Time-resolved X-ray holographic imaging of the light-induced phase transition in vanadium dioxide

We demonstrate the first femtosecond-temporal and nanometer-spatial resolution imaging of a light induced phase transition, the insulator-to-metal transition of vanadium dioxide, using time-resolved resonant X-ray holography [1] at the Pohang Accelerator Laboratory X-ray free electron laser. Thin films of VO 2 deposited onto silicon nitride membranes were masked for holography [2] and heated to 325 K, where domains of the rutile metallic phase began to nucleate. Femtosecond 800 nm pulses then drove the system through the phase transition, with the domain growth and nucleation dynamics monitored by coherent X-ray scattering at the vanadium L 2,3 and oxygen K edges ( Fig 1 ). Dynamics at a range of time-scales are observed in the real space videos of the process: we observe nanometer sized domains forming within 200 fs, followed by both sub-picosecond and picosecond dynamics. Real space imaging-spectroscopy was performed on the long-lived transient state at 20 ps time-delay [3] , allowing us to comment on the existence of a proposed nanoscale monoclinic metallic precursor in the ultrafast phase transition [4] , [5] .