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.