Phonon sidebands of intersubband absorption in AlGaN/GaN high-electron-mobility transistors

Femtosecond two-color pump-probe experiments in the mid-infrared are performed on intersubband transitions in a GaN/Al0.8Ga0.2N heterostructure. In these experiments, we observe around zero time delay distinct phonon sidebands, which disappear because of spectral diffusion with a time constant of 80 fs. The signal itself decays with a time constant of 380 fs. The independent boson model agrees very well with all observed results.