Ultrafast hole burning in intersubband absorption lines of GaN/AlN superlattices

The authors present evidence for a distinct optical phonon progression in the nonlinear intersubband absorption spectra of electrons in a GaN/AlN superlattice. Femtosecond two-color pump-probe experiments in the near infrared show spectral holes separated by the longitudinal optical (LO) phonon frequency and a homogeneous line broadening of approximately 50 meV. The nonlinear bleaching signal decays with a time constant of 160 fs due to intersubband scattering of delocalized electrons, followed by a weak picosecond component attributed to the relaxation of electrons from longer-lived localized states.