We investigated the optical nonlinearity around the excitonic resonance region of ZnO thin films with thickness of about 200 nm grown with laser molecular 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 exciting pulses. It is suggested, through the dependence, that the induced annihilation of a two-photon coherent state of biexcitons plays a negligibly small role in the FWM spectra compared with the excitonic population-grating effect.
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