Probing thermal evanescent fields with a near-field microscope

We demonstrate probing thermally activated surface waves with a passive near-field microscope (wavelength λ: ~14.5 μm) without external illumination. A clear passive image of a room temperature object is achieved with a spatial resolution of 60 nm (λ/240). The decay characteristic of thermal evanescent waves on Au is quantitatively consistent with a numerical prediction based on the theories considering the thermal energy localization and the scattering efficiency of the near-field. In addition, the sample-size and temperature dependence of the near-field thermal signals agrees well with the theoretical calculations.