We directly measure the photoinduced insulator-to-metal transition in ${\mathrm{VO}}_2$ using time-resolved near-edge x-ray absorption. Picosecond pulses of synchrotron radiation are used to detect the redshift in the vanadium $L_3$ edge at 516 eV, which is associated with the transient collapse of the low-temperature band gap. We identify a two-component temporal response, corresponding to an ultrafast transformation over a 50 nm surface layer, followed by 40 m/s thermal growth of the metallic phase into the bulk.

• Received 19 September 2003

DOI:https://doi.org/10.1103/PhysRevB.69.153106