Coherent phonon spectroscopy at LCLS: extreme frequency and momentum resolution with broadband pulses

Program Description

I will show that broadband, hard x-ray pulses from LCLS can be used to probe the vibrational spectrum (phonon dispersion) of a solid with sub-meV energy resolution over the entire Brillouin zone. Our pump-probe approach exploits a two-phonon mechanism to generate coherent oscillations in the rms displacements (squeezed phonons) across the entire Brillouin zone. The time-dependent rms displacements are then measured via diffuse scattering with femtosecond hard-x-ray pulses. The sub-meV resolution is achieved by a Fourier transform of the oscillatory dynamics. Using this method we obtain the full transverse acoustic phonon dispersion in germanium. I will discuss some of the requirements for this approach, both in the generation of coherent two-phonon states by ultrafast optical pulses and their detection via x-ray scattering. I will finally comment on recent results on the anharmonic phonons of thermoelectric PbTe using this method.