Researchers from the Illinois Grainger College of Engineering are the first to demonstrate a simple and tunable method for realizing asymmetric couplings in integrated photonics. Their findings, published in Physical Review Letters and selected as an Editor’s Pick, provide insight into topological physics while introducing a new approach for optical non-reciprocity and photonic gyration.
Experimental systems in photonics are often considered to be closed and reciprocal implying that no energy is exchanged with the wider universe around the device. However, recent studies on non-Hermiticity have shown that opening a system to outside interactions can produce surprising new phenomena. One such phenomenon is an asymmetric interaction of the type proposed theoretically by Hatano and Nelson in the mid-1990s, which gives rise to unique effects that seem to violate the reciprocity principle, such as the Non-Hermitian Skin Effect. However, such asymmetric interactions do not occur in natural materials.
Engineers have previously circumvented similar constraints by using metamaterials that can emulate and even go beyond the properties of real materials. Metamaterials can be built using simple building blocks at a much larger scale and routinely allow researchers to study and develop a better understanding of materials that have been theoretically proposed or that might be synthesized some day in the future.
“While asymmetric Hatano-Nelson type couplings have been produced in electronic and mechanical metamaterials, nobody has yet discovered a generalizable approach that can be deployed independent of the type of metamaterial,” said Gaurav Bahl, the George B. Grim Professor of Mechanical Science and Engineering and senior author of the paper. “Certainly, there has been very little work on this in optics. Optical devices can be very tricky to work with and previous experiments that generated asymmetric interactions have required optical gain, which is not at all available in most materials.”
Instead, Bahl’s lab discovered that the variation of a material index in both time and space can produce this asymmetry quite easily. To test this hypothesis, they built a two-resonator ‘photonic molecule’ using lithium niobate (a material in which modulation can be induced by applying a voltage). By controlling the amplitude, phase, and frequency of the modulations, they showed that the interaction between the resonators could be modified dynamically, achieving both symmetric or asymmetric couplings on demand.
“It’s a very interesting approach that we’ve developed,” Bahl said. “The modulation simply changes the resonators in a periodic manner, resulting in an effect that looks precisely like the Hatano-Nelson coupling. The best part is that the method is completely agnostic to the system used and can even be used in electronics, acoustics, or even superconducting quantum devices.”
The original goal of the study was to get this asymmetry to a point of perfect isolation—that is, where there is zero interaction in one direction. They successfully achieved this goal by demonstrating a giant optical isolation effect, where the propagation of light in one direction was a million times easier than in the opposite direction.
But while exploring their test devices, the engineers encountered a surprise. Their approach was so efficient that they could even get past the isolation point to where the sign of the coupling simply flipped and the phase became direction dependent. This was something that has not been seen before in time modulated coupling and is an easy path to photonic gyration.
Going forward, the Illinois researchers will work to expand their findings. They are working with their partners specializing in condensed matter to explore how longer and more elaborate chains of resonators with this kind of tunable couplings could answer fundamental questions on topological physics. Simultaneously, from an engineering standpoint, they aim to create a pure gyrator which is a universal building block of many nonreciprocal devices.
This work was spearheaded by Dr. Ogulcan Orsel (currently at Analog Photonics), Dr. Jiho Noh (currently at Sandia National Labs), and Dr. Penghao Zhu (currently at Ohio State University).