Optical nonlinearities in VO2 nanoparticles and thin films

Z-scan and pump-probe measurements with ultrafast, 800nm laser pulses were used to compare the ultrafast optical nonlinearities of VO2 nanoparticles and thin films in both semiconducting and metallic states. In the metallic state, both the nanocrystals and thin films exhibit a positive, intensity-dependent nonlinear index of refraction. However, the nonlinear effects are relatively larger in the VO2 nanocrystals, which also reveal a saturable nonlinear absorption. When the semiconductor-to-metal phase transition is induced by the laser pulse, VO2 thin films exhibit a negative equivalent nonlinear index of refraction while the nanocrystals exhibit a smaller but still positive index. Both the VO2 nanocrystals and thin films undergo the phase transition within 120fs.