Phonon generation by femtosecond pulsed laser excitation of an aluminium nitride/gallium nitride superlattice

We have used bolometric detection techniques to study the phonons; generated by ultrafast optical excitation of a gallium nitride/aluminium nitride superlattice (SL) structure. For excitation wavelengths shorter than 435 nm, we observe a strong longitudinal acoustic phonon signal. Angular dependence measurements show that this signal enhancement becomes channelled closer to the SL growth direction for excitation wavelengths shorter than the peak photoluminescence wavelength. The results are consistent with similar measurements using GaAs/AlAs SL's, where ultrafast optical excitation resulted in the generation of monochromatic phonons when the structure was resonantly excited.