Ultrafast photoinduced transition of an insulating VO2 thin film into a nonrutile metallic state

Using time-resolved photoemission spectroscopy, we have investigated the nonequilibrium electronic structures of a VO2 thin film upon photoexcitation. We employed the high-harmonic generation method, which was crucial in obtaining the results. Irradiation by a 170-fs optical pulse at a fluence above ∼6 mJ/cm2 rapidly converts the insulating VO2 thin film into a metallic state, and we show that the transition is accompanied by a spectral-weight redistribution over a 1-eV scale. This observation provides direct spectroscopic evidence of an ultrafast insulator-to-metal transition. The transient metallic state has a unique spectrum that deviates from the static spectrum of the rutile metal phase. We also observe an anomalous spreading of spectral weight up to ∼0.4 eV above the Fermi level as soon as the transient metallic state emerges. The temporal evolution of the spectral weight near the Fermi level exhibits an ultrafast increase and a subsequent slower increase over ∼3 ps. We suggest that a broadening mechanism related to the excitation of phonon modes is responsible for the spreading of the spectral weight and that the slower increase of the spectral weight is associated with the expansion of the metallic region after photoinduced nucleation. In addition, the initial nucleation of the metallic state appears to be spatially inhomogeneous even near the surface. The results of this study highlight the uniqueness of the nonequilibrium metallic state in comparison to the equilibrium rutile metallic state from an electronic-state perspective.