Plasma effects on harmonic spectra generated from moderately relativistic laser-plasma interactions

When intense p-polarized laser light is incident on a plasma with an electron density many times the critical density, the flux of fast electrons created by Brunel absorption excites plasma oscillations. These oscillations may in turn affect the spectrum of high harmonics by modulating the spectrum at the plasma frequency, ω(p), and by coupling to the radiation field through the steep density gradient at the plasma-vacuum interface, so generating plasma line emission (PLE) at ω(p) and harmonics of ω(p). Both aspects depend sensitively on a range of plasma and laser pulse parameters, including the initial electron density, the density profile at the plasma-vacuum interface, and the intensity, pulse shape, and pulse length of the incident laser light. These various dependences have been characterised for moderately relativistic laser-plasma interactions by means of a series of particle-in-cell (PIC) simulations.