ABSTRACT
The first experimental evidence of the higher-order harmonic radiation generated by periodically modulated targets (gratings) irradiated by relativistic, ultrashort (<30 fs), high intensity [Iλ(2)=10(20) (W/cm(2)) µm(2)] laser pulse is presented. The interference effects on the grating surface lead to the emission of high harmonics up to 45th order along the target surface when the laser beam is focused onto a grating target close to normal incidence (5°). By means of numerical simulations we demonstrate the possibility of controlling the composition of the higher harmonic spectrum and we prove the influence of the laser pulse parameters in the interaction area (laser focusing and wavefront curvature) on the emission angle of a certain high harmonic order.
ABSTRACT
When a laser pulse hits a solid surface with relativistic intensities, XUV attosecond pulses are generated in the reflected light. We present an experimental and theoretical study of the temporal properties of attosecond pulse trains in this regime. The recorded harmonic spectra show distinct fine structures which can be explained by a varying temporal pulse spacing that can be controlled by the laser contrast. The pulse spacing is directly related to the cycle-averaged motion of the reflecting surface. Thus the harmonic spectrum contains information on the relativistic plasma dynamics.
