September 8, 2016
UT researchers are taking the concept of one-way mirrors to the extreme by manipulating light waves to make objects invisible.
UT Cockrell School of Engineering professor Andrea Alu and his research team are working on exploring the properties of light and sound waves that could make technologies like cloaking and one-way soundproof walls possible.
Alu and his research team won a grant from the National Science Foundation in order to design new kinds of devices that use different mechanisms of signal transport. The grant is part of a four-year program with a total of nine engineering teams aimed at researching and inventing new, innovative ways of using acoustic and light wave propagation.
When waves travel through a natural medium, then they look the same from both sides of the medium, a property called reciprocity. This applies to any kind of wave, including light and sound. The team wants to take advantage of breaking reciprocity, which would cause waves to act differently between the source of the wave and the receiver.
“The best way to explain reciprocal is: if I see you, you see me,” said UT physics professor Mark Raizen. “The same reciprocity applies to sound waves: if I can hear you, you can hear me. We take that for granted.”
Raizen said that breaking reciprocity is similar to a one-way mirror, but what people think of as one-way mirrors don’t truly break reciprocity because they are just an optical illusion caused by darkening one room.
“There is a possibility to make devices that will really break reciprocity; in other words, make a true one-way mirror,” he said. “A true one-way mirror would mean that you could have two people separated in rooms by that mirror, both are lit the same way, and one person could see the other person but the other person couldn’t see the first one.”
Although the concept of breaking reciprocity isn’t really new, Raizen said there is a lot of new research about non-reciprocity because researchers have developed the ability to control materials on smaller scales, making it possible to build devices specifically to break reciprocity.
This research is related to a design for a non-reciprocal antenna that Alu and his team designed earlier this year. That work is one of the reasons why they were awarded the grant, Alu said.
“Our idea now is to actually build a material platform that can allow very unusual signal transport by using a singular approach,” he said. “In this new project, we can build material structures that can change the way that photons and phonons travel in materials.”
Alu’s research team works with metamaterials, or artificial materials. Metamaterials can cause non-reciprocity because they can be built and designed in ways that changes the nature of waves passing through them. The material needs to cause a nonlinear interaction, which means that the input into the material doesn’t correspond to the output, according to Raizen.
“In order to make a one-way wall for any kind of wave, you need some nonlinear conversion that will change the nature of the wave like its frequency or color,” Raizen said. “If you can do that, you can break the reciprocity.”
In addition to cloaking and one-way walls, breaking wave reciprocity has many applications in various fields, according to Alu.
“Some of the areas that we have envisioned that might be impacted by this effort are enhanced data rates, spectrum efficiency for the telecom industry, enhanced acoustic imaging for healthcare, and better sensing for civil engineering and the defense industry,” he said. “These are some of the directions that we have been looking at where we see that these new materials may have an impact. [This research] pushes the frontiers farther.”