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

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 10(16) W/cm(2) for a typical laser pulse with a length of 100 fs and a spot size of 30 mum. 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. (C) 2002 American Institute of Physics.