LAUSR

Intense, ultrafast photoexcitation of semiconducting materials produces several discrete but inter-related energy relaxation processes that span large spatial and temporal scales [1]. Initial charge-carrier excitation, and generation of a dense electron-hole plasma, produces hypersonic plasma waves that propagate outward from the excitation zone. Related to this, myriad scattering processes (electronelectron, electron-phonon, etc.) occur within the first few picoseconds following photoexcitation, ultimately leading to the launch of coherent propagating strain waves [2]. In thin membranes of semiconducting materials, propagating strain waves display Lamb-type symmetries and hypersonic velocities (depending on mode type and membrane thickness) [3]. In addition to often-used ultrafast spectroscopic methods, femtosecond X-ray and electron scattering has been used to study both hypersonic plasma waves and acoustic-phonon wavetrains [4,5]. Observations from these experiments, combined with knowledge of the associated spatiotemporal scales, lead to questions about preferred nucleation points/regions, propagation directions, and time-varying phase-velocity dispersion behaviors.