Thin crystal layers in superstrong laser fields: Dynamics and coherent x-ray generation

Coherent x-ray generation is investigated for a thin solid layer or any other periodic atomic structure with limited dimensions exposed to a short, superintense laser pulse. We single out a selective range of parameters at which the impact of nonlinear collective processes on ion dynamics is reduced. This allows us to consider an ionic structure that is quasiregular during the brief interaction, which we prove also by particle-in-cell simulations. In the relativistic regimes of the interaction, when the transversal oscillation amplitude of the electron in the laser field exceeds the layer transversal width, the multiphoton scattering of electrons dominates over recombination to bound states. Electrons accelerated by the laser field penetrate through the periodic ionic structure, emitting coherent bremsstrahlung. Hard x rays are shown to amplify along the interaction length due to stimulated coherent bremsstrahlung. The high gain of the scheme compensates for the short interaction length, yielding a gain-length product competitive with other coherent x-ray sources. Limitations on the gain due to the energy spread of the electrons and the short time of the interaction are estimated