J. Appl. Phys. 121, 014901 (2017)

The ways in which we use wireless devices seem limitless, but less so the options for charging them. Either we connect our mobile marvels to a cable, or we place them on a pad, where they couple to a magnetic near-field. David Smith and co-workers now argue that we might have a third option before too long: focused microwave beams.

The idea of beaming power from a source to a receiver is not new. The efficiency of the approach, however, is typically too low to be practically useful, because in the far field the aperture is limited by diffraction, making it difficult to target a given region.

To get around this problem, Smith et al. propose to work in the Fresnel zone, where a focus can be created. Their analysis shows that dynamically reconfigurable Fresnel lenses can be produced using flat metamaterials, known as metasurfaces. These could be conveniently mounted on walls or ceilings and should make it possible to focus energy onto several spots simultaneously. Also, current technologies based on liquid crystals or semiconductors might be used to build the required apertures at low cost.