Study of exciton–polariton modes in nanocrystalline thin films of ZnO using reflectancespectroscopy

Abstract

We report detailed reflectance studies of the exciton–polariton structure of thinfilm nanocrystalline ZnO at low temperatures and compare these data to bulkcrystal data. The reflectance spectra are modelled using a two-band dielectricresponse function with a number of different models involving reflected waves in thethin film and/or excitonic dead layers. We present matrix forms for the solutionof these models, enabling computation of the reflected intensity and other fieldcomponents. The reflectance of nanocrystalline ZnO differs substantially from that ofbulk material, with Fabry–Perot oscillations at energies below the transverseA exciton and abovethe longitudinal B exciton. Between these energies we see no evidence of anomalous waves because the stronginteraction of the damped exciton with the photon leads to polaritons with substantialdamping such that the Fabry–Perot oscillations are eliminated. Good agreement is foundbetween the model and data, and the importance of the polariton viewpoint inunderstanding the reflectance data for nanocrystalline material is clearly seen. Thefits provide parameter values that can be compared to bulk crystal parameters,providing a method for quantitative analysis of the films and their potential forapplications such as thin film random lasing or polariton lasing in microcavities.