Ultrafast optical dynamics in VO2

Transient reflection spectroscopy and degenerate four wave mixing technique were employed to study the light-induced insulator-to-metal phase transition (PT) and ultrafast relaxation dynamics in VO2. Spectral reflectivity during light- and thermally-induced PT shows close proximity in the relative change. The relaxation dynamics is strongly dependent on the film morphology, laser pump energy and substrate material. After light-induced PT the recovery time demonstrates a near-exponential dependence on the pump power. The recovery owing to cooling is considerably faster for VO2 films deposited on single-crystal Al2O3 or MgO substrates compare to VO2 on amorphous glass. The noticeable transient nonlinear optical response of metallic VO2 was observed and interpreted in terms of electronic-polaron and hole-polaron clustering.