Amplitude- and Phase-Resolved Near-Field Mapping of Infrared Antenna Modes by Transmission-Mode Scattering-Type Near-Field Microscopy

M. Schnell, A. Garcia-Etxarri, A. J. Huber, K. B. Crozier, A. Borisov, J. Aizpurua, and R. Hillenbrand

Journal of Physical Chemistry C 114, p.7341 (2010)
We describe transmission-mode scattering-type near-field optical microscopy (s-SNOM) with interferometric detection. Using this technique, we map the near-field modes of infrared antennas in both amplitude and phase. The use of dielectric probing tips, higher-harmonic demodulation, and a complex-valued subtraction of residual background yields accurate near-field images of the antenna modes. We map metallic nanorods, disks, and triangles, designed for antenna resonance at mid-infrared frequencies, in good agreement with numerical calculations of the modal field distribution. Furthermore, we show that transmission-mode s-SNOM can map the z-component of the antenna near fields. Our results establish a basis for future near-field characterization of complex antenna structures for molecular sensing and spectroscopy.