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

TL;DR

This paper develops a formalism using kinematic diffraction theory to simulate dark-field X-ray microscopy images of both coherent and incoherent phonons, enabling detailed study of phonon dynamics and interactions at the nanoscale.

Contribution

It introduces a new formalism to relate phonon dynamics to DFXM-measured strains, extending imaging capabilities to incoherent phonons relevant for thermal transport.

Findings

Simulated DFXM images of coherent phonons in diamond.

Extended formalism to describe imaging of high-frequency incoherent phonons.

Discussion of optimal sampling strategies for real-time, reciprocal, and spatial domains.

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.