A smart wall for cell phones and tablets

Most of our offices and homes reverberate with unseen microwaves that are emitted by a Wi-Fi box or other base station and, hopefully, interact with our wireless devices such as cell phones, laptops, and tablets. Sophisticated antennas in those devices help to capture the multiply scattered waves as they zip by (see the article by Steve Simon and coauthors, Physics Today, September 2001, page 38 ). Still, reception is often spotty at best. To optimize reception, Mathias Fink, Geoffroy Lerosey, and their colleagues at Institut Langevin in Paris are looking at the environment. Using ideas from time-reversal acoustics (see Fink’s article in Physics Today, March 1997, page 34 ) and spatial light modulators in optics, they engineered tunable metamaterial panels that focus wireless signals onto a wireless device, say a cell phone. Their prototype spatial microwave modulator (SMM) has 102 unit-cell “pixels,” each with two resonators and a feedback loop to the cell phone. When a wave at a resonant frequency impinges on a pixel, the secondary resonator adjusts the pixel to reflect with a phase shift of either 0 or π, depending on how the feedback loop is set by the cell phone. The researchers carried out tests with a 0.4-m² SMM mounted on one wall in a complex reverberant office room. The SMM enhanced the overall signal centered on the cell phone (at the origin in the figure) by more than an order of magnitude even when the SMM, the source, and the phone were out of each other’s line of sight. When the SMM flips the phases, the waves cancel at the phone. According to Fink, not only can such a “smart wall” reduce power needs for wireless communication, but SMMs can enable microwave wavefront shaping for fundamental physics. (N. Kaina et al., Sci. Rep. 4, 6693, 2014, doi:10.1038/srep06693 .)