Polarization and wave-vector-dependent measurements by four-wave mixing in ZnO: valence-band ordering and biexcitons

Abstract

We have investigated free excitons and biexcitons of bulk ZnO by time-integrated and spectrally resolved four-wave mixing (FWM). The polarization- and wave-vector-dependent FWM signals for A and B excitons demonstrate clearly that the valence-band ordering is A– Γ 9 , B– Γ 7 , and C– Γ 7 . Also, we gave qualitative explanation of the FWM signals induced by excitons and biexcitons in ZnO. For negative delay times, bound and unbound AB-biexcitons (A exciton–B exciton correlation), which have been considered to play a minor role in GaAs heterostructures, was found to play an important role and give a significant qualitative impact to the optical response of ZnO.