Spectral frustration and spatial coherence in thermal near-field spectroscopy

Scattering scanning near-field microscopy recently provided spectroscopic access to the fundamentally distinct spatial, spectral, and coherence properties of the thermal near field. Using SiC as an example, we study its thermal surface phonon polariton (SPhP) response. In contrast to the strongly surface-confined thermal near field of localized vibrational modes, an extended exponential distance dependence is observed reflecting the spatial coherence of the SPhP thermal field. In addition, we observe pronounced spectral frustration with spectral shifts ranging from $\ensuremath{\sim}5$ to $\ensuremath{\sim}50$ ${\mathrm{cm}}^{\ensuremath{-}1}$ that cannot be explained by conventional tip-sample dipole coupling alone. We present an alternative model describing an effective medium change by the tip. The results highlight the possibility for local spectral and spatial tuning of the thermal SPhP resonance for control of the light-matter interaction using thermal near-field radiation.