激光能量对气体等离子体产生太赫兹波的影响

双色激光脉冲激励气体等离子体产生太赫兹波是得到高强度宽频带太赫兹波的重要方法,本文利用光电流模型研究了该方法中激光能量对产生太赫兹波的影响。理论计算表明,太赫兹波随激光能量的增大而增强,而太赫兹波的频谱结构不受激光能量的影响。分析了双色激光能量影响太赫兹波强度的原因,并利用自由电子浓度和电子电流密度诠释了该影响的内在物理机制。该研究为提高太赫兹辐射强度提供了一种有效的途径。     

双色激光偏振夹角对产生太赫兹辐射的影响

基于改进的二维光电流模型,研究了双色激光偏振夹角对太赫兹辐射产生的影响.通过改变基频光以及其对应的倍频光的偏振夹角发现,在激光等离子体中辐射的太赫兹波的强度与双色激光的偏振夹角呈周期性变化,并且最佳偏振夹角和不同激光强度也有关系.在相对较低的激光强度下(≤2×1014 W/cm2),当两束激光具备相同的偏振方向,即偏振夹角为0°时,太赫兹幅值达到最大;然而,当激光强度足够高(＞2×1014 W/cm2)时,最佳的偏振夹角会随着激光强度的增加而变大.从电子密度的角度来分析产生这种现象的原因,并用剩余漂移电流来揭示其潜在的物理机制.研究表明,剩余漂移电流是激光诱导等离子体产生太赫兹波的本质根源,对太赫兹波的产量起着决定性作用.     

双色激光偏振夹角对产生太赫兹辐射的影响（英文）

基于改进的二维光电流模型,研究了双色激光偏振夹角对太赫兹辐射产生的影响.通过改变基频光以及其对应的倍频光的偏振夹角发现,在激光等离子体中辐射的太赫兹波的强度与双色激光的偏振夹角呈周期性变化,并且最佳偏振夹角和不同激光强度也有关系.在相对较低的激光强度下(≤2×1014 W/cm2),当两束激光具备相同的偏振方向,即偏振夹角为0°时,太赫兹幅值达到最大;然而,当激光强度足够高(2×1014 W/cm2)时,最佳的偏振夹角会随着激光强度的增加而变大.从电子密度的角度来分析产生这种现象的原因,并用剩余漂移电流来揭示其潜在的物理机制.研究表明,剩余漂移电流是激光诱导等离子体产生太赫兹波的本质根源,对太赫兹波的产量起着决定性作用.     

基于共线型双等离子体的双色场辐射太赫兹波频谱调制

提出了一种通过共线型双等离子体的简单方法实现双色场作用下辐射太赫兹波的频谱调制。利用修正的光电流模型,模拟了共线型双等离子体在双色场条件下辐射太赫兹波的频谱分布。通过调节体系的参数,研究了两等离子体间距及激光光强对太赫兹波频谱调制的影响。结果表明,太赫兹频谱分布与两等离子体间距及双色激光强度有关。等离子体间距影响太赫兹波频谱的峰值个数和频移;激光光强虽会影响单个等离子体辐射太赫兹波的峰值频率,但是对于调制的频谱而言只会改变峰值的幅值。     

激光在大气中驱动的强太赫兹辐射的理论和实验研究

两束双色激光脉冲能在大气中产生MV/cm的强太赫兹波.本文主要介绍了我们最近的三项理论和实验工作,澄清了双色激光方案的物理机制这个长期存在的问题,并对该方案进行了推广.为了在气体中有效地产生太赫兹波,在广泛研究的双色激光方案中两束激光的频率比ω_2/ω_1总是被取为1:2.首先从理论上预测采用其他频率比时,此方案仍能有效地工作,并通过实验进行证实.实验上观察到在新的频率比ω_2/ω_1=1:4,2:3下,也能有效地产生太赫兹波;观察到通过旋转较长波长的激光脉冲的偏振方向,能够有效地调节太赫兹波的偏振,但是旋转波长较短的激光脉冲的偏振方向,太赫兹波的偏振几乎没有变化,这违背了多波混频理论中极化率张量对称性的要求;采用不同的频率比时,太赫兹能量定标率并没有显示出明显的区别,这与多波混频理论预测的能量定标率不符.这些实验结果与等离子体电流模型及粒子模拟结果符合得很好.因此,该研究不仅对双色激光方案进行了推广,而且证实了其物理机制应该归结为等离子体电流模型.     

氦原子在双色强激光驱动下产生的太赫兹波辐射

基于量子力学的数值求解二维含时薛定谔方程的方法,研究了双色激光场驱动氦原子辐射太赫兹波的特点.研究结果表明,两束激光的相位差对太赫兹波的产额具有显著的调制作用,在高强度激光驱动下,当相位差分别为-0.5π和0.55π时,太赫兹波有最大产额,与光电流模型得到的结论一致.比较光电流模型和量子力学结果表明,在高强度激光驱动下,原子实的库仑场对形成电流的自由电子影响较小;同时,通过调节双色场的相位差,能够从氦原子发射的宽频太赫兹波得到时域空间脉宽在30飞秒以下的半周期波形.     

气体辐射太赫兹波产额与驱动激光强度的依赖关系

利用光电流模型研究了气体辐射太赫兹波产额与驱动激光峰值强度的依赖关系.模拟表明,无论是单色激光脉冲还是双色激光场,当激光较弱时,太赫兹产额随着激光强度的增加而迅速增大;而当气体电离接近或达到饱和后,进一步增强激光场强度,太赫兹产额出现了振荡变化.与通常采用较弱激光强度相比,更高的激光强度(致使气体达到完全电离)会进一步增强太赫兹波的辐射强度.分析各个电离事件所产生电子的个数和运动轨迹的对称性,可以很好地解释这些结果.     

激光参数影响太赫兹辐射的数值模拟

 利用2维PIC程序模拟了聚焦激光脉冲垂直入射非均匀等离子体,在界面激发超强太赫兹辐射的物理过程,研究了激光强度、脉宽和焦斑半径对太赫兹辐射功率和频率的影响,并和模式转换理论作了对比。结果表明:激光强度是影响太赫兹辐射功率的主要因素;当脉宽在等离子体波长附近时,激起的静电波振幅最大,此时对应的太赫兹辐射最强;同时存在一个最优的焦斑半径,使得辐射功率最大。     

电场诱导水的太赫兹透射特性研究

许多生物分子自身的转动、振动或分子团的整体振动模式都位于太赫兹波段内,因此可以利用太赫兹光谱技术对生物分子进行检测。同时又由于太赫兹波的光子能量仅为毫电子伏量级,不会对分子的内部结构造成破坏,所以太赫兹时域光谱技术在生物检测方面具有良好的应用前景。众所周知,绝大多数的生物分子只有在液体条件下才能发挥其生物活性,所以研究液体环境下生物分子之间的相互作用就非常必要。然而水分子的转动模式、振动模式以及和氢键有关的能量均处于太赫兹波段,从而对其产生强烈的吸收;另外,水分子为极性分子,而极性分子对太赫兹波有强烈的共振吸收,这就使利用太赫兹技术对生物分子活性进行动态表征产生了困难。因此在研究溶液中的生物分子与太赫兹波的相互作用时,最大限度地减小水分子对太赫兹波的吸收就成为近年来的研究热点。目前,减少水对太赫兹波吸收的主要方法有：在溶液样品中加入抑制氢键缔合的离子来减小水对太赫兹的吸收;通过改变溶液的温度来调节水对太赫兹的吸收;利用微流控芯片技术,通过减小被测样品与太赫兹波的作用距离来减小水对太赫兹波的吸收。另外,激光的激励、电场或磁场的处理,也能改变水对太赫兹波的吸收,将盛有去离子水的微流控芯片放于电场中,研究经电场处理不同时间的去离子水对太赫兹吸收强度的影响。结果发现,太赫兹波的透射强度随着去离子水在电场当中静置时间的增加而增强,当在电场中静置60 min时,太赫兹的频谱强度达到最大,与空气的频谱强度接近。由此可以推断外加电场使水分子的偶极矩发生了变化,从而对整体水分子的振动和转动产生了影响,并且改变了水中的氢键结构,导致了太赫兹透射光谱强度的增强。     

多脉冲激光增强太赫兹辐射的PIC模拟

 模拟了强激光和稀薄非均匀等离子体相互作用在界面辐射超强太赫兹波的物理过程,提出了利用多脉冲激光增强太赫兹辐射的方案,详细研究了多脉冲激光的脉冲个数（取１～４个）、脉冲间距等因素对太赫兹辐射功率和频率的影响。当入射激光包含4个脉冲时,辐射最强,此时的辐射功率是相同条件下单脉冲的6倍,可达到7.16 MW,辐射的太赫兹波的脉宽约为330 fs,总能量约为1 μJ。研究结果表明:多脉冲激光可以显著增强太赫兹辐射,且随着脉冲个数的增加,激起的电子静电波振幅变大,辐射功率也随之变大,直到尾流场饱和;当脉冲间距等于入射激光脉宽时辐射最强。     

Two-color laser wavelength effect on intense terahertz generation in air

Near-IR femtosecond lasers have been proposed to produce high-field terahertz radiation in the air via the laser-plasma interaction, but the physical mechanism still needs to be further explored. In this work, we theoretically investigate the effect of the two-color laser wavelength on the terahertz generation in the air based on a transient photocurrent model.We show that the long wavelength laser excitation can greatly enhance the terahertz amplitude for a given total laser intensity. Furthermore, we utilize a local current model to illustrate the enhancement mechanism. Our analysis shows that the terahertz amplitude is determined by the superposition of contributions from individual ionization events, and for the long wavelength laser excitation, the electron production concentrates in a few ionization events and acquires the larger drift velocities, which results in the stronger terahertz radiation generation. These results will be very helpful for understanding the terahertz generation process and optimizing the terahertz output.     

Enhancement and modulation of terahertz radiation by multi-color laser pulses

We study theoretically intense terahertz radiation from multi-color laser pulse with uncommon frequency ratios. Comparing the two-color laser scheme, of which the uncommon frequency ratio should be set to be a specific value, we show that by using multi-color harmonic laser pulses as the first pump component, the lasers as the second pump component can be adjusted in a continuous frequency range. Moreover, these multi-color laser pulses can effectively modulate and enhance the terahertz radiation, and the terahertz yield increases with the increase of the wavelength of the uncommon pump component and is stable to the laser relative phase. Finally, we utilize the electron densities and velocities of ionization events to illustrate the physical mechanism of the intense terahertz generation.     

传播效应对宽带极紫外连续谱的影响

宽带极紫外连续谱是获得阿秒脉冲的重要途径,在此过程中传播效应是影响宽带极紫外连续谱产生的重要因素,本文针对传播效应对宽带极紫外连续谱的影响,采用慢变波近似的一阶传播方程,研究了传播过程对双色场周期量级激光脉冲产生宽带连续谱及其对应的单个阿秒脉冲的影响.通过分析不同聚焦位置和介质长度的传播过程,发现介质位于焦点之后有利于产生连续谱,同时随着介质长度的变长,虽然单个阿秒脉冲峰值强度有所提高,但是产生的阿秒脉冲宽度也会增加.进一步的分析表明,在特定的双色场延迟优化下,传播效应不仅使得阿秒脉冲强度增强,还可以获得与单原子模型下具有同样脉宽的单个阿秒脉冲,而不会因为传播效应导致阿秒脉冲宽度变宽.     

Generation of electromagnetic waves in the terahertz frequency range by optical rectification of a Gaussian laser pulse in a plasma in presence of an externally applied static electric field

We propose a theoretical model for the generation of electromagnetic waves in the terahertz (THz) frequency range by the optical rectification of a Gaussian laser pulse in a plasma with an applied static electric field transverse to the direction of propagation. A Gaussian laser pulse can exert a transverse component of the quasi‐static ponderomotive force on the electrons at a frequency in the THz range by a suitable choice of the laser pulse width. This nonlinear force is responsible for the density oscillation. The coupling of this oscillation with the drift velocity acquired by electrons due to the applied static electric field leads to the generation of a nonlinear current density. A spatial Gaussian intensity profile of the laser beam enhances the generated THz yield by many folds as compared to a uniform spatial intensity profile.     

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.     

激光等离子体光丝中太赫兹频谱的调控

研究了双色激光场激发空气成丝产生太赫兹辐射频谱的变化规律.实验观察到随驱动光功率和光丝长度增加,太赫兹光谱主要发生红移的现象.分析表明,由于等离子体密度的增加,太赫兹辐射的趋肤深度减小,等离子体吸收主导了红移的发生.在光丝足够短的条件下,趋肤深度远大于光丝长度,从而产生等离子体振荡主导的太赫兹辐射光谱蓝移.本研究为超快宽带太赫兹辐射的频谱调控提供了新思路.     

Generation of an isolated sub-30 attosecond pulse in a two-color laser field and a static electric field

We theoretically investigate high-order harmonic generation(HHG) from a helium ion model in a two-color laser field,which is synthesized by a fundamental pulse and its second harmonic pulse.It is shown that a supercontinuum spectrum can be generated in the two-color field.However,the spectral intensity is very low,limiting the application of the generated attosecond(as) pulse.By adding a static electric field to the synthesized two-color field,not only is the ionization yield of electrons contributing to the harmonic emission remarkably increased,but also the quantum paths of the HHG can be significantly modulated.As a result,the extension and enhancement of the supercontinuum spectrum are achieved,producing an intense isolated 26-as pulse with a bandwidth of about 170.5 eV.In particular,we also analyse the influence of the laser parameters on the ultrabroad supercontinuum spectrum and isolated sub-30-as pulse generation.     

The dependence on optical energy of terahertz emission from air plasma induced by two-color femtosecond laser-pulses

The generation of terahertz （THz） emission from air plasma induced by two-color femtosecond laser pulses is studied on the basis of a transient photocurrent model. While the gas is ionized by the two-color femtosecond laser-pulses com- posed of the fundamental and its second harmonic, a non-vanishing directional photoelectron current emerges, radiating a THz electromagnetic pulse. The gas ionization processes at three different laser-pulse energies are simulated, and the corresponding THz waveforms and spectra are plotted. The results demonstrate that, by keeping the laser-pulse width and the relative phase between two pulses invariant when the laser energy is at a moderate value, the emitted THz fields are significantly enhanced with a near-linear dependence on the optical energy.     

THz radiation generation via the interaction of two‐colour ultra‐short laser pulses with SO2 and NH3 gases

In this work, using a two‐dimensional particle‐in‐cell Monte Carlo collision computation method, terahertz (THz) radiation generation via the interaction of two‐colour, ultra‐short, high‐power laser pulses with the polyatomic molecular gases sulphur dioxide (SO₂) and ammonia (NH₃) is examined. The influence of SO₂ and NH₃ pressures and two‐colour laser pulse parameters, i.e., pulse shape, pulse duration, and beam waist, on the THz radiation generation is studied. It is shown that the THz signal generation from SO₂ and NH₃ increases with the background gas pressure. It is seen that the THz emission intensity for both gases at higher laser pulse durations is higher. Moreover, for these polyatomic gases, the plasma current density increases with increase in the laser pulse beam waist. A more powerful THz radiation intensity with a larger time to peak of the plasma current density is observed for SO₂ compared to NH₃. In addition, many THz signals with small intensities are observed for both polyatomic gases. It is seen that for both SO₂ and NH₃ the generated THz spectral intensity is higher at higher gas pressures.