Near-infrared-to-visible photon upconversion in Mo-doped rutile titania

A broadband upconversion luminescence was observed from Mo-doped titania under a 978 nm diode laser excitation. The luminescence has been assigned to the transitions from the excited states 3T1, 3T2 to the ground state 1A1 of the [MoO4]2− radicle. The intermediate states available for upconversion are supposed to be related with Mo4+ and/or Mo5+ species. The temporal evolution of the upconversion luminescence and the excitation-power dependence of the steady-state emission intensity exhibit the typical fingerprint of a photon avalanche. The dynamical process is well described using the fluorescence ‘transfer function’ model.