Near-field imaging of optical antenna modes in the mid-infrared

TLDR

Optical antennas enhance coupling between free‑space light and localized emitters or receivers, underpinning many nanophotonic applications, and their functionality depends on understanding how geometry determines near‑field antenna modes. The study uses s‑SNOM with interferometric homodyne detection to probe vector near‑field components and investigate resonances of linear Au wire antennas designed for the mid‑IR. The authors use s‑SNOM with interferometric homodyne detection, supported by antenna theory and numerical electrodynamics simulations, to examine two coupling regimes that scale with gap width or reactive near‑field decay length. The experiments reveal that single Au wires exhibit an effective wavelength scaling of λ_eff = λ/(2.0 ± 0.2), that introducing a gap produces an effective multipolar mode, and that these observations confirm distinct optical‑frequency antenna behavior and theoretically predicted scaling laws compared to impedance‑matched RF antennas.

Abstract

Optical antennas can enhance the coupling between free-space propagating light and the localized excitation of nanoscopic light emitters or receivers, thus forming the basis of many nanophotonic applications. Their functionality relies on an understanding of the relationship between the geometric parameters and the resulting near-field antenna modes. Using scattering-type scanning near-field optical microscopy (s-SNOM) with interferometric homodyne detection, we investigate the resonances of linear Au wire antennas designed for the mid-IR by probing specific vector near-field components. A simple effective wavelength scaling is observed for single wires with lambda(eff) = lambda /(2.0+/- 0.2), specific to the geometric and material parameters used. The disruption of the coherent current oscillation by introducing a gap gives rise to an effective multipolar mode for the two near-field coupled segments. Using antenna theory and numerical electrodynamics simulations two distinct coupling regimes are considered that scale with gap width or reactive near-field decay length, respectively. The results emphasize the distinct antenna behavior at optical frequencies compared to impedance matched radio frequency (RF) antennas and provide experimental confirmation of theoretically predicted scaling laws at optical frequencies.