Spatiotemporal Response of Crystals in X-ray Bragg Diffraction

TL;DR

This paper develops a theoretical framework for understanding the spatiotemporal response of crystals in x-ray Bragg diffraction, applicable to various geometries, and discusses applications in ultrafast x-ray laser techniques.

Contribution

It extends previous analysis to include asymmetric geometries and provides a comprehensive model for the crystal's spatiotemporal response in x-ray diffraction.

Findings

Response characterized by a few key parameters like extinction length and Bragg angle

Amplitude modulation occurs due to angular dispersion and Bragg's law

Application potential in femtosecond-resolution ultrafast imaging

Abstract

The spatiotemporal response of crystals in x-ray Bragg diffraction resulting from excitation by an ultra-short, laterally confined x-ray pulse is studied theoretically. The theory presents an extension of the analysis in symmetric reflection geometry [1] to the generic case, which includes Bragg diffraction both in reflection (Bragg) and transmission (Laue) asymmetric scattering geometries. The spatiotemporal response is presented as a product of a crystal-intrinsic plane wave spatiotemporal response function and an envelope function defined by the crystal-independent transverse profile of the incident beam and the scattering geometry. The diffracted wavefields exhibit amplitude modulation perpendicular to the propagation direction due to both angular dispersion and the dispersion due to Bragg's law. The characteristic measure of the spatiotemporal response is expressed in terms of a few parameters: the extinction length, crystal thickness, Bragg angle, asymmetry angle, and the speed of light. Applications to self-seeding of hard x-ray free electron lasers are discussed, with particular emphasis on the relative advantages of using either the Bragg or Laue scattering geometries. Intensity front inclination in asymmetric diffraction can be used to make snapshots of ultra-fast processes with femtosecond resolution.