Softening of the insulating phase near Tc for the photo-induced insulator-to-metal phase transition in vanadium dioxide

We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of the photoinduced insulator-to-metal (IM) transition in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence required to drive the IM transition is observed, consistent with a softening of the insulating state due to an increasing metallic volume fraction (below the percolation limit). This phase coexistence facilitates the growth of a homogeneous metallic conducting phase following superheating via photoexcitation. A simple dynamic model using Bruggeman effective medium theory describes the observed initial condition sensitivity.Comment: accepted for publication in Physical Review Letter

Contribution ea l'etude de l'implantation d'ions (Ti C N Ni) dans le fer et dans des aciers (m2 304l)

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 77822 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Optical Properties Tuning of SnO2 Films by Metal Incorporation (Pt,Pd): Correlation with Microstructure Change

International audienceIn this work, we report on the effect of noble metal doping (namely Pd or Pt) on the optical properties of SnO2 thin films. The optical constants (n and k) of the films, as a function of noble metal nature and content, were obtained using variable angle spectroscopic ellipsometry in the ultraviolet-visible-near infrared (UV-vis-NIR) regions. Ellipsometry analysis showed that we can tune the optical constants of SnO2 films by changing Pt or Pd doping concentration. In particular, their refractive index increases from 1.6 to similar to 2 while varying Pt content from 3 to 12 at. %. The origin of this optical behaviour was correlated to the microstructure change induced by metal doping. X-ray diffraction (XRD) was used to investigate the effect of doping on SnO2 lattice parameter, on crystallite size and on film preferential orientation. Atomic force microscopy (AFM) was used to estimate the surface roughness of the films. A metal concentration of similar to 3 at.% (for both Pt and Pd), which is known to yield the highest SnO2 gas sensing response, was found to correspond to the highest contraction of the lattice parameter of the films. Finally, the energy band gap of undoped SnO2 thin films (estimated to 4 eV) was found to shift to lower value while increasing doping concentrations. (C) 2009 The Japan Society of Applied Physics DOI: 10.1143/JJAP.48.07250