Backscattering of ultrashort high intensity laser pulses from solid targets at oblique incidence

Abstract

Backreflection of short, intense laser pulses at oblique incidence on solid targets is explained with a model where a periodic electron density modulation acts as a diffraction grating. The pump and reflected electromagnetic waves drive through the ponderomotive force the grating, and the coupled system becomes parametrically unstable. The basic equations governing this situation are given. A linearized stability analysis is used to obtain the instability growth rate in a homogeneous plasma and the convective gain coefficients for the inhomogeneous case. The results support the feasibility of the suggested mechanism. An absolute instability is predicted to set on at a threshold intensity of about 1016 W/cm2 for a typical laser pulse with a length of 100 fs and a spot size of 30 μm. The instability is shown to saturate at a level of a few percent, because the higher harmonics in the electron density modulation make the diffraction more diffuse, thus reducing both the ponderomotive force and the backreflection coefficient.