Nonthermal nature of photo-induced insulator-to-metal transition in NbO2

We study the photo-induced metallization process in niobium dioxide NbO2. This compound undergoes the thermal insulator-to-metal transition at the remarkably high temperature of 1080 K. Our optical pump ¬– terahertz probe measurements reveal the ultrafast switching of the film on a sub-picosecond timescale and the formation of a metastable metallic phase when the incident pump fluence exceeds the threshold of ~10 mJ/cm2. Remarkably, this threshold value corresponds to the deposited energy which is capable of heating NbO2 only up to 790 K, thus, evidencing the non-thermal character of the photo-induced insulator-to-metal transition. We also observe an enhanced formation of the metallic phase above the second threshold of ~17.5 mJ/cm2 which corresponds to the onset of the thermal switching. The transient optical conductivity in the metastable phase can be modeled using the Drude-Smith model confirming its metallic character. The present observation of non-thermal transition in NbO2 can serve as an important test bed for understanding photo-induced phenomena in strongly correlated oxides.