Huygens Optical Elements and Yagi – Uda Nanoantennas Based on Dielectric Nanoparticles

593 1. INTRODUCTION Investigation of optical nanoantennas is a new, rapp idly developing field of modern optics. Nanoantennas are devices that transform freely propagating optical radiation into the field that is strongly localized in a certain region and vice versa [1–9]. They are interestt ing primarily because they make it possible to increase the efficiency of controlling and governing electroo magnetic fields at the nanometer scale. Optical electromagnetic fields are traditionally controlled by lenses, optical waveguides, and diffracc tion elements, i.e., devices with sizes much larger than the wavelength of controlled radiation. At the same time, rf and microwave radiations are usually conn trolled by antennas, which are devices comparable with sizes comparable to the wavelength. Although the optical frequency range is of great importance, the diss cussion of particular designs of nanoantennas and their implementation began only recently because the characteristic sizes of an optical antenna are deterr mined by the working wavelength of radiation, which is hundreds of nanometers. For this reason, there is a technological problem of reproducing an object of this size with a nanometer accuracy. Since the nanometerr length scale has become more available for experii mental investigations, the number of works concernn ing this subject has recently increased sharply. The main types of nanoantennas and their experimental implementations were reviewed in [8]. In particular, designs of Yagi–Uda nanoantennas have been recently proposed in [1, 2, 4–9]. Similar to a classical Yagi– Uda antenna, these nanoantennas have a reflector and one or several directors. However, unlike their classical analog, the narrow radiation pattern and high gain