Spectral dependence of transient reflectance in a ZnO epitaxial film at room temperature

Abstract

We demonstrated the spectral dependence of time-resolved reflectance in a ZnO epitaxial film at room temperature. The ultrafast thermalization time increases as increasing the excitation photon energy accompanied with the hot phonon effect. The obtained recovery time of renormalized bandgap, associated with the nonradiative decay or the free exciton formation, is independent of photon energy. Based on a theoretical model to calculate the carrier-induced change of refractive index in ZnO, the spectral dependent transient reflectance can be successfully analyzed as a result of combined effects of band-filling (BF) and bandgap renormalization (BGR). The measured transient reflectance decreases with decreasing the photon energy for excitation above the bandgap states revealing the significance of competition of the BF and BGR effects.