Engineering the optical response of nanodipole antennas using equivalent circuit representations of core-shell particle loads

The feasibility of tuning the optical response of a dipole nanoantenna using plasmonic core-shell particles is demonstrated. The proposed scheme consists of a two-step tuning process. First, it is demonstrated that when the gap between the nanodipole arms is loaded by a homogeneous dielectric sphere, the configuration exhibits effective material properties that can be described by a mixing rule, thereby allowing it to be effectively mapped onto an equivalent circuit topology. An additional degree of tunability is introduced by substituting the load consisting of a homogeneous sphere with one represented by a two-layer core-shell particle. An electrically small core-shell particle offers the advantage that it functions as a tunable nanocircuit element with properties that depend on the material constitution of the particle as well as its volume fraction. Effective medium theory is employed, and through rigorous analysis the resulting core-shell nanoparticle properties are then mapped onto an equivalent circuit topology. The complete derivation is presented for the total equivalent circuit model that corresponds to this two-step tuning process. Full-wave numerical predictions of the nanoantenna's extinction cross section are presented that validate the results obtained through the equivalent circuit-based representation of the loaded antenna. The proposed methodology illustrates the inherent tuning capabilities that core-shell particles can offer. Additionally, a novel compact and efficient scheme is introduced in order to map general nanoantenna loads onto equivalent circuit representations. The proposed approach permits the examination of the nanodipole antenna's performance in its transmitting mode; therefore it completely eliminates the need for time-consuming full-wave simulations of loaded nanoantenna structures. At the same time, it provides the optical designer with the capability to custom-engineer the nanoantenna's response through fast and accurate circuit-based analysis. (C) 2013 Optical Society of America