New Approach to Controlling Light Signals
A new way to marshal light within optical devices has been demonstrated experimentally by researchers in China. They have been able to induce light to organize itself into specific patterns of pulses as it circulates within a pair of optical fiber loops using a version of a phenomenon—called the non-Hermitian skin effect (NHSE)—that has been predicted but not observed previously [1]. The effect could be used to control light signals in photonic devices such as switches and routers.
In the standard theory for electron behavior in a metallic crystal, the periodic atomic structure leads to so-called Bloch waves—electron quantum states that spread across the entire crystal. But in recent years, theorists have found surprising results for a scenario in which one assumes that a particle such as an electron hops between neighboring sites in a periodic lattice asymmetrically—say, rightward hopping is more probable than leftward hopping. The particle’s quantum states become localized at the edge or surface of the lattice rather than spreading across it. This localization is the NHSE.
Testing the theory proved difficult, but in 2020, researchers demonstrated the NHSE with light rather than electrons [2]. They used an optical fiber circuit comprising two loops of slightly different length that passed through a shared optical coupler. This circuit created a “temporal photonic lattice”—a sequence of light pulses equally spaced in time, with the spacing determined by the difference in propagation times around the two loops. The asymmetric “hopping” was produced by different loss and gain conditions for the signal strength in each loop, which allowed pulses to switch between loops asymmetrically. Imagining this temporal lattice as a lattice in space, the central pulses in the train can be considered to be an interface between two regions with different asymmetry, and the team found that, regardless of where a pulse began, it would always move toward this interface.
Now Bing Wang and colleagues at the Huazhong University of Science and Technology in China have demonstrated a version of the effect that appears under nonlinear conditions, meaning that the intensity of light in the optical medium isn’t simply proportional to the energy in the initial pulse. Nonlinearities allow the propagation of self-sustaining “solitary” waves (solitons). Unlike ordinary light pulses, these waves are not easily scattered or dispersed inside an optical medium, so they are potentially more robust and thus useful for photonic information transmission and processing.
Wang and colleagues created a nonlinear response in the fiber loops by splitting off part of the light in each fiber and sending it into an amplifying circuit that then interacted with and modified the original pulses. Using one part of a signal to modify another part is a common technique for producing nonlinearity. In this way the researchers could control and tune the strength of the nonlinearity—something rarely possible when a nonlinearity arises from some inherent property of the optical medium.
The nonlinear NHSE was manifested as a concentration of intensity into particular pulses in the train. Under some conditions of nonlinearity, the soliton appears in the pulses at the start or end (“edges”) of the train, while under other conditions, it can relocate to the middle (“bulk”).
Wang says that this ability to controllably confine the light to specific positions in a pulse train might be used in optical switches. To illustrate this potential application, the team designed an optical router with multiple output ports that could be opened at a specific moment to admit some element of the temporal lattice of pulses. Which of these temporal ports received the soliton could be controlled by varying the strength of the nonlinearity.
Optical physicist Konstantinos Makris of the University of Crete in Greece says the work is “very exciting.” This demonstration of “a new class of solitary waves,” he says, could have applications for signal processing and photonic communications.
“This is fundamental research that shows a novel phenomenon that was completely unknown only a few years ago,” says optical physicist Alexander Szameit of the University of Rostock in Germany, who led the team that demonstrated linear NHSE in 2020. “The non-Hermitian skin effect is really cool, and now they’ve shown a nonlinear version.” Szameit agrees that the effect might be used in devices such as optical switches and thinks that a similar phenomenon might be found in other systems that show nonlinear effects, such as ultracold atoms. He stresses, however, that the immediate value of such work is in its potential to generate new ideas for ways to manipulate light.
–Philip Ball
Philip Ball is a freelance science writer in London. His latest book is How Life Works (Picador, 2024).
References
- S. Wang et al., “Nonlinear non-Hermitian skin effect and skin solitons in temporal photonic feedforward lattices,” Phys. Rev. Lett. 134, 243805 (2025).
- S. Weidemann et al., “Topological funneling of light,” Science 368, 311 (2020).