Illinois-led team wins NSF grant for physics-defying research

Bill Bowman, MechSE Communications

Today’s electronic, photonic, and acoustic devices utilize waves that are limited by the classic laws of physics. But tomorrow’s machines may break free of these constraints, thanks to a research team led by Gaurav Bahl, an assistant professor of mechanical science and engineering (MechSE) at Illinois.

Artist visualization of slow light, fast light, and one-way light blocking using BSIT in a series of silica microresonators (slow/red, fast/blue; central yellow shows blocking effect).
Artist visualization of slow light, fast light, and one-way light blocking using BSIT in a series of silica microresonators (slow/red, fast/blue; central yellow shows blocking effect).
The National Science Foundation (NSF) has awarded $2 million to the team, comprised of Bahl (lead principal Investigator) and Taylor Hughes, an associate professor of physics at Illinois, with Steven Cummer (electrical engineering, Duke), and Hailin Wang (physics, Oregon). They make up one of nine research groups of engineering-led, interdisciplinary researchers working to break the conventional ways in which light and sound waves propagate. Their proposal is titled, “Reconfigurable pathways and directionality for sound using time-varying engineered materials.”

“We are very grateful for this opportunity offered to us by the National Science Foundation, as it is through this kind of large collaborative grant, spanning multiple subjects, that we can truly cross-stimulate new ideas,” Bahl said. “This grant is also a big victory for the University of Illinois as it establishes our leadership role in a frontier area of research.”

The NSF program supporting this research is called NewLAW (New Light and Acoustic Wave Propagation: Breaking Reciprocity and Time-Reversal Symmetry) and is funded through the NSF Emerging Frontiers in Research and Innovation (EFRI) program. EFRI's broad mission is to push the absolute frontiers in scientific research in specific areas of great importance. The EFRI teams—comprising 37 researchers at 17 institutions—are each funded for the next four years representing an $18 million investment by NSF on the topic.

Gaurav Bahl
Gaurav Bahl
“Our grant is a collaborative effort with an amazing team of engineer-scientists with expertise spanning nonlinear and quantum optics, metamaterials, condensed matter physics, and acoustics,” Bahl said.

Propagating waves in the form of sound, light, and radio energy are the fundamental phenomena used in a wide range of communication, computation, signal processing, and sensing systems. Natural materials offer a well understood set of symmetric rules on how these waves propagate. However, recent efforts by Bahl’s team and a few other groups around the world are showing that waves can be compelled to move in unnatural ways, with broken symmetries, using engineered materials.

“This is a topic of research at the absolute frontiers of knowledge as we are only just beginning to learn how to break these symmetries,” Bahl said. “It is going to be an exciting time ahead as there may be wild new applications that we have not even dreamed of!”

Bahl’s team has chosen to work on a series of methods to design reconfigurable directional pathways for sound waves at various lengthscales. Initially, these could enable better environmental noise reduction, improvements in ultrasound imaging in healthcare, nondestructive sound-based testing of materials, and even signal processing for communication systems. Further in the future, these techniques could enable waveguides that transmit information without loss, even when damaged.

Taylor Hughes
Taylor Hughes
In many ways, the project is as multi-faceted as a full research center, including goals of increasing diversity and inspiring young students through community and regional outreach.

“Our team spanning mechanical engineering, electrical engineering, and physics will offer graduate students a unique interdisciplinary training opportunity,” Bahl said. “Additionally, we will be aiming to broaden participation of women and minority students in research, and develop several innovative educational and scientific outreach activities with significant involvement of undergraduate students.”

The wave-based research will focus on breaking the physical principles of reciprocity and time-reversal symmetry. These are physical concepts applying to natural materials and systems at rest, implying that all waves travel with the same properties in both the forward and reverse directions. In other words, if a wave behaves a certain way in one direction, it should behave in the exact same way when fully reversed in space and time.

"Imagine a completely still body of water," said Massimo Ruzzene, who coordinated EFRI NewLAW during his rotation as an NSF program officer. "It takes the same amount of effort to swim from point A to point B and back. That's a reciprocal system."

But if there is flow or a disturbance in the water, the reciprocity of the system can be broken.

"We want to break reciprocity on purpose," Ruzzene said. "The idea is to take waves and make them do things that were physically not possible before, such as bending light, radio waves or sound around an object, guiding them along a specific path, or completely absorbing them."

The current reliance on reciprocal systems limits the ways in which light, sound, and radio waves can be employed today in our technologies and everyday life. The ability to intentionally break reciprocity and time reversal symmetry could generate unprecendented new applications in areas ranging from ultrasound imaging and environmental noise reduction, to more efficient wireless communications systems and photonic circuits.