High order harmonic beams emitted from plasma gratings

When focusing an ultra-intense femtosecond laser pulse (I>10W/cm) onto a solid target, it is ionized at the very beginning of the laser pulse. The resulting dense plasma specularly reflects the laser: this is a plasma mirror. The nonlinear response of the plasma to the ultra-intense laser field results in the generation of trains of attosecond pulses associated, in the frequency domain, to high-order harmonics of the laser frequency. Two main HHG processes on plasma mirrors have been observed for different interaction conditions. At moderate laser intensities (I<10W/cm), Brunel electrons, accelerated into the target, excites plasma oscillations in the high density plasma gradient, which radiate at the local plasma frequency. It is the Coherent Wake Emission (CWE) process. At ultra-high intensity (I>10W/cm), the electron density oscillates normally to the target with a relativistic velocity. It periodically distorts the reflected field by a Doppler effect: this is the Relativistic Oscillating Mirror (ROM) process. These two processes carry rich information on the laser-plasma interaction. For instance, the spatial curvature of each emitted attosecond pulse directly results from the interaction properties: the spatial curvature either of the plasma oscillations or of the plasma surface under radiation pressure.