Key points are not available for this paper at this time.
The rapid advancement of x-ray free-electron laser technology facilitates the attainment of relativistic attosecond x-ray laser pulses in the future. It could serve as an ideal source to explore x-ray-driven high-energy density physics, such as particle acceleration and secondary radiation at solid densities. Here, utilizing particle-in-cell simulations, we systematically investigate high-order harmonic generation (HHG) based on intense x-ray lasers interacting with crystal-like structures. We find that the nonlinear interaction induces significant HHGs that are highly modulated due to the periodic target structure. By varying the spacing of the density spikes, the resonant conditions are tuned such that the efficiency of HHGs is drastically different. While each order of harmonic has multiple propagating directions, we find that the intensity can be enhanced significantly in the non-Bragg condition case than that in the Bragg case. The results indicate an approach to manipulate HHG emission driven by strong-field x-rays.
Cai et al. (Fri,) studied this question.