Making walls transparent to waves

Faced with an obstacle that reflects most of the waves, researchers from Rennes together with Austrian colleagues present a counter-intuitive solution: just add a second obstacle to the first one! The result is almost perfect transmission. Publication in Nature (July 13, 2022)
Simulation de trajectoire d'ondes - Image IETR/C. Ferise, M. Davy

Try to imagine a concrete wall nearly transparent to Wi-Fi or 5G, or a coat of paint that becomes translucent when covered with a specially engineered thin coating. This novel perspective has been revealed by researchers from Rennes at IETR (CNRS/Université de Rennes 1) in collaboration with Austrian colleagues from TU Wien.

A counter-intuitive hypothesis

On the Beaulieu campus of the Université de Rennes 1, in a small room of the new technology hall of IETR, lies an experimental set-up: a thin, long metallic case, equipped at one end with an array of transmitting antennas, and at the other with a detector of radio waves. Between the two, small cylinders randomly arranged constitute a "disordered medium", an obstacle to the transmission of waves. This is the device that allowed the verification of the hypotheses involved in this discovery.

The waves follow random paths in this maze, so that their progress is strongly hindered. They only reach the detector to a limited extent, most of the energy being reflected back to the transmitting antennas.

"An analogy we could use would be a thick concrete wall separating two rooms. A Wi-Fi transmitter located in one of them will have its signal reflected and randomly scattered by the wall, and this will reduce transmission to the next room"

Matthieu Davy, assistant professor in electronics at the Université de Rennes 1 and researcher at IETR

Surprisingly, it is not by removing, but by adding an additional obstacle on the path of the waves that the scientists from Rennes achieve an almost perfect transmission. They only needed to strategically place some additional cylinders before the initial obstacle.

"This additional obstacle allows to guide the waves in the initial maze to follow totally transmitted paths, eliminating any reflection, regardless of the direction of illumination".

Setting up the second obstacle

Of course, there is a trick, or better to say three of them:

  • The structure of the obstacles added to the wave path has to be carefully designed: it has to match the reflection pattern of those same waves by the initial obstacle. This optimization is performed at TU Wien in Austria by Stefan Rotter's team;
  • The transparency is only valid at a single frequency of the incident wave; reflection remains strong at other, distinct frequencies.
  • At this stage, the experiment mimics only obstacles in two dimensions, for waves spreading along a surface (i.e. a waveguide). If we consider a volume of material, the determination of the very precise configuration of the added obstacles requires too much computer time at this point.

Nevertheless, the researchers have succeeded in making an obstacle nearly transparent to waves, even though this obstacle initially stopped all of them. Well, not really all of them, actually: recent results have shown that a wave carefully engineered in phase and amplitude could pass through a material that was generally reflective. However, this was only valid for a small variety of waveforms and specific to a given material. The discovery made in Rennes overcomes these limitations.

A result that can be applied to all types of waves

This counter-intuitive result was published at the beginning of July 2022 in the prestigious scientific journal Nature. This technique is remarkable because it does not require any previous knowledge of the obstacle’s internal structure. You need only to scan the reflection of waves at its surface.

This result is applicable to all types of waves and to a whole range of frequencies: from radio waves to visible light, from seismic waves to sound waves.

Two additional advances

In addition, this work brings two important advances:

  • An innovative tool for calculating the additional obstacle, developed at TU Wien, which allows to obtain fast results regardless of the two-dimensional complexity of the additional obstacle to mode
  • It has been found, surprisingly, that the wave energy stored in the initial obstacle is much higher when the additional obstacle is optimized and results in almost perfect transmission.

Perspectives

This discovery offers many perspectives in the field of telecommunications, energy transfer, detection of disturbances or biomedical imaging. It could be possible to improve the performance of wireless chargers, Wi-Fi coverage in a building, or even to recharge a pacemaker through skin and muscles.

Référence

Anti-reflection structure for perfect transmission through complex media
Michael Horodynski, Matthias Kühmayer, Clément Ferise, Stefan Rotter, Matthieu Davy
Nature (2022.07) doi: s41586-022-04843-6