Scattering scanning near-field optical microscopy on anisotropic dielectrics using a free-electron laser light source

Scattering scanning near- field optical microscopy (s-SNOM) is based on the interaction between an optically scattering nano-particle (AFM tip) and a dielectric sample. The size of the scatterer defines the optical resolution of the microscope, which is on the order of a few nanometers. On that scale, the optically anisotropic properties of most samples have to be taken into account [1]. To examine the influence of optical anisotropy on the scattering signal, we excite a ferroelectric sample close to its phonon resonance in the mid infrared regime. As the precisely tunable light source at infrared wavelengths we used a free electron laser (FEL). We have measured the near-field signal at several wavelengths while scanning the sample, as well as the tip-sample distance dependence of the scattered light signal for the 1st, 2nd, and 3rd harmonic signal. The anisotropy is revealed for different sample orientations. Not only are we presenting the first tunable IR near-field measurements on ferroelectric lithium niobate and barium titanate single crystals, but furthermore are our measurements in excellent accordance with recent calculations of optical anisotropy in such systems [1]. [1] S. Schneider, et al., Phys. Rev. B 71, 115418 (2005)