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 100x faster. The ability to perform
such rapid evaluations enables, for the first time, large-scale
single- and multi-objective optimizations. A series of optimizations
reveal that electrically small super-directive 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.Paper submitted for review on 03/20/2018. The authors would like to acknowledge the Spanish Ministry of Education - Commission Fulbright Program “Salvador de Madariaga” (PRX14/00320) for sponsoring the joint research collaboration. This work has also been partially financed by the Spanish and Andalusian research programs TEC202016-79214-C3-3-R and P12-TIC-1442 as well as by the Center for Nanoscale Science, an NSF Materials Research Science and Engineering Center, under the award DMR-1420620
