Andrea Alu

UT researchers have built a nonlinear mirror that can detect harmful gases unseen to the naked eye. 

Photo Credit: Erik E. Zumalt | Daily Texan Staff

Cockrell School of Engineering researchers have designed and built a "nonlinear mirror" that could improve laser technology in fields such as chemical detection and biomedical research.

“The significance of the work is that it will, in the long term, make it possible to build very small optical devices, including laser sources,” said Ahmed Tewfik, electrical and computer engineering department chair. “This, in turn, will enable the deployment of well-understood and useful optical innovations into a host of new application areas, including medical applications, scientific instruments and defense.”

The nearly yearlong research study was led by engineering professors Andrea Alu and Mikhail Belkin along with engineering graduate students. The Technical University of Munich also worked on the study with the UT researchers.

Unlike traditional nonlinear crystals, the nonlinear mirror is made up of materials that are not found in nature, called metamaterials, and can be specifically designed to have uncommon optical properties, according to Belkin. 

The research team built a 400-nanometer thick mirror that doubles the frequency of the light that is shone at it by using light input intensity as small as that of a laser pointer. The nonlinear materials produce light intensities that are 1 million times more intense than those of natural materials, which helps researchers sense chemicals that they were not able to before.

“The beauty of our design is that unlike traditional nonlinear optical crystals which are limited by what nature gives you, we can play with the thickness and engineer this material to produce extremely large nonlinear optical response and to work at various wavelengths,” Belkin said.

Their research is groundbreaking in what it has shown about the use of engineered materials in extremely small laser designs, Alu said.

“This work opens a new paradigm in nonlinear optics by exploiting the unique combination of exotic wave interaction in metamaterials and of quantum engineering in semiconductors,” Alu said.

Along with possible biomedical applications, the mirror helps sense dangerous chemicals and gases that are not visible to the naked eye. This sensing is done in the mid-infrared spectral range, according to Belkin.

“If you are able to see in the mid-infrared range, then all the dangerous gases will have very specific colors, and so if you have a laser source with very specific wavelengths, you can use it to detect the presence of gas in the air,” Belkin said. “The nonlinear mirror can help us to generate these specific wavelengths.”

The metamaterials were grown with the help of collaborators at the Walter Schottky Institute of the Technical University of Munich, but much of the research and work that went into designing, building and testing the nonlinear mirror took place at the electrical and computer engineering department at UT as well as the Pickle Research Center, according to Tewfik.

According to Belkin, though this invention is a big step forward for nonlinear technology, there is still a lot of work to be done and many improvements still to be made.

Associate electrical and computer engineering professor Andre Alu and his team of researchers at the Cockrell School of Engineering have built an unnamed device that can manipulate acustic sound waves by breaking time reversal symmetry.

Photo Credit: MichelleToussaint | Daily Texan Staff

Researchers at the Cockrell School of Engineering have built a new device that can manipulate acoustic sound waves with a technique that has never been used before.

The device, which the researchers have yet to officially name, is the first of its kind and uses a new technique in order to break “time reversal symmetry.” Time reversal symmetry is the theory that, if a wave is sent in one direction, another wave can be sent back the same way.

Andrea Alu, associate electrical and computer engineering professor and project’s leader, said there are many ways to break time reversal symmetry, which is crucial to maintain efficiency in wireless devices.

“This device is very useful for many devices we use today for wireless communications,” Alu said. “The standard way to break time reversal symmetry is using magnetic materials. They are expensive [and] based on materials we don’t have in the U.S. We have to buy them from China and Russia. They have several drawbacks.”

Wireless devices contain objects called circulators, in which waves travel in one direction but are not transmitted back. Romain Fleury, an electrical and computer engineering graduate student working with Alu, said it is necessary for circulators to break time reversal symmetry in wireless devices. 

“Circulators make possible the use of only one antenna for both emitting and receiving signals,” Fleury said. “Without them, you would have no choice but to use two antennas — one for emission, one for reception — and, therefore, the system would be a lot more bulky and expensive.”

Alu said his team considered whether there was a different principle that could be used to do the same job. Fleury said the new device uses cheaper and more efficient tactics. 

“We already have two patents on these ideas.” Fleury said. “I think it’s a pretty major discovery.” 

Fleury said the experiment itself was straightforward, but developing the idea was the hardest part. Dimitrios Sounas, a postdoctoral fellow, assisted in the construction of the device.

“Together with [Fleury], I designed the experimental setup and performed the theoretical analysis,” Sounas said.

According to Alu, the same functionality of the device his team created could be implemented for other types of waves and solve many problems in the future.

Alu said the device has received widespread attention and was featured on the cover of Science Magazine. Fleury said it has the potential to be manufactured into a product for companies within the next few years.