Temperature dependence of four-wave-mixing spectra in ZnO thin films on sapphire substrates grown with laser MBE

We investigated the optical nonlinearity around the excitonic resonance reg ion of ZnO thin films with thickness of about 200 nm grown with laser molec ular beam epitaxy (MBE) method by using four-wave-mixing (FWM) spectroscopy . It is found that the FWM signal can be confirmed in the temperature range (10-294 R) and that its spectra significantly depends on polarization of e xciting pulses. It is suggested, through the dependence, that the induced a nnihilation of a two-photon coherent state of biexcitons plays a negligibly small role in the FWM spectra compared with the excitonic population-grati ng effect. (C) 2000 Elsevier Science B.V. All rights reserved.