Real-space mapping of mid-infrared near-field of Yagi-Uda antenna in the emission mode.

By using transmission-mode, scattering-type scanning near-field optical microscopy, we characterize the mid-infrared near-field properties of a Yagi-Uda antenna in the emission mode. The underlying near-field properties, including the near-field dipole-dipole coupling between antenna elements, are clearly observed. Moreover, even though most of the radiation energy is emitted into the substrate, by adopting two detector antennas, we managed to verify the unidirectionality and frequency-selectivity of the Yagi-Uda antenna in the air side. All the experimental results presented in this work are in good qualitative agreement with our numerical simulations. Our work on the Yagi-Uda antenna could help lead to novel methods for mid-infrared material analysis and bio-sensing. It should also be applicable in all-optical processing like radiation routers or a chromatic discriminator.