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

Infrared nano-spectroscopy based on scattering-type scanning near-field optical microscopy (s-SNOM) is commonly employed to probe the vibrational fingerprints of materials at the nanometer length scale. However, due to the elongated and axisymmetric tip shank, s-SNOM is less sensitive to the in-plane sample anisotropy in general. In this article, we report an easy-to-implement method to probe the in-plane dielectric responses of materials with the assistance of a metallic disk micro-antenna. As a proof-of-concept demonstration, we investigate here the in-plane phonon responses of two prototypical samples, i.e. in (100) sapphire and x-cut lithium niobate (LiNbO3). In particular, the sapphire in-plane vibrations between 350cm(-1) to 800cm(-1) that correspond to LO phonon modes along the crystal b- and c-axis are determined with a spatial resolution of

Automated summary

s-SNOM is a powerful tool commonly used for infrared nano-spectroscopy, but it lacks sensitivity to in-plane sample anisotropy. Authors introduce a method involving a metallic microdisk antenna to improve sensitivity by probing in-plane phonon responses.