Evidence for Photoinduced Insulator-to-Metal transition in B-phase vanadium dioxide

Ultrafast optical studies have been performed on epitaxial films of the novel B-phase of vanadium dioxide using temperature-dependent optical pump-probe technique. Signature of temperature-driven metal-to-insulator transition was distinctly observed in the ultrafast dynamics - the insulating phase showed two characteristic electronic relaxation times while the metallic phase showed only one. Beyond a threshold value of the pump fluence, the insulating state collapses into a 'metallic-like' phase which can be further subdivided into two regimes according to the lengths of the fast characteristic time. The first regime can be explained by lattice heating due to the optical pump; the other cannot be accounted by simple lattice heating effects alone, and thus offers evidence for a true photoinduced phase transition.