Maximum absorption by homogeneous magneto-dielectric sphere.

Abstract

In order to obtain a benchmark for electromagnetic energy harvesting, we investigate the maximum absorption efficiency by a magneto-dielectric homogeneous sphere illuminated by a plane wave, and we arrive at several novel results. For electrically small spheres we show that the optimal relative permeability and permeability of materials where ϵ(r)', μ(r)'≥1 is (1+i3) independent of sphere size, while that of metamaterials is (-2+iδ), where the imaginary part δ decreases strongly with decreasing sphere size. For larger spheres we show that while maximum absorption efficiency occurs at the resonances of the spherical modes, there exists a wide plateau of high absorption efficiency when material intrinsic impedance is constant; in the case of free-space intrinsic impedance and electrical radius κ=1, the absorption efficiency becomes 2.8. The investigation is analytic/numerical and based on the Lorenz-Mie theory combined with the optical theorem.