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

Abstract

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.

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

“…A reasonable question to consider is whether it is possible to redirect this mode towards a different direction. A recent study in [9] has shown that by adding capacitors, the loop may radiate superdirectively in different directions. The full Pareto fronts of the directivity at different angles versus frequency with various loading arrangements are shown in Figure 4.…”

“…Additional applications of optimizations to the study and design of nanoloops can be found in [9,[16][17].…”

“…Using these definitions, Werner and Lu et al developed a comprehensive, analytical model of the loop antenna valid from the RF to the optical regime [7]. Later this work, in collaboration, was extended to include the effects of loading [8,9] following the methods of Harrington [10] as well as mutual coupling using the induced-EMF method [11]. Furthermore, given the analytical nature of the model, thin loop antennas and their interactions can be modeled more efficiently while using much less RAM than a fullwave solver would require.…”

On Unsupervised Artificial Intelligence-Assisted Design of Antennas for High-Performance Planar Devices

“…For reliability reasons, this is most often performed using full-wave electromagnetic (EM) simulation models [3]. The literature offers a plethora of optimization techniques, including local [4] (both gradient-based [5][6][7] and derivative free [8]), global [9][10][11][12], multi-criterial [13][14][15], but also specialized methods for uncertainty quantification (statistical analysis [16], robust design [17,18]). As the computational cost of EM-driven design is high, often prohibitive when using conventional methods [19], considerable research efforts have been directed towards acceleration.…”