Intersubband electrons coupled to zone-folded coherent acoustic phonons in a GaN/AlN superlattice

We present spectrally-resolved resonant pump-probe measurements on a strongly polar GaN/AlN superlattice. The transmitted and reference probe spectra are detected at the same read out rate. Analysis of the normalized probe spectra reveals electronic contributions as well as the first three folded longitudinal acoustic phonons, matching calculated frequency values predicted by the elastic continuum model. The observed modes couple strongly to intersubband electrons and modulate the spectral width and position of the intersuband absorption. We conclude that this electron-phonon coupling takes place via piezoelectric effects and a phonon-induced modulation of the subband effective mass.