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

Impedance loading is a common technique traditionally used in the RF to enhance the performance of an antenna, but its application in the optical regime is not as rigorously studied. This is mainly due to a lack of exact analytical expressions that can be used to rapidly predict the radiation properties of loaded nanoantennas. This paper will derive a set of useful analytical expressions for the far-field radiation properties of loop antennas loaded with an arbitrary number of lumped impedances that are valid from the RF-to-optical regimes. The analytical expressions will be validated with full-wave solvers and can be evaluated more than $100\times $ faster. The ability to perform such rapid evaluations enables, for the first time, large-scale single-objective and multi-objective optimizations. A series of optimizations reveal that electrically small superdirective antennas can be achieved at a variety of far-field angles through capacitive loading, paving the way for a pattern reconfigurable antenna. In addition, gains of greater than 3 dB can be achieved for electrically small antennas over a fractional bandwidth of 28%. Finally, it is shown that impedance loading can be used to achieve circularly polarized radiation from a single loop.