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Second harmonic microscopy of poled x-cut thin film lithium niobate: Understanding the contrast mechanism
摘要： Thin film lithium niobate has been of great interest recently, and an understanding of periodically poled thin films is crucial for both fundamental physics and device developments. Second-harmonic (SH) microscopy allows for the noninvasive visualization and analysis of ferroelectric domain structures and walls. While the technique is well understood in bulk lithium niobate, SH microscopy in thin films is largely influenced by interfacial reflections and resonant enhancements, which depend on film thicknesses and substrate materials. We present a comprehensive analysis of SH microscopy in x-cut lithium niobate thin films, based on a full three-dimensional focus calculation and accounting for interface reflections. We show that the dominant signal in backreflection originates from a copropagating phase-matched process observed through reflections, rather than direct detection of the counterpropagating signal as in bulk samples. We simulate the SH signatures of domain structures by a simple model of the domain wall as an extensionless transition from a ?χ(2) to a +χ(2) region. This allows us to explain the main observation of domain structures in the thin-film geometry, and, in particular, we show that the SH signal from thin poled films allows to unambiguously distinguish areas, which are completely or only partly inverted in depth.
关键词： lithium niobate,thin films,ferroelectric domain structures,Second-harmonic microscopy,nonlinear optics
更新于2025-09-12 10:27:22
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Mechanism of formation of noiselike pulses in passively mode-locked fiber lasers
摘要： We present the results of an analysis of noiselike pulses generated in passively mode-locked ?ber lasers with anomalous dispersion. The master equation used for the analysis takes into account the gain saturation, the quadratic frequency dispersion of the gain and the refractive index, and the cubic-quintic nonlinearity of the losses and refractive index. Reasons for the stabilization of noiselike pulses are investigated. It is found that the space-time localization of a noiselike pulse is associated with the suppression of single solitons located outside this pulse. The mechanism of such competition between single solitons and a noiselike pulse is determined.
关键词： noiselike pulses,passively mode-locked ?ber lasers,solitons,anomalous dispersion,nonlinear optics
更新于2025-09-12 10:27:22
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[IEEE 2019 18th International Conference on Optical Communications and Networks (ICOCN) - Huangshan, China (2019.8.5-2019.8.8)] 2019 18th International Conference on Optical Communications and Networks (ICOCN) - A novel kind of pulsating soliton in mode-locked fiber lasers
摘要： We find a novel kind of pulsating soliton with broadened Kelly sidebands. By simulation, we find the wavelength of the soliton oscillates due to the interaction with the unsynchronized resonant dispersive wave.
关键词： Fiber lasers,Optical solitons,Nonlinear optics
更新于2025-09-12 10:27:22
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Doubly-resonant plasmonic hot spot-exciton coupling enhances second harmonic generation from Au/ZnO hybrid porous nanosponges
摘要： We introduce zinc oxide (ZnO) functionalized porous gold nanoparticles which exhibit strong second harmonic (SH) emission due to an efficient coupling of localized surface plasmons to ZnO excitons. The nanosponges are perforated with a random network of 10-nm sized ligaments, localizing plasmons in a high density of hot spots. We use a broadband, few-cycle ultrafast laser to probe coherent nonlinear emission from individual bare gold and ZnO-functionalized sponges. While the third harmonic spectrum of the hybrid particles redshifts with respect to that of bare gold sponges, a distinct blueshift is seen in their SH spectra. SH emission around 390 nm, slightly below the ZnO band gap, is enhanced by ten times. We attribute this to doubly-resonant plasmon-exciton interactions: the laser drives nanosponge plasmon hot spot resonances and this locally enhanced field induces two-photon excitation of localized ZnO excitons. This opens a path towards the design of efficient coherent nonlinear optical sources by combining randomly-disordered nanoantennas with semiconductor gain materials.
关键词： exciton-plasmon coupling,Surface plasmons,third harmonic generation,zinc oxide,disordered nanoantennas,plasmon localization,nonlinear optics
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Novel InN-Based SESAMs with Ultra-Short Time Response
摘要： Semiconductor saturable absorbers are becoming a matter of interest since they are keystone elements of pulsed lasers, leading to ultrashort pulses with high peak intensities and wide optical spectra. This kind of ultrafast lasers are empowering new applications in the fields of optical telecommunications and nonlinear optics. In order to improve the radiation source with regard to pulse energy and temporal duration, new saturable absorbers are still under development. In this work, we study semiconductor saturable absorbers (S1 and S2) consisting of an active InN layer grown by molecular beam epitaxy directly on a 10-μm-thick GaN-on-sapphire template. We compare structures with an InN thickness of 400 nm (S1) and 900 nm (S2). The energy band gap of the samples extracted from optical transmission measurements is Eg1 ～ 0.69 eV (1782 nm) and Eg2 ～ 0.70 eV (1767 nm) for samples S1 and S2, respectively. The nonlinear absorption of the samples at 1550 nm has been measured by the Z-scan method, using a radiation source which delivers 250 fs pulses with an average power of 35 mW and a repetition rate of 5.2 MHz. To increase the power density impinging the sample, the laser is focused with a lens with a focal length of 3 cm in the Z-scan scheme, attaining an energy fluence of E = 5.2 mJ/cm2. The nonlinear optical response of the samples is obtained by measuring the optical transmittance as a function of the incident peak intensity on the sample. In both samples, a huge nonlinear change is observed. The linear transmittance at 1550 nm has been estimated in Tlin = 17 % for S1 and Tlin = 2.6 % for S2, and the modulation depth is ΔT (S1) = 22.6 % and ΔT (S2) = 21.8 %, respectively. Thus, sample S1 exhibits a nonlinear transmittance change (Tnl/Tlin) above ～ 240% at the maximum peak intensity (optical bleaching), whereas for S2 the total change in transmittance reaches 815%. These results reveal that the thickness of S2 is particularly adapted for the fabrication of saturable absorbers at 1550 nm, since it is thick enough to get a low linear transmittance but keeping the maximum modulation depth. Semiconductor saturable mirrors (SESAMs) have been implemented by deposition of an aluminium mirror directly on the InN sample surface. Optical fiber mode-locked lasers have been developed with these SESAMs. The AC traces and optical spectra for lasers using samples S1 and S2 are plotted. The traces have been fitted to Gaussian curves with a temporal width of 210 fs for S1 and 150 fs for S2. In conclusion, we present a new SESAM technology based on InN thin films which displays huge nonlinear effects. Optimized InN layer thickness results in more than 800% change in transmittance, which makes it possible to generate ultrashort pulses with temporal duration as low as 150 fs.
关键词： Semiconductor saturable absorbers,InN,ultrafast lasers,SESAMs,nonlinear optics
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Efficient Nonlinear Metasurfaces using Multiresonant High-Q Plasmonic Arrays
摘要： Nonlinear optical processes are important in many fields of photonics, ranging from biomedical imaging to ultrafast spectroscopy. Thanks to recent progress in nanophotonics and metamaterials, there is a growing demand for smaller and more efficient nonlinear optical components. Unfortunately, it is very challenging to satisfy this demand by using traditional materials, which motivates the search for alternative approaches. Nonlinear plasmonics has recently emerged as a potential solution for enabling more efficient nanoscale nonlinear optics. However, it is not yet clear how nonlinear responses of metamaterials can be enhanced sufficiently to enable practical nonlinear applications. Recent attempts to find the solution to this issue have been focused on two enhancement strategies, which are both based on resonance engineering. First, nanoparticle arrays associated with collective and narrow plasmon resonances with relatively high quality factors (Q-factors) known as surface lattice resonances (SLRs) have been utilized. Second, multiply-resonant nanostructures have been investigated where the resonance enhancement occurring both at the input and output wavelengths results in strong nonlinear response. Here, we combine these two concepts to enable even higher overall enhancement. We numerically study the enhancement of second-harmonic generation (SHG) in multiply-resonant plasmonic metasurfaces consisting of L-shaped aluminium nanoparticles. The array is designed to support two simultaneous high-Q SLRs, termed SLR(cid:90) and SLR2(cid:90), peaking at the fundamental and SHG wavelengths, respectively. The Q-factors of SLR(cid:90) and SLR2(cid:90) were estimated to be 800 and 250, respectively. We perform SHG simulations using an approach based on the nonlinear discrete-dipole approximation, and predict an enhancement of over six orders-of-magnitude of the emitted SHG intensity at doubly-resonant conditions.
关键词： metasurfaces,surface lattice resonances,second-harmonic generation,Nonlinear optics,plasmonics
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Engineering the Enhancement of Nonlinear Optical Properties of 2D Structures
摘要： The challenge of nonlinear optics stems from the negligible interaction among electromagnetic waves in free space, motivating substantial efforts to identify materials with sufficient anharmonic response as to enable optical nonlinearities at low light powers. On the nanoscale, near-field intensity enhancement through optical resonators has been widely explored to design active photonic devices operating on subwavelength scales. Further interest in nanoscale nonlinear optics has arisen with the isolation of graphene and other atomically-thin materials that combine a large electro-optic response with strong intrinsic nonlinearities. Despite their large nonlinear optical susceptibilities, the response of 2D materials is limited by their inherently small volumes. Optical resonances provide a promising way for circumventing this limitation, as they can enhance the driving electric field, thus boosting the effective nonlinear response. In the present work we introduce a figure-of-merit (FoM) that quantifies the enhancement of the nonlinear optical response that can be achieved in 2D materials by means of resonant cavities. As concrete examples, we apply our FoM to explore the extrinsic near-field enhancement provided by (1) localized nanoparticle plasmons acting on a 2D material; (2) the extrinsic enhancement in a 2D material produced by Mie-like resonances in dielectric nanoparticles; (3) the intrinsic enhancement of optical nonlinearity in a dielectric nanoparticle by its supported Mie resonances; (4) the effect of lattice resonances on the nonlinear response for a periodic 2D array of dielectric nanodisks. We focus in particular on the enhancement in second- and third-harmonic generation, as well as the Kerr nonlinearity, quantifying the yield of these nonlinear processes in each of the systems. Our FoM provides an efficient tool to explore the dependence of the nonlinear response enhancement by optical resonances on the size-to-wavelength ratio, composition, and geometry, thus facilitating the design of structures with optimal nonlinear yields.
关键词： Kerr nonlinearity,optical resonances,harmonic generation,nonlinear optics,2D materials,figure-of-merit
更新于2025-09-12 10:27:22
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Random mode coupling assists Kerr beam self-cleaning in a graded-index multimode optical fiber
摘要： In this paper, we numerically investigate the process of beam self-cleaning in a graded-index multimode optical ?ber, by using the coupled-mode model. We introduce various models of random linear coupling between spatial modes, including coupling between all modes, or only between degenerate ones, and investigate the e?ects of random mode coupling on the beam self-cleaning process. The results of numerical investigations are in complete agreement with our experimental data.
关键词： Nonlinear optics,Kerr e?ect,Multimode ?bers,Beam self-cleaning
更新于2025-09-12 10:27:22
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1 kHz dual sub-pulse train picosecond radially polarized beam KGW Raman generator capable of achieving multiple optical communication wavelength
摘要： A dual sub-pulse train 1064 nm picosecond radially polarized beam pump KGW single-pass Raman generator was designed. The Stokes lines of both the 768 cm?1 and 901 cm?1 vibrational modes of the KGW Raman-active crystal were excited simultaneously. The Raman generator radiated up to seven Stokes lines in the infrared spectrum, which covered multiple optical communication channels. The overall Stokes average power reached 1.01 W, and the maximum Raman overall conversion efficiency was 23.1%. The interaction between the optical field and the Raman crystal was discussed from two perspectives of line width compression and beam quality.
关键词： Multi-wavelength KGW Raman laser,Nonlinear optics,Dual sub-pulse train picosecond laser,Infrared spectrum,Radially polarized beam
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Nonlinear Enhancement with Graphene Heterostructures
摘要： Nonlinear nanoplasmonics provides precise control and manipulation of light. Graphene sustains electrically tunable and long-lived plasmons. By combining these platforms, we observe a 102 enhancement in the third-harmonic generation in graphene heterostructures.
关键词： Nonlinear optics,Plasmonics,Quantum optics
更新于2025-09-11 14:15:04