Scattering-type scanning near-field optical microscopy (s-SNOM) has become a powerful tool to directly visualize the optical properties of solids on ultrasmall length scales, far beyond the diffraction limit of light. Using this technology fascinating phenomena like propagating phonon [1] or plasmon-polaritons [2,3], the formation of metallic puddles in vanadium dioxide [4], and even mid-infrared absorption bands of single molecules [5] could be visualize with <10nm spatial resolution.