Analysis of thin-wire nanoloops as superdirective antennas

Superdirectivity has been a topic of active research in antennas at radio frequencies since it was first postulated almost a hundred years ago. Despite its potential use in solar cells and optical communications, the topic of superdirectivity on the nano-scale is a relatively new field of study. Examples of superdirective nano-antennas include phased arrays and notched dielectric spheres. In this communication, we show that superdirectivity naturally occurs in a simple thin-wire nanoloop without the need for phase shifters or the introduction of inclusions. A closed-form expression for the directivity at end-fire will be presented which allow the designer to easily predict when this phenomenon will occur. The physical mechanism producing superdirectivity will be explored by studying the currents and performing a multipole expansion.