Ultrafast insulator-to-metal switching by photoinduced Mott transition

We demonstrated the ultrafast photoinduced Mott transition from a charge transfer (CT) insulator to a metal in a halogen-bridged Ni chain compound by visible-mid IR pump-probe reflection spectroscopy. Upon the excitation of the CT band, the spectral weight of the gap transition is transferred to the inner-gap region. When the excitation density exceeds 0.1 per Ni site, the Drude-like high-reflection band appears in the infrared region, indicating the formation of a metallic state. The photogeneration of the metallic state and the subsequent recovery of the original insulating state occur within a few picoseconds.