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

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–image, B–image, and C–image. 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.