Existence of Superdirective Radiation Modes in Thin-Wire Nanoloops

Abstract

The problem of obtaining broadband superdirective radiation from an electrically small, easy to manufacture antenna is among the most challenging and elusive problems in electromagnetics. Superdirective arrays tend to be narrowband and sensitive to tolerancing, while a single superdirective radiator typically requires a very complicated and difficult to manufacture design. In this Article, we report on a new and transformative discovery, the fact that broadband superdirectivity naturally occurs for a single thin-wire nanoloop of the appropriate material composition and size. Full-wave simulations have revealed end-fire directivity of above 4.0 (6 dBi) for a nanoloop with radius less than 0.2 wavelength. This surprising phenomenon is explained in two ways: by comparison with two-element superdirective arrays and via a spherical multipole decomposition. This finding offers a solution to the problem of the inherently short-range communication of nanodevices and thus had the potential to strongly impact the fields of sensors, electronics, and wireless communications.

Optimization of Far-Field Radiation From Impedance-Loaded Nanoloops Accelerated by an Exact Analytical Formulation

“…In his expressions the superdirective limit approaches the directivity of a Huygen's source as approaches zero. It has recently been discovered theoretically that superdirectivity can be achieved by electrically small gold nanoloops [42][43]. A series of multi-objective studies revealed the trade-offs between directivity, gain and for unloaded gold nanoloops [44].…”

“…Fig. 6 shows one of the main results of [42][43], namely that a gold nanoloop with exhibits high directivity along over an extremely broad bandwidth, which is below the plasma frequency. As can be seen, a thin ( loop exhibits superdirective performance according to the Geyi and Harrington limit at .…”

The Analytical Foundations of Loop Antennas and Nano-Scaled Rings

“…Nano-rings have been found useful for high-definition imaging [61], concentrating energy using tiny Fresnel lenses [62], tuning antennas in the THz region [59], controlling loop resonances with dielectric materials [63], shaping the radiation beam pattern [64], emitting single photons [65], bio-sensing [66], capturing light more effectively in solar cells [67], super-scattering [68], and super-directivity [69]. Might not many more uses for these rings be found once we probe ring behaviour more fully?…”

Analytical Formulation for Loop Antennas Valid from the RF to Optical Regime: A Review

“…The analytical nature of the model presented allows for rapid exploration of the possible design space. Such exploration has revealed that it is possible for a single nanoloop, in isolation, to exhibit superdirectivity [13].…”

“…Regardless, the thick nanoloop provides a robust, broadband gain nanoantenna with a simple, fabrication friendly geometry. A further discussion of this material involving spherical multipole decomposition, comparison to other nanoantennas, and the effect of a substrate on the radiation pattern can be found in [13]. Given the analytical form of the theory, optimizations of dispersive nanoloops have now become highly tractable.…”