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Second harmonic generation divergence—a new method for domain size evaluation of 2D materials

Jingwen Deng, ZiHan Xu, Zhihao Yu, and Junrong Zheng

DOI: 10.1364/OL.409642 Received 09 Sep 2020; Accepted 24 Nov 2020; Posted 24 Nov 2020  View: PDF

Abstract: Single-atomic-layered materials are important for future electronics. They allow optoelectronic devices to be fabricated at the single-atomic layer level. A single-atomic-layered two-dimensional (2D) transition metal dichalcogenides (TMDs) film is usually composed of randomly orientated single-crystalline domains, and the size distribution of the domains on a large-area film has significant impacts on the applications of the film but difficult to characterize. We report an approach to evaluate the size of the single-crystalline domains by measuring the second harmonic generation (SHG) divergence caused by domains of different orientations. Using this method, domain size mapping on an 8 * 8 mm2 region of a continuous MoS2 film is achieved. This method provides a fast and efficient way of domain size characterization across a large area in a non-destructive and transfer-free manner for single-atomic-layered TMDs films.

Asymmetric excitable phase triggering in an optically injected semiconductor laser

Michael Dillane, Evgeny Viktorov, Ilya Dubinkin, Nikita Fedorov, Bryan Kelleher, and Benjamin Lingnau

DOI: 10.1364/OL.410085 Received 15 Sep 2020; Accepted 24 Nov 2020; Posted 24 Nov 2020  View: PDF

Abstract: One of the defining characteristics of excitability is the existence of an excitable threshold: The minimum perturbation amplitude necessary to produce an excitable response. We analyse an optically injected dual state quantum dot laser, previously shown to display a dual state stochastic excitable dynamic. We show that deterministic triggering of this dynamic can be achieved via optical phase perturbations. Further, we demonstrate that there are in fact two asymmetric excitable thresholds in this system corresponding to the two possible directions of the optical phase. For fast enough perturbations an excitable interval arises and there is a limit to the perturbation amplitude above which excitations no longer arise, a phenomenon heretofore unobserved in studies of excitability. We also highlight potential applications of the phenomenon including a novel filtering mechanism.

Enhancing the photodetection performance of MAPbI3 perovskite photodetectors by a dual functional interfacial layer for color imaging

Zhong Ji, Yujin Liu, and Wenjie Mai

DOI: 10.1364/OL.408510 Received 26 Aug 2020; Accepted 23 Nov 2020; Posted 24 Nov 2020  View: PDF

Abstract: The color imaging capacity of recently developed perovskite photodetectors (PDs) are not fully explored. In this letter, we fabricate a CH3NH3PbI3 (MAPbI3) PD as color imaging sensor mainly due to its almost flat spectral response in full visible light region. To enhance the photodetection performance, we introduce a dual functional interfacial TiO2 layer by atomic layer deposition (ALD), reducing dark current to 12 pA from 13 nA and improving photocurrent to 1.87 μA from 20 nA, resulting in a ~105 folds enhancement of ON/OFF ratio. Since having obtained satisfactory color images, we believe that MAPbI3 perovskite PD is an ideal photosensitive device for color imaging.

Chirality and band enhancement in optical waveguide lattices

Stefano Longhi

DOI: 10.1364/OL.412440 Received 13 Oct 2020; Accepted 23 Nov 2020; Posted 23 Nov 2020  View: PDF

Abstract: Light escape from an optical waveguide side-coupled to a waveguide lattice provides a photonic analogue of the spontaneous emission process of an excited two-level atom in a one-dimensional array of cavities. According to the Fermi golden rule the decay process is prevented when the atomic resonance frequency falls in a stop band of the lattice, while time-reversal symmetry ensures that the spontaneously emitted photon has equal probability to propagate in opposite directions of the array. This scenario is drastically modified when the quantum emitter drifts along the lattice at a constant speed. In the waveguide optics analogue the atomic drift is emulated by the introduction of a slight geometric tilt of the waveguide axis from the lattice axis. In this setting light excitation in the array is chiral, i.e. light propagates in a preferred direction of the lattice, and coupling is allowed even thought the waveguide is far detuned from the tight-binding lattice band,.

Few-layer metamaterials for spontaneous emission enhancement

Ling Li, Changjun Min, and Xiaocong Yuan

DOI: 10.1364/OL.413268 Received 26 Oct 2020; Accepted 23 Nov 2020; Posted 23 Nov 2020  View: PDF

Abstract: Multilayer hyperbolic metamaterials consisting of alternating metal and dielectric layers have important applications in spontaneous emission enhancement. In contrast to the conventional choice of at least dozens of layers in multilayer structures to achieve tunable Purcell effect on quantum emitters, our numerical calculations reveal that multilayers with fewer layers and thinner layers would outperform in Purcell effect. These discoveries are attributed to the negative contributions by an increasing layer number to the imaginary part of the reflection coefficient and the stronger coupling between surface plasmon polariton modes on a thinner metal layer. This work could provide fundamental insights and a practical guide for optimizing the local density of optical states enhancement functionality of ultrathin and even two-dimensional photon sources.

Focused Gaussian beam in the paraxial approximation

Ankur Das, navid soltani, and Mario Agio

DOI: 10.1364/OL.414302 Received 05 Nov 2020; Accepted 23 Nov 2020; Posted 23 Nov 2020  View: PDF

Abstract: Focusing a Gaussian beam represents a fundamental problem in optics, which is of highly practical importance in many areas of optics and photonics. Here, we derive analytical expressions for a focused Gaussian beam in the paraxial approximation and discuss the electric field in the focal region.

Microfluidic tuning of the linear and nonlinear absorption in the graphene oxide liquid crystal

Milad REZAEI MIRGHAED, Maqsoud Arshadi Pirlar, Mohammadmahdi Jahanbakhshian, and Rouhollah Karimzadeh

DOI: 10.1364/OL.408816 Received 02 Sep 2020; Accepted 23 Nov 2020; Posted 23 Nov 2020  View: PDF

Abstract: Manipulation of the nonlinear optical response of materials plays an important role in photonics applications. The difficulty in manipulating the nonlinear optical response includes irreversible, untunable, and uncontrollable. Here, we present a mechanical-hydrodynamical method for changing nonlinear absorption response of graphene oxide liquid crystals by using micro-channel. In graphene oxide liquid crystal, the optical properties depend on the orientation of the flakes. This feature led us to study empirically the dependence of the nonlinear absorption coefficients to external hydrodynamical force using the Z-scan method. Our experimental results show that by increasing the flow rate in the micro-channel, both linear and nonlinear absorption coefficients increase and the transmittance of incident laser light through the sample decreases. It is observed that the percentage change in the nonlinear absorption coefficient of the sample due to the flow rate is significant.

Narrowband nonlinear optical spectroscopy with spatially chirped broadband pulses

Hui Li, Yaying Zhao, Ying Li, and Weitao Liu

DOI: 10.1364/OL.410335 Received 18 Sep 2020; Accepted 22 Nov 2020; Posted 22 Nov 2020  View: PDF

Abstract: Nonlinear optical vibrational spectroscopies are powerful experimental tools for inspecting material properties that are difficult to acquire otherwise. As ultrafast lasers used in such experiments are typically of much broader bandwidth than vibrational modes, narrowband filtering is usually essential, and the utility of laser energy is often highly inefficient. Here we introduce an experimental scheme to break this trade-off. A broadband beam is spatially chirped as reaching the sample, and generates sum-frequency signals upon overlapping with another broadband, unchirped beam. A narrow band spectrum can then be retrieved from the spatially dispersed image of signals, with the full energy of both broadband pulses utilized. The scheme is also readily employed as a spatially resolved spectroscopy technique without scanning, and can be easily extended to other wave-mixing experiments.

Nonlinear accelerated orbiting motions of optical trapped particles through two-photon absorption

Xiaohe Zhang, Guanghao Rui, jun He, Yiping Cui, and Gu Bing

DOI: 10.1364/OL.411216 Received 28 Sep 2020; Accepted 21 Nov 2020; Posted 22 Nov 2020  View: PDF

Abstract: Vortex beams carrying optical angular momentum (AM) could drive the orbital motion of a small particle around the optical axis. In general, the orbital rotation speed of trapped particles increases linearly with the increasing of the laser power. Beyond the linear optics regime, in this work, we investigate both the optical force and torque on a two-photon absorbing Rayleigh particle produced by the tightly focused femtosecond-pulsed circularly polarized vortex beam. Different from the trapping dynamics of particles without two-photon absorption (TPA), it is shown that the orbital motion of trapped particles with TPA accelerates nonlinearly as the laser power increases. Moreover, the orbital motion acceleration of trapped particles is proportional to the TPA coefficient. The corresponding underlying mechanism is discussed in detail. Our results may find interesting applications in the characterization of the optical nonlinearity of a single nanoparticle, and AM manipulation and particle transportation in the nonlinear optics regime.

Biomimetic curved compound-eye camera with a high resolution for the detection of distant moving objects

Huangrong Xu, Yuanjie Zhang, Dengshan Wu, Geng Zhang, Ziyuan Wang, Xiangpeng Feng, Bing-liang Hu, and Weixing Yu

DOI: 10.1364/OL.411492 Received 08 Oct 2020; Accepted 21 Nov 2020; Posted 22 Nov 2020  View: PDF

Abstract: In this work, we demonstrate the design and fabrication of a biomimetic curved compound-eye camera (BCCEC) with a high resolution for detecting distant moving objects purpose. In contrast to previously reported compound eye cameras, our BCCEC has two distinct features. One is that the ommatidia of the compound eye are deployed on a curved surface which makes a large field of view (FOV) possible. The other is that each ommatidium has a relatively large optical entrance and long focal length so that a distant object can be imaged. To overcome the mismatch between the curved focal plane formed by the curved compound eye and the planar focal plane of the CMOS image sensor (CIS), an optical relay subsystem is introduced in between the compound eye and the CIS. As a result, a BCCEC with 127 ommatidia in the compound eye is designed and fabricated to achieve a large FOV of up to 98° × 98°. Experimental results show that objects with a size of 100 mm can be clearly resolved at a distance of 25 m. The capture of the motion trajectories of a moving object is also demonstrated, which makes it possible to detect and track the moving targets in a very large FOV for security surveillance purposes.

Pump-shaping of non-collinear and non-degenerate entangled photons

Ohad Lib and Yaron Bromberg

DOI: 10.1364/OL.411606 Received 02 Oct 2020; Accepted 21 Nov 2020; Posted 22 Nov 2020  View: PDF

Abstract: Free-space quantum key distribution is gaining increasing interest as a leading platform for long range quantum communication. However, the sensitivity of quantum correlations to scattering induced by turbulent atmospheric links limits the performance of such systems. Recently, methods for compensating the scattering of entangled photons have been demonstrated, allowing for real-time optimization of their quantum correlations. In this Letter, we demonstrate the use of wavefront shaping for compensating the scattering of non-collinear and non-degenerate entangled photons. These results demonstrate the applicability of wavefront shaping schemes for protocols utilizing the large bandwidth and emission angle of the entangled photons.

Fourth-harmonic generation of orbital angular momentum light with cascaded quasi-phase matching crystals

Zheng Ge, Zhiyuan Zhou, Yan Li, Chen Yang, Shikai Liu, and Bao-Sen Shi

DOI: 10.1364/OL.406162 Received 21 Aug 2020; Accepted 21 Nov 2020; Posted 22 Nov 2020  View: PDF

Abstract: Orbital angular momentum (OAM) light, combined with nonlinear process to expand the frequency range, has drawn research increasing interests in recent years. Here, we implement the first experimental fourth-harmonic generation (FHG) of OAM light with two cascaded quasi-phase-matching (QPM) crystals. A Laguerre-Gaussian beam was transmitted through a duplet crystals system and frequency-doubled twice by two separate second-harmonic generation (SHG) processes, which transduces the frequency of OAM beam from telecom band to visible band and then to ultraviolet band (UV), the topological charge of the OAM beam is increased substantially in this cascaded frequency conversion processes. In this experiment, we verify the OAM conservation by utilizing a specially designed interferometer, and the results correspond well with the numerical simulation. This work provides an effective way for the generation of UV OAM beam with high topological charges.

Anomalous Motion of Particle Levitated by Laguerre-Gaussian Beams

Yang Li, Nan Zhao, and Leiming Zhou

DOI: 10.1364/OL.405696 Received 01 Sep 2020; Accepted 21 Nov 2020; Posted 23 Nov 2020  View: PDF

Abstract: Abstract Laguerre-Gaussian (LG) beams have orbital angular momentum (OAM). A particle trapped in an LG beam will rotate about the beam axis, due to the transfer of OAM. The rotation of the particle is usually in the same direction as that of the beam OAM. However, we discovered that when the LG beam is strongly focused, the rotation of the particle and the beam OAM might be in the opposite direction. This anomalous effect is caused by the negative torque on the particle exerted by the focused LG beam, which is similar to the optical pulling force in the linear case. We calculated the scattering force distribution of a micro-particle trapped in an optical tweezer formed by the strongly focused LG beam, and showed that there exist stable trajectories of the particle that are controlled by the negative torque. We proposed several necessary conditions for observing the counter-intuitive trajectories. Our work reveals that the strongly trapped micro-particle exhibits diversity of motion patterns.

Orbital Angular Momentum beams generation from a 61 channels Coherent Beam Combining femtosecond Digital Laser

Matthieu Veinhard, severine bellanger, Louis Daniault, Ihsan Fsaifes, Jerome Bourderionnet, Christian Larat, Eric Lallier, Arnaud Brignon, and Jean-Christophe Chanteloup

DOI: 10.1364/OL.405975 Received 19 Aug 2020; Accepted 21 Nov 2020; Posted 24 Nov 2020  View: PDF

Abstract: We report on the use of a 61 beamlets Coherent Beam Combination femtosecond fiber amplifiers as a digital laser source to generate high power Orbital Angular Momentum beams. Such approach opens the path for higher order non-symmetrical user-defined far field distributions.

Observation of second-order interference beyond the coherence time with true thermal photons

Gyu-Hyeok Lee, Dong-Gil Im, Yosep Kim, U-Shin Kim, and Yoon-Ho Kim

DOI: 10.1364/OL.413287 Received 22 Oct 2020; Accepted 20 Nov 2020; Posted 22 Nov 2020  View: PDF

Abstract: It has recently been shown that counter-intuitive Franson-like second-order interference can be observed with a pair of classically-correlated pseudo thermal light beams and two separate unbalanced interferometers (UIs): the second-order interference visibility remains fixed at 1/3 even though the path length difference in each UI is increased significantly beyond the coherence length of the pseudo thermal light [Phys, Rev. Lett. 119, 263603 (2017)]. However, as the pseudo-thermal beam itself was originated from a long-coherence laser (and by using a rotating ground disk), there exists the possibility of a classical theoretical model to account for second-order interference beyond the coherence time on the long coherence time of the original laser beam. In this work, we experimentally explore this counter-intuitive phenomenon with a true thermal photon source generated via quantum thermalization, i.e., obtaining a mixed state from a pure two-photon entangled state. This experiment not only demonstrates the unique second-order coherence properties of thermal light clearly, but may also open up remote sensing applications based on such effects.

Watt-level visible laser in double-clad Pr3+-doped fluoride fiber pumped by a GaN diode

esrom kifle, Florent Starecki, Pavel Loiko, Solenn Cozic, Franck Joulain, Thibaud Berthelot, Thierry Georges, dragan stojcevski, Damien Deubel, and Patrice Camy

DOI: 10.1364/OL.413673 Received 28 Oct 2020; Accepted 20 Nov 2020; Posted 20 Nov 2020  View: PDF

Abstract: We report on a red Praseodymium fiber laser delivering 1.07 W at 634.5 nm with a slope efficiency of 21.6%, a laser threshold of 0.55 W and a single-mode output (M2x,y <1.1) in the quasi-continuous-wave regime. It is based on a 0.6 mol% Pr3+-doped ZBLAN double-clad fiber with a 5.5 μm core, a double D-shaped (diameters: 115/125 μm) inner cladding and a length of 5.0 m. The fiber is pumped by a multimode 443-nm GaN diode. The laser design is optimized using a numerical model. The proposed concept is suitable for the development of diode-pumped high brightness watt-level visible Praseodymium fiber lasers.

Compact high flux hard X-ray source driven by femtosecond mid-infrared pulses at a 1 kHz repetition rate

Azize Koç, Christoph Hauf, Michael Woerner, Lorenz von Grafenstein, Dennis Ueberschaer, Martin Bock, Uwe Griebner, and Thomas Elsaesser

DOI: 10.1364/OL.409522 Received 08 Sep 2020; Accepted 20 Nov 2020; Posted 20 Nov 2020  View: PDF

Abstract: A novel table-top hard X-ray source driven by femtosecond mid-infrared pulses provides 8-keV pulses at a 1 kHz repetition rate with an unprecedented flux of up to 1.5×10¹² X-ray photons/s. Sub-100 fs pulses at a center wavelength of 5 μm and multi-millijoule energy are generated in a four-stage optical parametric chirped-pulse amplifier (OPCPA) and focused onto a thin Cu tape target. Electrons are extracted from the target, accelerated in vacuum up to 100 keV kinetic energy during the optical cycle, and generate a highly stable Kα photon flux from the target, which outperforms state-of-the-art table-top sources by a factor of 30.

Enhancement of the response time in organic photorefractive composites using alkoxy-substituted PDCST as a nonlinear optical chromophore

Jongwan Choi, Moon Il Kim, Felix Kim, Nakjoong Kim, Jin-Woo Oh, and Jong-Sik Moon

DOI: 10.1364/OL.409743 Received 09 Sep 2020; Accepted 19 Nov 2020; Posted 20 Nov 2020  View: PDF

Abstract: The ease of the molecular orientation of a chromophore has an important effect on the electro-optical properties of polymeric photorefractive composites. A derivative of 4-piperidinobenzylidene-malononitrile (PDCST) with an alkoxy group added as a side branch was synthesized to improve the molecular orientation characteristics. Electrophoresis was performed on the polymeric photorefractive composite to which the improved PDCST had been added. The optical properties and response times were examined to evaluate the effects of the substitution of the alkoxy group. PDCST substituted with the alkoxy group showed enhanced electro-optical properties and photorefractive grating formation rate.

Arbitrarily directed emission of integrated cylindrical vector vortex beams by geometric phase engineering

Yoel Sebbag and Uriel Levy

DOI: 10.1364/OL.412026 Received 09 Oct 2020; Accepted 19 Nov 2020; Posted 20 Nov 2020  View: PDF

Abstract: Integrated cylindrical vector vortex (CVV) emitters have been introduced and studied for their potential applications in classical optics and quantum optics technologies. In this work we demonstrate that the emission angle of integrated CVV emitters can be engineered by taking advantage of the geometrical phase of a microring resonator. Two methods to superimpose an arbitrary phase profile on top on the integrated emitters are presented and compared. Angled emission of integrated vector vortex beams enables the use of chip-scale emitters for integrated nonlinear optics and for beam steering applications with orbital angular momentum.

1.1 J Yb:YAG Picosecond Laser at 1 kHz Repetition Rate

Yong Wang, Han Chi, Cory Baumgarten, KRISTIAN DEHNE, Alexander Meadows, Aaron Davenport, Gabe Murray, Brendan Reagan, Carmen Menoni, and Jorge Rocca

DOI: 10.1364/OL.413129 Received 22 Oct 2020; Accepted 19 Nov 2020; Posted 22 Nov 2020  View: PDF

Abstract: We demonstrate the generation of 1.1 J pulses of picosecond duration at 1 kHz repetition rate (1.1 kW average power) from a diode-pumped chirped pulse amplification Yb:YAG laser. The laser employs cryogenically cooled amplifiers to generate λ=1030 nm pulses with average power of up to 1.26 kW prior to compression with excellent beam quality. Pulses are compressed to 4.5 ps duration with 90% efficiency. This compact picosecond laser will enable a variety of applications that require high energy ultrashort pulses at kHz repetition rates.

Coherent beam combining with micro-lens arrays

Maike Prossotowicz, Andreas Heimes, Daniel Flamm, Florian Jansen, Hans-Jurgen Otto, Aleksander Budnicki, Alexander Killi, and Uwe Morgner

DOI: 10.1364/OL.414388 Received 10 Nov 2020; Accepted 18 Nov 2020; Posted 19 Nov 2020  View: PDF

Abstract: A novel concept for coherent beam combining is presented based on a simple setup with micro-lens arrays. These standard components are used in a proof-of-principle experiment for both beam splitting and combination of 5x5 beams. Here a combination efficiency above 90% is achieved. We call this novel concept mixed aperture.

Pixel-super-resolved complex field reconstruction using adaptive smoothing function

Yunhui Gao and Liangcai Cao

DOI: 10.1364/OL.409697 Received 08 Sep 2020; Accepted 18 Nov 2020; Posted 18 Nov 2020  View: PDF

Abstract: Digital holographic imaging can achieve a high numerical aperture, yet the resolution is limited by the finite size of the sensor pixels. To overcome this sampling limit, pixel super-resolution (PSR) techniques have been developed; however, the inherent non-convexity of the PSR phase retrieval problem can potentially degrade the reconstruction quality, by causing the iterations to tend toward a false local minimum. Furthermore, the ill-posedness of the up-sampling procedure renders the PSR algorithms highly susceptible to noise. In this Letter, we propose a heuristic PSR algorithm featuring an adaptive smoothing function (ASF-PSR), to achieve high-fidelity reconstruction. By automatically adjusting the intensity constraints on the estimated field, the algorithm can effectively locate the optimal solution and converge with a high reconstruction quality, pushing the resolution toward the diffraction limit. The proposed method is verified experimentally within a coherent modulation phase retrieval framework, achieving a twofold improvement in resolution. The ASF-PSR algorithm can be further applied to other propagation-based phase retrieval methods.

V- and W-band microwave generation and modulation using optically injected semiconductor lasers at period-one nonlinear dynamics

Chin-Hao Tseng, Chun-Ting Lin, and Sheng-Kwang Hwang

DOI: 10.1364/OL.412327 Received 15 Oct 2020; Accepted 18 Nov 2020; Posted 18 Nov 2020  View: PDF

Abstract: Microwave generation and modulation over the V and W bands are investigated using a semiconductor laser subject to both comb-like (CL) optical injection and direct modulation. The former not only excites period-one (P1) nonlinear dynamics for tunable microwave generation, but also improves the stability and purity of such generated microwaves. The latter upconverts data onto the generated microwaves by superimposing the data effectively only onto the lower oscillation sideband of the P1 dynamics, which prevents the data from dispersion-induced degradation over fiber distribution. As a result, microwaves that are continuously tunable from 40 to 110 GHz with a 3-dB linewidth of less than 1 Hz and with phase noise better than -95 dBc/Hz at 10-kHz offset are generated. A bit-error ratio better than the forward error correction limit, 3.8×10^(-3), is achieved for 12-Gb/s 16-quadrature amplitude modulation data after 25-km fiber distribution.

Wide-range optical carrier recovery via broadened Brillouin filters

Atiyeh Zarifi, Moritz Merklein, Yang Liu, Amol Choudhary, Benjamin Eggleton, and Bill Corcoran

DOI: 10.1364/OL.411482 Received 30 Sep 2020; Accepted 17 Nov 2020; Posted 17 Nov 2020  View: PDF

Abstract: Stimulated Brillouin Scattering has great potential forwide wavelength range optical carrier recovery as itcan act as a parametrically-defined narrowband gain filter.However, due to the dispersion of the Brillouinfrequency shift, prior demonstrations have been limitedin wavelength range. Here, we demonstrate thatfrequency modulating the pump light for a gain filterbased on stimulated Brillouin scattering enables opticalcarrier recovery for a broad range of input wavelengths.We demonstrate highly selective (<150 MHzbandwidth) amplification for optical carriers over an18-nm wide wavelength range in the optical communicationsC-band, a ~6x improvement over using an unmodulatedpump. Measurements of the noise propertiesof these spectrally-broadened gain filters, in bothamplitude and phase, indicate the noise performanceand SNR is maintained over a wide wavelength range.Our technique provides a potential solution for highly selective,wavelength agnostic optical carrier recovery.

Coherent synthetic aperture imaging for visible remote sensing via reflective Fourier ptychography

MENG XIANG, An Pan, Yiyi Zhao, Xuewu Fan, Hui Zhao, Chuang Li, and Baoli Yao

DOI: 10.1364/OL.409258 Received 02 Sep 2020; Accepted 17 Nov 2020; Posted 18 Nov 2020  View: PDF

Abstract: Synthetic aperture radar (SAR) can measure the phase of microwave with antenna, which cannot be directly extended to visible light imaging due to phase lost. In this letter, we reported an active remote sensing with visible light via reflective Fourier ptychography (FP), termed coherent synthetic aperture imaging (CSAI), achieving high resolution, wide field-of-view (FOV), and phase recovery. A proof-of-concept experiment was reported with laser scanning and a collimator for the infinite object. Both smooth and rough objects are tested, and the spatial resolution increased from 15.6 μm to 3.48 μm with a factor of 4.5. The speckle noise can be suppressed obviously, which is important for coherent imaging. Meanwhile, the CSAI method can tackle the aberration induced from the optical system by one-step deconvolution and shows the potential to replace the adaptive optics for aberration removal of atmospheric turbulence.

Beyond the 2D limit: Étendue-squeezing line-focus solar concentrators

Håkon Johnsen, Astrid Aksnes, and Jan Torgersen

DOI: 10.1364/OL.406280 Received 01 Sep 2020; Accepted 17 Nov 2020; Posted 17 Nov 2020  View: PDF

Abstract: Line-focus solar concentrators are commonly designed by extruding a two-dimensional concentrator in the third dimension. These concentrators are, by the nature of their design, fundamentally limited by the two-dimensional solar concentration limit of 212x. This limit is orders of magnitude lower than the 45 000x concentration limit for three-dimensional solar concentrators. Through the use of étendue-squeezing, we conseptually show that it is possible to design line-focus solar concentrators beyond this 2D limit. This allows a concentrator to benefit from a line-focus suitable for heat-extraction through a tubular receiver, while reaching concentration ratios and acceptance angles previously unseen for line-focus concentrators. We show two design examples, achieving a simulated 75x concentration and 218x concentration ratio respectively, with a ±1º acceptance angle. For comparison, the 2D concentration limit is 57x at this acceptance angle. Étendue-squeezing line-focus solar concentrators, combined with recent developments in tracking-integration, may enable the development of a new class concentrated solar power (CSP) system.

Cascaded metamaterial polarizers for the visible region

PAWARAT BOOTPAKDEETAM, Hafez Hemmati, and Robert Magnusson

DOI: 10.1364/OL.411234 Received 29 Sep 2020; Accepted 17 Nov 2020; Posted 17 Nov 2020  View: PDF

Abstract: Polarizers serve many application fields such as imaging, display technology, and telecommunications. Focusing on the visible spectral region, we provide design and fabrication of compact high-efficiency resonant polarizers in the crystalline silicon-on-quartz (SOQ) material system. We experimentally verify the improved efficiency attained by a cascaded dual-module polarizer assembled with building blocks of elemental subwavelength grating structures. We obtain a measured extinction ratio (ER) of ~3000 in a 2-mm-thick stacked prototype device across a bandwidth of ~110 nm in the 570-680 nm spectral-domain. The ridge-width of the constituent nanograting is ~84 nm. Computed results show that the ER is based on the low-loss nature of crystalline silicon in the visible range, thus enabling cascaded metasurfaces while preserving high transmission.

High-accuracy automatic target recognition scheme based on photonic analog to digital converter and convolutional neural network

Jun Wan, Shaofu Xu, and Weiwen Zou

DOI: 10.1364/OL.411214 Received 06 Oct 2020; Accepted 17 Nov 2020; Posted 20 Nov 2020  View: PDF

Abstract: We propose a high-accuracy automatic target recognition (ATR) scheme based on a photonic analog-to-digital converter (PADC) and a convolutional neural network (CNN). The adoption of the PADC enables wideband signal processing up to several giga-hertz and thus high-resolution range profiles (RPs) are attained. Besides, the CNN guarantees high recognition accuracy based on such RPs. Using unmanned aerial vehicles (UAVs) and corner reflectors (CRs) as targets, the performance of the proposed ATR scheme based on the PADC and CNN is experimentally tested in different range resolution cases. The recognition result reveals that high range resolution leads to high ATR accuracy. It is proved that when dealing with centimeter-sized small targets, the ATR scheme can acquire a much better recognition accuracy than other RP ATR solutions based on electronic schemes. Analysis results also show the reason why higher recognition accuracy is attained with higher-resolution RPs.

Light-field multispectral radiation thermometry

Yinsen Luan, Di Mei, and kirin shi

DOI: 10.1364/OL.408437 Received 25 Aug 2020; Accepted 17 Nov 2020; Posted 19 Nov 2020  View: PDF

Abstract: This letter proposes a light-field multi-spectral radiation thermometry (MRT) based on unfocused light-field camera which can simultaneously record directions and intensities of incident rays. In this method, the direction information of rays is substituted by radiation spectrums via placing an array of filter in front of camera main lens, such that the image sensor can simultaneously acquire spectrums and intensities of rays. By decoupling a raw multi-spectral light field (MSLF) image and utilizing traditional multi-wavelength pyrometer (MWP) algorithms, the scalar field of surface temperature distribution can be achieved. To verify the method, measurement errors of different temperature levels on several typical areas of MSLF image are analyzed. In addition, validation experiment demonstrates that accurate surface temperature measurement can be achieved with a single lens, single monochromatic image sensor and just one snapshot in the proposed method.

A low-cost single-point optoacoustic sensor for spectroscopic measurement of local vascular oxygenation

Antonios Stylogiannis, Lucas Riobo, Ludwig Prade, Sarah glasl, sabine Klein, Giulia Lucidi, Martin Fuchs, Dieter Saur, and Vasilis Ntziachristos

DOI: 10.1364/OL.412034 Received 09 Oct 2020; Accepted 16 Nov 2020; Posted 19 Nov 2020  View: PDF

Abstract: Optical sensors developed for oxygen assessment in tissue microvasculature, such as those based on near-infrared spectroscopy (NIRS), are limited in application by light scattering. Optoacoustic methods are insensitive to light scattering and therefore can provide higher specificity and accuracy when quantifying local vascular oxygenation. However, there is currently no low-cost, single point, optoacoustic sensor for the dedicated measurement of oxygen saturation in tissue microvasculature. This work introduces a spectroscopic optoacoustic sensor (SPOAS) for the non-invasive measurement of local vascular oxygenation in real time. SPOAS employs continuous wave laser diodes and measures at a single point, making it low-cost and portable. The performance of SPOAS was benchmarked with blood phantoms and showed excellent correlation (R2=0.98) with a Blood Gas Analyzer. Subsequent measurements of local vascular oxygenation in living mice during an oxygen stress test correlated well with simultaneous readings from a reference instrument.

Tm3+ and Ho3+ colasing in codoped ultrafast laser inscribed in-band pumped waveguides

Esrom Kifle, Pavel Loiko, Carolina Romero, Javier Vazquez de Aldana, Viktor Zakharov, Julia Gurova, Andrei Veniaminov, Valentin Petrov, Uwe Griebner, romain thouroude, Mathieu Laroche, Magdalena Aguilo, Patrice Camy, Francesc Diaz, and Xavier Mateos

DOI: 10.1364/OL.399546 Received 04 Jun 2020; Accepted 16 Nov 2020; Posted 17 Nov 2020  View: PDF

Abstract: We report on the first in-band pumped Tm3+,Ho3+ codoped waveguide (WG) laser. A depressed-index surface channel WG (type III) with a 50 μm half-ring cladding is fabricated in a 5 at.% Tm3+, 0.5 at.% Ho3+:KLu(WO4)2 crystal by femtosecond pulse direct laser writing. Under in-band pumping by a 1679-nm Er Raman fiber laser, Tm3+ and Ho3+ colasing is observed in the WG and explained by bidirectional energy transfer. The maximum total output power at ~1942 nm (Tm3+) and 2059 nm (Ho3+) is 448 mW with a slope efficiency of 40.6%, record-high for this type of WG lasers. The maximum Ho3+ output power reaches 144 mW.

Tunable Metamaterial-Based Silicon Waveguide

Yuxi Han, Jie Lin, and Yu-Sheng Lin

DOI: 10.1364/OL.414129 Received 03 Nov 2020; Accepted 16 Nov 2020; Posted 18 Nov 2020  View: PDF

Abstract: A tunable metamaterial (MM) based silicon (Si) waveguide is presented, which is composed of MM nanodisk array on silicon-on insulator (SOI) substrate. It could be realized a significant modulation efficiency of transmission intensity by elevating individually or simultaneously the column number of MM nanodisks. For convenient description, MM-based Si waveguide with one, two, three, four, and five columns of MM nanodisks are denoted as MM-1, MM-2, MM-3, MM-4, and MM-5, respectively. Transmission intensity of MM-based Si waveguide could be switched between on to off states by driving different columns of MM nanodisks on Si waveguide surface. Transmission intensities could be attenuated from 100% to 56%, 24%, 6%, 1%, and 0% for MM-1, MM-2, MM-3, MM-4, and MM-5, respectively at the wavelength of 1.525 μm. Furthermore, MM-5 device is exposed on ambient environment with different refraction indexes. It exhibits a linear relationship of resonance dips and refraction indexes. The proposed design of MM-based Si waveguide provides the potential possibilities in optical switch, variable optical attenuator (VOA), and sensor applications.

Doppler effect as a tool for ultrashort electric field reconstruction

Pierre Bejot, Ester Szmygel, Antoine Dubrouil, Franck Billard, B. Lavorel, Olivier Faucher, and edouard hertz

DOI: 10.1364/OL.402935 Received 17 Jul 2020; Accepted 16 Nov 2020; Posted 16 Nov 2020  View: PDF

Abstract: We present a novel method for characterizing ultrashortlaser pulses by spectral-shearing interferometry. Thisoriginal approach, called Doppler Effect E-field Replication(DEER), exploits the rotational Doppler effect forproducing the frequency shear and provides a spectralshearingin the absence of frequency conversion thatbreaks the technological limitations of operation in theultraviolet spectral range. Evaluation of the DEER-SPIDERset-up reveals a phase reconstruction of greatreliability. Possible improvements, benefits, and worthwhileprospects of the method are discussed.

Adjustable and stable beam profile generation in a Yb:YAG thin-disk laser

enmao song, tianrui dai, Guangzhi Zhu, Hailin Wang, Kozlov Aleksei, and Xiao Zhu

DOI: 10.1364/OL.412599 Received 14 Oct 2020; Accepted 16 Nov 2020; Posted 16 Nov 2020  View: PDF

Abstract: We present adjustable and stable beam profile generation from a Yb:YAG thin-disk laser. A double-cavity geometry with an acousto-optic modulator (AOM) makes the spatial separation of gain area, in which the central and external part are used for LG00 and LG01 mode generation, respectively. By changing the loss of AOM, the beam profile is adjustable continuously from annularity to flat-top. At input pump power of 44 W, 8.5 W annular, 11.1 W shallow crater-shaped and 12.1 W flat-toped modes were obtained. Moreover, the thin-disk architecture with multi-pass pumping scheme can not only improve thermal management, but also minimize depolarization loss, allowing the optimization of mode stability and laser performances. Our work implies that a high-power laser with adjustable and stable beam profile can be implemented via this scheme.

Spectral broadening of ultraviolet dispersive waves in gas-filled hollow-core fiber using pump pulse modulation

Callum Smith, ABUBAKAR ISA ADAMU, Mattia Michieletto, and Ole Bang

DOI: 10.1364/OL.412652 Received 15 Oct 2020; Accepted 16 Nov 2020; Posted 16 Nov 2020  View: PDF

Abstract: Ultraviolet (UV) supercontinuum laser sources based on resonant dispersive wave (RDW) generation in gas-filled hollow core (HC) fibers offer an attractive architecture for numerous applications. However, the narrow UV spectral peak inherent to RDW generation limits the suitability for applications that require broad spectral coverage within the UV region such as spectroscopic scatterometry techniques. In this Letter, we demonstrate how the UV spectrum can be shaped by modulating the peak power of the pump pulses driving the RDW generation, thereby creating a broadened and flattened UV spectrum. Using an argon (Ar)-filled anti-resonant (AR) HC fiber, we generate a UV spectrum with a center wavelength of 3 .6 nm with a full-width half-maximum (FWHM) of 51.7 nm, corresponding to a relative bandwidth of 16.1 %.

Long-wave mid-infrared time-resolved dual-comb spectroscopy of short-lived intermediates

Pei-Ling Luo

DOI: 10.1364/OL.413754 Received 02 Nov 2020; Accepted 15 Nov 2020; Posted 17 Nov 2020  View: PDF

Abstract: In this Letter, an electro-optic dual-comb spectrometer with a central tunable range of 7.77−8.22 μm is demonstrated to perform transient absorption spectroscopy of the simplest Criegee intermediate (CH₂OO), a short-lived species involved in many key atmospheric reactions, and its self-reaction product via comb-mode-resolved spectral sampling at microsecond temporal resolution. By combining with a Herriott-type flash photolysis cell, CH₂OO can be probed with a detection limit down to ~1×10¹¹ molecules/cm³. Moreover, pressure broadening of CH₂OO absorption lines can be studied with spectrally interleaved dual-comb spectroscopy. This work holds promise for high-resolution precision measurements of transient molecules, and especially for study of large molecules in complex systems.

Surface tension and viscosity measurement of oscillating droplet using rainbow refractometry

Qimeng Lv, Yingchun Wu, Can Li, Xuecheng Wu, Linghong Chen, and Kefa Cen

DOI: 10.1364/OL.412498 Received 13 Oct 2020; Accepted 14 Nov 2020; Posted 16 Nov 2020  View: PDF

Abstract: We extend rainbow refractometry to quantify the oscillations of a droplet in its fundamental mode. The oscillation parameters (frequency and amplitude damping), extracted by the time-resolved rainbow angular shift, are utilized to measure surface tension and viscosity of the liquid. Proof-of-concept experiments on oscillating droplets stream produced by a monodisperse droplet generator are conducted. Results show that the relative measurement errors of surface tension and viscosity are 1.5% and 8.4% for water, and 5.3% and 2.5% for ethanol. This approach provides an alternative mean for characterizing liquid surface properties, e.g., dynamic surface tension and viscosity measurement, especially for liquids with a low Ohnesorge number.

Dual-longitudinal-mode CW self-sweeping operation in Er-doped fiber laser

Ivan Lobach, Sergey Kablukov, and Ekaterina Kashirina

DOI: 10.1364/OL.412781 Received 19 Oct 2020; Accepted 14 Nov 2020; Posted 18 Nov 2020  View: PDF

Abstract: A new type of sweeping operation – dual-mode continuous wave (CW) self-sweeping - is demonstrated in an Erbium-doped fiber laser with a sweeping range of 2.8 nm in a region of 1605 nm. The laser generates two adjacent longitudinal modes of equal intensity, but at some moments of time one of the modes with lower frequency begins to vanish and a new one with even higher frequency starts to grow. As a result, the self-sweeping of lasing frequency with CW intensity dynamics is observed.

Visual data detection through side-scattering in a multimode optical fiber

Daniel Marima, Barak Hadad, Sahar Froim, Avishay Eyal, and Alon Bahabad

DOI: 10.1364/OL.408552 Received 01 Sep 2020; Accepted 14 Nov 2020; Posted 16 Nov 2020  View: PDF

Abstract: Light propagation in optical fibers is accompanied by random omnidirectional scattering. The small fraction of coherent guided light that escapes outside the cladding of the fiber forms a speckle pattern. Here, visual information imaged into the input facet of a multi-mode fiber with a transparent buffer is retrieved, using a Convolutional Neural Network, from the side-scattered light, at several locations along the fiber. This demonstration can promote the development of distributed optical imaging systems and optical links interfaced via the sides of the fiber.

All-fiber laser source at 1645~nm for lidar measurement of methane concentration and wind velocity

Philippe Benoit, Simon LM, Julien Le Gouët, and Guillaume Canat

DOI: 10.1364/OL.409327 Received 02 Oct 2020; Accepted 14 Nov 2020; Posted 16 Nov 2020  View: PDF

Abstract: We report on the realization of an all-fiber laser source that delivers single-frequency pulses at 1645~nm, on a linearly polarized single-mode beam, based on stimulated Raman scattering in passive fibers. The pulse energy reaches 14 µJ, for a repetition rate of 20~kHz, and the spectral linewidth is 10~MHz, which is almost Fourier limited for 100~ns square pulses. To the best of our knowledge, this energy is the highest reported at 1645~nm in an all-fiber laser source. Our method consists in reducing the stimulated Brillouin scattering (SBS) gain for the pump and signal pulses, respectively by sweeping the optical frequency of the pump beam, and by applying a strain gradient on the amplification fiber. This compact laser source is now used in a transportable lidar system to measure simultaneously wind velocity and methane (CH4) concentration.

Detector self-tomography

Alfredo Luis and Raul Consul

DOI: 10.1364/OL.410265 Received 16 Sep 2020; Accepted 13 Nov 2020; Posted 16 Nov 2020  View: PDF

Abstract: We present an intuitive model of detector self-tomography. Two identical realisations of the detector are illuminated by an entangled state that connects the joint statistics in a way in which each detector sees the other as a kind of mirror reflection. A suitable analysis of the statistics reveals the possibility of fully characterizing the detector. We apply this idea to Bell-type experiments revealing their nonclassical nature.

High-sensitivity, high-resolution polymer fiber Bragg grating humidity sensor harnessing microwave photonic filtering response analysis

Kun Zhu, Xin Cheng, zhiyong zhao, and Chao Lu

DOI: 10.1364/OL.411221 Received 30 Sep 2020; Accepted 12 Nov 2020; Posted 12 Nov 2020  View: PDF

Abstract: Polymer optical fiber Bragg gratings (POFBGs) provide an efficient, robust, and cost-effective approach for humidity sensing. In this letter, we propose and experimentally demonstrate a high-sensitivity and high-resolution POFBG-based humidity sensor that utilizes microwave photonic filtering response analysis. A POFBG and a silica FBG are used as the sensing probe, and a two-tap microwave photonic filter is developed to enable the high precision interrogation. Since only the poly (methyl methacrylate) (PMMA) based fiber is sensitive to humidity, the wavelength shift due to the humidity change is converted to the change of frequency response of the microwave photonic filter. A relationship between the humidity and the free spectral range (FSR) of the microwave photonic filter is established as the interrogation technique. Compared with the traditional approach by directly monitoring the wavelength shift using optical spectrum analyzer, our proposed scheme can increase the humidity sensor resolution by two orders of magnitude, which greatly increases the accuracy of the humidity sensor, making it possible and feasible in many potential high-precision applications.

Optical mode conversion in coupled Fabry-Pérot resonators

Mark Stone, Aziza Suleymanzade, Lavanya Taneja, David Schuster, and Jonathan Simon

DOI: 10.1364/OL.400998 Received 15 Sep 2020; Accepted 12 Nov 2020; Posted 13 Nov 2020  View: PDF

Abstract: Low-loss conversion amongst a complete and orthogonal set of optical modes is important for high bandwidth quantum and classical communication. In this letter, we introduce tunable impedance mismatch between coupled Fabry-Pérot resonators as a powerful tool for manipulation of the spatial and temporal properties of optical fields. In the single-mode regime, frequency dependent impedance matching enables tunable finesse optical resonators. Introducing the spatial dependence of the impedance mismatch enables coherent spatial mode conversion of optical photons at near-unity efficiency. We experimentally demonstrate a NIR resonator whose finesse is tunable over a decade, and an optical mode converter with efficiency >75% for the first six Hermite-Gauss modes. We anticipate that this new perspective on coupled multimode resonators will have exciting applications in micro- and nano- photonics and computer-aided inverse design.

Phase-sensitive, angle-resolved light-scattering microscopy of single cells

Robert Draham, Kaitlin Dunn, and Andrew Berger

DOI: 10.1364/OL.409345 Received 02 Oct 2020; Accepted 12 Nov 2020; Posted 13 Nov 2020  View: PDF

Abstract: We report what is to our knowledge the first use of Fourier phase microscopy (FPM) to estimate diameters of individual single-micron beads and to classify cells based upon changes in scatterer size distribution. FPM, a quantitative phase imaging (QPI) method, combines the planar illumination typically used in off-axis QPI (ideal for Mie theory analysis) with the common-path geometry typically used in on-axis QPI (ideal for optimizing angular scattering range). Low-spatial-frequency imaging artifacts inherent to FPM have negligible impact upon these angular-domain applications. The system is simple to align, stable, and requires no external reference beam. Angular scattering patterns obtained from single one-micron polystyrene beads in glycerol (δn = 0.10) display unprecedented fidelity to Mie theory, produce diameter estimates consistent with the manufacturer’s specifications, and offer precision on the scale of tens of nanometers. Measurements of macrophages at different stages of antibody-dependent cellular phagocytosis demonstrate the ability to detect changes in a cell’s scattering caused by the presence of phagocytosed material within.

Sputtering AlN/InxAl1-xN Distributed Bragg Reflector across the Full Visible Range on Arbitrary Substrates

Linyun Long, tao li, Zelin Hu, WenQing Song, Lei Zhang, and Liancheng Wang

DOI: 10.1364/OL.413264 Received 22 Oct 2020; Accepted 12 Nov 2020; Posted 13 Nov 2020  View: PDF

Abstract: III-nitrides based Distributed Bragg Reflector (DBR) are advantageous to in-situ be integrated in III-nitrides devices and the bandgap and their other corresponding optical parameters are tunable. However, growing nitride DBR with low strain and high reflectivity remains as a challenge. Here we demonstrate AlN/InxAl1-xN DBR grown on various substrates by reactive radio-frequency (RF) magnetron sputtering. Reflectance wavelengths covering the whole visible regions of the visible spectrum were achieved by rationally tuning the indium composition in InxAl1-xN and each layer’s thickness of AlN/InxAl1-xN DBR. This work should advance the design and fabrication of nitrides optical and optoelectrical devices by incorporating AlN/InxAl1-xN DBR, such as VCSEL and RC LEDs.

High purity optical vortex generation in fiber Bragg grating inscribed by femtosecond laser

li yali, Zhiyong Bai, liu Zhao, zhu guoxuan, Kaiming Yang, Jian Yu, Chen Jiayan, Cailing Fu, Changrui Liao, and Yiping Wang

DOI: 10.1364/OL.410277 Received 23 Sep 2020; Accepted 12 Nov 2020; Posted 13 Nov 2020  View: PDF

Abstract: In this letter, a method for orbital angular momentum (OAM) modes generation is proposed and experimentally demonstrated using fiber Bragg grating (FBG) and off-axis incidence. The FBG fabricated by femtosecond laser was used to couple the incidence beam into backwards high order modes. The generated modes were then reformed into ring-shape OAM modes by adjusting the off-axis displacement of the input beam. The intensity distribution, phase vortex and mode purity of the output light were experimentally investigated. Results indicates that the order of the generated OAM modes is depended on the resonant wavelength of FBG, and the sign of the OAM topological charge is determined by the displacement value of the off-axis incident light. In the experiment, ±1- and ±2-order OAM modes were achieved and confirmed, with purities as high as 90%, 91%, 89% and 88%, respectively.

Storage and retrieval of slow-light dark solitons

Guoxiang Huang and Chong Shou

DOI: 10.1364/OL.412247 Received 13 Oct 2020; Accepted 10 Nov 2020; Posted 10 Nov 2020  View: PDF

Abstract: We show that stable slow-light dark solitons with finite continuous-wave background can be generated in a Λ-type atomic system via electromagnetically induced transparency (EIT). We also show that such dark solitons can be stored and retrieved with high efficiency and fidelity. Moreover, an optical routing of them can be realized via the EIT in the system with a double-Λ-type level configuration.

Continuously tunable high-power single-longitudinal-mode Ho:YLF laser around P12 CO2 absorption line

Yunpeng Wang, Youlun Ju, Tongyu Dai, Dong Yan, Ying Chen, Xinyu Fang, Xiaoming Duan, and Baoquan Yao

DOI: 10.1364/OL.412617 Received 15 Oct 2020; Accepted 10 Nov 2020; Posted 10 Nov 2020  View: PDF

Abstract: A continuously tuned single-longitudinal-mode (SLM) Ho:YLF laser around P12 CO₂ absorption line was demonstrated. The continuous tuning range of 5.75 pm within one longitudinal mode spacing of the Ho:YLF resonator was realized by using a new intra-cavity wedge prism device fixed on the piezoelectric transducer (PZT) as the cavity length controller. High SLM power of 11.3 W was obtained from a master oscillator power amplifier (MOPA) by matching laser mode twice at the pump power of 31.8 W and seed power of 3 mW, corresponding to a gain of 15.44 dB and an optical-to-optical conversion efficiency of 34.5%. The beam quality factors M² of the SLM Ho:YLF amplifier in the x and y directions were estimated to be 1.03 and 1.04, respectively.

An Individual Transducer Impulse Response Characterization Method to Improve Image Quality of Array-based Handheld Optoacoustic Tomography

Kaushik Chowdhury, Maximilian Bader, Christoph Dehner, Dominik Jüstel, and Vasilis Ntziachristos

DOI: 10.1364/OL.412661 Received 16 Oct 2020; Accepted 10 Nov 2020; Posted 10 Nov 2020  View: PDF

Abstract: The physical properties of each transducer element play a vital role in the quality of images generated in optoacoustic (photoacoustic) tomography using transducer arrays. Thorough experimental characterization of such systems is often laborious and impractical. A shortcoming of the existing impulse response correction methods however is the assumption that all transducers in the array are identical and, therefore, share one electrical impulse response (EIR). In practice, the EIR of the transducer elements in the array vary, and the effect of this element-to-element variability on image quality has not been investigated so far. We hereby propose a robust EIR derivation for individual transducer elements in an array using sparse measurements of the total impulse response (TIR) and by solving the linear system for the temporal convolution. Thereafter, we combine a simulated spatial impulse response (SIR) with the derived individual EIRs to obtain a full characterization of the TIR, which we call individual synthetic TIR (isTIR). Correcting for individual transducer responses we demonstrate significant improvement in isotropic resolution, which further enhances the clinical potential of array-based handheld transducers.

Lithography-free disordered metal-insulator-metal nanoantennas for colorimetric sensing

Zeinab Eftekhari, Amir Ghobadi, and Ekmel Ozbay

DOI: 10.1364/OL.410213 Received 15 Sep 2020; Accepted 10 Nov 2020; Posted 11 Nov 2020  View: PDF

Abstract: The colorimetric detection of bio-agent targets has attracted considerable attention in nanosensor designs. This platform provides an easy to use, real-time, and rapid sensing approach as the color change can be easily distinguished by the naked eye. In this paper, we propose a large scale compatible fabrication route to realize colorimetric optical nanosensors. For this purpose, we design and fabricate a tightly-packed disordered arrangement of metal-insulator-metal (MIM) nanoantennas with a resonance frequency at visible light wavelengths. In this design configuration, the adsorbed bio-agent changes the effective refractive index of the cavity and this causes a shift in the resonance wavelength. The experimental data shows that the proposed design can have sensitivity values >70 nm/refractive index unit (RIU). Unlike other optical sensing schemes that mainly rely on hot spot formation and field enhancement, this design has a large active area with relatively uniform patterns that make it a promising approach for low-level and reliable bio-detection.

Phase-only modulation of light

Vladimir Kesaev and Alexei Kiselev

DOI: 10.1364/OL.410450 Received 16 Sep 2020; Accepted 10 Nov 2020; Posted 11 Nov 2020  View: PDF

Abstract: We disclose the method to obtain polarization insensitive phase-only modulation that preserves both the state and the degree of polarizationof light modulated using a medium with controlled birefringence.We find that, in the double-pass configuration involving reflection from the Faraday rotator mirror, such a medium acts as the phase-only modulator. The experimental data measured in the Michelson interferometer based setup for deformed-helix ferroelectric liquid crystal cells are found to be in good agreement with the theoretical results. For such cells, we experimentallydemonstrate high-frequency (4 kHz modulation rate) 2π phase-onlylight modulation governed by the average phase shift and solve the problem of optical axes switching.

PAExM: Label-free hyper-resolution photoacoustic expansion microscopy

Hyojin Kim, Jinwoo BAIK, Seungwan Jeon, Jin Kim, and Chulhong Kim

DOI: 10.1364/OL.404041 Received 29 Jul 2020; Accepted 10 Nov 2020; Posted 12 Nov 2020  View: PDF

Abstract: Reflection-mode ultraviolet photoacoustic microscopy (UV-PAM) is capable of imaging cell nuclei in thick tissue without complex preparation procedures, but it is challenging to distinguish adjacent nuclei due to the limited spatial resolution. Tissue expansion technology has recently been developed to exceed the diffraction-limited fluorescence microscopies, but it is accompanied by limitations including additional staining. Herein, photoacoustic expansion microscopy (PAExM) is presented, which is an advanced histologic imaging strategy combining advantages of fast label-free reflection-mode UV-PAM and the tissue expansion technology. Clustered cell nuclei in an enlarged volume of a mouse brain section can be visually resolved without staining, demonstrating a great potential of the system to be widely used for histologic applications throughout biomedical fields.

Multi-wavelength Phase Retrieval

Erik Malm, Anders Mikkelsen, and Edwin Fohtung

DOI: 10.1364/OL.408452 Received 01 Sep 2020; Accepted 09 Nov 2020; Posted 10 Nov 2020  View: PDF

Abstract: Phase retrieval is a numerical procedure concerned with the recovery of a complex-valued signal from measurements of its amplitude. We describe a generalization of this method for multi-wavelength data acquired from a coherent diffractive imaging experiment. It exploits the wavelength-dependent scaling of the real-space spatial coordinate to recover separate reconstructions for each wavelength providing new possibilities for coherent diffractive imaging experiments. Limitations on the number of wavelengths are discussed through the adaptation of the constraint ratio and the method's performance is investigated as a function of the source spectrum, sample geometry and degree of complexity through numerical simulations.

Dispersion tailoring of femtosecond laser written chirped fiber Bragg gratings by selective femtosecond laser post-processing

Timothy Imogore, Ria G. Krämer, Thorsten A. Goebel, Christian Matzdorf, Daniel Richter, and Stefan Nolte

DOI: 10.1364/OL.411679 Received 05 Oct 2020; Accepted 09 Nov 2020; Posted 11 Nov 2020  View: PDF

Abstract: We present the tuning of the dispersion properties of a femtosecond (fs) laser inscribed chirped fiber Bragg grating (CFBG), realized by selectively modifying the refractive index of the already inscribed CFBG by fs laser post-processing. This work demonstrates for the first time to the best of our knowledge, a flexible approach for tailoring higher order dispersion terms of a fs inscribed CFBG via fs post-processing of selected grating regions, thus paving the way e.g. for applications in dispersion management of ultrashort pulse fiber lasers.

Single-Pixel Pattern Recognition with Coherent Nonlinear Optics

ting bu, Santosh Kumar, He Zhang, Irwin Huang, and Yuping Huang

DOI: 10.1364/OL.411564 Received 01 Oct 2020; Accepted 09 Nov 2020; Posted 10 Nov 2020  View: PDF

Abstract: We propose and experimentally demonstrate a nonlinear-optics approach to pattern recognition with single-pixel imaging and deep neural network. It employs mode selective image up-conversion to project a raw image onto a set of coherent spatial modes, whereby its signature features are extracted nonlinear-optically. With 40 projection modes, the classification accuracy reaches a high value of 99.49% for the MNIST handwritten digit images, and up to 95.32% even when they are mixed with strong noise. Our experiment harnesses rich coherent processes in nonlinear optics for efficient machine learning, with potential applications in online classification of large-size images, fast lidar data analyses, complex pattern recognition, and so on.

Coherent Silicon Photonic Interferometric Biosensor with Inexpensive Laser Source for Sensitive Label-Free Immunoassays

Jonas Leuermann, Vladimir Stamenkovic, Patricia Ramirez-Priego, Alejandro Sánchez-Postigo, Adrian Gavela, Cole Chapman, Ryan Bailey, Laura Lechuga, Ezequiel Perez-Inestrosa, Daniel Collado, Robert Halir, and I. Molina-Fernández

DOI: 10.1364/OL.411635 Received 02 Oct 2020; Accepted 09 Nov 2020; Posted 10 Nov 2020  View: PDF

Abstract: Over the last two decades integrated photonic sensors have been of major interest to the optical biosensor community due to their capability to detect low concentrations of molecules with label-free operation. Among these, interferometric sensors can be read-out with simple, fixed-wavelength laser sources and offer excellent detection limits, but can suffer from sensitivity fading when not tuned to their quadrature point. Recently, coherently detected sensors were demonstrated as an attractive alternative to overcome this limitation. Here we show, for the first time, that this coherent scheme provides sub-nanogram per milliliter limits of detection in C-reactive protein immunoassays, and that quasi-balanced optical arm lengths enable operation with inexpensive Fabry-Perot type lasers sources at telecom wavelengths.

Thermal lens astigmatism in glass and in cubic crystals with [001] orientation

Ilya Snetkov and Alexey Yakovlev

DOI: 10.1364/OL.412108 Received 12 Oct 2020; Accepted 09 Nov 2020; Posted 10 Nov 2020  View: PDF

Abstract: Thermal lens was investigated in CaF2 and TGG cubic crystals with [001] crystallographic axes orientation and in MOG103 glass by the method of phase-shifting interferometry. It was demonstrated experimentally that the thermal lens has astigmatism determined by the incident radiation polarization state and by the optical anisotropy parameter ξ. A method of ξ determination by measuring thermal lens astigmatism in cubic crystals with [001] orientation was proposed and verified. It was shown that thermally induced depolarization and the amplitude of phase astigmatism depend on the position of the crystal with [001] orientation in antiphase.

The highly-symmetric Platonic-Gauss beams

Rodrigo Gutiérrez-Cuevas and Miguel Alonso

DOI: 10.1364/OL.405988 Received 25 Aug 2020; Accepted 09 Nov 2020; Posted 09 Nov 2020  View: PDF

Abstract: A class of self-similar beams, the Platonic-Gauss beams, are introduced by identifying the vertices of the five Platonic solids with points in a Majorana representation. Different orientations of these solids correspond to beams connected through astigmatic transformations. The many rotational symmetries of the Platonic solids translate into invariances to specific transformations. While these beams can be considered as the least ray-like for their given total order, aspects of the ray formalism can be applied to determine their overall intensity distribution, and a ray-based field estimate is possible.

A high sensitivity magnetic sensor based on the evanescent scattering by a magnetorheological film

Hau Ping Chan, Binghui Li, Kazi Tanvir Ahmmed, Liangjun He, Shuyan Zhu, and Qiang Wu

DOI: 10.1364/OL.411542 Received 01 Oct 2020; Accepted 08 Nov 2020; Posted 09 Nov 2020  View: PDF

Abstract: We present a simple concept to implement a magnetic sensor that uses evanescent scattering by a suspended magnetorheological (MR) film above a planar waveguide. The soft MR film embedded with ferromagnetic particles is to induce scattering on the evanescent field of a planar waveguide at a proximity distance. This distance can be controlled precisely by a magnetic field. Consequently, the waveguide output power changes in response to the magnetic intensity. Two sensor prototypes of different film thickness were designed and tested showing a tradeoff between the sensitivity and dynamic sensing range. A maximum sensitivity of ~2.62 dB/mT was obtained while the minimum limit of detection (LOD) value was ~0.016 mT. Compared with optical MEMS, the presented sensors feature simple design, easy fabrication, low cost and potentials of large-scale production and miniaturization to be integrated into portable devices.

Hyperspectral two-photon excitation microscopy using visible wavelength

Toshiki Kubo, Kenta Temma, Nicholas Smith, Kai Lu, Tomoki Matsuda, Takeharu NAGAI, and Katsumasa Fujita

DOI: 10.1364/OL.413526 Received 28 Oct 2020; Accepted 08 Nov 2020; Posted 11 Nov 2020  View: PDF

Abstract: We demonstrate hyperspectral imaging by visible-wavelength two-photon excitation microscopy using line illumination and slit-confocal detection. A femtosecond pulsed laser light at 530 nm was used for the simultaneous excitation of fluorescent proteins with different emission wavelengths. The use of line illumination enabled efficient detection of hyperspectral images and achieved simultaneous detection of three fluorescence spectra in the observation of living HeLa cells with an exposure time of 1 ms per line, which is equivalent to about 2µs per pixel in point scanning, with 160 data points per spectrum. On combining linear spectral unmixing techniques, localization of fluorescent probes in the cells was achieved. A theoretical investigation of the imaging property revealed high-depth discrimination property attained through the combination of nonlinear excitation and slit detection.

Real-time continuous calibration method for UV camera

Kuijun Wu, Yutao Feng, Yuanhui Xiong, Weimin Duan, Yu Guangbao, and Faquan Li

DOI: 10.1364/OL.410635 Received 22 Sep 2020; Accepted 08 Nov 2020; Posted 12 Nov 2020  View: PDF

Abstract: The accuracy of SO2 cameras is significantly determined by the ability to obtain an accurate calibration. This work presents a real-time continuous calibration method for SO2 cameras with a moderate resolution spectrometer by taking realistic radiative transfer into account. The effectiveness and accuracy of the proposed method have been verified through simulations and experiments. The calibration error can be reduced by about 20-80 % compared with the commonly used cell calibration, especially for situations of long distance, poor visibility or optically thick plumes.

Ge-on-insulator lateral p-i-n waveguide photodetectors for optical communication

CHIN-YUAN CHENG, Cheng-Hsun Tsai, PO-LUN YEH, SHENG-FENG HUNG, SHUYU BAO, Kwang Hong Lee, Chuan Seng Tan, and Guo-En Chang

DOI: 10.1364/OL.409842 Received 10 Sep 2020; Accepted 07 Nov 2020; Posted 10 Nov 2020  View: PDF

Abstract: We report high-performance lateral p-i-n Ge waveguide photodetectors (WGPDs) on a Ge-on-insulator (GOI) platform that could be integrated with electronic-photonic integrated circuits (EPICs) for communication applications. The high quality Ge layer affords a low absolute dark current. A tensile strain of 0.144% in the Ge active layers narrows the direct bandgap to enable efficient photodetection over the entire range of C- and L bands. The low-index insulator layer enhances optical confinement, resulting in a good optical responsivity. These results demonstrate the feasibility of planar Ge WGPDs for monolithic GOI-based EPICs.

High-efficiency chirped grating couplers on lithium niobate on insulator

Kang Shuting, ru zhang, Zhenzhong Hao, JIA DI, Feng Gao, Fang Bo, Guoquan Zhang, and Jingjun Xu

DOI: 10.1364/OL.412902 Received 23 Oct 2020; Accepted 07 Nov 2020; Posted 10 Nov 2020  View: PDF

Abstract: High-efficiency chirped grating couplers (GCs) with coupling efficiencies (CE) approaching 90%/coupler were designed by using particle swarm optimization algorithm. These GCs were fabricated on Z-cut lithium niobite on insulator (LNOI) with an Au layer on the lithium niobite substrate. The maximum CEs for transverse electric and transverse magnetic polarization input were measured up to ~72.0 %/coupler and ~61.6 %/coupler, respectively, which are the state-of-the-art values for LNOI GCs as far as we know. These GCs contribute to the realization of high-efficiency LNOI on-chip integrated optics.

Non-Hermitian optics for coherent perfect absorption, invisibility and lasing with different orbital angular momentums

Lai Yun, Weixin LU, jie luo, Hongchen Chu, and Ping Bai

DOI: 10.1364/OL.409690 Received 02 Oct 2020; Accepted 07 Nov 2020; Posted 09 Nov 2020  View: PDF

Abstract: We propose and demonstrate that the phenomena of coherent perfect absorption, invisibility and lasing can be simultaneously realized in a composite structure with both lossy and gain components. The three distinct functions can be independently triggered by incident waves with different orbital angular momentums. For instance, a triple-layer cylinder is demonstrated to simultaneously achieve perfect absorption, invisibility, and lasing under cylindrical monopolar, dipolar, and quadrupolar incidence, respectively. Our work demonstrates that total attenuation, elimination of scattering and great amplification can all be constructed in a single non-Hermitian photonic device, which utilizes the orbital angular momentum as a controlling mechanism for customized functions.

Optimization of N₂+ air lasing by waveform-controlled polarization-skewed pulses

Hanxiao Li, Shengzhe Pan, Fei Chen, Fenghao Sun, Zhaohui Li, Huailiang Xu, and Jian Wu

DOI: 10.1364/OL.410153 Received 14 Sep 2020; Accepted 07 Nov 2020; Posted 10 Nov 2020  View: PDF

Abstract: Optical ionization of N₂ and subsequent population redistribution among the ground and excited states of N₂+ in intense laser field are commonly accepted to be fundamentally responsible for the generation of N₂+ air lasing, which holds great promise for atmospheric sensing and standoff spectroscopy. By finely controlling this two-step process, the optimization of N₂+ air lasing is possibly achieved. Here we design a waveform-controlled polarization-skewed (PS) pumping pulse, in which the leading and falling edges are orthogonal-polarized and their relative field strength and phase can be well controlled. We demonstrate a precise manipulation of the N₂+ air lasing at both 391 nm and 428 nm emissions with the optimization occurring at the phase of nπ. Orders of magnitude enhancement in the N₂+ lasing intensity is observed as the phase changes from (n+1/2)π to nπ, indicating the important role of the optical phase of the driving pulse. The PS pulse with controllable spatiotemporal waveform provides us a robust and straightforward tool to efficiently steer the air lasing emission.

400 MHz ultrafast optical coherence tomography

Dongmei HUANG, Feng Li, Zijian HE, Zihao Cheng, Shang Chao, and Ping Kong Wai

DOI: 10.1364/OL.409607 Received 08 Sep 2020; Accepted 06 Nov 2020; Posted 09 Nov 2020  View: PDF

Abstract: An ultrafast time-stretched swept source with a sweep rate of 400 MHz is demonstrated based on the buffering of a 100 MHz femtosecond laser pulse train. To the best of our knowledge, this is the highest sweep rate of swept sources for optical coherence tomography (OCT) that had been reported. With a 10-dB sweep range of ~100 nm, an axial resolution of 19 μm is obtained in the OCT. OCT imaging of high-speed rotating disks is demonstrated. A composite complex apodization method is proposed and demonstrated to enhance the signal to noise ratio in the OCT imaging. The ultrafast swept source will find applications in video rate volumetric OCT imaging and laser detection and ranging of high-speed moving objects.

Ultra strong parametric nonlinearities in AlGaAs-on-insulator waveguides

Marlon Placke and Sven Ramelow

DOI: 10.1364/OL.406152 Received 25 Aug 2020; Accepted 06 Nov 2020; Posted 06 Nov 2020  View: PDF

Abstract: Aluminum gallium arsenide has highly desirable properties for integrated parametric optical interactions: large material nonlinearities, maturely established nanoscopic structuring through epitaxial growth and lithography, and a large bandgap for broadband low-loss operation. However, its full potential for record-strength nonlinear interactions is only released when the semiconductor is embedded within a dielectric cladding to produce highly confining waveguides. From simulations of such, we present second and third order pair generation that could outperform state-of-the-art quantum optical sources and make novel regimes of strong parametric photon-photon nonlinearities accessible.

Generation and manipulation of polarization twisting dual-pulse with a high degree of freedom

Hao-Keng Wei, Ito Hironori, Kazuhiko Misawa, and Chih Wei Luo

DOI: 10.1364/OL.409672 Received 13 Oct 2020; Accepted 06 Nov 2020; Posted 06 Nov 2020  View: PDF

Abstract: A polarization twisting dual-pulse (PTDP) system is demonstrated using a modified Michelson interferometer (MI), in which a pellicle beamsplitter is inserted into each arm. By tuning the positions of the end mirrors and pellicle beamsplitters in the MI, the polarization twisting frequency, the helicity, and the interval between two pulses can be individually manipulated. This PTDP generation system has a high degree of freedom in terms of tuning, and has applications in the study of helicity dynamics in quantum matter, particularly in the THz regime.

Photonic spin Hall effect by anisotropy induced polarization gradient in momentum space

Lei Cai, Shuang Zhang, Wenguo Zhu, Hao Wu, Huadan Zheng, Jianhui Yu, Yongchun Zhong, and Zhe Chen

DOI: 10.1364/OL.409946 Received 14 Sep 2020; Accepted 06 Nov 2020; Posted 06 Nov 2020  View: PDF

Abstract: Abstract: We demonstrate theoretically and experimentally a novel photonic spin Hall effect (PSHE) at an interface between air and uniaxial crystal, whose optical axis is within the interface plane. Owing to the anisotropy of crystal, partial cross polarization conversion occurs. For a horizontally polarized paraxial Gaussian beam incident, linear polarization gradient forms along the in-plane wavevector in the reflected beam, allowing us to achieve spin separation in real space. The spin separation can be tuned by rotating the optical axis of crystal. A maximum spin-dependent displacement up to 0.45 times of the incident beam waist is obtained at Brewster incidence. This novel anisotropy-induced PSHE deepens the understanding of spin-orbit interaction and provides a new way for control of spin photons.

Fano resonances in optical spectra of semiconductor quantum wells dressed by circularly polarized light

Oleg Kibis, Stanislav Kolodny, and Ivan Iorsh

DOI: 10.1364/OL.410091 Received 16 Sep 2020; Accepted 05 Nov 2020; Posted 06 Nov 2020  View: PDF

Abstract: Optical properties of semiconductor quantum wells irradiated by a strong circularly polarized electromagnetic field are studied theoretically. Since the field can induce the composite electron-light states bound at repulsive scatterers, it drastically modifies all optical characteristics of the system. Particularly, it is demonstrated that the quantum interference of the direct interband optical transitions and the transitions through the light-induced intermediate states leads to the Fano resonances in the optical spectra, which can be detected in the state-of-the-art measurements.

Resonance photogeneration of hot electrons through Tamm surface states

Alexander Uskov, Igor Protsenko, Igor Smetanin, and Morten Willatzen

DOI: 10.1364/OL.411789 Received 06 Oct 2020; Accepted 05 Nov 2020; Posted 06 Nov 2020  View: PDF

Abstract: Internal surface photoemission of electrons from 1D-crystal into barrier with participation of Tamm state at the interface crystal-barrier is considered theoretically for the first time. It is shown that resonant tunneling of electrons through a Tamm state could lead to substantial enhancement of the quantum efficiency and lowering the red border to a value defined by the Tamm state. In contrast to the Fowler quadratic law, the photocurrent scales linearly with photon energy near the red border. The results suggest that the efficiency of hot electron generation with plasmonic metal nanoparticles could reach several tens of percents that is very attractable for application in energy conversion technologies such as water splitting.

Profilometry of Optical Microfiber Based on Modal Evolution

Mohammad Zibaii, forugh jafary, omid Ranjbar-Naeini, and Hamid Latifi

DOI: 10.1364/OL.411767 Received 06 Oct 2020; Accepted 05 Nov 2020; Posted 05 Nov 2020  View: PDF

Abstract: The waist diameter of a Tapered Optical Fiber (TOF) has been determined using the modal evolution during the tapering process of a single-mode optical fiber (SMF28) through the Short-Time Fourier Transform (STFT) analysis. The STFT was utilized to calculate the cut-off moment of the different modes. By the knowledge of the cut-off diameter, the final diameter of the waist with accuracy better than 5 nm was measured. The TOF shape depends on the flame parameters, the material properties, and the stretching conditions. By calculating the deformation rate of the TOF, the diameter of TOFs near the waist has been measured with an accuracy of 6.1%, moreover, the TOFs were fabricated with a non-uniform flame.

Optical analog computing of two-dimensional spatial differentiation based on Brewster effect

Dingyu Xu, Shanshan He, Junxiao Zhou, Shizhen Chen, Shuangchun Wen, and Hailu Luo

DOI: 10.1364/OL.413104 Received 20 Oct 2020; Accepted 05 Nov 2020; Posted 05 Nov 2020  View: PDF

Abstract: Optical analog computing has garnered great interest in recent decades due to it has some advantages of lower consumption, higher efficiency and real-time imaging in image processing. Here, we propose a two-dimensional optical analog computing scheme based on the Brewster effect. We experimentally demonstrate two-dimensional edge detection with high efficiency. By combining microscopy or high contrast imaging, our approach may develop some significant applications in cellular and molecular imaging.

Enhanced light extraction of light-emitting diode with micro patterns by femtosecond laser micromachining for visible light communication

Fan Zhang, Cong Xu, Kai Yin, and Jian Duan

DOI: 10.1364/OL.411206 Received 28 Sep 2020; Accepted 05 Nov 2020; Posted 09 Nov 2020  View: PDF

Abstract: A significant enhancement of light extraction of light-emitting diodes (LED) with micro patterns has been experimentally investigated. The micro patterns on surface of polymer layer are fabricated by femtosecond laser Bessel beam for obtaining microhole arrays with large depth, resulting in reduction of photon loss by total internal reflection (TIR) at the surface of LED. The light output power of the LED is apparently increasing by introducing the array patterns without influencing its current-voltage (I-V) characteristics. Moreover, the electroluminescence spectra of multi-color LED and its angular radiation profiles with orthogonal and hexagonal patterns also have been explored. In addition, the optical field distributions of the micro patterns simulated by the finite difference time domain (FDTD) method have expressed the modulation effect of the array depth. Finally, the patterned LED as a transmitter is embedded in visible light communication (VLC) system for evaluating the transmission signal quality.

Single-frame reconstruction for improvement of off-axis digital holographic imaging based on image interpolation

YiWei Liu, Zhuqing Jiang, YIBO WANG, QIUYA SUN, and HAO CHEN

DOI: 10.1364/OL.405578 Received 18 Aug 2020; Accepted 04 Nov 2020; Posted 04 Nov 2020  View: PDF

Abstract: We present a method of single-frame reconstruction in off-axis digital holography with Kronecker product interpolation, to maximize spatial frequency extraction. The Kronecker-product interpolation operated on one-frame off-axis digital hologram can generate several aliasing spectrums in its Fourier domain, where the zero-order aliasing spectrums in the extrapolation region can be suppressed effectively. After adding all aliasing spectrum regions and substituting each aliasing spectrum in place with their sum, spectrum interception with a larger scope for filtering can be made in one of aliasing spectrum regions. The experimental results demonstrate that the method is valid to improve the resolution in off-axis digital holography.

Fast, sub-pixel accurate, displacement measurement method: Optical and terahertz systems

Min Wan, John Healy, and John Sheridan

DOI: 10.1364/OL.413011 Received 19 Oct 2020; Accepted 04 Nov 2020; Posted 10 Nov 2020  View: PDF

Abstract: A novel, fast method to measure in-plane object motion in 1-D with sub-pixel accuracy, which complements the correlation technique, is proposed. The method is verified experimentally using both visible and terahertz (THz) images. The absolute sum of grey level accumulated change is used to quantify object motion. The method requires calibration for each target, but only addition and subtraction operations. This results in a decrease of two order of magnitude in the computation time.

Long-range Bloch Surface Waves in Photonic Crystal Ridges

Tommaso Perani and Marco Liscidini

DOI: 10.1364/OL.412625 Received 16 Oct 2020; Accepted 03 Nov 2020; Posted 03 Nov 2020  View: PDF

Abstract: We theoretically study light propagation in guided Bloch surface waves (BSWs) supported by photonic crystal ridges. We show that low propagation losses can be achieved just by a proper design of the multilayer to obtain photonic band gaps for both light polarizations. We present an optimization strategy based on a Fourier analysis that allows one to obtain intrinsic losses as low as 5 dB/km for a structure operating in the visible spectral range. These results clarify the limiting factors to light propagation in guided BSWs and represent a fundamental step towards the development of BSW-based integrated optical platforms.

Noise-tolerant single-photon imaging with a superconducting nanowire camera

Lingdong Kong, Qingyuan Zhao, Kai Zheng, Haiyangbo Lu, Shi Chen, Xu Tao, Hui Wang, Hao Hao, Chao Wan, xuecou tu, Labao Zhang, Xiaoqing Jia, Lin Kang, Jian Chen, and Peiheng Wu

DOI: 10.1364/OL.394087 Received 06 Apr 2020; Accepted 03 Nov 2020; Posted 04 Nov 2020  View: PDF

Abstract: The quality of an image is limited to the signal-to-noise ratio of the output from sensors. As the background noise increases much more than the signal, which can be caused by either a huge attenuation of light pulses after a long-haul transmission or a blinding attack with a strong flood illumination, a LiDAR stops working properly. Here, we built a superconducting single-photon infrared camera of negligible dark counts and 60 ps timing resolution. Combining with an adaptive 3D slicing algorithm that gives each pixel an optimal temporal window to distinguish clustered signal photons from a uniformly distributed background, we successfully reconstructed 3D single-photon images at both a low signal level (~ 1 average photon per pixel) and extremely high noise background (background-to-signal ratio = 200 within a period of 50 ns before denoising). Among all detection events, we were able to remove 99.45 % of the noise photons while keeping the signal photon loss at 0.74 %. Our work is a direct outcome of quantum-inspired imaging that asks for a codevelopment of sensors and computational methods. We envision it can increase the working distance of existing LiDARs or defend the camera from blinding attacks.

Multi-wavelength Pumped Upconversion Enhancement Induced by Cu2−xS Plasmonic Nanoparticles in NaYF4@Cu2-xS Core-shell Structure

Donglei Zhou, Li Tao, Jiannan Jiao, Junhua Hu, and Wen Xu

DOI: 10.1364/OL.403901 Received 28 Jul 2020; Accepted 02 Nov 2020; Posted 04 Nov 2020  View: PDF

Abstract: Cu2−xS nanoparticles (NPs) demonstrate unique tunable localized surface plasmon resonance (LSPR) and nonlinear optical properties, which are promising materials for photoelectric and display devices. In this work, we represent the highly improved upconversion luminescence (UCL) in the NaYF4:Yb3+, Er3+ @NaYF4:Yb3+ , Nd3+ @Cu2−xS core-shell structure. The UCL enhancement were systemically studied under excitation of multi-wavelengths, 808 , 980 and 1540 nm, due to the broad band nature of Cu2−xS LSPR. Two different mechanisms synergistically contribute to the UCL enhancement, namely, LSPR effect and two-photon effect, which lead to the extraordinary power-dependence of UCL. UCL enhancement as high as 12 folds was achieved in the core-shell upconversion nanoparticles (UCNPs). The core-shell NPs were printed on the paper substrate using the nano-printing technique, displaying different colors irradiated by different near-infrared light, which have potential applications in anti-counterfeiting, encryption, and display fields. These findings provide a method to design and optimize luminescent materials and demonstrate potential applications of plasmonic semiconductor and UCNPs.

Anisotropic nonlinear optical response in graphene oxide-gold nanohybrid

Rajesh Yadav, J. Aneesh, Rituraj Sharma, TUHIN MAJI, DEBJANI KARMAKAR, and KV Adarsh

DOI: 10.1364/OL.412536 Received 14 Oct 2020; Accepted 02 Nov 2020; Posted 05 Nov 2020  View: PDF

Abstract: In this letter, we report for the first-time anisotropic optical response in graphene oxide (GO)-gold (Au) nanohybrid. Polarization-sensitive nonlinear optical absorption measurements revealed that nanohybrids are highly anisotropic ( ≈ 28 cm/GW), which is more than one order magnitude higher than that of control GO (2 cm/GW). The first principle analysis of absorbance at nanohybrid interfaces with varying functional ligand concentrations corroborates with the experimentally observed intrinsic linear anisotropy. Thus, our work enables new routes to realize smart and high-performing nonlinear optical systems selectively and directionally, such as tunable optical limiters and optical data processing devices.

Efficient high-energy 5-µm Fe:CdTe laser

Peter Fjodorow, M. Frolov, Yuriy Korostelin, Vladimir Kozlovsky, Stanislav Leonov, and Yan Skasyrsky

DOI: 10.1364/OL.411559 Received 02 Oct 2020; Accepted 01 Nov 2020; Posted 04 Nov 2020  View: PDF

Abstract: The results of research aimed at energy scaling-up of a Fe:CdTe laser by pumping with long pulses are reported. High-energy laser operation of a Fe2+-doped single-crystal CdTe is demonstrated at the temperature of 77K. Pumped with 250-µs pulses of a 4.08-µm Fe:ZnSe laser, the Fe:CdTe laser produced an output of 0.35 J in the 5-µm spectral region with an input energy slope efficiency of 44%. Using an intracavity prism, the laser was tuned from 4.86 to 5.37 µm. In a nonselective cavity, the laser demonstrated a temperature tunability from 5.03 µm at 77K to 5. µm at 215K. At the later operation point reachable using a Peltier element, the laser still produced 0.15 J of output energy with an efficiency of 22%.

Polarization independent and temperature tolerant AWG based on a silicon nitride platform

Sylvain GUERBER, Carlos Alonso Ramos, Xavier Le Roux, Nathalie Vulliet, Eric Cassan, Delphine Marris-Morini, Frederic Boeuf, and Laurent Vivien

DOI: 10.1364/OL.411332 Received 01 Oct 2020; Accepted 01 Nov 2020; Posted 03 Nov 2020  View: PDF

Abstract: Polarization tolerant optical receiver is a key building block for the development of wavelength division multiplexing (WDM) based high-speed optical data links. However, the design of polarization independent demultiplexer (DEMUX) is not trivial. In this work, we report on the realization of a polarization tolerant arrayed waveguide grating (AWG) on a 300-mm silicon nitride (SiN) photonics platform. By introducing a series of individual polarization rotators in the middle of the waveguide array, the polarization dependence of the AWG has been substantially reduced. Insertion losses below 2.2dB and a crosstalk level better than -30dB has been obtained for TE and TM polarizations on a four channel coarse AWG. The AWG temperature sensitivity has also been evaluated. Thanks to the low thermo-optical coefficient of SiN, a thermal shift below 12pm/°C has been demonstrated.

Integrated silicon reconfigurable optical transmitter

Wei Liu, Yu Yu, and Sidong Fu

DOI: 10.1364/OL.412017 Received 12 Oct 2020; Accepted 01 Nov 2020; Posted 03 Nov 2020  View: PDF

Abstract: We demonstrate an integrated silicon reconfigurable optical transmitter based on the reconfigurability of Mach-Zehnder interferometer (MZI). By incorporating modulators into the tunable MZI structure and manipulating the operation states, different modulation formats, including amplitude/phase modulated binary/quaternary signals, as well as polarization multiplexed signals, can be generated as required, to accommodate different transmission links. For a proof-of-concept demonstration, the microring modulators are adopted and we experimentally generate 10 GBaud on-off keying signal, four level pulse amplitude signal and polarization division multiplexing on-off keying signal using the same transmitter. The device is promising for next-generation intelligent optical link.

Controllable entanglement of two distant quantum dots bridged by dark whispering gallery modes

Yanhui Zhao

DOI: 10.1364/OL.408938 Received 02 Sep 2020; Accepted 01 Nov 2020; Posted 02 Nov 2020  View: PDF

Abstract: We present a scheme to create and control the entanglement between two distant semiconductor quantum dots (QDs) embedded in separated photonic crystal (PhC) nanobeam cavities. When bridged by a largely detuned microring cavity, photonic supermodes between two distant nanobeam cavities are formed via the whispering gallery modes (WGMs). Due to the large detuning, WGMs in the microring exhibit almost no photonic excitation, showing the "dark WGMs". With the dyadic Green's functions of the nano-structure and the resolvent operators of the Hamiltonian, we numerically investigate the entanglement dynamics of two distant QDs. Furthermore, we prove that the entanglement can be controlled by adjusting the distances between the cavities. Such a scheme paves an efficient way for realizing a scalable quantum network in a solid-state system.

Nonlinear optical propagation near the Bragg resonance in heliconical cholesteric liquid crystals

Francesco Simoni

DOI: 10.1364/OL.411356 Received 01 Oct 2020; Accepted 01 Nov 2020; Posted 02 Nov 2020  View: PDF

Abstract: We show here that the light-induced tuning of the Bragg reflection recently demonstrated in heliconical cholesterics opens new perspectives to nonlinear optical propagation in liquid crystals. We highlight that, by properly adjusting the static electric field that stabilizes the heliconical structure, a dramatic change of the refractive index of the circularly polarized resonant mode can be achieved. Additionally, a stop band for a definite range of light intensity is obtained that can be tuned in order to get the conditions of self-induced transparency.

Enhanced rotation sensing and exceptional points in a parity-time-symmetric coupled-ring gyroscope

Matthew Grant and Michel Digonnet

DOI: 10.1364/OL.399985 Received 12 Jun 2020; Accepted 31 Oct 2020; Posted 02 Nov 2020  View: PDF

Abstract: A study is performed of the enhancement in rotation sensitivity achieved by a parity-time-symmetric gyroscope consisting of a ring with gain coupled to a lossy ring, operated below laser threshold and in the vicinity of its exceptional point (EP). A conventional readout system is used to measure the large rotation-induced shifts in resonance frequency known to occur at this structure’s EP. A complete model of rotation sensitivity is derived that accounts gain saturation due to the large circulating signal power. Compared to a single-ring gyro, the rotation sensitivity is enhanced by factor of 304 when the inter-ring coupling is tuned to its EP value κEP, and 2412 when it is decreased from κEP, even though the Sagnac frequency shift is then much smaller. ~40% of this 2412-fold enhancement is assigned to a new sensing mechanism where gain saturation changes in response to a rotation. These results show that with a suitable readout system, this compact gyro has a far greater sensitivity than a conventional ring gyro, and that this improvement arises mostly from the gain compensating the loss, as opposed to the enhancement in Sagnac frequency shift from the EP. This gyro is also shown to be much more stable against gain fluctuations than a single-ring gyro with gain.

Confocal air-coupled ultrasonic optical coherence elastography probe for quantitative biomechanics

Fernando ZVIETCOVICH, Achuth Nair, Yogeshwari Sanjayrao Ambekar, Manmohan Singh, Salavat Aglyamov, Michael Twa, and Kirill Larin

DOI: 10.1364/OL.410593 Received 21 Sep 2020; Accepted 31 Oct 2020; Posted 02 Nov 2020  View: PDF

Abstract: We present an air-coupled ultrasonic radiation forceprobe co-focused with a phase-sensitive optical coherencetomography system (ACUS-OCE) for quantitativewave-based elastography. A custom-made 1 MHzspherically focused piezo-electric transducer with aconcentric 10 mm wide circular opening allowed forconfocal micro-excitation of waves and phase-sensitiveOCT imaging. Phantom studies demonstrated the capabilitiesof this probe to produce quasi-harmonic excitationup to 4 kHz for generation of highly localizedelastic waves. Experimental results in ocular tissuesshowed highly localized 2D and 3D elasticity mappingusing this approach with great potential for clinicaltranslation.

Coherently coupled vector black solitons in a quasi-isotropic cavity fiber laser

XIAO HU, Jun Guo, Luming Zhao, Jie Ma, and Dingyuan Tang

DOI: 10.1364/OL.410830 Received 23 Sep 2020; Accepted 31 Oct 2020; Posted 03 Nov 2020  View: PDF

Abstract: We report the first experimental observation of coherently coupled black-black solitons in a quasi-isotropic cavity fiber laser with normal cavity dispersion. The properties of the vector solitons accord well with the theoretical predictions based on the coherently coupled nonlinear Schrodinger equations. Numerical studies and experimental results clearly confirmed the existence of the highly robust vector black solitons in the fiber laser system.

Nonlinear Photoacoutic Response of Suspensions of Laser-Synthesized Plasmonic Titanium Nitride Nanoparticles

Anderson S. Gomes, Andrei Kabashin, Melissa Maldonado Cantillo, avishek das, Sergey Klimentov, and Anton Popov

DOI: 10.1364/OL.404304 Received 31 Jul 2020; Accepted 31 Oct 2020; Posted 10 Nov 2020  View: PDF

Abstract: Nonlinear photoacoustic response from solutions of 40 nm plasmonic titanium nitride (TiN) nanoparticles (NPs) synthesized by laser ablation in liquid environment (acetone) is reported. Using photoacoustic Z-scan based on 5 ns pumping pulses, values of optically excited nonlinear response coefficients βPA,eff arising from effective nonlinear absorption were measured and found to be 3.27×10-8 cm/W, 6.41×10-8 cm/W and 3.22×10-8 cm/W for pumping wavelengths of 600 nm, 700 nm and 800 nm, respectively. The influence of nonlinear scattering was negligible, as absorption-dependent photoacoustic measurements were simultaneously compared to the optical Z-scan, and similar values were obtained. The origin of the effective absorptive nonlinearity is discussed based on combined nonlinear absorption in the TiN NPs, heating and bubble generation in the suspension triggered by the TiN nonlinear absorption. Potential applications will be biomedical diagnostics and therapy.

Asymmetric dissipative solitons in a waveguide lattice with non-uniform gain-loss distributions

Changming Huang, Liangwei Dong, and Zhang Xiao

DOI: 10.1364/OL.409577 Received 07 Sep 2020; Accepted 30 Oct 2020; Posted 02 Nov 2020  View: PDF

Abstract: We address the existence and stability of two types of asymmetric dissipative solitons, including fundamental and dipole solitons, supported by a waveguide lattice with non-uniform gain-loss distributions. Fundamental solitons exist only when the linear gain width is greater than or equal to the linear loss width, while dipole solitons only exist when the linear gain width is less than or equal to the linear loss width. With an increase in the relative gain depth, the effective width of the soliton gradually decreases. In addition, we find that both asymmetric fundamental and dipole solitons are completely stable in a considerable part of their lower edge of existence regions, and solitons beyond this range are dissipative. Our results provide, to the best of our knowledge, the first example of stable dissipative solitons in a waveguide lattice with non-uniform gain-loss distributions.

Blind Source Separation with Integrated Photonics and Reduced Dimensional Statistics

Philip Ma, Alexander Tait, Weipeng Zhang, Emir Karahan, Thomas Ferreira de Lima, Chaoran Huang, Bhavin Shastri, and Paul Prucnal

DOI: 10.1364/OL.409474 Received 07 Sep 2020; Accepted 29 Oct 2020; Posted 29 Oct 2020  View: PDF

Abstract: Microwave communications have witnessed an incipient proliferation of multi-antenna and opportunistic technologies in the wake of an ever-growing demand for spectrum resources, while facing increasingly difficult network management over widespread channel interference and heterogeneous wireless broadcasting. Radio frequency (RF) blind source separation (BSS) is a powerful technique for demixing mixtures of unknown signals with minimal assumptions, but relies on frequency dependent RF electronics and prior knowledge of the target frequency band. We propose photonic BSS with unparalleled frequency agility supported by the tremendous bandwidths of photonic channels and devices. Specifically, our approach adopts an RF photonic front-end to process RF signals at various frequency bands within the same array of integrated microring resonators, and implements a novel two-step photonic BSS pipeline to reconstruct source identities from the reduced dimensional statistics of front-end output. We verify the feasibility and robustness of our approach by performing the first proof-of-concept photonic BSS experiments on mixed-over-the-air RF signals across multiple frequency bands. The proposed technique lays the groundwork for further research in interference cancellation, radio communications, and photonic information processing.

Omnidirectional field enhancements drive giant nonlinearities in epsilon-near-zero waveguides

Gordon Li, C. Martijn de Sterke, and Alessandro Tuniz

DOI: 10.1364/OL.412761 Received 16 Oct 2020; Accepted 29 Oct 2020; Posted 02 Nov 2020  View: PDF

Abstract: Bulk materials with a relative electric permittivity ε close to zero exhibit giant Kerr nonlinearities. However, harnessing this response in guided-wave geometries is not straightforward, due to the extreme and counter-intuitive properties of such epsilon-near-zero materials. Here we investigate, through rigorous calculations of the Kerr nonlinear coefficient, how the remarkable nonlinear properties of such materials can be exploited in several structures, including bulk films, plasmonic nanowires, and metal nanoapertures. We find the largest nonlinear response when the modal area and the group velocity are simultaneously minimized, leading to omnidirectional field enhancement. This physical insight will be key for understanding and engineering nonlinear nanophotonic systems with extreme nonlinearities and point to new design paradigms.

Snapshot spectral polarimetric light field imaging using a single detector

Xiaobo Lv, Yiwei Li, shuaishuai zhu, Xinmin Guo, Jianlong Zhang, Jie Lin, and Peng Jin

DOI: 10.1364/OL.409476 Received 04 Sep 2020; Accepted 27 Oct 2020; Posted 02 Nov 2020  View: PDF

Abstract: In this letter, we investigate a snapshot spectral-polarimetric-volumetric imaging (SSPVI) system using a single detector. Through compressed acquisition and reconstruction, SSPVI can achieve spectral imaging, polarization imaging, and light-filed imaging simultaneously. The newly discovered performance is showcased by attaining the spectral-polarimetric-volumetric video and different laboratory accuracy experiments. These never-seen-before capacities of the camera open new prospects for many applications, such as biological analysis, object recognition, and remote sensing.

A Multi-Terawatt Two-Color Beam for High-Power Field-Controlled Nonlinear Optics

Matthew Edwards, Nicholas Fasano, Tim Bennett, Alec Griffith, Nikita Turley, Brad O Brien, and Julia Mikhailova

DOI: 10.1364/OL.403806 Received 06 Aug 2020; Accepted 27 Oct 2020; Posted 29 Oct 2020  View: PDF

Abstract: Two-color laser beams are instrumental in light-field control and enhancement of high-order harmonic, spectral supercontinuum, and terahertz radiation generated in gases, plasmas, and solids. We demonstrate a multi-terawatt two-color beam produced using a relativistic plasma mirror, with 110 mJ at 800 nm and 30 mJ at 400 nm in a 25 fs pulse. Both color components have high spatial quality and can be simultaneously focused, provided that the plasma mirror lies within a Rayleigh range of the driving fundamental beam. Favorable scaling of second harmonic generation by plasma mirrors at relativistic intensities suggests them as an excellent tool for multi-color waveform synthesis beyond the petawatt level.

Extending the 3D scanning range of digital-micromirror-device-based scanners for femtosecond lasers

Yu Wang, Huaming Li, Qinglei Hu, Ruixi Chen, Xiaohua Lv, and Shaoqun Zeng

DOI: 10.1364/OL.409862 Received 11 Sep 2020; Accepted 26 Oct 2020; Posted 26 Oct 2020  View: PDF

Abstract: Digital micromirror devices (DMDs) have shown their potential in two-photon imaging and microfabrication as diffractive scanners for femtosecond lasers. However, the scanning range of a DMD-based scanner is decreased by the spatial filter used to block undesired diffraction orders. Instead of a spatial filter, we present a method of introducing and correcting aberration (ICA) to reduce the effects of these undesired diffraction orders. In ICA, aberrations are introduced by optical elements (e.g., cylindrical lenses), and only the aberration of the desired diffraction order is corrected by adding a compensatory phase to the scanning phase. The scanning ranges in the y and z directions can be nearly doubled when the spatial filter is removed. We demonstrate that ICA can be conveniently applied to a previously constructed DMD-based two-photon microscope, and the field of view can be extended at different axial positions.

Ramsey interferometry through coherent A²Πu−X²Σg+−B²Σu+ coupling and population transfer in N₂+ air laser

Yu Hung Kuan, Xiangxu Mu, Zhiming Miao, Wen-Te Liao, Chengyin Wu, and Zheng Li

DOI: 10.1364/OL.401800 Received 03 Jul 2020; Accepted 26 Oct 2020; Posted 26 Oct 2020  View: PDF

Abstract: Motivated by the hot debate on the mechanism of the laser like emission at 391nm from N₂ gas irradiated by a strong 800nm pump laser and a weak 400nm seed laser, we theoretically study the temporal profile, optical gain and periodic modulation of the 391nm signal from N₂+ . Our calculation sheds light on the long standing controversy on whether population inversion is indispensable for the optical gain, and demonstrate the Ramsey fringes of the emission intensity at 391nm formed by additionally injectinganother 800nm pump or 400nm seed, which provides strong evidence for the coherence driven modulation of transition dipole moment and population transfer between the A²Πu(ν = 2)-X²Σg+ states and the B²Σu+ (ν = 0) - X²Σg+ states. Our results show that the 391nm optical gain is susceptible to the degree of population inversion within N₂+ states manipulated by the Ramsey technique, and thus clearly reveal their symbiosis. This study provides with not only the physical picture of producing population inversion in N₂+ but also versatile methods for coherent control of N₂+ air laser.

Computational nanosensing from defocus in single particle interferometric reflectance microscopy

Celalettin Yurdakul and M. Selim Ünlü

DOI: 10.1364/OL.409458 Received 21 Sep 2020; Accepted 25 Oct 2020; Posted 26 Oct 2020  View: PDF

Abstract: Single particle interferometric reflectance (SPIR) microscopy has been studied as a powerful imaging platform for label-free and highly-sensitive biological nanoparticle detection and characterization. SPIR's interferometric nature yields a unique 3D defocus intensity profile of the nanoparticles over a large field-of-view. Here, we utilize this defocus information to recover high signal-to-noise ratio nanoparticle images with a computationally and memory efficient reconstruction framework. Our direct inversion approach recovers this image from a 3D defocus intensity stack using the vectorial-optics-based forward model developed for sub-diffraction limited dielectric nanoparticles captured on a layered substrate. We demonstrate proof-of-concept experiments on silica beads with a 50 nm nominal diameter.

Realization of strong coupling between 2D excitons and cavity-photons at room temperature

Xinchao Zhao, Yanhong Yan, Zhuangzhuang Cui, Feng Liu, Shao-Wei Wang, Liaoxin Sun, Yuwei Chen, and Wei Lu

DOI: 10.1364/OL.401330 Received 30 Jun 2020; Accepted 23 Oct 2020; Posted 26 Oct 2020  View: PDF

Abstract: Two-dimensional semiconductor materials demonstrate strong interaction with light, which can be further engineered by optical microcavities. This research investigates the strong light-matter coupling between monolayer Tungsten Disulphide (WS₂) excitons and microcavity photons at room temperature. A Rabi splitting of 36 meV is observed in angle-resolved reflectance spectra, which agrees well with the theoretical results simulated by using the transfer matrix method. It provides an effective way to study the room temperature strong coupling between cavity photon and 2D excitons.

Strong Exciton-Plasmon Coupling in Dye-Doped Film on a Planar Hyperbolic Metamaterial

Ekembu Tanyi, Nina Hong, Teresa Sawyer, Jonathan Van Schenck, Gregory Giesbers, Oksana Ostroverkhova, and Li-Jing Cheng

DOI: 10.1364/OL.402210 Received 20 Jul 2020; Accepted 22 Oct 2020; Posted 22 Oct 2020  View: PDF

Abstract: We experimentally demonstrate the direct strong coupling between the S0→S1 absorption transition of Rhodamine 6G (R6G) dye molecules and the surface plasmon polaritons of a hyperbolic metamaterial (HMM) substrate. The surface plasmon mode was excited by a guided mode of the R6G-doped polymer thin film on the HMM. The coupling strengths of the interactions between the surface plasmon and two molecular exciton modes are greater than the average linewidths of the individual modes indicating a strong coupling regime. This is the first experimental demonstration of the direct strong coupling between the resonance mode supported by the HMM and the dye molecules on the HMM surface, not embedded in the HMM structure. The study may provide the foundation for the development of novel planar photonic or electronic devices.

A multifunctional optical fiber sensor for simultaneous measurement of temperature and salinity

Hongkun Zheng, Ri-Qing Lv, Yong Zhao, Ruijie Tong, Ziting Lin, Xixin Wang, Yifan Zhou, and Qiang Zhao

DOI: 10.1364/OL.409233 Received 03 Sep 2020; Accepted 22 Oct 2020; Posted 02 Nov 2020  View: PDF

Abstract: A multifunctional optical fiber sensor fabricated by asymmetric offset splicing is proposed in this paper. The light is divided into serval parts at the offset interface, among which the transmitted light forms the Mach-Zehnder interference spectrum while the reflected light forms the Fabry-Perot interference spectrum. The online monitoring system is built to make a better light distribution at the offset interface. Theoretical analysis and experimental verification are carried out. The experiment result shows that the proposed sensor has good characteristics to salinity and temperature, and the salinity sensitivity can up to -2.65385 nm/‰ in the range of 20~40 ‰, the temperature sensitivity is better than 2.17 nm/℃ in the range of 28~48 ℃. The two interferometers involved have different responses to temperature and salinity, contributing to the effective elimination of cross-sensitivity. The proposed optical fiber sensor boasts the merits of compact size, high sensitivity, and multispectral measurement function.

3kW high OSNR 1030nm single mode monolithic fiber amplifier with 180pm linewidth

Chu Chu, Qiang Shu, Yu Liu, Rumao Tao, Donglin Yan, Honghuan Lin, Jianjun Wang, and Feng Jing

DOI: 10.1364/OL.405386 Received 18 Aug 2020; Accepted 21 Oct 2020; Posted 02 Nov 2020  View: PDF

Abstract: In this letter, a theoretical model which takes simulate amplified spontaneous emission (ASE) light falling in range of Raman light as the Raman seed has been used to optimize the power scaling capability of 1030nm fiber amplifiers. It shows that SRS effect seeded by the ASE is the main limiting factor for the fiber amplifiers operating at 1030nm, and >3kW output power with high OSNR can be achieved by proper parameter designing of the fiber laser system. A 1030nm monolithic narrow linewidth fiber amplifier, which delivers 3kW output power with OSNR being 37dB and 0.18nm spectrum linewidth, has been demonstrated. At the maximal 3kW output power, the SRS light peak is obviously higher than ASE light, which agrees with the theoretical predictions. No stimulated Brillouin scattering effect or thermal induced mode instability effect have been observed at ultimate power level, and the beam quality factor M2 is measured to be less than 1.2. To the best of our knowledge, this is the highest power ever reported so far operating at 1030nm.

Two-stage transient stimulated Raman chirped pulse amplification in KGd(WO₄)₂ with compression to 145 fs

Paulius Mackonis, Augustinas Petrulenas, Aleksej Rodin, Vytenis Girdauskas, and Andrejus Michailovas

DOI: 10.1364/OL.408478 Received 26 Aug 2020; Accepted 20 Oct 2020; Posted 21 Oct 2020  View: PDF

Abstract: We report efficient amplification of chirped supercontinuum pulses in a two-stage stimulated Raman amplifier based on KGd(WO₄)₂ crystals, pumped with 1.2 ps transform-limited pulses at 1030 nm wavelength. The second stage demonstrates a conversion efficiency of 55% with an output pulse energy of 0.6 mJ at 1135 nm wavelength. The amplified Stokes bandwidth is ten times the pump bandwidth, providing 145 fs pulses after compression.

Core crosstalk in ordered imaging fibre bundles

Helen Parker, Antonios Perperidis, James Stone, Kevin Dhaliwal, and Michael Tanner

DOI: 10.1364/OL.405764 Received 02 Sep 2020; Accepted 17 Oct 2020; Posted 19 Oct 2020  View: PDF

Abstract: Coherent fibre bundles are widely used for imaging. Commonly, disordered arrays of randomly sized fibre cores avoid proximity between like-cores which would otherwise result in increased core crosstalk and a negative impact on imaging. Recently, stack-and-draw fibre manufacture techniques have been used to produce fibres with a controlled core layout to minimise core crosstalk. However, one must take manufacturing considerations into account during stack-and-draw fibre design in order to avoid impractical or unachievable fabrication. Through characterisation of core crosstalk patterns, this Letter aims to aid the understanding of limitations imposed by compromises made in the core layout.

Volumetric two-photon fluorescence imaging of live neurons using a multimode optical fiber

Raphaël Turcotte, Carla Schmidt, Martin Booth, and Nigel Emptage

DOI: 10.1364/OL.409464 Received 08 Sep 2020; Accepted 16 Oct 2020; Posted 19 Oct 2020  View: PDF

Abstract: Multimode optical fibers (MMFs), combined with wavefront control methods, have achieved minimally-invasive in vivo imaging of neurons in deep-brain regions with diffraction-limited spatial resolution. Here, we report a method for volumetric two-photon fluorescence imaging with a MMF-based system requiring a single transmission matrix measurement. Central to this method is the use of a laser source able to generate both continuous wave light and femtosecond pulses. The chromatic spreading of pulses generated an axially elongated excitation focus, which we used to demonstrate volumetric imaging of neurons and their dendrites in live rat brain slices through a 60 μm-core MMF.

Characterization and application of nonlinear plastic materials for post-CPA pulse compression

Issa Tamer, Marco Hornung, Luise Lukas, Marco Hellwing, Sebastian Keppler, Reece Van Hull, Joachim Hein, Mathew Zepf, and Malte Kaluza

DOI: 10.1364/OL.409637 Received 08 Sep 2020; Accepted 15 Oct 2020; Posted 15 Oct 2020  View: PDF

Abstract: We demonstrate the three-fold post-CPA pulse compression of a high peak power laser pulse using CR39 (allyl diglycol carbonate), which was selected as the optimal material for near field self-phase modulation out of a set of various nonlinear plastic materials, each characterized with respect to their nonlinear refractive indices and optical transmission. The investigated materials could be applied for further pulse compression at high peak powers, as well as for gain narrowing compensation within mJ-class amplifiers. The post-CPA pulse compression technique was tested directly after the 1st CPA stage within the POLARIS laser system, with the compact setup containing a single 1 mm thick plastic sample and a chirped mirror pair that enabled a substantial shortening of the compressed pulse duration and, hence, a significant increase in the laser peak power without any additional modifications to the existing CPA chain.

AI-enabled real-time dual-comb molecular fingerprint imaging

Thibault Voumard, Victor Brasch, Thibault Wildi, Raúl Gutiérrez Álvarez, Germán Vergara Ogando, and Tobias Herr

DOI: 10.1364/OL.410762 Received 23 Sep 2020; Accepted 15 Oct 2020; Posted 16 Oct 2020  View: PDF

Abstract: Hyperspectral imaging provides spatially resolved spectral information. Utilising dual frequency combs as active illumination sources, hyperspectral imaging with ultra-high spectral resolution can be implemented in a scan-free manner when a detector array is used for heterodyne detection. Here, we show that dual-comb hyperspectral imaging can be performed with an uncooled near-to-mid-infrared detector by exploiting the detector array's high framerate, achieving 10 Hz acquisition in 30 spectral channels across 16'384 pixel. Artificial intelligence enables real-time data reduction and imaging of gas concentration based on characteristic molecular absorption signatures. Owing to the detector array's sensitivity from 1 to 5 µm wavelength, this demonstration lays the foundation for real-time versatile imaging of molecular fingerprint signatures across the infrared wavelength-regime with high-temporal resolution.

Analysis of active medium parameters of lasing from molecular nitrogen ions in ambient air

Ilya Zyatikov, Valery Losev, Dmitriy Lubenko, and Evgeniy Sandabkin

DOI: 10.1364/OL.403577 Received 24 Jul 2020; Accepted 13 Oct 2020; Posted 15 Oct 2020  View: PDF

Abstract: Results of studying lasing pulse duration from molecular nitrogen ions under various focusing conditions of a femtosecond radiation pulse with wavelength of 950 nm are presented. The parameters of active medium are analyzed and the mechanism for the formation of picosecond lasing pulse duration at femtosecond seed pulse duration is proposed.

Green-pumped continuous-wave optical parametric oscillator based on fanout-grating MgO:PPLN

Sukeert ., Chaitanya Kumar Suddapalli, and Majid Ebrahim-Zadeh

DOI: 10.1364/OL.404979 Received 07 Aug 2020; Accepted 09 Oct 2020; Posted 12 Oct 2020  View: PDF

Abstract: We report the first green-pumped continuous-wave (cw) optical parametric oscillator (OPO) based on MgO:PPLN in a fanout grating design. Pumped by a single-frequency cw laser at 532 nm, the OPO provides tunable radiation across 813-1032 nm in the signal and 1098-1539 nm in the idler by simple mechanical translation at a fixed temperature of 55 °C. By deploying a 25-mm-long crystal to minimize thermal effects and using output coupling for the signal wave, we generate a total output power of up to 714 mW at 30% extraction efficiency in excellent Gaussian beam quality with M2<1.1 and high output stability. Simultaneous measurements of signal and idler power result in passive stability of 2.8% rms and 1.8% rms, respectively, over 1 hour. Strong thermal effects contribute to the high stability and excellent beam quality, while linear and green-induced infrared absorption limit the power scaling capabilities of the OPO. The output signal is single-mode with an instantaneous linewidth of ~3 MHz and frequency stability of ~84 MHz over 72 seconds.

Suppression of Decoherence tied to Electron-Phonon Coupling in Telecom-Compatible Quantum Dots: Low-threshold Reappearance Regime for Quantum State Inversion

Ajan Ramachandran, Grant Wilbur, Sabine ONeal, Dennis Deppe, and Kimberley Hall

DOI: 10.1364/OL.403590 Received 27 Jul 2020; Accepted 01 Oct 2020; Posted 05 Oct 2020  View: PDF

Abstract: We demonstrate suppression of dephasing tied to deformation potential coupling of confined electrons to longitunidal acoustic (LA) phonons in optical control experiments on large semiconductor quantum dots (QDs) with emission compatible with the low-dispersion telecommunications band at 1.3 μm. By exploiting the sensitivity of the electron-phonon spectral density to the size and shape of the QD, we demonstrate a four-fold reduction in the threshold pulse area required to enter the decoupled regime for exciton inversion using adiabatic rapid passage (ARP). Our calculations of the quantum state dynamics indicate that the symmetry of the QD wave function provides an additional means to engineer the electron-phonon interaction. Our findings will support the development of solid-state quantum emitters in future distributed quantum networks using semiconductor QDs.

A 40 W, 780 nm laser system with compensated dual beam splitters for atom interferometry

Minjeong Kim, Remy Notermans, Chris Overstreet, Joseph Curti, Peter Asenbaum, and Mark Kasevich

DOI: 10.1364/OL.404430 Received 17 Aug 2020; Accepted 29 Sep 2020; Posted 29 Sep 2020  View: PDF

Abstract: We demonstrate a narrow-linewidth 780 nm laser system with up to 40 W power and a frequency modulation bandwidth of 0 MHz. Efficient overlap on nonlinear optical elements combines two pairs of phase-locked frequency components into a single beam. Serrodyne modulation with a high-quality sawtooth waveform is used to perform frequency shifts with > 96.5 % efficiency over tens of MHz. This system enables next-generation atom interferometry by delivering simultaneous, Stark-shift-compensated dual beam splitters while minimizing spontaneous emission.

Reconfigurable Terahertz Switch Using Flexible L-shaped Metamaterial

Fangyuan Lu, Huiliang Ou, and Yu-Sheng Lin

DOI: 10.1364/OL.402949 Received 16 Jul 2020; Accepted 27 Sep 2020; Posted 02 Oct 2020  View: PDF

Abstract: A design of reconfigurable terahertz (THz) switch by using flexible L-shaped metamaterial (FLM) is presented, which is composed of dual-layer L-shaped metamaterials on PDMS substrate, which has three resonances at 0.57 THz, 1.05 THz, and 1.52 THz. By stretching the FLM along x-axis direction, the transmission intensity is increased gradually at transverse electric (TE) and reduced at transverse magnetic (TM) mode, respectively. Reversely, by stretching the FLM along y-axis direction, the transmission intensity is reduced gradually at TE mode and increased at TM mode, respectively. These electromagnetic responses of FLM provide the optical-logic behaviors with programmable characteristics by stretching FLM at different polarized light. It indicates the proposed FLM could be used for the dual/triple-band switching, polarization switching, and programmable switching applications.

Vertically-Stacked RGB LEDs with Optimized Distributed Bragg Reflectors

Haotian Jin, Liang Chen, Xiaoshuai An, Yiping Wu, Ling Zhu, Hanxiang Yi, Kwai Hei Li, and Jing Li

DOI: 10.1364/OL.408416 Received 25 Aug 2020; Accepted 18 Sep 2020; Posted 03 Nov 2020  View: PDF

Abstract: The design and fabrication of a vertically-stacked RGB light-emitting diode (LED) with novel wavelength-selective distributed Bragg reflectors (DBRs) is demonstrated. The two DBRs are optimized to achieve selective reflectance in the RGB spectral region through theoretical calculations and simulation modeling. The insertion of optimal DBRs into the stack structure can effectively reflect downward emission from the upper chip without filtering the emission from the lower chips, thereby increasing the luminous efficiency for white emission with a color temperature range of 3000-8000 K by 1.6-7.4 %. The optical performances of stacked devices with and without DBRs are thoroughly studied, verifying the effectiveness of the proposed wavelength-selective DBR structure.

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