Insulator-to-metal phase transformation of VO2 films upon femtosecond laser excitation

Light-induced insulator-to-metal phase transition of vanadium dioxide films was studied by ultrafast optical pump-probe spectroscopy. The transient optical reflection measurement shows that both heating and laser illumination contribute to the phase transition of VO₂. Within 10⁻¹¹–10⁻⁹ sec, these two mechanisms are competitive. Excited-state dynamics were found to be strongly dependent on the concentration of structural defects and the pump laser power as well. Comparison of the transient reflection of VO₂ films deposited on different substrates suggests that the excitonic-controlled light-induced insulator-to-metal phase transition in VO₂ proceeded through an intermediate state. The transient reflection measurement of VO₂ in metallic phase shows a three-stage relaxation process. A polaron excitation model is introduced to describe the dynamical process for metallic VO₂.