Generation and Modulation of Harmonic Emission Spectra from Intense Laser-Plasma Interactions

We report results from recent investigations of the interaction of intense laser light with overdense plasma with particular emphasis on the harmonic emission spectrum generated in these interactions. The harmonic spectrum is governed by both the photon and electron densities, though dependence on the latter has been largely overlooked. To illustrate the effects of the plasma density we examine the characteristics of low-order harmonics by means of a simplified one-dimensional model and compare the spectra with particle-in-cell (PIC) simulations. Recent observations record a spectrum of harmonics showing distinct, if variable, modulation of between two and four harmonics. This spectral feature proved highly sensitive to the incident intensity, with more pronounced modulation at higher intensities. In this paper we report on features of the emission spectrum across a wide parameter range and its dependence on the detailed structure of the plasma density profile. We find clear evidence of a modulated spectrum and identify the plasma frequency as the modulation frequency