Generating optical supercontinuum and frequency comb in tenuous plasmas

TL;DR

This paper investigates how near-relativistic laser pulses in tenuous plasma generate broad optical spectra through nonlinear effects, specifically four-wave mixing and forward Raman scattering, leading to supercontinuum and frequency combs.

Contribution

It identifies the dominant nonlinear mechanisms for spectrum broadening in tenuous plasma and describes their effects on laser pulse modulation.

Findings

FWM dominates with short, intense pulses, producing symmetric supercontinuum.

FRS dominates with long, weaker pulses, creating a frequency comb.

The spectrum broadening involves frequency modulation without pulse compression.

Abstract

There are several mechanisms by which the frequency spectrum of a laser broadens when it propagates at near-relativistic-intensity in tenuous plasma. Focusing on one dimensional effects, we identify two strong optical nonlinearities, namely, four-wave mixing (FWM) and forward Raman scattering (FRS), for creating octave-wide spectra. FWM dominates the interaction when the laser pulse is short and intense; Its combination with phase modulation produces a symmetrically broadened supercontinuum. FRS dominates when the laser pulse is long and relatively weak; It broadens the laser spectrum mainly towards lower frequencies and produces a frequency comb. The creation of the supercontinuum and frequency combs only frequency modulates, but does not compress, the laser pulse.