Wavefront shaping is not limited to optical waves. Similar techniques can be used for any kind of wave for which one can control dynamically the phase over a large number of independent elements. In [N. Kaina et al., Sci. Rep. (2014)], the authors demonstrate the use of their Spatial Microwave Modulator (SMM) to control the propagation of radiofrequency waves inside a room to improve the WiFi signal at any chosen position. The system is passive as there is no energy transfer from the modulator to the WiFi signal, it only controls the local phase of the waves reflected off the modulator. The device is thin and has the typical size of a small poster, it can be conveniently placed on the wall of a typical room without any loss of space.
The authors built what is the closest equivalent of a spatial light modulator for microwaves; an array of 102 independent electromagnetic reflectors at the working frequency of 2.47 GHz. The phase of each element can be controlled electronically. It is 1.5 mm thick and the total surface is ~0.4 m², so it can be hidden behind (or be!) a decorative element. They decided to use it as a binary phase modulator, i.e. they can switch the phase of the wave on each element from 0 to \(\pi\). To do so, each element of this so-called "metasurface" is a resonant cell. The phase of the reflection can be changed by switching the resonant frequency above or below 2.47 GHz (well, it is slightly more complicated than that but it is the main idea).
Similar to previous optical experiments, the phase modulator can be used to focus the energy at a given target in a complex medium. Instead of a purely random scattering medium, they chose a more realistic environment for microwaves; a complex medium with both reverberations and scattering, i.e. a typical office room. Reverberations are due to the walls and scattering comes from the presence of various objects in the room (chair, desk, ...). A schematic representation of the room used is shown in Figure 1.
Figure 1. Schematic view of the setup and its surrounding environment. Image from [N. Kaina et al., Sci. Rep. (2014)].
To improve the signal at one position, a receptor is placed there and all the elements of the SMM are initially set to the 0 phase state. Iteratively, each element is switched to \(\pi\), the state is kept if it improves the signal at the target position or switched back to 0 otherwise. In a nutshell, the contributions coming from all the elements are put in phase to create a constructive interference at the target position.