Coherent acoustic phonons in GaN and GaN/InGaN heterostructures

Abstract

We discuss the generation and detection of coherent acoustic phonons in GaN/InGaN superlattices, multiple quantum wells, and epilayers via ultrafast laser excitation. We show that the generation of the acoustic phonons is driven by the ultrafast photoexcitation of electron-hole pairs in the InGaN layers. Under typical conditions, a complicated microscopic theory including the effects of strain induced piezoelectric fields and valence band mixing can be mapped onto a simpler problem: a loaded uniform string with a non- uniform loading function. The string model allows one to obtain analytic solutions under a variety of conditions. We find that in the superlattices and multi-quantum wells, the frequency of oscillation is related to the superlattice period, whereas in the epilayers and single quantum wells, the frequency of oscillation is related to the velocity of sound and the wavelength of the probe laser. In epilayers, we show that the coherent phonons are actually localized wavepackets that can be used as a powerful probe of nano- scale structures. Finally, we look at the application of multiple laser pulses a a means to coherently control the dynamics of the acoustic phonons.