Picosecond charge transport in rutile at high carrier densities studied by transient terahertz spectroscopy

In rutile the charge carriers form polarons with high effective mass owing to the strong electron-phonon coupling. In this paper we studied ultrafast terahertz photoconductivity in rutile under strong optical excitation. In this regime the polaron mobility depends on the polaron density; as a consequence, highly complex transport phenomena occur on a picosecond scale involving fine interplay between diffusion and recombination. We developed a general model to disentangle these phenomena and we determined dynamical properties of electron and hole polarons. At room temperature internal degrees of freedom of polarons are also observed through a dramatic increase of the transient permittivity in photoexcited state.