Optical antennas, which are analogs of microwave and radio antennas, are defined by their ability to efficiently convert localized electromagnetic energy into optical radiation modes, and vice versa. The conversion process involves the concentration, absorption, and radiation of light at the nanometer scale. The majority of optical antennas work via the localized surface plasmon resonances (LSPRs) of metallic structures. In particular, the roles of LSPRs and geometrical dimensions have been addressed regarding the optical properties of optical antennas. The characteristic sizes of optical antennas are on the scale of light wavelengths, i.e., the dimensional feature sizes are less than tens of nanometers. The advent of nanofabrication and characterization technologies have enabled optical antennas in significant plasmonic-optics devices.
Optical antennas are used in a wide range of forms, including single
nanospheres and nanorods (discussed in Chapter 3). In addition to metallic materials, semiconductor and dielectric materials have been adopted in antenna designs, but this chapter only discusses the former, wherein LSPR modes are excited by visible light or near-infrared wavelengths.
To begin, the elementary antenna theory inspired by traditional antennas in the microwave region are discussed, followed by the characterizing properties of optical antenna. Examples of optical antennas and their applications are provided for photovoltaic power, photodetection, and the emerging nanoscale imaging.
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