Size-dependent optical properties of VO2 nanoparticle arrays

The size effects on the optical properties of vanadium dioxide nanoparticles in ordered arrays were investigated. It was observed that the optical contrast between the semiconducting and metallic phases is enhanced in the visible region, presenting size-dependent optical resonances and size-dependent transition temperatures. The collective optical response as a function of temperature was found to present an enhanced scattering state during the evolving phase transition. These observations were suggested to provide critical insights into statistical processes occurring in materials at the nanoscale

Semiconductor to metal phase transition in the nucleation and growth of VO2 nanoparticles and thin films

X-ray diffraction (XRD) and Rutherford backscattering were used for investigating the morphological and optical properties of vanadium dioxide nanoparticles and thin films during their nucleation and growth phases. The processing parameters were correlated in accordance with the temperature and sharpness of the transition. Grain growth and improved crystallinity resulted from thermal annealing. Because of fewer nucleating defects within the volume, the improved crystal perfection led to a large hysteresis. The effects of grain size and crystallinity determined the shape and width of the hysteresis cycle

Metal-insulator phase transition in a VO2 thin film observed with terahertz spectroscopy

We investigate the dielectric properties of a thin VO2 film in the terahertz frequency range in the vicinity of the semiconductor-metal phase transition. Phase-sensitive broadband spectroscopy in the frequency region below the phonon bands of VO2 gives insight into the conductive properties of the film during the phase transition. We compare our experimental data with models proposed for the evolution of the phase transition. The experimental data show that the phase transition occurs via the gradual growth of metallic domains in the film, and that the dielectric properties of the film in the vicinity of the transition temperature must be described by effective-medium theory. The simultaneous measurement of both transmission and phase shift allows us to show that Maxwell-Garnett effective-medium theory, coupled with the Drude conductivity model, can account for the observed behavior, whereas the widely used Bruggeman effective-medium theory is not consistent with our findings. Our results show that even at temperatures significantly above the transition temperature the formation of a uniform metallic phase is not complete.Peter Uhd Jepsen, Bernd M. Fischer, Andreas Thoman, Hanspeter Helm, J. Y. Suh, René Lopez, and R. F. Haglund, Jr

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

Using a Semiconductor-to-metal transition to control optical transmission through subwavelength hole arrays

We describe a simple configuration in which the extraordinary optical transmission effect through subwavelength hole arrays in noble-metal films can be switched by the semiconductor-to-metal transition in an underlying thin film of vanadium dioxide. In these experiments, the transition is brought about by thermal heating of the bilayer film. The surprising reverse hysteretic behavior of the transmission through the subwavelength holes in the vanadium oxide suggest that this modulation is accomplished by a dielectric-matching condition rather than plasmon coupling through the bilayer film. The results of this switching, including the wavelength dependence, are qualitatively reproduced by a transfer matrix model. The prospects for effecting a similar modulation on a much faster time scale by using ultrafast laser pulses to trigger the semiconductor-to-metal transition are also discussed

Fabricating arrays of vanadium dioxide nanodisks by focused ion-beam lithography and pulsed laser deposition

Vanadium dioxide undergoes a structural (monoclinic to tetragonal) insulator-to-metal transition at 70°C, accompanied by large changes in electrical and optical properties. By combining focused ion-beam lithography and pulsed laser deposition, patterned nanoscale arrays of vanadium dioxide nanoparticles are created that can be used for studies of linear and nonlinear optical physics, as well as demonstrating the potential for a variety of applications