Generation of terahertz monochromatic acoustic phonon pulses by femtosecond optical excitation of a gallium nitride/aluminium nitride superlattice

Abstract

We describe the generation of pulses of monochromatic acoustic phonons by ultrafast laser excitation of a gallium nitride/aluminium nitride superlattice (SL). The SL response to the optical excitation was studied using femtosecond pump-probe measurements. Superconducting bolometers were used to detect the phonons that leak out of the SL and propagate across the substrate. We see that, for excitation wavelengths around 430 nm, a pronounced enhancement in the longitudinal acoustic phonon signal occurs. The results are consistent with similar measurements using GaAs∕AlAs SL’s, where propagating monochromatic phonons were generated under resonant photoexcitation [Appl. Phys. Lett. 81, 3497 (2002)].