  Intense electromagnetic evanescent fields are thermally excited in near
fields on material surfaces (at distances smaller than the wavelength of peak
thermal radiation). The property of the fields is of strong interest for it is
material-specific and is important for understanding a variety of
surface-related effects, such as friction forces, Casimir forces, near-field
heat transfer, and surface-coupled molecular dynamics. On metal surfaces,
relevance of surface plasmon polaritons (SPlPs), coupled to collective motion
of conduction electrons, has attracted strong interest, but has not been
explicitly clarified up to the present time. Here, using a passive terahertz
(THz) near-field microscope with unprecedented high sensitivity, we unveil
detailed nature of thermally generated evanescent fields
(wavelength:lamda0~14.5micron) on metals at room temperature. Our experimental
results unambiguously indicate that the thermal waves are short-wavelength
fluctuating electromagnetic fields, from which relevance of SPlPs is ruled out.
