Insulator-to-metal phase transition and recovery processes in VO2 thin films after femtosecond laser excitation

Insulator-to-metal phase transition (PT) was produced by femtosecond light radiation in VO2 thin films deposited on substrates with different thermal conductivities. Optical pump-probe techniques were applied to study the PT dynamics on a 10(-11)-10(-9) s time scale. Transient grating measurements conducted at different laser pump fluences indicate that the contribution to insulator-to-metal PT on subnanosecond time scale by thermal effect due to phonon absorption is rather small as compared to effect by electronic mechanism. After laser irradiation, during the recovery process of metal-to-insulator PT, both the thermal and nonthermal contributions were identified in ultrathin films when different laser fluences are applied. For VO2 films deposited on single-crystal or glass substrates, the thermal recovery is considerably different due to difference in substrate thermal conductivity. For VO2 films on Al2O3 or MgO single-crystal substrates, the heat sink effect from film to the substrate can be described consistently in terms of a thermal boundary resistance model.