Femtosecond Time-Resolved Nano-Imaging of the Insulator-to-Metal Transition in Vanadium Dioxide

We have preformed femtosecond time-resolved and nanometer spatially resolved measurements of the insulator-to-metal transition in Vanadium Dioxide (VO$_{\mathrm{2}})$. In order to make this work possible, we have devised and implemented a method for artifact-free nano-imaging with pulsed laser sources. We observe that the transient metallic state is highly inhomogeneous. Following an ultrafast pumping event, a homogeneous increase in near-field signal occurs, which signifies that the initial injection of conduction electrons is homogeneous. This is followed by the inhomogeneous insulator-to-metal transition, which evolves over two distinct timescales from tens to hundreds of picoseconds. Our advances pave a pathway to study a wide range of systems with inhomogeneities properties on the nanoscale with nanoscopic spatial, and ultrafast temporal resolution.