To explain these striking differences observed on the harmonics beams resulting from the two generation mechanisms, we then investigated the properties of these beams both experimentally and theoretically –in particular their phase properties (e.g. their spatial and temporal phase). We described how the phase of CWE harmonics depends on the laser intensity, leading to significant spatial and temporal phases of these harmonics, which significantly affect their properties [Que08,Tha08]. Later on, we investigated the phase properties of ROM harmonics, which are mostly determined by the deformation of the plasma mirror surface see figure 2), resulting from the inhomogeneous radiation pressure and ponderomotive force exerted by the incident laser field [Vin14]. The different dependences of the phase of these two types of harmonics on laser intensity explain why their properties are observed to be so different in experiments. A review of all these results is provided in [Tha10].
Figure 2: Focusing of high order harmonics by a curved relativistic plasma mirror in a 2D PIC simulation. The laser-induced curvature of the plasma mirror surface tends to focus the reflected light in front of the plasma mirror. The right panel shows a spatial map of the plasma electron density ne(x,y) at the maximum of the laser pulse in dark red scale. A zoom on the surface is shown in the left panel. The multicolor map shows the intensity of the train of attosecond pulses obtained by filtering ROM harmonics from order 4 to 8, at three different times during its propagation away from the PM. Focusing of this train in vacuum, in front of the plasma mirror surface, is clearly observed.
[Que08] Quéré et al, Phys. Rev. Lett. 100, 095004 (2008)
[Tha08] Thaury et al, Nature Physics 4, 631 (2008)
[Tha10] Thaury & Quéré, J. Phys. B. 43, 213001(2010)
[Vin14] Vincenti et al, Nature Communications 5, 3403 (2014)