We use apertureless scattering near-field optical microscopy (SNOM) toinvestigate the nanoscale optical response of vanadium dioxide (VO2) thin filmsthrough a temperature-induced insulator-to-metal transition (IMT). We compareimages of the transition at both mid-infrared (MIR) and terahertz (THz)frequencies, using a custom-built broadband THz-SNOM compatible with bothcryogenic and elevated temperatures. We observe that the character of spatialinhomogeneities in the VO2 film strongly depends on the probing frequency. Inaddition, we find that individual insulating (or metallic) domains have atemperature-dependent optical response, in contrast to the assumptions of aclassical first-order phase transition. We discuss these results in light ofdynamical mean-field theory calculations of the dimer Hubbard model recentlyapplied to VO2.
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