Optical nonlinearities in VO ₂ nanoparticles and thin films

Z-scan and pump-probe measurements with ultrafast, 800 nm laser pulses were used to compare the ultrafast optical nonlinearities of VO ₂ 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 VO ₂ nanocrystals, which also reveal a saturable nonlinear absorption. When the semiconductor-to-metal phase transition is induced by the laser pulse, VO ₂ thin films exhibit a negative equivalent nonlinear index of refraction while the nanocrystals exhibit a smaller but still positive index. Both the VO ₂ nanocrystals and thin films undergo the phase transition within 120 fs.