  We study the photo-induced metallization process in niobium dioxide NbO$_2$.
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/cm$^2$. Remarkably, this threshold
value corresponds to the deposited energy which is capable of heating NbO$_2$
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/cm$^2$
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 NbO$_2$ can serve as an important test bed for understanding
photo-induced phenomena in strongly correlated oxides.
