Tracking the evolution of electronic and structural properties of VO2during the ultrafast photoinduced insulator-metal transition

We present a detailed study of the photoinduced insulator-metal transition in VO₂ with broadband time-resolved reflection spectroscopy. This allows us to separate the response of the lattice vibrations from the electronic dynamics and observe their individual evolution. When we excite VO₂ above the photoinduced phase transition threshold, we find that the restoring forces that describe the ground-state monoclinic structure are lost during the excitation process, suggesting that an ultrafast change in the lattice potential drives the structural transition. However, by performing a series of pump-probe measurements during the nonequilibrium transition, we observe that the electronic properties of the material evolve on a different, slower time scale. This separation of time scales suggests that the early state of VO₂, immediately after photoexcitation, is a nonequilibrium state that is not well defined by either the insulating or the metallic phase.