Introduction

In order for antennas to operate in the visible and near-IR wavelength range (optical antennas), the devices need to be subwavelength in size. Recently, nanofabrication tools have been developed to create optical antennas with unprecedented properties which have enabled many applications [202]. For example, optical antennas can be used as nanoscale energy transmitters or scatterers for SNOM and spectroscopy with subwavelength resolution and directional emission of single photons [68, 143, 146, 256]. The antennas can also operate as receivers to collect and concentrate EM energy into nanoscale volumes for photovoltaics, photo-detection and nonlinear optical devices [34, 171, 201, 435, 668].

Over the past decade, a variety of optical antenna designs have been investigated for different applications. These structures include: (i) metal NPs (NPs) that support LSPRs, which can act as receivers to enhance optical absorption for active materials as well as transmitters to enhance emission rates of nearby dipole emitters (see Fig. 16.1a) [68]. (ii) NP dimers that can result in significant field enhancements of the incident light in the nanoscale gap separating the NPs (see Figs. 16.1b–d) [34, 167, 171]. (iii) nanoscale apertures in a metallic film that can also operate as receivers to convert optical energy from propagating waves into nano-localized spots. (see Fig. 16.1e) [669]. (iv) nano-rod arrays that can function as miniaturized Yagi–Uda antennas and result in directional radiation (see Fig. 16.1f) [143, 146].