Critical behavior and size effects in light-induced transition of nanostructured VO2 films

The light-induced insulator-to-metal phase transition was investigated by femtosecond pump-probe techniques in nanocrystalline VO2 films. The size of VO2 nanoparticles and the VO2 film morphology were found to be critical for the ultrafast light-induced phase-transition dynamics. Experimental measurements of the third-order nonlinear susceptibility chi((3)) for the insulating phase at different excitation levels and transient grating experiments show a size-dependent threshold behavior which is related to the light-induced transition process. Optical properties of VO2 nanoparticles, vertical bar chi((3))vertical bar data, and transition dynamics at subnanosecond time scale also demonstrate a pronounced size dependence. The rate of structural phase transition increases due to confinement effect as particle size decreases. Nucleation of the metallic phase shows a critical behavior with formation of a metastable state in VO2. A kinetic model for the metallic phase growth and transient grating signal evolution upon ultrafast laser excitation is developed for VO2 thin film.