Inverse Compton scattering from solid targets irradiated by ultra-short laser pulses in the $10^{22}-10^{23}\,\mathrm{W/cm^2}$ regime

Emission of high energy gamma rays via the non-linear inverse Compton scattering process (ICS) in interactions of ultra-intense laser pulses with thin solid foils is studied using particle-in-cell simulations. It is shown that the angular distribution of the ICS photons has a forward-oriented two-directional structure centred at an angle $\vartheta = \pm 30^\circ$, a value predicted by a theoretical model based on a standing wave approximation to the electromagnetic field in front of the target, which only increases at the highest intensities due to faster hole boring, which renders the approximation invalid. The conversion efficiency is shown to exhibit a super-linear increase with the driving pulse intensity. In comparison to emission via electron-nucleus bremsstrahlung, it is shown that the higher absorption, further enhanced by faster hole boring, in the targets with lower atomic number strongly favours the ICS process.