A metamaterial power harvester

Metamaterials are composite materials whose electromagnetic or acoustic properties are quite different from those of the subwavelength building blocks they comprise. By carefully tailoring small constituent pieces, researchers can create large-scale exotic phenomena such as negative indices of refraction, superlensing, and invisibility cloaking (see the articles by John Pendry and David Smith, Physics Today, June 2004, page 37 , and by Martin Wegener and Stefan Linden, Physics Today, October 2010, page 32 ). Metamaterials have also shown promise for more familiar applications, including harmonic generation and nonlinear wave mixing. Allen Hawkes, Alex Katko, and Steve Cummer at Duke University have now demonstrated that metamaterials can function well as power harvesters, rectifying incident RF energy to deliver DC power to integrated components. The building block of the Duke metamaterial is a split-ring resonator: a nearly closed square metallic loop, 40 mm on a side, designed to resonantly couple to the magnetic field from incident microwaves at 900 MHz. The researchers integrated into the resonator additional circuit elements—capacitors and diodes—to harness and rectify the AC current induced in the loop. A metamaterial (shown in the figure) consisting of an array of five such resonators wired together achieved a power-conversion efficiency of 37%, in close agreement with the team’s simulations. Since the collective response of the resonators can be finely engineered, the team expects that metamaterial power harvesters could find wider application than general antenna-based implementations. (A. M. Hawkes, A. R. Katko, S. A. Cummer, Appl. Phys. Lett. 103, 163901, 2013, doi:10.1063/1.4824473 .)