Application Of Detuned Laser Beatwave For Generation Of Few-Cycle Electromagnetic Pulses
An approach to compressing high-power laser beams in plasmas via coherent Raman sideband generation is described. The technique requires two beams: a pump and a probe detuned by a near-resonant frequency Omega < omega(p). The two laser beams drive a high-amplitude electron plasma wave (EPW) which modifies the refractive index of plasma so as to produce a periodic phase modulation of the incident laser with the laser beat period tau(b) = 2pi/Omega. After propagation through plasma, the original laser beam breaks into a train of chirped beatnotes (each of duration tau(b)). The chirp is positive (the longer-wavelength sidebands are advanced in time) when Omega < omega(p) and negative otherwise. Finite group velocity dispersion (GVD) of radiation in plasma can compress the positively chirped beatnotes to a few-laser-cycle duration thus creating in plasma a sequence of sharp electromagnetic spikes separated in time by tau(b). Driven EPW strongly couples the laser sidebands and thus reduces the effect of GVD. Compression of the chirped beatnotes can be implemented in a separate plasma of higher density, where the laser sidebands become uncoupled.