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We use scattering-type apertureless near-field optical microscopy to demonstrate distance-controlled spectral resonance and phase shifts of a single near-field coupled system. In our experiment a metal tip optically interacts with a SiC surface at continuously varied nanometric distances. For illumination we use mid-infrared wavelengths (10 μm) where phonon polaritons in SiC resonantly enhance the near-field interaction. The phase of the scattered light increases and the resonance peak red shifts with decreasing gap width, in accordance with theoretical modeling.
Taubner et al. (2004) studied this question.