Femtosecond x-ray diffraction study of multi-THz coherent phonons in $SrTiO_{3}$

We report generation of ultra-broadband longitudinal acoustic coherent phonon wavepackets in $SrTiO_{3}$ (STO) with frequency components extending throughout the first Brillouin zone. The wavepackets are efficiently generated in STO using femtosecond infrared laser excitation of an atomically flat 1.6 nm-thick epitaxial $SrRuO_{3}$ film. We use femtosecond x-ray diffraction at the European X-Ray Free Electron Laser Facility to study the dispersion and damping of phonon wavepackets. The experimentally determined damping constants for multi-THz frequency phonons compare favorably to the extrapolation of a simple ultrasound damping model over several orders of magnitude.