12/10/2025 Taylor Parks
Written by Taylor Parks
Professor Gaurav Bahl’s investigation of nonreciprocity in integrated photonics has been recognized as a “Top 30 Development in Optics” in this year’s special issue of Optics & Photonics News. This is the fifth time that Bahl’s work has received this recognition and the latest since 2022.
“It’s really wonderful that in different contexts, and over different types of research projects, our efforts have been recognized by the optics and photonics community,” Bahl said. “It’s particularly nice for the students to receive this recognition for all the energy that they put into this work over the years.”
For the past forty years, OPN’s annual feature has reviewed submissions of recently published research in optics and photonics and compiled a peer-reviewed list of top contenders. Bahl’s article, with first author Dr. Ogulcan Orsel and coauthors Dr. Jiho Noh, Dr. Penghao Zhu, postdoctoral researcher Dr. Jieun Yim, physics professor Taylor Hughes, and Professor Ronny Thomale from the University of Würzburg, focuses on the implementation of asymmetric, or nonreciprocal, energy exchange interactions in photonic systems. Several of the coauthors were previously or are currently members of Bahl’s research group—Orsel is a recent graduate and Noh was previously a postdoctoral researcher.
Reciprocity is a fundamental property in optical devices, meaning that the transmission of light is symmetric between any pair of optical components. Even so, non-reciprocal optical devices are essential to protect systems from undesirable reflections and to maintain one-way propagation. Bahl’s team has found a new approach to solving this problem.
“[In our latest work], we demonstrated that the coupling between two optical devices can be made non-reciprocal by simply using a particular time variation recipe,” he said.
The behavior of coupled optical devices can perhaps be easily visualized by imagining a series of empty buckets in a line, with each connected to the next via tubes at the bottom. Any water poured into the left-most bucket will evenly distribute among all the rest. The same will happen if water is poured into the right-most bucket.
Now, imagine adding a valve (i.e., one-way non-reciprocal device) inside each tube to block the flow of water one way. When all the valves are set to block leftward flow, any water poured into the right-most bucket will stay there. However, water poured into the left-most bucket will still evenly distribute among the buckets because rightward flow is permitted.
Bahl’s team found a scenario that, in the bucket analogy, occurs when there are no valves present and the buckets are simply raised and lowered in a particular, synchronous pattern. The team found that doing so causes the entire system to act like a pump, generating a unique form of flow in only one direction. In this case, water added to the left-most bucket would flow through to the right-most and collect there, while water added directly to the right-most bucket would stay in that bucket. The directionality may be switched by changing the pattern.
While the properties of light are different than those of fluids, light similarly tends to “fill up” matched optical devices that are coupled to each other. Bahl’s team found that their special time-variation recipe produces a similar one-way pumping effect for light even without any one-way devices being present in the system.
“We simply applied a periodic modulation to optical resonators that are set up in a linear chain and showed that the light only flows one way,” he said. “This can be applied to any optical platform irrespective of technology.”
While liquids support waves on their surface, light is itself a wave and therefore also has a phase property. This is where the bucket analogy fails and another surprising effect appears. The team showed that, when the periodic modulation is set in the correct regime, the phase response for light propagation can be made to exhibit strong nonreciprocity. In this regime, light propagates normally in one direction but exhibits a [pie] phase flip, equivalent to a sign flip, in the opposite direction. In other words, positive fields become negative.
“A device that exhibits a non-reciprocal phase response is called a gyrator,” Bahl said. “Gyrators are special because they are a universal building block for all other non-reciprocal devices.”
As producing non-reciprocal effects without using special magnetic materials has proven quite challenging, the team’s findings are significant for modern optical device development.
“Here we’ve shown that, by using just time modulation, we can produce this incredibly useful asymmetric coupling and steer it into different regimes of operation,” Bahl said. “This approach to non-reciprocal coupling also opens the door to new dynamic meta-materials that can exhibit behaviors that are not found in nature.”
The investigation connects to Bahl’s ongoing efforts with the Multidisciplinary University Research Initiative (MURI) to explore high-order topological photonics.
________
Gaurav Bahl is a George B. Grim professor of mechanical science and engineering. He is an affiliate of the Department of Physics, the Department of Electrical and Computer Engineering, the Illinois Quantum Information Science and Technology Center (IQUIST), and the Holonyak Micro & Nanotechnology Laboratory (HMNTL).