Tracking the evolution of electronic and structural properties of VO<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mn>2</mml:mn></mml:msub></mml:math>during the ultrafast photoinduced insulator-metal transition

Abstract

We present a detailed study of the photoinduced insulator-metal transition in VO${}_{2}$ 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 VO${}_{2}$ 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 VO${}_{2}$, immediately after photoexcitation, is a nonequilibrium state that is not well defined by either the insulating or the metallic phase.