Optical properties of plasmonic nanoloop antennas

Abstract

Wireless communication systems are expected to play a crucial role in a variety of important technologies including tablets, smartphones, energy-harvesting devices, and medical devices . The need for compact systems with a high data rate means that nanotechnology-enabled devices will likely be the backbone of nextgeneration wireless communication systems [1]. In particular, these devices require high-performance nanoantennas which are capable of operating anywhere from the optical to terahertz regimes. Metallic devices at these frequencies can no longer be treated as perfect electric conductors (PECs). Instead, they exhibit significant dispersion and loss [3] which results in a drastic impact on the antenna parameters, including directivity and gain [4]. While closed-form analytical expressions are widely available for antennas operating in the RF regime , optical antennas are mostly designed using numerical simulation tools . Though these tools are very useful for high-fidelity simulations of complex structures, they typically require a large amount of computational resources and time. The availability of exact analytical expressions lead to a deeper intuitive understanding of the underlying physics and also allow for much faster, less computationally intensive design iterations. While there is a large amount of literature devoted to the analysis and design of nanodipole antennas , there has been less analytical study of the nanoloop antenna despite its potential applications in sensing , spectroscopy, and light harvesting in solar cells. This is due to the extremely complex form of the integrals which must be solved. This book chapter will present a summary of recent work on the theory of nanoloop antennas. First, Section 1.1 presents a theoretical formulation for the antenna parameters of nanoloops with an arbitrary number of impedance loads placed around the periphery. In particular, useful exact analytical expressions will be derived for the current, input impedance, far-zone electric field, radiated power, directivity, and gain. These expressions will be specialized for the case of a simple closed loop with no additional impedance loads. Next, Section 1.2 presents analytical expressions for evaluating the coupling between two loops at arbitrary locations. Array factor theory can be employed to compute the resulting far-field properties of arrays of nanoloops. After the theoretical derivations have been presented, a variety of interesting results will be showcased. Section 1.3 presents a surprising discovery, the fact that superdirectivity over a broad bandwidth occurs for a nanoloop comprised of the appropriate material and size. A physical explanation of this phenomenon will be presented, along with comparisons to dielectric nanoantennas. Then, Section 1.4 shows a variety of trade-off curves between electrical size, directivity and gain. The geometries under consideration will include closed and impedance-loaded nanoloops, along with arrays of nanoloops. Finally, Section 1.5 introduces an exciting new research topic, elliptical nanoloop antennas. The analytical theory and some interesting results are presented.