Blue photoluminescence of highly photoexcited rutile TiO2: Nearly degenerate conduction-band effects

We report the observation of blue photoluminescence (PL) from highly photoexcited rutile TiO2 single crystals at room temperature. Under extremely high-density excitation, we found an anomalous dependence of the PL dynamics and intensity on the excitation density. By considering the nearly degenerate conduction bands of rutile, we revealed that the PL behavior is determined by the electron density in the lower-energy conduction band. The PL decay dynamics is well explained by the simple model involving two-body radiative recombination and three-body Auger recombination processes. We discuss the origin of the blue PL band.