Ultrafast physics in nitrides

We present an investigation of the excitonic resonances in wurtzite GaN by ultrafast linear, and nonlinear optical spectroscopy. The exciton lifetime and the cooling of the exciton distribution are investigated by time-resolved photoluminescence (PL) studies of the LO-phonon replicas. We obtain a biexponential decay of the exciton number with time and find that excitons cool mainly by scattering with acoustic phonons via deformation potential interaction. Transient four-wave-mixing (FWM) studies yield the homogeneous broadening of the A-exciton and the strengths of exciton–exciton and exciton–phonon interactions. These values are comparable to other direct gap semiconductors. The biexciton binding-energy is (5.7±0.3) meV. The energy splitting between A- and B-exciton is obtained with very high accuracy from quantum-beat spectroscopy.