Existence of Superdirective Radiation Modes in Thin-Wire Nanoloops

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.