T. Taubner, F. Keilmann, and R. Hillenbrand
Nano letters 4, 1669 (2004)
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.