Kalmykov Serguei, Shvets Gennady
Department of Physics and Institute for Fusion Studies, The University of Texas at Austin, One University Station C1500, Austin, Texas 78712, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2006 Apr;73(4 Pt 2):046403. doi: 10.1103/PhysRevE.73.046403. Epub 2006 Apr 18.
The near-resonant beat wave excitation of an electron plasma wave (EPW) can be employed for generating the trains of few-femtosecond electromagnetic (EM) pulses in rarefied plasmas. The EPW produces a comoving index grating that induces a laser phase modulation at the difference frequency. As a result, the cascade of sidebands red and blue shifted by integer multiples of the beat frequency is generated in the laser spectrum. The bandwidth of the phase-modulated laser is proportional to the product of the plasma length, laser wavelength, and amplitude of the electron density perturbation. When the beat frequency is lower than the electron plasma frequency, the redshifted spectral components are advanced in time with respect to the blueshifted ones near the center of each laser beat note. The group velocity dispersion of plasma compresses so chirped beat notes to a few-laser-cycle duration thus creating a train of sharp EM spikes with the beat periodicity. Depending on the plasma and laser parameters, chirping and compression can be implemented either concurrently in the same, or sequentially in different plasmas. Evolution of the laser beat wave and electron density perturbations is described in time and one spatial dimension in a weakly relativistic approximation. Using the compression effect, we demonstrate that the relativistic bistability regime of the EPW excitation [G. Shvets, Phys. Rev. Lett. 93, 195004 (2004)] can be achieved with the initially subthreshold beat wave pulse.
电子等离子体波（EPW）的近共振拍波激发可用于在稀薄等离子体中产生一系列飞秒级电磁（EM）脉冲。EPW产生一个共动折射率光栅，该光栅以差频诱导激光相位调制。结果，在激光光谱中产生了由拍频的整数倍红移和蓝移的边带级联。相位调制激光的带宽与等离子体长度、激光波长和电子密度扰动幅度的乘积成正比。当拍频低于电子等离子体频率时，红移的光谱分量相对于每个激光拍频中心附近的蓝移分量在时间上提前。等离子体的群速度色散将如此啁啾的拍频压缩到几个激光周期的持续时间，从而产生一系列具有拍频周期性的尖锐EM尖峰。根据等离子体和激光参数，啁啾和压缩可以在同一等离子体中同时实现，也可以在不同等离子体中顺序实现。在弱相对论近似下，在时间和一个空间维度上描述了激光拍波和电子密度扰动的演化。利用压缩效应，我们证明了对于初始低于阈值的拍波脉冲，可以实现EPW激发的相对论双稳区[G. Shvets，《物理评论快报》93，195004（2004）]。
