu$m wavelength range. Specifically, our OPA scheme allows ready tunability in the molecular fingerprint regime and is tailored for strong-field excitation and coherent control of both stretch and bend (or torsional) vibrational modes in molecules. The OPAs are pumped and directly seeded (via supercontinuum generation) by a 2-$\mu$m, 3-ps Ho:YLF regenerative amplifier. The compressibility of the OPA output is demonstrated by a representative measurement of the near-Gaussian temporal profile of a dispersion-compensated 105-fs idler pulse at a central wavelength of 5.1 $\mu$m, corresponding to $\sim 6$ optical cycles. Detailed numerical simulations closely corroborate the experimental measurements, providing a benchmark and a platform to further explore the parameter space for future design, optimization and implementation of high-energy, ultrafast, mid-infrared laser schemes.

Broad-bandwidth High-temporal-contrast Carrier-envelope-phase stabilized laser seed for 100 PW lasers

Yuxin Leng, Beijie Shao, Yanyan Li, Yujie Peng, Pengfei Wang, Junyu Qian, and Ruxin Li

DOI: 10.1364/OL.390110 Received 07 Feb 2020; Accepted 09 Mar 2020; Posted 10 Mar 2020  View: PDF

Abstract: A high-temporal-contrast carrier-envelope phase (CEP) stabilized broadband laser seed based on cascaded optical parametric amplification (OPA) and second harmonic generation (SHG) is demonstrated. With perfect dispersion and gain bandwidth compensation, temporal cleaned pulses centered at 910 nm with a full bandwidth of >200 nm and energy of 170 µJ are obtained. The achieved pulse duration is ~15 fs and the temporal contrast reaches over 10¹². The carrier envelope phase is passively stabilized with 340 mrad. The laser can be used as the seed for the DKDP based 100 PW laser systems and will be integrated into the SEL facility as the seed laser in the near future.

Laser-induced suspension of microbubble in liquid-filled fiber micro-cavity for large range tilt sensing

Xiang Ji, Yi Liu, KunJian Cao, Yingying Liao, Yan Li, and ShiLiang Qu

DOI: 10.1364/OL.390579 Received 14 Feb 2020; Accepted 09 Mar 2020; Posted 10 Mar 2020  View: PDF

Abstract: We proposed a compact fiber tilt sensor based on a microbubble suspending in a liquid-filled micro-cavity at the end of single-mode fiber (SMF). By coupling a single-frequency laser with enough power, the microbubble could suspend in the micro-cavity due to the Marangoni effect, which constitutes a Fabry-Perot interferometer (FPI). When the tilt angle changes, the microbubble’s position changes consequently which causes the variation of the dominant frequency of the interference fringes in the spectrum. The experimental results show that the tilt angle sensitivity of the sensor reaches 3.64×10-4 nm-1/Deg at a wide sensing range from –45° to 45° with a good repeatability.

Long-wavelength-infrared laser filamentation in solids in the near-single-cycle regime

Shizhen Qu, Garima Nagar, Wenkai Li, Kun Liu, xiao zou, hl seck, Dennis Dempsey, Kyung-Han Hong, Qijie Wang, Ying Zhang, Bonggu Shim, and Houkun Liang

DOI: 10.1364/OL.389456 Received 29 Jan 2020; Accepted 09 Mar 2020; Posted 11 Mar 2020  View: PDF

Abstract: We experimentally demonstrate the long-wavelength-infrared (LWIR) femtosecond filamentation in solids. Systematic investigations of supercontinuum (SC) generation and self-compression of the LWIR pulses assisted by laser filamentation are performed in bulk KrS-5, and ZnSe, pumped by ~ 145 fs, 9 μm, 10 µJ pulses from an optical parametric chirped pulse amplifier operating at 10 kHz of repetition rate. Multi-octave SC spectra are demonstrated in both materials. While forming the stable single filament, the 1.5-cycle LWIR pulses with 4.5 µJ output pulse energy are produced via soliton-like self-compression in a 5-mm thick KrS-5. The experimental results quantitatively agree well with the numerical simulation based on the unidirectional pulse propagation equation. This work shows the experimental feasibility of the high-energy, near-single-cycle LWIR light bullet generation in solids.

Single frequency DBR Nd-doped fiber laser at 1120 nm with narrow linewidth and low threshold

yafei wang, jiamin wu, Qilai Zhao, weiwei wang, Jing Zhang, Zhongmin Yang, Shanhui Xu, and Mingying Peng

DOI: 10.1364/OL.386477 Received 19 Dec 2019; Accepted 09 Mar 2020; Posted 10 Mar 2020  View: PDF

Abstract: We report a narrow linewidth and low threshold single frequency distributed Bragg reflector (DBR) fiber laser at 1120 nm based on a short 1.5 cm long Nd-doped silica fiber, which to our best knowledge is the first demonstration of Nd-doped fiber-based single frequency fiber laser with a wavelength greater than 1100 nm. A stable single-longitudinal-mode laser operation with signal-to-noise ratio (SNR) greater than 67 dB was verified by a scanning Fabry–Perot interferometer. The laser threshold is as low as 10 mW. The DBR fiber laser has a maximum output power of 15 mW and optical-to-optical efficiency for the lunched pump power reaches to more than 8 %. The narrow linewidth of 71.5 kHz is obtained in such single frequency fiber laser. Our result is expected to offer an exciting new opportunity to realize high performance single frequency fiber laser above 1100 nm.

Broadband thulium-doped fiber ASE source

Jan Aubrecht, Pavel Peterka, Pavel Honzatko, Ondrej Moravec, Michal Kamradek, and Ivan Kasik

DOI: 10.1364/OL.389397 Received 29 Jan 2020; Accepted 09 Mar 2020; Posted 09 Mar 2020  View: PDF

Abstract: We report on a flat and broadband thulium-doped amplified spontaneous emission fiber source working around 1.85 microns in the eye-safe spectral region. Core-pumped thulium fibers were prepared in house using the modified chemical vapor deposition method. The amplified spontaneous emission source in backward direction with respect to the pump and in a single-ended configuration, produces stable, non-polarized radiation with an output power of up to 280 mW, corresponding to a slope efficiency of about 36 % with respect to the pump power. To our knowledge, the device reported herein is the broadest ASE source based on Tm-doped fiber without internal spectral filtering with an output power exceeding 90 mW and full width at half maximum of the spectrum greater than 155 nm.

Widely tunable cavity-enhanced frequency combs

Myles Silfies, Grzegorz Kowzan, Yuning Chen, Neomi Lewis, Ryan Hou, Robin Baehre, Tobias Gross, and Thomas Allison

DOI: 10.1364/OL.389412 Received 29 Jan 2020; Accepted 09 Mar 2020; Posted 09 Mar 2020  View: PDF

Abstract: We describe the cavity-enhancement of frequency combs over a wide tuning range of 450-700 nm (>7900 cm¯¹), covering nearly the entire visible spectrum. Tunable visible frequency combs from a synchronously-pumped optical parametric oscillator are coupled into a 4-mirror, dispersion-managed cavity with a finesse of 600 to 1400. An intracavity absorption path length enhancement greater than 190 is obtained over the entire tuning range, while preserving intracavity spectral bandwidths capable of supporting sub-200 fs pulse durations. These tunable cavity-enhanced frequency combs can find many applications in nonlinear optics and spectroscopy.

A new tunable narrow-linewidth 226nm laser for hypersonic flow velocimetry

shutao dai, tao jiang, Hongchun Wu, Zhi Zhang, LIXIA WU, hongming gong, wen weng, jing deng, hui zheng, and Wenxiong Lin

DOI: 10.1364/OL.390347 Received 19 Feb 2020; Accepted 09 Mar 2020; Posted 19 Mar 2020  View: PDF

Abstract: We report the development and application of a novel all-solid-state tunable narrow-linewidth 226 nm UV laser system. The laser system consists of three parts: a tunable single-frequency Ti:sapphire 787 nm laser, a single-frequency long-pulse-width flat-top shape 532 nm laser, and a nonlinear frequency transformation system. The 532 nm laser is sum-frequency mixed with the second harmonic of the 787 nm laser to produce 226 nm laser. Maximum output pulse energy at 226 nm is 3 mJ. Then nitric oxide (NO) planar laser-induced fluorescence (PLIF) velocimetry is demonstrated in the China Aerodynamics Research & Development Center’s (CARDC) FD14 hypersonic shock tunnel using this 226 nm laser system. It is proven that this laser is convenient for high resolution molecular tagging fluorescence spectroscopy.

Single-shot diagnosing of spatio-temporal couplings in ultrashort laser pulses by spatio-spectral imaging of a third order non-linear process

Thomas Oksenhendler, Jean-Baptiste Margoto, Zhao Cheng, and Rodrigo Lopez-Martens

DOI: 10.1364/OL.388676 Received 21 Jan 2020; Accepted 08 Mar 2020; Posted 09 Mar 2020  View: PDF

Abstract: The impact of spatio-temporal couplings (STC) on the focal-spot intensity of ultra-intense femtosecond lasers scales up with the larger beam sizes and shorter pulse duration typically implemented to reachPWintensities.The repetition rate of such facilities (<10 Hz) makes single-shot diagnosing of STC mandatory. We present here a new spatio-temporal diagnostic technique based on the observation of the spatio-spectral intensity of athird-order nonlinear effect, in this case cross-polarised wave generation (XPW), with an imaging spectrometer. The measured XPW spectrum displays specific couplings that depend on the STC of the initial pulse to becharacterized. Measurements were carried out on a titanium:sapphire chirped pulse amplifier and confirm the ability of our approach to diagnose common STC in a laser pulse on the fly.

A simple and fully CMOS-compatible low-loss fiber coupling structure for silicon photonics platform

Yuriko Maegami, Makoto Okano, Guangwei Cong, Keijiro Suzuki, Morifumi Ohno, Toshihiro Narushima, Nobuyuki Yokoyama, Miyoshi Seki, Minoru Ohtsuka, Shu Namiki, and Koji Yamada

DOI: 10.1364/OL.388267 Received 15 Jan 2020; Accepted 08 Mar 2020; Posted 09 Mar 2020  View: PDF

Abstract: A simple low-loss fiber coupling structure consisting of a Si inverted-taper waveguide and a 435-nm-wide and 290-nm-thick SiN waveguide was fabricated with fully CMOS-compatible processes. The small SiN waveguide can expand to the optical field corresponding to a fiber with a mode field diameter of 4.1 μm. The fiber-to-chip coupling losses were 0.25 and 0.51 dB/facet for quasi-TE and quasi-TM modes at a 1550-nm wavelength. Polarization dependent losses of the conversion in the Si-to-SiN waveguide transition and the fiber-to-chip coupling were less than 0.3 and 0.5 dB, respectively, in the wavelength range of 1520–1580 nm.

All-fiber polarization maintaining mode-locked laser operated at 980 nm

Svetlana Aleshkina, Andrei Fedotov, Dmitry Korobko, Dmitry Stoliarov, Denis Lipatov, Vladimir Vel'miskin, Valery Temyanko, leonid kotov, Regina Gumenyuk, and Mikhail Likhachev

DOI: 10.1364/OL.391193 Received 20 Feb 2020; Accepted 08 Mar 2020; Posted 16 Mar 2020  View: PDF

Abstract: For the first time we present an all-fiber polarization maintaining (PM) a passively mode-locked picosecond laser operated at 980 nm. The laser cavity had a ring configuration with a SESAM-based mode-locking element. As an active medium we used a specially designed cladding pumped Yb-doped fiber with reduced cladding-to-core diameter ratio. The laser was self-starting and operated in the net cavity normal dispersion regime, where a spectral profile of the gain medium acted as a filter element. By intracavity spectral filtering, we achieved about 40 dB dominance of the signal wavelength at 980 nm over 1 µm emission in a highly stable picosecond pulsed regime. The corresponding simulation was performed to extend the knowledge about the laser operation.

Chirping Fibre Bragg gratings within additively manufactured polymer packages

John Canning and ASma Ziyani

DOI: 10.1364/OL.387371 Received 06 Jan 2020; Accepted 08 Mar 2020; Posted 12 Mar 2020  View: PDF

Abstract: Fibre Bragg gratings are embedded within 3D printed polymer packages. Information about both induced and applied stresses, and operator error, can be determined from the observed spectral shifts and chirping. A novel way to produced packaged broadband gratings, with Δλ > ~ 7nm/cm, is proposed and demonstrated.

Quadratic soliton mode-locked degenerate optical parametric oscillator

Shu-Wei Huang and Mingming Nie

DOI: 10.1364/OL.389568 Received 04 Feb 2020; Accepted 07 Mar 2020; Posted 09 Mar 2020  View: PDF

Abstract: By identifying the similarities between the coupled-wave equations and the parametrically driven nonlinear Schrödinger equation, we unveil the existence condition of the quadratic soliton mode-locked degenerate optical parametric oscillator in the previously unexplored parameter space. We study the nature of the quadratic solitons and divide their dynamics into two distinctive branches depending on the system parameters. Origin of the quadratic soliton perturbation is identified and strategy to mitigate its detrimental effect is developed. Terahertz comb bandwidth and femtosecond pulse duration are attainable in an example periodically poled lithium niobate waveguide resonator. Design rules of the quadratic soliton mode-locking are summarized, and the principle can be further extended to other material platforms, making it a competitive ultrashort pulse and broadband comb source architecture at the midinfrared.

100 MHz frequency comb for low-intensity multi-photon studies: intra-cavity velocity-map imaging of xenon

Janko Nauta, Jan-Hendrik Oelmann, Alexander Ackermann, Patrick Knauer, Ronja Pappenberger, Andrii Borodin, Isa Shams Muhammad, Hans Ledwa, Thomas Pfeifer, and José Crespo López-Urrutia

DOI: 10.1364/OL.389327 Received 30 Jan 2020; Accepted 06 Mar 2020; Posted 09 Mar 2020  View: PDF

Abstract: We raise the power from a commercial 10 W frequency comb inside an enhancement cavity and perform multi-photon ionization of gas-phase atoms at 100 MHz for the first time. An intra-cavity velocity-map-imaging setup collects electron-energy spectra of xenon at rates several orders of magnitude higher than those of conventional laser systems. Consequently, we can use much lower intensities ~10^12 W/cm² without increasing acquisition times above just a few seconds. The high rate and coherence of the stabilized femtosecond pulses are known to be transferred to the actively stabilized cavity and will allow studying purely perturbative multi-photon effects, paving the road towards a new field of precision tests in nonlinear physics.

Anisotropic asymmetric transmission of circularly polarized terahertz waves in a three dimensional spline assembly

C. K. Amaljith, Chakravarthy Venkateswaran, Venkatachalam Subramanian, Zhengbiao Ouyang, and Yogesh Natesan

DOI: 10.1364/OL.388646 Received 27 Jan 2020; Accepted 06 Mar 2020; Posted 09 Mar 2020  View: PDF

Abstract: Asymmetric transmission (AT) for circularly polarized (CP) electromagnetic (e-m) waves in chiral metamaterial (CMM) is a well known phenomenon. However most of the CMMs exhibit AT along only one direction. In this work, AT for CP waves with the magnitude of more than 0.5 along three principal directions of a newly realized three dimensional spline assembly is reported at terahertz frequencies. Surface current analysis is presented to explain the mechanism of AT for CP waves in the proposed 3-D assembly.

Temporal contrast enhancement of ultrashort pulses using a spatiotemporal plasma-lens filter

Ping Zhu, Arie Zigler, Xinglong Xie, Dongjun Zhang, Qingwei Yang, Meizhi Sun, Evgeny (Jenya) Papeer, Jun Kang, Gao Qi, Xiao Liang, Haidong Zhu, Ailin Guo, LIANG YAN, Shengzhe Ji, Lei Ren, LIU HUIYA, Ning Kang, Yao Zhao, and Jianqiang Zhu

DOI: 10.1364/OL.388391 Received 24 Jan 2020; Accepted 06 Mar 2020; Posted 06 Mar 2020  View: PDF

Abstract: High temporal contrast is one of the most important properties of ultrashort laser pulses in the laser-matter interaction physics experiments. The spatiotemporal plasma-lens filter proposed here enhances the temporal contrast of the laser by combining plasma optics and spatial filtering. Experimentally, the spatiotemporal plasma-lens filter has improved the temporal contrast by two orders of magnitude with 80% laser transmission efficiency under 1Hz repetitive laser operation. Not only the pre-pulse and pedestal were cleaned out, but also the rising edge of the main pulse was sharpened.

Ultra-large local field enhancement effect of isolated thick triangular silver nanoplates on a silicon substrate in the green waveband

Huimin Feng, Jianjie Dong, Xianxin Wu, Fengyou Yang, Lijun Ma, Xinfeng Liu, and Qian Liu

DOI: 10.1364/OL.389241 Received 27 Jan 2020; Accepted 06 Mar 2020; Posted 06 Mar 2020  View: PDF

Abstract: The local field enhancement in plasmonic nanostructures is vital for surface enhanced Raman scattering (SERS). However, it remains a challenge to achieve a large local field enhancement at an illumination wavelength in the green waveband so far. Here, we report on an ultra-large local field enhancement effect of isolated thick triangular silver nanoplates (ITTSNPs) on a silicon substrate at an illumination wavelength in the green waveband. We show that when the thickness of the ITTSNP is larger than a critical thickness, a large local field enhancement with an enhancement factor (EF) greater than 350 can be achieved at an illumination wavelength in the green waveband, which is due to the excitation of strong localized surface plasmon polaritons only at three top apexes of the ITTSNP. This finding makes it possible to achieve a larger SERS EF at an excitation wavelength in the green waveband. Furthermore, we experimentally demonstrate that at an excitation wavelength of 514.5 nm, the average SERS EF of the ITTSNPs can exceed 10^11 and the sensitivity for the detection of Rhodamine 6G molecules can reach 10^−12 M.

Revealing photonic Symmetry-Protected Modes by Finite-Difference-Time-Domain method

Ayman HOBLOS, Miguel Suarez, blandine guichardaz, Nadège COURJAL, Fadi BAIDA, and Maria Pilar Bernal

DOI: 10.1364/OL.386845 Received 01 Jan 2020; Accepted 06 Mar 2020; Posted 06 Mar 2020  View: PDF

Abstract: This letter is devoted to point out a specific character of the Finite-Difference-Time-Domain method through the study of nano-structures supporting geometrical symmetry-protected modes that can not be excited at certain conditions of illumination. The spatial discretization performed in the FDTD algorithm naturally leads to break this symmetry and allows the excitation of these modes. The quality factor of the corresponding resonances are then directly linked to the degree of the symmetry breaking i.e. the spatial grid dimension even though the convergence criteria of the FDTD are fulfilled. This finding shows that the FDTD must be handled with a great care and, more importantly, that very huge quality-factor resonances could be achieved at the cost of nanometer-scale mastered fabrication processes.

Optical analog computing of spatial differentiation and edge detection with dielectric metasurfaces

Tianhua Feng, Lei Wan, Danping Pan, Shuaifeng Yang, Wei Zhang, Alexander Potapov, Xia Wu, Weiping Liu, and Li Zhaohui

DOI: 10.1364/OL.386986 Received 16 Jan 2020; Accepted 06 Mar 2020; Posted 06 Mar 2020  View: PDF

Abstract: We propose and demonstrate that optical analog computing of spatial differentiation and edge detection can be realized with a single layer of dielectric metasurface. The optical transfer function for second-order derivation is obtained by engineering the spatial dispersion of electric dipole resonance supported by the silicon nanodisks in the metasurface. Benefiting from this unique mechanism, spatial differentiation can be performed for two dimensions and arbitrary polarization with a large spatial bandwidth and high efficiency at the visible wavelength. Explicitly, We have numerically validated the application with one-dimensional spatial functions as well as an image and the results show excellent performance. Our study can facilitate the research of optical computing with artificial nanostructures.

Enhancing Extraction Efficiency of Quantum Dot Light-Emitting Diodes Introducing a Highly Wrinkled ZnO Electron Transport Layer

Dan-Dan Zhang and Jian-Long Xu

DOI: 10.1364/OL.390266 Received 11 Feb 2020; Accepted 06 Mar 2020; Posted 13 Mar 2020  View: PDF

Abstract: Light extraction efficiency is crucial for achieving highly efficient and bright quantum dot light-emitting diodes (QLEDs), and current efforts towards introducing light outcoupling nanostructures always need complicated procedures. An extremely simple and efficient method to introduce light outcoupling nanostructures in ZnO electron transport layer (ETL) is demonstrated by adopting a certain heating rate during the annealing process. The ultimate device exhibits a current efficien-cy of 9.1 cd/A, giving a 50% efficiency improvement compared to the control device with flat ZnO ETLs. This arises from the increased light extraction efficiency in-duced by random nanostructures formed on wrinkled ZnO ETL, which could also be controlled by adjusting the heating rate during the annealing process. This study not only provides a simple and efficient method to in-troduce light outcoupling nanostructures, but also shows lots of room for further performance enhance-ment of QLEDs with the guideline of light extraction.

Spectroscopic optical coherence refraction tomography

Kevin Zhou, Ruobing Qian, Sina Farsiu, and Joseph Izatt

DOI: 10.1364/OL.389703 Received 03 Feb 2020; Accepted 05 Mar 2020; Posted 06 Mar 2020  View: PDF

Abstract: In optical coherence tomography (OCT), the axial resolution is often superior to the lateral resolution, which is sacrificed for long imaging depths. To address this anisotropy, we previously developed optical coherence refraction tomography (OCRT), which uses images from multiple angles to computationally reconstruct an image with isotropic resolution, given by the OCT axial resolution. On the other hand, spectroscopic OCT (SOCT), an extension of OCT, trades axial resolution for spectral resolution and hence often has superior lateral resolution. Here, we present spectroscopic OCRT (SOCRT), which uses SOCT images from multiple angles to reconstruct a spectroscopic image with isotropic spatial resolution limited by the OCT lateral resolution. We experimentally show that SOCRT can differentiate differently sized beads based on Mie theory at simultaneously high spectral and isotropic spatial resolution. We also applied SOCRT to a biological sample, achieving axial resolution enhancement limited by the lateral resolution.

Direct generation of the lowest-order vortex beam using a spot defect mirror in the ultraviolet region

Yuuki Uesugi, Shunichi Sato, and Yuichi Kozawa

DOI: 10.1364/OL.389174 Received 27 Jan 2020; Accepted 05 Mar 2020; Posted 05 Mar 2020  View: PDF

Abstract: The creation of ultraviolet optical vortex beams with the topological charge of |l| = 1 at the wavelength of 325 nm was demonstrated from a He-Cd metal vapor laser with a spot defect mirror.The measured M² factor was close to the theoretical value of 2 of the LG₀₁ Laguerre-Gaussian mode.Some interference experiments showed that the obtained vortex beams were stable enough for practical applications such as holographic lithography.

Cellular-Resolution Retinal Structural Imaging with Visible-Light Optical Coherence Tomography

Yali Jia, Shaohua Pi, Tristan Hormel, Xiang Wei, William Cepurna, and John Morrison

DOI: 10.1364/OL.386454 Received 20 Dec 2019; Accepted 05 Mar 2020; Posted 06 Mar 2020  View: PDF

Abstract: In vivo high-resolution images are the most direct way to understand retinal function and diseases. Here, we report the use of visible-light optical coherence tomography to achieve cellular-level retinal structural imaging in rats covering the entire thickness of the retina. Vitreous fibers, nerve fiber bundles, and vasculature were clearly revealed, as well as at least three laminar sub-layers in the inner plexiform layer. We also successfully visualized ganglion cell somas in the ganglion cell layer, bipolar cells in the inner nuclear layer, and photoreceptors in the outer nuclear layer and ellipsoid zone. This technique provides a new means to visualize the retina in vivo at cellular resolution, and may provide clinically promising insights.

Efficient solar-blind ultraviolet detection based on Sn2+ ions activated fluosilicate glass

Ruilin Zheng, Kai Xu, Jianyong Ding, Yongjin Gao, Qidong Feng, Xuan Chen, LiLi Fu, and Wei Wei

DOI: 10.1364/OL.387375 Received 07 Jan 2020; Accepted 05 Mar 2020; Posted 05 Mar 2020  View: PDF

Abstract: As an effective method to improve the sensitivity of detection, spectral conversion technology has been widely used in solar-blind ultraviolet (UVC) communication. However, it remains an outstanding challenge in the field of UVC detection to produce a spectral converter with high efficiency, short decay lifetime and excellent stability. In this work, a kind of Sn2+ activated fluosilicate (SFS) glass as a spectral converter for UVC detection has been prepared, which has a short lifetime (5.65 μs) and a high quantum efficiency (85%). The SFS decorated Si-based PIN photodiode enables strong absorption and shifts subtly UVC to emission band optimized for the detector. Moreover, the outstanding performance of detection with responsivity of 27 μA/mW@278 nm, frequency limit up to 50 KHz, and the rise/fall time scale of 15.14 μs/18.71 μs@10 KHz is achieved. The comprehensive performance shows that SFS glass has the potential for efficient UVC detection in a scalable and low-cost strategy.

The role of surface passivation in integrated sub-bandgap silicon photodetection

Rivka Gherabli, Meir Grajower, Joseph Shappir, Noa Mazurski, Menachem Wofsy, Naor Inbar, Uriel Levy, and Jacob Khurgin

DOI: 10.1364/OL.388983 Received 24 Jan 2020; Accepted 03 Mar 2020; Posted 05 Mar 2020  View: PDF

Abstract: We study experimentally the effect of oxide removal on the sub-bandgap photodetection in silicon waveguides at the telecom wavelength regime. Depassivating the device allows for enhancing the quantum efficiency by about 2-3 times. Furthermore, the propagation loss within the device is significantly reduced by the oxide removal. Measuring the device 60 days after the de-passivation shows slight differences. We provide a possible explanation for these observations. Clearly, passivation and de-passivation plays an essential role in the design and the implementation of such sub-bandgap photodetectors devices for applications such as on-chip light monitoring.

Intracavity supercntinuum generation in mode-locked Er-doped fiber laser based on Mamyshev mechanism with highly nonlinear fiber

Luo Xing, Tong Tuan, Takenobu Suzuki, and Yasutake Ohishi

DOI: 10.1364/OL.389779 Received 05 Feb 2020; Accepted 03 Mar 2020; Posted 04 Mar 2020  View: PDF

Abstract: An all-fiber supercontinuum laser source with a piece of highly nonlinear fiber inserted into a mode-locked fiber laser is experimentally demonstrated. This laser achieves mode-locking based on Mamyshev mechanism and realized supercontinuum generation spanning from 1330 nm to 2030 nm directly. Mode-locking based on Mamyshev mechanism can be obtained easily and the influence of the parameters of the laser cavity on the supercontinuum laser source is investigated. This supercontinuum laser source has a simple structure and no amplifier stages are required. It opens up a new way for intracavity supercontinuum generation and exploits the operation of mode-locked fiber laser based on Mamyshev mechanism further.

1 W, 10.1 μm, CdSe optical parametric oscillator with continuous-wave seed injection

Yi Chen, gaoyou liu, Chao Yang, baoquan yao, Ruixue Wang, Shuyi Mi, ke Yang, Tongyu Dai, Xiaoming Duan, and Youlun Ju

DOI: 10.1364/OL.391547 Received 25 Feb 2020; Accepted 02 Mar 2020; Posted 05 Mar 2020  View: PDF

Abstract: We demonstrated a 2.1 µm nanosecond laser pumped, 2.6 µm continuous-wave (CW) seed injected, cadmium selenide (CdSe) signal singly resonant optical parametric oscillator (OPO). A maximum average power of 1.05 W was obtained corresponding to a pulse energy of 1.05 mJ at the idler wavelength of 10.1 µm and optical-to-optical conversion efficiency of 4.69%, beam quality of M2x = 2.25, M2y = 2.12 and pulse width of 24.4 ns. To the best of our knowledge, this is the first time to achieve 10-12 μm laser with watt-level average power using OPO technology.

Amplitude-splitting pulse-front-preserving split-delay system with sub-nanoradian relative pointing stability

Haoyuan Li, Yanwen Sun, Mark Sutton, Paul Fuoss, and Diling Zhu

DOI: 10.1364/OL.389977 Received 07 Feb 2020; Accepted 02 Mar 2020; Posted 03 Mar 2020  View: PDF

Abstract: We present the design and analysis of a hard x-ray split-delay optical arrangement that combines diffractive and crystal optics. Transmission gratings are employed to achieve the much-desired amplitude splitting and recombination of the beam. Asymmetric channel-cut crystals are utilized to tune the relative delay time. The use of a dispersion-compensation arrangement of the crystals allows the system to achieve sub-nanoradian pointing stability during a delay scan. It also minimizes wave-front distortion and preserves the pulse front and pulse duration. We analyze the performance of a prototype design that can cover a delay time range of 15ps with a sub-20fs time resolution at 10keV. We anticipate that this system can fully satisfy the very demanding stability requirements for performing split-pulse x-ray photon correlation spectroscopy measurements for the investigation of fast atomic scale dynamics in complex disordered matter.

Transmissive resonant fiber optic gyroscope employing Kagome hollow-core photonic crystal fibers (HCPCF) resonator

xinxin suo, haicheng yu, and xudong wu

DOI: 10.1364/OL.388274 Received 17 Jan 2020; Accepted 02 Mar 2020; Posted 04 Mar 2020  View: PDF

Abstract: A novel scheme of double closed-loop resonant fiber optic gyroscope (R-FOG) employing high-performance transmissive Kagome hollow-core photonic crystal fibers (HCPCF) resonator is proposed. We use the specially designed Kagome HCPCF and meniscus lens module to form a resonator, whose finesse is 58.2 with the length of 5.6m and diameter of 13cm,for a R-FOG and the theoretical sensitivity of the RFOG is better than 0.05o/h. Based on the novel Kagome HCPCF resonator, a double closed-loop R-FOG is set up and the performance of the R-FOG system is experimentally studied. It demonstrates that the white noise dominates in the output at the integration time of 200 seconds, the bias stability of 0.15o/h and the angle random walk coefficient (ARWC) of 0.04o/h1/2 is achieved. Under the dynamic range of -100 o/s~100o /s, the scale factor nonlinearity of the FOG is 310ppm, which shows a significant improvement comparing with single closed loop R-FOG system. The novel double closed-loop R-FOG proposed is quite feasible for tactical grade applications.

Observation on temperature and strain dependency of Brillouin dynamic grating in few-mode fiber with a ring-cavity configuration

Yinping LIU, Guangyao Yang, Ning Wang, Lin Ma, Juan Carlos Alvarado Zacarias, Jose Antonio-Lopez, Pierre Sillard, Adrian Amezcua, Rodrigo Amezcua Correa, Xinyu Fan, Zuyuan He, and Guifang Li

DOI: 10.1364/OL.388739 Received 20 Jan 2020; Accepted 02 Mar 2020; Posted 04 Mar 2020  View: PDF

Abstract: We experimentally conduct Brillouin dynamic grating (BDG) operation over a 1 km-long 4-mode fiber. By employing a simplified ring-cavity configuration with single-end pumping, the BDG is effectively generated in LP01 mode within a range of 250 meters and three higher order modes, namely LP11b, LP21a, and LP02 modes are chosen as probes to analyze the BDG with a spatial resolution of 1 meter. To the best of our knowledge, this is the first time to characterize the responses of BDG frequency to temperature and strain for different modes in a conventional few mode fiber. By employing the pump-probe pair of LP01- LP02 mode, the highest temperature and strain sensitivities of 3.20 MHz/ºC and -0.0384 MHz/µε have been achieved. Besides, the performance of simultaneous distributed temperature and strain sensing based on BDG are evaluated.

Attitude metrology based on field-of-view effect of birefringence using high-speed polarimetry

Song Zhang, Hao Jiang, Honggang Gu, Xiuguo Chen, and Shiyuan Liu

DOI: 10.1364/OL.387626 Received 07 Jan 2020; Accepted 01 Mar 2020; Posted 02 Mar 2020  View: PDF

Abstract: A novel optical method using a high-speed polarimetry is proposed for the real-time attitude tracking in an ultra-large measurement range. The attitude metrology utilizes the field-of-view effect in birefringent crystals, which induces obvious birefringence variation versus the field-of-view angle (a comprehensive angle of the incidence and azimuth) of the incident polarized light. The basic principle of the metrology is presented via theoretical derivation and has been verified in the static retardance measurement experiments. With a resolution test, a temporal resolution of 0.4ms per attitude measurement and an angular resolution up to 0.0025°are achieved. Through dynamic experiments, all the attitude angles of an object attached with a birefringent waveplate are obtained in real time with an accuracy better than 0.02°. Additionally, the angular velocity and acceleration of the roll angle can be extracted simultaneously. Experimental results demonstrate that the proposed metrology has great potential and advantages in the real-time attitude sensing.

Parallel gas spectroscopy using mid-infrared supercontinuum from a single Si₃N₄ waveguide

Eirini Tagkoudi, Davide Grassani, Fan Yang, Clemens Herkommer, Tobias Kippenberg, and Camille-Sophie Brès

DOI: 10.1364/OL.390086 Received 07 Feb 2020; Accepted 01 Mar 2020; Posted 06 Mar 2020  View: PDF

Abstract: Efficient third-order nonlinear optical processes have been successfully integrated on silicon nitride (Si₃N₄) waveguides. In particular, owing to Si₃N₄ wide transparency window spanning from the visible to the middle infrared (mid-IR), efficient mid-IR dispersive-wave generation from a fiber laser has been recently demonstrated and its potential as a source for absorption spectroscopy of a single gas has been established. Here, we show that the system can be further engineered to broaden the coverage of a single DW, without losing efficiency, as to enable simultaneous and discrete detection of several gas-phase molecules within the 2900 cm-¹ and 3380 cm-¹ functional group region. We demonstrate quantitative detection of acetylene, methane and ethane using a simple direct-absorption spectroscopy scheme, achieving several hundreds of parts-per-million (ppm) noise-equivalent detection limit with a 5 cm long gas cell.

Rapid functional optoacoustic micro-angiography in a burst mode

Urs Alexander Tassilo Hofmann, Johannes Rebling, Héctor Estrada, Pavel Subochev, and Daniel Razansky

DOI: 10.1364/OL.387630 Received 10 Jan 2020; Accepted 28 Feb 2020; Posted 02 Mar 2020  View: PDF

Abstract: Optoacoustic microscopy (OAM) can image intrinsic optical absorption contrast at depths of several millimeters where state-of-the-art optical microscopy techniques fail due to intense light scattering in livingtissues. Yet, wide adoption of OAM in biology and medicine is hindered by the slow image acquisition speed, small field-of-view (FOV), and/or lack of spectral differentiation capacity of common system implementations. We report on a rapid acquisition functional optoacoustic micro-angiography (FOMA) approach that employs burst-mode laser triggering scheme to simultaneously acquire multi-wavelength 3D images over an extended FOV covering 50 mm by 50 mm in a single mechanical overfly scan, attaining 28 μm and 14 μm resolution in lateral and axial dimensions, respectively. Owing to an ultrawideband low-noise design featuring a spherically-focused olyvinylidene difluoride (PVDF) transducer, we demonstrate imaging of human skin and underlying vasculature at up to 3.8 mm depth when using per-pulse laser energies of only 25 μJ without employing signal averaging. Over-all, the developed system greatly enhances the performance and usability of OAM for dermatologic and micro-angiographic studies.

Precise 3D particle localization over large axial ranges using secondary astigmatism

Yongzhuang Zhou and Guillem Carles

DOI: 10.1364/OL.388695 Received 20 Jan 2020; Accepted 28 Feb 2020; Posted 28 Feb 2020  View: PDF

Abstract: We propose an analytical pupil phase function employing cropped secondary astigmatism for extended-depth nanoscale 3D-localization microscopy. The function provides high localization precision in all three dimensions, which can be maintained over extended axial ranges, customizable up to two orders of magnitude relative to the conventional, diffraction-limited imaging. This enables for example capturing nanoscale dynamics within a whole cell. The flexibility and simplicity in the implementation of the proposed phase function make its adoption in localization-based microscopy attractive. We demonstrate and validate its application to real-time imaging of 3D fluid flow over a depth of 40μm with a numerical aperture of 0.8.

Conservative Multi-Exponential Scheme for Solving the Direct Zakharov–Shabat Scattering Problem

Sergey Medvedev, Igor Chekhovskoy, Irina Vaseva, and Mikhail Fedoruk

DOI: 10.1364/OL.387436 Received 09 Jan 2020; Accepted 27 Feb 2020; Posted 27 Feb 2020  View: PDF

Abstract: The direct Zakharov–Shabat (ZS) scattering problem has recently gained significant attention in various applications of fiber optics. The development of accurate and fast algorithms with low computational complexity to solve the ZS problem remains an urgent problem in optics. In this paper, a fourth-order multi-exponential scheme is proposed for the ZS system. The construction of the scheme is based on a fourth-order three-exponential scheme and Suzuki factorization. This allows one to apply the fast algorithms with low complexity to calculate the ZS problem for a large number of spectral parameters. The scheme conserves the quadratic invariant for real spectral parameters, which is important for various telecommunication problems related to information coding.

Computational Cannula Microscopy of neurons using neural networks

Rajesh Menon, Ruipeng Guo, Zhimeng Pan, Andrew Taibi, and Jason Sheperd

DOI: 10.1364/OL.387496 Received 08 Jan 2020; Accepted 26 Feb 2020; Posted 02 Mar 2020  View: PDF

Abstract: Computational Cannula Microscopy is a minimally invasive imaging technique that can enable high-resolution imaging deep inside tissue. Here, we apply artificial neural networks to enable fast, power-efficient image reconstructions that are more efficiently scalable to larger fields of view. Specifically, we demonstrate widefield fluorescence microscopy of cultured neurons and fluorescent beads with field of view of 200mm (diameter) and resolution of less than 10mm using a cannula of diameter of only 220mm. In addition, we show that this approach can also be extended to macro-photography.

Effective nonlinearity of the new quaternary chalcogenide crystal BaGa2GeSе6

Kiyoshi Kato, Valeriy Badikov, Li Wang, Vladimir Panyutin, Konstantin Mitin, Kentaro Miyata, and Valentin Petrov

DOI: 10.1364/OL.388373 Received 15 Jan 2020; Accepted 25 Feb 2020; Posted 28 Feb 2020  View: PDF

Abstract: The trigonal nonlinear crystal BaGa2GeSe6 has been already successfully employed for down conversion of laser radiation into the mid-IR but unknown magnitude and relative signs of the nonlinear coefficients have hindered defining an optimum orientation for maximum efficiency. This issue is resolved in the present work obtaining d11 = + .6, d22 = -18.5, and d31 = +18.3 pm/V.

Analytical model for diffuse reflectance in Single Fiber Reflectance Spectroscopy

Dirk Faber, Anouk Post, Henricus Sterenborg, and Ton Van Leeuwen

DOI: 10.1364/OL.385845 Received 12 Dec 2019; Accepted 23 Feb 2020; Posted 24 Feb 2020  View: PDF

Abstract: Cancer progression leads to changing scattering properties of affected tissues. Single Fiber Reflectance (SFR) spectroscopy detects these changes at small spatial scales, making it a promising tool for early in situ detection. Despite its simplicity and versatility, SFR signal modeling is hugely complicated so that presently only approximate models exist. We use a classic approach from Geometrical Probability to derive exact analytical expressions for diffuse reflectance in SFR that shows strong improvement over existing models. We consider the case of limited collection efficiency and the presence of absorption. A Monte Carlo light transport study demonstrates we adequately describe the contribution of diffuse reflectance to the SFR signal. Additional steps are required to include semi-ballistic, non-diffuse reflectance also present in the SFR measurement. © 2019 Optical Society of America

Self vAlidating Mueller matrix Micro - Mesoscope (SAMMM) for the characterization of biological media

Ilyas Saytashev, Sudipta Saha, Joseph Chue-Sang, Pedro Lopez, Megan Laughrey, and Jessica Ramella-Roman

DOI: 10.1364/OL.387747 Received 20 Jan 2020; Accepted 21 Feb 2020; Posted 13 Mar 2020  View: PDF

Abstract: Mueller Matrix Polarimetry (MMP) is a promising linear imaging modality, which can enable noninvasive clinical screening of several health conditions. The origin of the polarimetric signature particularly when the modality is utilized in total reflectance in biological media is often unclear due to the impact of scattering. This limits the interpretation and understanding of the data and hence the translation of this modality to the clinical setting. We have implemented a new optical system that combines Muller Matrix reflectance microscopy, Muller Matrix digital confocal imaging and Nonlinear Microscopy. With this tool we can observe the evolution of polarized light as a function of depth, we can confirm the presence of a birefringence structure such as collagen with Second Harmonic Generation and observe its impact on retardation and depolarization. We demonstrate some capabilities of our system utilizing optical standards and excised biological tissue.

Concentric microcavities for cylindrical vector beam lasers

Shuang Zheng, xiang ma, Quanan Chen, Qiaoyin Lu, Wei-Hua Guo, and Jian Wang

DOI: 10.1364/OL.388974 Received 22 Jan 2020; Accepted 20 Feb 2020; Posted 24 Feb 2020  View: PDF

Abstract: Cylindrical vector (CV) beams with polarization singularities have attracted intense research interest because of their important applications in optical trapping and manipulation, imaging, and high-speed optical communication. In this Letter, we propose and design a high-speed integrated device to emit fundamental CV beams including both radially and azimuthally polarized beams. The device is composed of two grating-assisted concentric microcavities based on InP platform. The microcavity with only a second-order grating shallowly etched on the top is optimized and used for better azimuthally polarized CV beam emission. Another one with both triangular-shape side grating and rectangular-shape top grating is employed for radially polarized CV beam lasing. The proposed device is further analyzed in the simulation and holds great potential in CV beam-based high-speed wavelength-division multiplexed (WDM) and mode-division multiplexed (MDM) optical communication systems.

Adaptive optics for time-resolved Förster Resonance Energy Transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM) in vivo

Simao Coelho, Simon Ameer-Beg, Simon Poland, and Viviane Devauges

DOI: 10.1364/OL.385950 Received 13 Dec 2019; Accepted 13 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: Förster Resonance Energy Transfer (FRET) and Fluorescence lifetime imaging (FLIM) have been coupled with multiphoton microscopy to image in vivo dynamics. However, the increase in optical aberrations as a function of depth significantly reduces the fluorescent signal, reduces spatial resolution and fluorescence lifetime accuracy. We present the development of time-resolved FRET-FLIM imaging system with adaptive optics. We demonstrate the improvement of our AO-FRET-FLIM instrument over standard multiphoton FRET-FLIM imaging. We validate our approach using fixed cellular samples with FRET standards and in vivo with live imaging in a mouse kidney.

Half-Ring Point Spread Functions

Jacob Wirth, Abbie Watnik, and Grover Swartzlander

DOI: 10.1364/OL.376860 Received 20 Sep 2019; Accepted 11 Feb 2020; Posted 11 Feb 2020  View: PDF

Abstract: Point spread function engineering has been applied to design a reversible diffuser. Our design comprises single or multiple half-ring irradiance patterns satisfying two objectives: reduced peak irradiance in the focal plane, and high image fidelity of the numerically reconstructed image. Optical elements producing such half-ring point spread functions (HR-PSF's) may be useful for applications such as sensor protection or smart diffusers. Experimental and numerical techniques were used to demonstrate three orders of magnitude of suppression of the peak irradiance. Finally, we found a general power-law trend between the Strehl ratio and the light suppression factor.

Label-free characterization of different kinds of cells using optoelectrokinetic-based microfluidics

Wenfeng Liang, Xieliu Yang, Junhai Wang, Yuechao Wang, Hemin Zhang, Wenguang Yang, and Lianqing Liu

DOI: 10.1364/OL.384883 Received 04 Dec 2019; Accepted 10 Feb 2020; Posted 11 Feb 2020  View: PDF

Abstract: We report a novel method to rapidly characterize different kinds of cells and drug-treated cancer cells using a label-free biomarker of self-rotation in an optoelectrokinetics (OEK) -based microfluidic platform. OEK incorporates optics and electrokinetics into microfluidics, thereby offering a contact-free, label-free and rapid approach to the cellular manipulation community. Self-rotational behaviors of four different kinds of cells were experimentally investigated by the frequency-sweeping of an AC bias potential in an optically-induced non-uniform and irrotational electric field. The results revealed that these kinds of cells displayed a gaussian distribution versus the AC frequency as well as different self-rotational speeds under the same conditions. Furthermore, the peak self-rotational speed varied from one kind of cells to another, with that of cancer cells higher than that of normal cells. In addition, MCF-7 cells treated by various concentrations of drug showed remarkably different self-rotational speeds. This finding suggests a high potential of developing a new label-free biomarker to rapidly distinguish different kinds of cancer cells and quantitatively monitor the response of cancer patients to various treatments.

Indocyanine green provides absorption and spectral contrast for optical coherence tomography at 840 nm in vivo

Conrad Merkle, Marco Augustin, Danielle Harper, and Bernhard Baumann

DOI: 10.1364/OL.380051 Received 10 Oct 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: In recent years, there has been growing interest in the application of exogenous contrast agents to supplement the traditional strengths of optical coherence tomography (OCT) and provide additional biological information. Here we present how indocyanine green, a common fluorescent contrast agent approved by the United States Food and Drug Administration, can provide absorption and spectral contrast for OCT imaging in the mouse eye in vivo. We further demonstrate high stability of spectral contrast measurements for the long term monitoring of contrast agents in spite of fluctuations in intensity.

Ultrafast laser ablation assisted spatially-selective attachment of fluorescent sensors onto optical fibres

Vikram Kamaljith, Michael Tanner, Harry Wood, Kerrianne Harrington, Debaditya Choudhury, Mark Bradley, and Robert Thomson

DOI: 10.1364/OL.381018 Received 22 Oct 2019; Accepted 08 Dec 2019; Posted 14 Feb 2020  View: PDF

Abstract: A robust method to selectively attach specific fluorophores to the individual cores of a multicore fibre is demonstrated. The method is based on the use of ultrafast laser pulses to nanostructure the facet of the fibre core, followed by amine functionalization and sensor conjugation. This surface machining protocol not only enables precise spatial selectivity, but also facilitates high deposition densities of the sensor moieties. As a proof of concept, the successful deposition of three different fluorophores onto selected cores of a multicore-fibre is demonstrated. The protocol was developed to include attachment of a fluorescence based pH sensor using the ratiometric carboxynapthofluorescein.

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