  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.
