Polarization dependence of the excitonic optical Stark effect in GaN

The dynamic Stark effect of excitons in GaN was studied using femtosecond pump-probe spectroscopy with various polarization configurations and pump detunings at 10 K. In contrast to two-dimensional GaAs/AlGaAs quantum wells which have Bloch eigenstates similar to those of GaN and a large spin-orbit coupling, we observed that the Stark effect in GaN is strongly dependent on pump and probe relative linear polarizations. We found that this dependence results from the small spin-orbit splitting in GaN and a mixing of A and B valence bands induced by a linearly polarized pump. Using two different circular polarization configurations, we also observed splitting of degenerate excitons because of different optical Stark shifts. Our experimental results are explained by a simple theoretical model.