Influence of flying mirror features and time delay between two counterpropagating laser pulses on the generated attosecond pulse intensity in near-critical density plasmas

The attosecond pulse generation by the interaction of two counterpropagating ultrashort laser pulses with near-critical density plasma is simulated using two-dimensional particle in the cell method. Results of the simulations showed the flying mirror properties such as density and shape change, while moving through the plasma, behind the intense driver laser. We investigated the effects of the mirror features on the produced attosecond pulse intensity by setting various delay times between the driver and source pulses so that the source encounters the mirror at different points. It is demonstrated that the higher density of the mirror, particularly in its center (due to the Gaussian transverse profile of the source), in addition to its suitable curvature and surface smoothness, results in a more intense reflection. Moreover, a considerable size of the hole created in the mirror center due to the self-injection process has a destructive effect on the reflection efficiency. Finally, an efficient reflection can be obtained by controlling the delay time. The optimal delay for arbitrary parameters of the laser and plasma depends on the region in which the most efficient flying mirrors are created by the mutual interaction of the plasma density and the driver amplitude along with considering the pulse situation when reaching the mirror. By analyzing the electron phase space, it was found that the velocity of density spikes changes rapidly when passing through the plasma. The higher speed of the electrons of the mirrors contributing to the source reflection leads to the production of the higher upshifted frequency peak in different source delays.