Hard X-ray transient grating spectroscopy on bismuth germanate

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 regime. Xrays offer multiple advantages for TG: their large penetration depth allows probing bulk properties materials, element-specificity can address core-excited states, short wavelengths create excitation gratings with unprecedented momentum transfer spatial resolution. We demonstrate first TG hard range at 7.1 keV. In Bismuth Germanate (BGO), nonresonant generates optical phonons detected as function by diffraction an pulse. This experiment demonstrates ability materials paves way ultrafast techniques using probes involving nanoscale periods.