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.     

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。     

激光波长对双色激光等离子体产生太赫兹波的影响

基于瞬态光电流模型研究了激光波长对双色激光等离子体产生太赫兹波的影响.通过理论计算证明:太赫兹信号随激光波长的增大而增强,其变化趋势不会因为双色激光强度、脉宽、相位差、强度比的变化而改变;太赫兹波的频谱不会因为激光波长的改变发生变化.分析了双色激光波长影响太赫兹辐射强度的原因,并利用自由电子密度和漂移电流密度阐明了该影响的内在物理机制.     

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。     

飞秒超强激光驱动太赫兹辐射特性的实验研究

强太赫兹源是太赫兹科学技术发展的关键,其中大能量强场太赫兹脉冲源在超快物态调控、新型电子加速器等领域具有重要的应用前景.超快超强激光与等离子体相互作用是近年来发展起来的一种新型的强场太赫兹辐射产生途径.本文报道了利用超强飞秒激光脉冲与金属薄膜靶作用产生太赫兹辐射的实验结果,研究了激光能量和离焦量对靶后太赫兹辐射能量的影响,并通过监测激光背向散射光谱,定性揭示了其变化规律与不同光强下的电子加热机制的相关性.实验表征了太赫兹辐射的频谱、偏振及聚焦光斑情况.测量结果表明,实验产生了脉冲能量达458μJ、聚焦场强高达GV/m量级的超宽带太赫兹辐射,为开展极端太赫兹脉冲与物质相互作用研究提供了一种新的强场太赫兹光源.     

双色场诱导气体产生相干可控的四次谐波

利用钛宝石飞秒激光器输出的基频脉冲ω及其倍频脉冲2ω所构成双色场作用空气, 实验中检测到了中心波长处于真空紫外波段的四阶谐波. 在气体未发生电离的情况下, 四次谐波强度对双色场的能力依赖关系显示其产生是参量过程2ω+ω+ω→4ω的贡献. 当气体发生电离, 四次谐波强度与双色场相对相位有关, 可通过双色场相干控制. 实验研究了四次谐波对双色场相位的依赖性以及与太赫兹波的关联性, 其结果与数值模拟结果相符, 分析发现当气体发生电离时四次谐波的产生过程存在太赫兹辐射Ω_THz的参与, 是参量过程2ω+2ω±Ω_THz→4ω和2ω+ω+ω→4ω的共同贡献.     

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.     

两级喇曼压缩系统配偏振干涉仪诊断等离子体电子密度

 在星光Ⅱ铷玻璃激光装置上,采用两级喇曼压缩系统产生的波长为308 nm的紫外光作为探针束,配合Nomarski偏振干涉仪对金平面靶冕区激光等离子体进行诊断。308 nm光具有波长短、亮度高、脉冲时间短、相干性好的优点,作为探针束诊断冕区产生的等离子体电子密度,可以与高功率激光装置打靶激光同步,实现有效地脉冲压缩,同时避免等离子体中谐波分量的干扰。实验获得了308 nm紫外探针光偏振干涉条纹图,在研究Abel反演算法的基础上,利用自行研制的基于Windows操作系统的实验数据处理软件,对实验数据进行了处理和分析,得到了冕区等离子体电子密度的空间分布。结果表明：两级喇曼压缩系统配偏振干涉能有效抑制主束谐波影响,以更高时间分辨测量等离子体的更高密度区域。     

极化检测型铷原子磁力仪的研究

针对交变弱磁场的检测,研制了一种基于极化-检测双光束结构的激光抽运铷原子磁力仪.为了获得该磁力仪对磁场的响应特性,通过数值仿真分析了信号幅度随极化磁场强度、弛豫时间的变化关系,并进行了实验验证.最后通过选择合适的极化磁场使磁力仪对待测磁场的灵敏度最大.实验结果表明,优化后磁力仪灵敏度为0.2pT/(Hz)~(1/2),响应带宽3.5kHz,可用于弱磁场磁共振、高频异常物理现象等信号的检测.     

Second harmonic stimulated Raman scattering of laser radiation in aplasma

A high-power linearly polarized laser propagating through a plasma produces oscillatory electron velocity at the second harmonic due to-the ν&oarr;×B&oarr; force, This velocity couples a Langmuir wave (ω, k&oarr;) and an electromagnetic wave (ω₁, k&oarr;₁), where ω₁=ω-2ω₀ , k-2k&oarr;₀ and ω₀, k&oarr;₀ are frequency and wavenumber of the laser pump, causing second harmonic Raman scattering. The growth rate is maximum for side scattering. This process can occur above the quarter critical density, unlike the first harmonic stimulated Raman scattering which occurs below the quarter critical density     

On the effect of long-wavelength electron plasma waves onlarge-angle stimulated Raman scattering of short laser pulse in plasmas

Spectral features of a large-angle stimulated Raman scattering (LA SRS) of a short electromagnetic pulse in an underdense plasma, which are caused by the presence in a plasma of a given linear long-wavelength electron plasma wave (LW EPW), are investigated. It is shown that the LW EPW, whose phase velocity coincides with a group velocity of a pulse and a density perturbation normalized to a background electron density δn_LW/n₀ exceeds the ratio of the electron plasma frequency to the laser frequency ω_pe/ω₀ suppresses the well-known Stokes branch of the weakly coupled LA SRS. Under the same condition, the anti-Stokes band appears in the spectrum of the scattered radiation. Variation of a scattering angle and an electron temperature do not significantly modify qualitative features of the effect. In the case of strongly coupled LA SRS, the maximum of the increment is decreased by nearly one-half for δn_LW/n₀~(a₀ω_pe /ω₀)^2/3≫ω_pe /ω₀, where a₀ is an amplitude of an electron quiver velocity in the laser field normalized to a speed of light c, and it decreases further with an increase in plasma density perturbation in LW EPW     

Oscillating two-stream instability of a plasma wave in a plasmachannel

A long wavelength Langmuir wave (ω₀, k₀), propagating through a parabolic plasma density channel, can decay into a low-frequency mode (ω,k&oarr;) and two short wavelength Langmuir wave sidebands (ν_1,2,k&oarr;_1,2 ), via two-stream instability where ω_1,2=ω∓ω₀ and k&oarr;_1,2=k&oarr;∓k&oarr;₀. Depending on the mode number n, the growth rate maximizes in the range γ_max≃0.1ω_pi-0.4ω_pi for the range of k from 0.1(ω_pi/c_s) to 0.2(ω_pi/c_x) for ν₀~ν_th where ν₀ and ν_th are the oscillatory and thermal velocities of electrons, ω_pi is the ion plasma frequency on the axis, and c_s is the sound speed. The growth rate increases with the width a of the plasma density channel. It decreases with the mode number. The instability may be relevant to laser based charged particle accelerators     

Excitation of surface plasma waves over metallic surfaces by lasersand electron beams

A laser incident on a metal film (deposited on a glass substrate) from the glass side at a specific angle of incidence, excites a surface plasma wave (SPW) at the metal-free space interface. The ratio of the SPW field to the laser field increases with the laser spot size b attaining a value much greater than one at b>exp(2wα/c) where a is the film thickness and ω is the laser frequency. The SPW (ω, k_z,) can also he excited by a relativistic electron beam, propagating parallel to the interface in the free space region, via Cerenkov interaction when beam energy ϵb=(|ϵ|-1)mc² where ϵ is the effective metal permittivity, and mc² is the electron rest mass energy. When the surface has a ripple of wave number k₀, the SPW (ω, k_zz) can be excited at lower beam energy via sideband coupling, ω=(k_zz +k₀)v_b where v_bzˆ is the beam velocity. In both cases, however, the positioning of the beam in the close proximity of the interface is required. The scheme is useful for the generation of wavelengths longer than 1 μm     

空间非均匀啁啾双色场驱动下氦离子的高次谐波以及孤立阿秒脉冲的产生

本文理论上研究了初态为基态与第一激发态等权叠加的一维氦离子在空间非均匀啁啾双色场驱动下氦离子的高次谐波发射及孤立阿秒脉冲的产生. 研究表明, 一维氦离子在空间非均匀啁啾双色场驱动下发射的高次谐波相对于均匀场情况截止位置得到明显扩展, 得到了光滑的超连续谱,并应用半经典三步模型解释了高次谐波发射的物理机理. 通过小波变换的方法对连续谱进行了时频分析, 并且与电子的经典运动轨迹进行了对比分析, 结果显示在空间非均匀场中长量子轨道消失, 短量子轨道加强. 讨论了空间非均匀啁啾双色场中时间延迟对谐波和孤立阿秒脉冲产生的影响, 发现适当调整时间延迟值可以得到较大延展的光滑的超连续谐波谱, 本方案中时间延迟为t₀=1.6πup/ω₁时得到了最大延展, 通过对谐波中600次到680次(80次)谐波合成得到32 as的孤立脉冲.     

Plasma effects in a free electron laser

The introduction of a plasma and a strong guide magnetic field in a free electron laser (FEL) slows down the phase velocity of radiation, significantly reducing the requirements on beam energy for generating frequencies below the electron-cyclotron frequency (ω₁≲ω_c). Around plasma resonance (ω₁~ω_p), the FEL mode couples to two-stream instability (TSI), attaining a large growth rate, comparable to that of the wiggler-free TSI. At plasma densities comparable to beam density, the beam-induced local depression in the electron density of the plasma acts as a waveguide for guiding any high-frequency radiation when the beam current is ≳17 kA     

Tunable two-axis spin model and spin squeezing in two cavities

Multi-mode cavities have now attracted much attention both experimentally and theoretically. In this paper, inspired by recent experiments of cavity-assisted Raman transitions, we realize a two-axis spin Hamiltonian H = q(J_x~2+ χJ_y~2) + ω_0J_z in two cavities. This realized Hamiltonian has a distinct property that all parameters can be tuned independently. For proper parameters, the well-studied one- and two-axis twisting Hamiltonians are recovered, and the scaling of N~(-1) of the maximal squeezing factor can occur naturally. On the other hand, in the two-axis twisting Hamiltonian, spin squeezing is usually reduced when increasing the atomic resonant frequency ω_0. Surprisingly, we find that by combining with the dimensionless parameter χ(-1), this atomic resonant frequency ω_0 can enhance spin squeezing greatly. These results are beneficial for achieving the required spin squeezing in experiments.     

Dark and Bright Solitons of Extraordinary Waves in Ferromagnetic Systems

It is shown that the slowly varying amplitudes of both high and low frequency extraordinary waves propagating perpendicular to the dc magnetic field in ferromagnetic systems are described by the nonlinear SchrEdinger equation. The extraordinary waves are modulationally stable (unstable) and may evolve as dark (bright) solitons as the frequency ω satisfies ω₀ < ω < ω_b or ω > ω₁(0 < ω < ω₀ or ω_a ≤ ω < ω₁), where ω_a, ω_b, ω₀ and w₁ are given in the text.     

WKB solution for wave propagation in a time-varying magnetoplasmamedium: longitudinal propagation

The interaction of a right circularly polarized electromagnetic wave of frequency ω₀ with a switched-on-time-varying plasma medium, in the presence of a static magnetic field, is considered. Longitudinal propagation is assumed and ion motion is neglected. The electron density is assumed to vary slowly with time, and the solution is obtained through a WKB approximation. The main effect of switching the time-varying medium is the splitting of the original (incident) wave of frequency ω₀ into three new waves with time-varying frequencies. An exponentially increasing electron density profile is considered to illustrate the solution. The distinguishing feature of the presence of the static magnetic field is the creation of the third wave. The initial value of the instantaneous frequency ω₃ of this wave is equal to the gyrofrequency ω_b, and the final value depends on ω_b, ω₀, and the final value of the plasma frequency. ω₃ decreases with time for the profile under consideration