Probing subwavelength in-plane anisotropy with antenna-assisted infrared nano-spectroscopy*

Infrared nano-spectroscopy based on s-SNOM has been employed to probe the vibrational fingerprints at the nanometer scale. However, s-SNOM is less sensitive to the in-plane anisotropy in general. We report a method to probe the in-plane dielectric responses of materials with the help of a metallic disk. As a demonstration, in-plane phonon responses of m-sapphire along b and c axis are identified. The LO phonon frequencies between 350 cm^-1 to 800 cm^-1 are determined down to a length scale of at least λ/10 without fitting parameters. Our method can be implemented to determine the in-plane anisotropy with high sensitivity, which is difficult for conventional far-field techniques.