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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) - Comparative Study of High Order Harmonic Generation in Monolayer-Thick Semiconductors
摘要： In this contribution we report on the results of an experimental comparative study of high order harmonic generation (HHG) from strong mid-IR femtosecond laser pulses in two-dimensional (2D) materials consisting of a single layer. Beside their promising application in electronics and photonics, these materials allow to study microscopic light-condensed-matter interaction on the atomic scale. HHG in 2D materials, in comparison to a bulk, have several distinct qualitative features: single atom layer essentially excludes propagation effects and minimizes absorption, whereas weak charge screening results in significantly higher exciton binding energy. Here we report on comparative study of intensity and polarization dependence of HHG in polycrystalline single layers of MoS2 and WS2.
关键词： WS2,MoS2,mid-IR femtosecond laser pulses,2D materials,high order harmonic generation
更新于2025-09-16 10:30:52
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Time-resolved interference microscopy for studying nonideal plasma formed by high-power femtosecond laser pulses
摘要： Measurement results of characteristic size of plasma density inhomogeneity on the surface of bulk iron target formed under action of femtosecond laser pulses with intensity of 1016 W/cm2 are presented. Investigations are performed using time-resolved interference microscopy technique. Femtosecond laser pulses with high time contrast of 107 generated by chromium:forsterite laser system are applied. Efficacy of the chosen technique is demonstrated; the size of plasma inhomogeneity is shown to be less than 30 nm.
关键词： femtosecond laser pulses,time-resolved interference microscopy,plasma density inhomogeneity,nonideal plasma
更新于2025-09-16 10:30:52
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Enhanced Coherent Emission from Ionized Nitrogen Molecules by Femtosecond Laser Pulses
摘要： The forward emission spectra were experimentally measured for ionized nitrogen molecules by an 800-nm pump laser and a delayed seed laser. It was found that emission lines around both 428 nm and 391 nm are greatly enhanced with injecting either a 391-nm or 428-nm seed laser. The emission lines around 391 nm and 428 nm can be assigned to the rotational transitions of N2+ (B2Σu+, v'=0 → X2Σg+, v=0) and N2+ (B2Σu+, v'=0 → X2Σg+, v=1), respectively. They origin from seed-induced superfluorescence and resonant stimulated Raman scattering. The genetic algorithm was utilized to simulate the experimental observations and determine the relative population of B2Σu+(v'=0), X2Σg+(v=1) and X2Σg+(v=0). The result verifies that the vibrational population inversion is achieved between B2Σu+(v'=0) and X2Σg+(v=0) by the 800-nm pump laser. Our finding provides new insights into controlling the coherent emission of ionized nitrogen molecules, which has promising application in filamentation-based remote atmospheric sensing.
关键词： femtosecond laser pulses,ionized nitrogen molecules,coherent emission,population inversion,superfluorescence,stimulated Raman scattering
更新于2025-09-16 10:30:52
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Simulation of Self-focusing of Femtosecond Laser Pulses with Normal Dispersion in Air Using the Diffraction-Beam Tube Approach
摘要： Based on numerical simulation and qualitative analysis, the effect of the group velocity dispersion on the formation of light structures during self-focusing and filamentation of femtosecond Ti:sapphire laser pulses in air is studied. The main features of the filamentation have been determined at various pulse duration, initial beam radii, and peak radiation powers based on the results of numerical solutions of the nonlinear Schr?dinger equation in a Kerr-plasma dissipative dispersion medium and using the diffraction-beam tube approach. Dispersion is detected in the cases where the dispersion length is not a minimum at the scale of the process. It is shown that the relative (normalized to Rayleigh length) coordinate of the beginning of filamentation increases as the dispersion distortions of the pulse increase, and the filamentation channel length is reduced. For shorter laser pulses (tens of femtoseconds), the filamentation ceases as the laser beam radius increases. The size of the energy-replenishing diffraction-beam tube and the angular divergence of postfilamentation light channels increase for this class of pulses.
关键词： filamentation,self-focusing,femtosecond laser pulses,diffraction-beam tube,normal dispersion
更新于2025-09-12 10:27:22
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[IEEE 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - Paris, France (2019.9.1-2019.9.6)] 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) - THz response of metallic structures to femtosecond laser pulses
摘要： The talk is devoted to the nonlinear effects in metallic structures (like gratings and nanoparticle arrays) irradiated by femtosecond laser pulses. Possible mechanisms of delayed THz response and specific non-quadratic nonlinear regimes of conversion are analyzed. In contrast to previous models, only low-frequency currents inside the metal are considered without involving electron emission and acceleration. Special attention is paid to the role of plasmonic resonances at optical frequency in the enhancement of low-frequency nonlinear response.
关键词： femtosecond laser pulses,plasmonic resonances,metallic structures,nonlinear effects,THz response
更新于2025-09-12 10:27:22
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Plasma formation and relaxation dynamics in fused silica driven by femtosecond short-wavelength infrared laser pulses
摘要： Laser-induced plasma formation and subsequent relaxation in dielectric solids is the precursor to structural modi?cations serving as the basis for direct laser writing of functional optical micro- and nanostructures. Based on an experimental arrangement combining a time-resolved transmission measurement with a cross-phase modulation measurement, we isolate the plasma formation and relaxation dynamics in the bulk of amorphous fused silica excited by femtosecond short-wavelength infrared (k ? 2100 nm) laser pulses. Whereas the relaxation time of the generated subcritical electron-hole plasma was so far assumed to be constant, our ?ndings indicate an intensity-dependent relaxation time. We attribute this intensity dependence to vibrational activation of the medium, leading to detrapping of trapped carriers and a reduced trapping probability.
关键词： plasma formation,femtosecond laser pulses,relaxation dynamics,fused silica,short-wavelength infrared
更新于2025-09-12 10:27:22
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Precession damping in [Co <sub/>60</sub> Fe <sub/>40</sub> /Pt] <sub/>5</sub> multilayers with varying magnetic homogeneity investigated with femtosecond laser pulses
摘要： We report on the ultrafast magnetization dynamics of [Co60Fe40/Pt]5 multilayers studied with femtosecond laser pulses. The samples were grown at room temperature by DC magnetron sputtering with Ta capping and Pt buffer layers and present the same thickness and perpendicular magnetic anisotropy as determined by vibrating sample magnetometry. Controlled growth rate of the Pt buffer layer modified the anisotropy fields and magnetic domain sizes as measured by magnetic force microscopy (MFM). An estimation of the average magnetic domain sizes was obtained from the profile of the self-correlation transform of the MFM images. For multilayers having an average magnetic domain size of 490 nm, we report a damped precession of the magnetization which decays with a time constant of ～100 ps and which has a frequency which varies from 8.4 GHz to 17.0 GHz as the external field increases from 192 mT to 398 mT. Fitting the precession dynamics with the Landau-Lifshitz-Gilbert equation we evaluated the damping α, which decreases from 0.18 to 0.05 with increasing magnetic domain sizes (127 nm to 490 nm). These α values are higher than for single layers suggesting an enhanced scattering and spin pumping effects from the Pt adjacent layers. In addition, the precession frequency increases from 2.04 GHz to 11.50 GHz as the anisotropy field of the multilayers increases from 6.5 kOe to 13.0 kOe. Finally, a comparative analysis between micromagnetic simulations and MFM images allowed us to determine the exchange stiffness (Aex) in the [Co60Fe40/Pt]5 multilayers.
关键词： exchange stiffness,femtosecond laser pulses,Landau-Lifshitz-Gilbert equation,[Co60Fe40/Pt]5 multilayers,damping,magnetic force microscopy,ultrafast magnetization dynamics,perpendicular magnetic anisotropy
更新于2025-09-12 10:27:22
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Heat accumulation effects in laser processing of diamond-like nanocomposite films with bursts of femtosecond pulses
摘要： In this paper, we have investigated the burst mode (BM) ablation and surface structuring of diamondlike nanocomposite (DLN) a-C:H:Si:O films with femtosecond laser pulses (wavelength λ = 515 nm, pulse duration τ = 320 fs, and pulse repetition rate f = 100 kHz) under different scanning conditions (single spots and linear structures). The pulse separation in the bursts is 25 ns (intraburst frequency f = 40 MHz), and the pulse number is varied from 1 to 8. The ablation depth and specific ablation rates (μm3/μJ) are found to be higher for the burst mode compared to single-pulse irradiation, increasing with the pulse number in the burst. The obtained experimental data of the higher ablation efficiency are shown to correlate with computer simulations of the BM ablation. In correlation with the ablation findings, Raman spectra of single spots and microgrooves have evidenced a growing graphitization of the amorphous film structure with the pulse number in the bursts (at an equal energy deposited into the films). Contact-mode atomic force microscopy (AFM) is applied to reveal an influence of the BM processing on the surface properties (nanoscale relief, friction) of laser-structured films. Based on the ablation and Raman data analysis, AFM examination of ablated/redeposited layers, and computer simulations of the burst mode ablation, the heat accumulation is identified as the main factor responsible for the enhanced ablation efficiency during the BM processing of DLN films. In addition, results of the high precision surface microstructuring of DLN films in the burst mode are presented.
关键词： heat accumulation,diamondlike nanocomposite films,surface structuring,femtosecond laser pulses,burst mode ablation
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) - Terahertz Wave Generation in Air by Femtosecond Optical Vortex Pulses
摘要： Terahertz (THz) radiation is of great current interest due to many applications such as nonlinear THz spectroscopy and imaging. One of the compact and effective methods to obtain very high THz field strengths and extremely broadband spectral widths is THz wave generation from plasma filaments formed in air by focused bichromatic femtosecond laser pulses consisting of the first and the second harmonics (FH, SH). Here we investigate the effect of an azimuthal phase modulation of the SH carrying an optical vortex charge on the intensity and phase distribution of the THz pulse emitted from the laser induced plasma filament in air. We present both experimental and theoretical results.
关键词： Terahertz radiation,plasma filaments,femtosecond optical vortex pulses,bichromatic femtosecond laser pulses
更新于2025-09-11 14:15:04
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Nanodeformation in enstatite single crystals: Simulation of micrometeoroid impacts by femtosecond pulsed laser experiments
摘要： Space weathering by micrometeoroid bombardment is a cosmic phenomenon on atmosphere-free celestial bodies, a process that is expected to particularly overprint planetesimals and cosmic dust in debris discs. We reproduced micrometeoroid impact craters by femtosecond pulsed laser irradiation on oriented enstatite single crystals (En93Fs7) to investigate the deformation behavior and its orientation dependence. All microcraters show typical bowl shaped morphologies, a glass surface layer with splash like ejecta material and subsurface layering. Although we could reproduce melting and vaporization as typical space weathering effects in the enstatite experiments, there is no formation of agglutinate particles or metallic nanoparticles (npFe0). The shock effects in the deformation layer consist of planar structures like microfractures and cleavages, amorphous lamellae, stacking faults and clinoenstatite lamellae. Their activation and/or orientation depends on the shock direction. In special orientations we observe the activation of glide systems along specific low indexed crystallographic planes. Due to the short timescale and the high strain rates, the most prominent effect is the failure of enstatite by microfracturing along non-rational crystallographic planes. Common deformation mechanisms reported in meteorites like the formation of clinoenstatite lamellae via shearing along [001] (100) occur less frequently. Shear is apparently the dominant mechanism in the formation of the above-mentioned effects and causes also their modification by frictional heating. The wide-spread formation of amorphous lamellae is, for example, interpreted to be the result of this shear heating along planar structures. We interpret this unconventional deformation behavior as a consequence of the small spatial and temporal scale of the experiments, resulting in a short-lived spherical shock wave with high deviatoric stresses in contrast to a long pressure pulse and quasi-hydrostatic compression in large scale impacts that produce typical shock features.
关键词： Femtosecond laser pulses,Space weathering,Shock deformation,Enstatite,Micrometeoroid impact
更新于2025-09-11 14:15:04