Long-lived, coherent acoustic phonon oscillations in GaN single crystals

We report on coherent acoustic phonon (CAP) oscillations studied in high-quality bulk GaN single crystals with a two-color femtosecond optical pump-probe technique. Using a far-above-the-band gap ultraviolet excitation (similar to 270 nm wavelength) and a near-infrared probe beam (similar to 810 nm wavelength), the long-lived, CAP transients were observed within a 10 ns time-delay window between the pump and probe pulses, with a dispersionless (proportional to the probe-beam wave vector) frequency of similar to 45 GHz. The measured CAP attenuation corresponded directly to the absorption of the probe light in bulk GaN, indicating that the actual (intrinsic) phonon-wave attenuation in our crystals was significantly smaller than the measured 65.8 cm(-1) value. The velocity of the phonon propagation was equal to the velocity of sound in GaN. (c) 2006 American Institute of Physics.