A Multipolar Analysis of Near-Field Absorption and Scattering Processes

A multipolar formulation is adopted to investigate the absorption and scattering processes involved in near-field interactions. This approach allows one to determine the upper bounds for the absorbed and radiated powers that would be achieved by an ideal lossless sensor, which are of particular interest, for example, to wireless power transfer (WPT), wireless sensors and near-field coupled radiators. The multipolar formulation also helps to extricate the fundamental compromises that must be addressed in the design of such systems, as well as to identify strategies that could approach their best possible performances. The general theory is illustrated with an example consisting of a coated sensor illuminated by a Hertzian dipole, which is a representative example of any scattering or radiating system based on small resonators. The example also serves to compare the performance characteristics obtained with different phenomena such as multipolar resonances, phase-induced interference effects and cloaking.