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-resolv ed photoluminescence (PL) studies of the LO-phonon replicas. We obtain a bi exponential decay of the exciton number with time and find that excitons co ol mainly by scattering with acoustic phonons via deformation potential int eraction. Transient four-wave-mixing (FWM) studies yield the homogeneous br oadening of the A-exciton and the strengths of exciton-exciton and exciton- phonon interactions. These values are comparable to other direct gap semico nductors. The biexciton binding-energy is (5.7 +/- 0.3) meV. The energy spl itting between A- and B-exciton is obtained with very high accuracy from qu antum-beat spectroscopy. (C) 1999 Elsevier Science S.A. All rights reserved .