Hard X-ray Transient Grating Spectroscopy on Bismuth Germanate

TL;DR

This paper demonstrates the first use of hard X-ray transient grating spectroscopy at 7.1 keV to probe bulk properties of Bismuth Germanate, opening new avenues for ultrafast, element-specific material analysis.

Contribution

It introduces the first implementation of X-ray transient grating spectroscopy in the hard X-ray regime, enabling bulk and nanoscale investigations.

Findings

Successful demonstration of TG excitation at 7.1 keV in BGO

Detection of coherent optical phonons via diffraction

Potential for ultrafast, element-specific material studies

Abstract

Optical-domain Transient Grating (TG) spectroscopy is a versatile background-free four-wave-mixing technique used to probe vibrational, magnetic and electronic degrees of freedom in the time domain. The newly developed coherent X-ray Free Electron Laser sources allow its extension to the X-ray regime. Xrays offer multiple advantages for TG: their large penetration depth allows probing the bulk properties of materials, their element-specificity can address core-excited states, and their short wavelengths create excitation gratings with unprecedented momentum transfer and spatial resolution. We demonstrate for the first time TG excitation in the hard X-ray range at 7.1 keV. In Bismuth Germanate (BGO), the nonresonant TG excitation generates coherent optical phonons detected as a function of time by diffraction of an optical probe pulse. This experiment demonstrates the ability to probe bulk properties of materials and paves the way for ultrafast coherent four-wave-mixing techniques using X-ray probes and involving nanoscale TG spatial periods.