Formalism to Image the Dynamics of Coherent and Incoherent Phonon with Dark-Field X-ray Microscopy using Kinematic Diffraction Theory

Abstract: Dark-field X-ray microscopy (DFXM) is a novel X-ray imaging technique developed at synchrotrons to image along the diffracted beam with a real space resolution of ~100 nm and reciprocal space resolution of $10^{-4}$. Recent implementations of DFXM at X-ray free electron lasers (XFELs) have demonstrated DFXM's ability to visualize the real-time evolution of coherent GHz phonons produced by ultrafast laser excitation of metal transducers. Combining this with DFXM's ability to visualize strain fields due to dislocations makes it possible to study the interaction of GHz coherent phonons with the strain fields of dislocations, along with studying the damping of coherent phonons due to interactions with thermal phonons. For quantitative analysis of phonon-dislocation interactions and phonon damping, a formalism is required to relate phonon dynamics to the strains measured by DFXM. In this work, we use kinematic diffraction theory to simulate DFXM images of the specific coherent phonons in diamond that are generated by the ultrafast laser excitation of a metal transducer. We extend this formalism to also describe imaging of incoherent phonons of sufficiently high frequency, which are relevant for thermal transport, offering future opportunities for DFXM to image signals produced by thermal diffuse scattering. For both coherent and incoherent phonons, we discuss the optimal sampling of real space, reciprocal space and time, and the opportunities offered by the advances in DFXM optics.