Non-thermal nature of photo-induced insulator-to-metal transition in NbO$_2$

TL;DR

This study demonstrates that photo-induced insulator-to-metal transition in NbO$_2$ occurs via a non-thermal process on a sub-picosecond timescale, with a metastable metallic phase formed at energy levels below thermal heating thresholds.

Contribution

It provides evidence of a non-thermal, ultrafast insulator-to-metal transition in NbO$_2$, highlighting the role of photo-excitation in strongly correlated oxides.

Findings

Ultrafast switching occurs within sub-picoseconds.

Metastable metallic phase forms below thermal heating threshold.

Transition characterized by Drude-Smith optical conductivity model.

Abstract

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.