Nonequilibrium Phonon Dynamics

The interaction of hot carriers and phonons in solids is a fundamental problem with important technological implications. For example, in polar semiconductors, a bottleneck in the decay of certain phonon-modes leads to increased electron scattering from the nonthermal phonons having a direct effect on carrier mobility and heat removal in devices. Detailed knowledge of the nonthermal phonon distribution has been limited by the lack of probes with sufficient temporal and momentum resolution. PULSE researchers have made the first ever picosecond images of nonequilibrium phonons spanning the Brillouin-zone. The images are of photoexcited semiconductors and make use of time-resolved x-ray diffuse scattering. We find that the lattice remains significantly out of equilibrium with the delayed emission of transverse acoustic phonons up to a nanosecond after laser excitation, even though typical phonon lifetimes are only a few picoseconds. We hope to develop this into a broader ultrafast vibrational spectroscopic tool for understanding of dynamical properties of materials wherever specific phonons in the Brillouin-zone play a defining role. In particularl, the bright femtosecond x rays at LCLS will allow us to resolve the initial stages of energy relaxation in materials as well as measure ultrafast changes in the interatomic potential during a strongly driven phase transition.

 PULSE Research Ultrafast Materials Science • Publications • Scientific Staff