Thanks to this improved controlled of the interaction, we could start investigating the generation of high-order harmonics that occurs when very intense fs laser pulses reflect on plasma mirrors. We first identified a new generation mechanism, called coherent wake emission (CWE) [Que06], where the harmonic emission results from collective electron oscillations in the dense plasma, triggered by attosecond bunches of fast electrons injected by the laser field from the plasma surface into its bulk. CWE remains efficient for laser intensities down to a few 1015 W/cm2. This is an ubiquitous mechanism in all harmonic generation experiments on plasma mirrors.
By increasing the peak intensity in our experiments, we were then able to simultaneously observe harmonics due to the CWE and Relativistic oscillating mirror (ROM) mechanisms (see figure 1) [Tha07]. A remarkable feature is that these two families of harmonics can be very clearly distinguished in experiments, in particular through to their different spectral widths and divergences.
Figure 1: Raw images of HHG spectra, obtained on a plastic target at a laser intensity of 8.1018W/cm2. Harmonics below the 15th are due to CWE, and those above to ROM, leading to a striking difference in spectral width. From [Tha07].