Ultrafast dynamical diffraction with nanobeams, simulations on thin Au crystals

Abstract

The diffraction of x-rays in quasi-perfect thin crystals of elements with high Z can generate multiple diffracted beams at the exiting surface of a crystal. These beams propagate in the free space parallel to each other and share the same divergence and monochromatic properties. These beams have a spatial separation that varies between few nm and couples of μm. Due to the different path that the photons follow, these x-ray beams present a temporal delay between each other in the order of the fs. It is for these that the x-ray beams generated by this ultrafast diffraction process are so-called x-ray echoes. The x-ray echoes can only be described using the dynamical diffraction theory formalism. Here, we present simulations with the expected diffracted wave-fronts both in the diffracted and forward direction in several compounds, such as Ni, GaAs, InSb and in particular Au. This work presents also the dependence with energy and thickness of the x-ray echoes. The properties of these x-ray beams can produce ambiguous results while performing temporal studies, that became obvious with fs and sub-fs pulse facilities. Moreover, the spatial overlapping of the echoes can mislead the scientist to think that speckles from a small crystal could be generated from different centers of diffraction, while in reality are a dynamical diffraction effect.