Introduction

Optical antennas have added a new aspect to the field of light–matter interactions by efficiently coupling localized fields to propagating radiation [202, 203]. Most of their properties can be described in terms of Maxwell equations, which can be solved numerically even for complex antenna geometries (see Chapter 10). The constantly improving understanding of optical antennas has led to a large number of proposed applications that can only be realized by making use of high-precision state-of-the-art nanofabrication tools and techniques, as well as of a subsequent detailed characterization using optical methods to thoroughly verify the intended properties.

Upon illumination, resonant optical antennas can provide very large near-field intensities, resulting from LSPRs that lead to enhanced local surface charge accumulation. Such resonantly enhanced optical fields are the basis for the improved light–matter interaction afforded by optical antennas. Optical antennas are thus exploited in the context of optical spectroscopy, e.g. involving multi-photon processes [33, 34, 350, 353], harmonic generation [171, 329] or Raman scattering [481, 543]. Other applications include the creation of point-like light sources for super-resolved near-field imaging [145, 544] and lithography [545]. Moreover, nanoantennas can act as highly-efficient absorbers in solar-cell and photon-detector technology [435] and they are the ideal interface between far-field propagating photons and guided modes in plasmonic nanocircuitry [546].

Plasmon resonant nanoantennas also exhibit enhanced scattering due to resonantly enhanced plasmonic currents. This property can be exploited in far-field experiments for sensing applications in conjunction with the large sensitivity of the antenna resonance condition to the local dielectric environment [547].