Strong biexcitonic effects and exciton-exciton correlations in ZnO

We have studied experimentally biexcitonic states of ZnO of rather high quality in the low exciton density limit by means of time-integrated and spectrally-resolved four-wave mixing (FWM). FWM emission signals due to biexcitons consisting of two A-hole excitons (AA biexcitons), an A-hole and a B-hole exciton (AB biexcitons), and two B-hole excitons (BB biexcitons) have been clearly observed according to polarization selection rules. The obtained binding energies for AA, AB, and BB biexcitons are 15.6, 16.6, and 4.7 meV, respectively. A brief discussion is also given on the contribution of biexciton and two-pair continuum resonances to the FWM signal.