Third- and fifth-harmonic generation by mid-infrared ultrashort pulses: beyond the fifth-order nonlinearity

Third- and fifth-harmonic generation by ultrashort laser pulses in the mid-infrared (mid-IR) reveals nonlinear-optical effects beyond the fifth-order nonlinearity and enables, because of an extraordinarily long coherence length, efficient multiplex frequency upconversion of ultrashort mid-IR pulses. We show that harmonic generation by mid-IR pulses provides an access to the key optical constants of gas media, allowing metrology of linear and nonlinear-optical susceptibilities in the mid-IR and offering a tool for the remote sensing of the atmosphere.     

晶体环境下高次谐波谱的截止频率分析

通过数值求解中红外飞秒光脉冲与一维单电子多阱势相互作用的含时薛定谔方程, 研究了晶体环境下的高次谐波发射光谱, 发现了晶体谐波谱的截止位置公式.研究发现, 不同于气体环境下的谐波谱截止频率规律, 晶体谐波谱的截止频率与激光电场的峰值振幅及晶格参数成线性关系. 鉴于准经典力学的三步模型对于气体谐波截止频率的解释, 进一步采用准经典力学方法验证了晶体谐波截止位置规律的正确性. 关键词： 中红外飞秒光脉冲 晶体 高次谐波 截止频率     

Generation of intense highly coherent femtosecond pulses in the mid infrared

Highly coherent mid-infrared femtosecond pulses, tunable between 2.5 and 4.4 mum, and with an average energy and duration between 2.6 and 3.6 microm of 11 microJ and 200fs, respectively, have been produced by an optical parametric amplifier setup driven at 1kHz by a 400- microJ , 800-nm pulse from a Ti:sapphire amplifier. In this system, first tunable moderate-energy femtosecond pulses in the near infrared are produced by continuum amplification in beta -barium borate, and subsequently the near-infrared pulses are amplified and frequency mixed with 800-nm radiation in potassium titanyl phosphate to produce intense tunable mid-infrared pulses. The time-bandwidth product of the mid-infrared pulses is 0.28+/-0.02 over the whole high-energy tuning range. Experimental results are compared with numerical simulations and a simple model.     

A systematic investigation of high harmonic generation using mid-infrared driving laser pulses

We report on a systematic investigation of the influences of gas pressure,focal position and focusing geometry on high harmonic generation by use of mid-infrared femtosecond laser pulses. We also discuss the spatial characteristics of harmonics under different focusing conditions. By optimizing the parameters,we experimentally observed the generation of 1 kHz,low divergence coherent X-ray beams in the water-window region.     

Generation of infrared radiation by stimulated Raman scattering in para-hydrogen crystal at 5 K

We report the preliminary results of our experiments with stimulated Raman scattering in para-hydrogen crystal aimed at developing a continuously tunable laboratory laser source of mid-infrared radiation. With laser pulses at 532 nm, a conversion efficiency for the first Stokes beam of as much as 20% in the forward direction was observed through a single-pass, 5-cm-long crystal. Generation of mid-infrared pulses at 4.5 microm was achieved by use of the output of a near-infrared pulsed laser (1.6 microm, 3 ns), and an absorption spectrum of gaseous CD4 molecules was successfully recorded. These results suggest use of the solid para-hydrogen Raman shifter as a promising light source for mid-infrared spectroscopy.     

Nonsequential double ionization of Xe by mid-infrared laser pulses

With the three-dimensional semiclassical ensemble model, we studied the correlated electron dynamics in strong-field nonsequential double ionization of Xe by mid-infrared laser pulses over a wide range of laser intensities. Numerical results show that the correlated electron momentum distribution exhibits the strong back-to-back pattern at the lower laser intensity. It evolves into the side-by-side behavior as the laser intensity increases and presents the experimentally observed crosslike shape at high laser intensity. Different from the case of near-infrared region where recollision mainly occurs at the first returning, at the mid-infrared region recollision dominantly occurs at the later returnings. The windows of initial transverse velocity for the various multiple-returning recollision trajectories are different and are unambiguously determined by this semiclassical ensemble model.     

双色光场驱动产生单个阿秒脉冲过程中的宏观效应

通过数值模拟三维传播方程研究了双色场产生高次谐波过程中宏观效应的影响,重点分析了阿秒脉冲的时域形状.在6 fs/800 nm和21.3 fs/400 nm组成的双色场中,通过调节双色场间的相对延时,双色场各焦点相对于气体盒子入口的间距和双色场束腰半径和气体压强等参数,可以在平台区得到超宽带连续谱.对该连续谱中420—540 eV的谐波直接进行滤波后可以得到时间同步性很好的单个阿秒脉冲.这种性质对实验上通过双色场方案产生超短的阿秒脉冲非常有利. 关键词： 传播效应 阿秒脉冲 时间同步性     

Femtosecond infrared pulses tunable from 9 to 18 mum at an 88-MHz repetition rate

Femtosecond mid-infrared laser pulses that are continuously tunable in the wavelength range from 9 to 18mum are demonstrated. These nearly bandwidth-limited pulses are generated by phase-matched difference-frequency mixing within the broad spectrum of 20-fs pulses from a mode-locked Ti:sapphire laser in GaSe. A direct determination of the pulse duration at 11.5mum gives a value of 140 fs. The average mid-infrared power of 1muW is ~100 times greater than that for infrared generation by non-phase-matched optical rectification.     

Extended phase matching of high harmonics driven by mid-infrared light

We demonstrate that phase-matched frequency upconversion of ultrafast laser light can be extended to shorter wavelengths by using longer driving laser wavelengths. Experimentally, we show that the phase-matching cutoff for harmonic generation in argon increases from 45 to 100 eV when the driving laser wavelength is increased from 0.8 to 1.3 microm. Phase matching is also obtained at higher pressures using a longer-wavelength driving laser, mitigating the unfavorable scaling of the single-atom response. Theoretical calculations suggest that phase-matched high harmonic frequency upconversion driven by mid-infrared pulses could be extended to extremely high photon energies.     

New trends in high-order harmonics generation using the mid-infrared pulses propagating through the laser-produced plasmas

Various recently emerged applications of mid-infrared pulses for high-order harmonic generation in laser plasmas produced on the surfaces of various materials are reviewed. Among them are (a) the highorder harmonic generation in Ag, Sn, fullerene, and graphene nanoparticle-containing plasmas using tunable two-color mid-infrared pulses, (b) efficiency dependence on the generating particle properties in the case of two-color high-harmonic generation of the longer-wavelength radiation in the plasmas, and (c) high-order sum and difference frequency generation using tunable two- and three-color commensurate and incommensurate mid-infrared pumps of graphite plasma. These studies demonstrate the perspectives of the frequency conversion of mid-infrared radiation in the extended laser-produced plasmas.     

Near- and mid-infrared laser monitoring of industrial processes, environment and security applications

The advantages of infrared laser monitoring in terms of sensitivity, selectivity and the ability of non-intrusive detection of gases are reviewed. Emphasis is laid on direct absorption spectroscopy and evanescent-field spectroscopy. The performance of the latter for gas detection in the near-infrared is demonstrated for the analysis of volcanic gases. For industrial process control, direct mid-infrared absorption spectroscopy is used to detect CO in the high-temperature atmosphere of a glass melting furnace. For both applications portable, stable, rugged and easy-to-handle laser systems are needed. Mid-infrared absorption spectroscopy is also applied to detect different explosives. Material evaporation is achieved by plasma generation with a pulsed laser at high repetition rate. Energetic materials contain high concentrations of nitrogen; therefore NO is present in the generated plasma. However, the rate at which NO is produced varies in a highly characteristic manner for different energetic materials. This enables the distinction between different types of explosives.     

Phase-Matched High-Order Harmonics from N2 Using kHz Femtosecond Laser Pulses

We report the phase-matched high-harmonic generation from N2 driven by kHz femtosecond laser pulses. By varying the position of the laser focus in the gas tube, we observe the radiation of different harmonics. Our results provide a potential source for studying ultrafast molecular dynamics.     

基于光场离化电流机制产生强太赫兹辐射的参数优化研究

基于超短激光脉冲与气体作用通过光场离化电流产生太赫兹(THz)辐射的模型, 研究了用双色激光脉冲的方法产生强THz辐射的优化参数条件. 数值计算表明, 导致THz辐射产生的离化电流主要是由一阶电离过程产生的, 高阶离化对该电流产生的贡献很小. 通过调节基频光与倍频光的配比、相位差都能增大离化电流, 从而可以提高THz辐射振幅. 将激光波长拓展到中红外波段, 也有利于提高离化电流. 此外,改变作用气体的种类也能改变离化电流. 在激光和密度参数相等的情况下, 在氦气中可以产生高于氮气中2倍左右的离化电流.     

超强激光场物理学

首先,评述了超强激光场的理论结果;其次,描述了超短光脉冲在密的气体和光学介质传播中的自作用;第三,评述了强场离化无碰撞等离子体中高度离化的离子产生,及高功率超短激光脉冲巨大强度的电场可用于电子加速;第四,分析了超短声脉冲和无线电脉冲的产生和应用,讨论了在不同条件下,激光辐射谐波和 Ｘ 射线激光的产生,进一步评述了超强激光脉冲与凝聚靶相互作用可以产生接近星体物质参数的高温、超密、强磁场和巨大压力等离子体;最后,简要叙述了激光激发核、核反应,高能电子─光子相互作用的可能效应及可能进行的实验。     

Mid-infrared difference-frequency generation of ultrashort pulses tunable between 3.2 and 4.8 microm from a compact fiber source

We report single-pass difference-frequency generation of mid-infrared femtosecond pulses tunable in the 3.2-4.8 microm range from a two-branch mode-locked erbium-doped fiber source. Average power levels of up to 1.1 mW at a repetition rate of 82 MHz are obtained in the mid infrared. This is achieved via nonlinear mixing of 170 mW, 65 fs pump pulses at a fixed wavelength of 1.58 microm, with 11.5 mW, 40 fs pulses tunable in the near-infrared range between 1.05 and 1.18 microm. These values indicate that the tunable near-infrared input component is downconverted with a quantum efficiency that exceeds 30%.     

超强激光场物理学

首先,评述了超强激光场的理论结果;其次,描述了超短光脉冲在密的气体和光学介质传播中的自作用;第三,评述了强场离化无碰撞等离子体中高度离化的离子产生,及高功率超短激光脉冲巨大强度的电场可用于电子加速;第四,分析了超短声脉冲和无线电脉冲的产生和应用,讨论了在不同条件下,激光辐射谐波和 Ｘ 射线激光的产生,进一步评述了超强激光脉冲与凝聚靶相互作用可以产生接近星体物质参数的高温、超密、强磁场和巨大压力等离子体;最后,简要叙述了激光激发核、核反应,高能电子─光子相互作用的可能效应及可能进行的实验。     

Studies on the mechanisms of powerful terahertz radiations from laser plasmas

A survey on the mechanisms of powerful terahertz (THz) radiation from laser plasmas is presented.Firstly,an analytical model is described,showing that a transverse net current formed in a plasma can be converted into THz radiations at the plasma oscillation frequency.This theory is applied to explain THz generation in a gas driven by two-color laser pulses.It is also applied to THz generation in a tenuous plasma driven by a chirped laser pulse,a few-cycle laser pulse,a DC/AC bias electric field.These are well verified by particle-in-cell simulations,demonstrating that THz radiations produced in these approaches are nearly single-cycles and linear polarized.In the chirped laser scheme and the few-cycle laser scheme,THz radiations with the peak field strength of tens of MV/cm and the peak power of gigawatt can be achieved with the incident laser intensity less than 10 17 W/cm 2.     

整形飞秒激光脉冲的成丝超连续辐射控制

飞秒激光成丝超连续辐射具有高强度和高时空相干性等优点,作为一种超宽带光源在很多领域都具有广泛的应用前景.本文提出一种结合微透镜阵列的空间调制和基于液晶空间光调制器的时域整形的飞秒激光脉冲整形方式,利用基于遗传算法的反馈优化控制,实现了飞秒激光在熔融石英中成丝产生的超连续辐射强度的调制,得到了在一定范围内光谱强度可控的超连续辐射光谱;光谱的能量密度可以从0.03μJ/nm调制到0.09μJ/nm,其能量密度变化达到了初始值的3倍.计算了典型迭代代数对应的整形脉冲时域包络,分析了超连续光谱随迭代代数的演化趋势,结果表明,脉冲包络的峰值强度和波形分布是影响超连续光谱展宽和强度的主要物理原因.     

Single attosecond pulse generation with intense mid-infrared elliptically polarized laser pulses

We have studied theoretically high-harmonic-order and single attosecond pulse generation with elliptically polarized laser pulses at wavelengths ranging from the visible to the mid-infrared. Results of ab initio simulations of the time-dependent Schr?dinger equation show that the ellipticity dependence of the high-harmonic signal intensifies with increasing wavelength of the driving pulse and saturates in the mid-infrared. The isolation of single attosecond pulses using the polarization gating method in the mid-infrared is due to an effective suppression of side pulses as compared with an operation at Ti:sapphire wavelengths.     

Mutual coherence of supercontinuum pulses collinearly generated in bulk media

We report the collinear generation of white light pulses in bulk media by phase-locked and time-delayed intense laser pulses. The mutual phase coherence of the pump pulses is fully transferred to the two supercontinua, which thus show a highly-modulated two-pulse spectrum extending from all of the visible down to the near-infrared. We have investigated the effects of changing the relative delay between the pulses as well as the role of imbalances in the pump energy, and we have found that, at least in a limited energy interval, the mutual phase coherence of the two white-light pulses is surprisingly robust.