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 ϵr, μr1 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.

© 2014 Optical Society of America

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