A new approach to control light in photonic circuits

Researchers at Washington University in St. Louis have developed a simple, efficient way to enable one-way light transmission in photonic chips

Leah Shaffer 05.05.2025

A new method developed by WashU researchers can reconfigure transmission directions of light in a way that will lower expenses and simplify photonic circuits. (Image: Micro/Nano Photonics Lab).

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To improve photonic and electronic circuitry used in semiconductor chips and fiber optic systems, researchers at the McKelvey School of Engineering at Washington University in St. Louis tinkered with the rules of physics that govern movement of light over time and space. They have introduced a new way to manipulate light transmission, opening possibilities for advanced optical devices.

Their method causes a “mirror-flip of the system,” said Lan Yang, the Edwin H. & Florence G. Skinner Professor of Electrical & Systems Engineering and senior author of the research, now published in Science Advances.

Using parity-time (PT) symmetric photonic waveguides, they can manipulate the light waves to “reverse time” so the system behaves the same as before, Yang added.

“PT symmetry enables new ways to control light which are especially useful for applications in optical communications, lasers and sensors,” she said.

Postdoctoral researchers Wenbo Mao and Fu Li, co-first authors of the study, described their systems as enabling “reconfigurable transmission.”

Mao uses an optical polarizer as an analogy to explain this system.

“Think of a polarizer as a door that is slightly open – it only allows light with a specific polarization direction, aligned with the slit, to pass through, while light polarized perpendicular to the slit is blocked,” Mao said.

“Then, imagine a series of polarizers. The first one is aligned with the initial polarization of the light, while each subsequent polarizer gradually rotates until the last one is completely perpendicular to the original polarization. If light first encounters the properly aligned polarizer, the gradual rotation allows it to pass through the entire system. However, if light first encounters the final polarizer, which is perpendicular to its polarization, it gets blocked right away.”

This effect enables light to travel through the system in only one direction. This “asymmetric transmission” is one of the key outcomes of this work because it can simplify telecommunication systems.

Telecom systems contain components designed to ensure that data flows in only one direction, such as from a server to a client. When someone is streaming a movie, for instance, they want that stream of data to move steadily toward their device; the user doesn’t want their home data to move back to the source. Asymmetric transmission is how they address that.

In optical communication, isolators are typically used to direct light by applying magnetic fields, ensuring it travels in one direction and preventing interference. The system demonstrated in this study uses carefully designed waveguides to control light transmission, eliminating the need for complex and bulky isolator components and providing a simpler, more efficient solution for one-way light propagation.

In the work, the researchers carefully designed photonic chips to construct a periodical system, so-called Floquet non-Hermitian physics, to mimic that rotated polarizers effect as described by Mao. The system allows the light propagation in one way when it experiences non-Hermitian phase transitions.

Li described the light like “a dancer twirling across a room filled with carefully placed traps.”

When the dancer moves forward (left to right), each step is precisely guided to avoid the traps. However, when the dancer tries to move backward, every step lands directly into a trap, preventing smooth movement in that direction. In the experiment, light acts as the dancer, while tiny pieces of metal along the waveguides serve as traps, ensuring light moves efficiently only in one direction.

“Our structure is very simple but delivers great performance which can be used to reduce the cross-talking between different optical components,” said Li, who added that this mechanism has potential application for many telecommunication devices.

“We simply adjust the design of the device itself,” Yang said. “This allows us to actively achieve asymmetrical light transmission that will carry information in the preferred direction.”

Mao W, Li F, Zhang Q, Xu W, Masud Awan K, Yang L. On-chip reconfigurable transmission in spatially chirped Floquet parity-time symmetric photonics. Sci. Adv. 11(2025).

DOI:10.1126/sciadv.adu4653

This project is supported in part by the National Science Foundation (EFMA1641109).

The McKelvey School of Engineering at Washington University in St. Louis promotes independent inquiry and education with an emphasis on scientific excellence, innovation and collaboration without boundaries. McKelvey Engineering has top-ranked research and graduate programs across departments, particularly in biomedical engineering, environmental engineering and computing, and has one of the most selective undergraduate programs in the country. With 165 full-time faculty, 1,420 undergraduate students, 1,614 graduate students and 21,000 living alumni, we are working to solve some of society’s greatest challenges; to prepare students to become leaders and innovate throughout their careers; and to be a catalyst of economic development for the St. Louis region and beyond.