Observation of coherent acoustic phonon oscillations in bulk gallium nitride

Summary form only given. Progress in femtosecond lasers and ultrafast spectroscopy technology has enabled us to generate and directly observe the coherent oscillation of phonon modes. Coherent optical phonon oscillations have been observed in bulk GaAs, bulk Ge, and a number of other bulk materials. For acoustic phonons, it is much harder to selectively excite a specific coherent acoustic phonon mode in bulk materials. Higher frequency coherent acoustic phonon oscillations were recently observed in AlAs/GaAs superlattices, InGaN/GaN multiple-quantum-wells, and PbTe/PbS quantum dots. In this presentation, we report our observation of high frequency coherent acoustic phonon oscillations in a bulk material, a highly strained bulk GaN film. The longitudinal interference of an ultraviolet femtosecond pump pulse was used to create periodic carrier distribution in the bulk GaN thin film. The periodic carrier distribution screened out the strain-induced piezoelectric field and initiated the coherent acoustic phonon oscillations corresponding to the carrier periods. The decay time of the initiated coherent phonon oscillation is longer than 250 ps. This long decay time and the traveling wave nature of the generated acoustic phonons provide the hope for the realization of the first phonon laser.