In front of the optical bench, from left: NTU physics PhD student Zheng Hao, Nanyang Assistant Professor Su Rui and NTU physics PhD student Jin Feng
Many systems are traditionally described under the assumption of perfect balance, where energy is conserved and isolated from the environment. Non-Hermitian systems break this assumption by describing open systems in which energy can flow in and out. In such systems, certain behaviours remain remarkably stable even when the system is deformed. This observation naturally leads to the concept of topology, which studies properties that remain unchanged when an object is stretched or squeezed. In non-Hermitian systems, topology acts as stabilising principle that preserves specific physical effects in the presence of gain and loss.
When light interacts strongly with matter, hybrid quasiparticles known as exciton-polaritons are formed. These systems naturally involve both gain and loss, making them ideal for studying non-Hermitian physics and topology. Discovering non-Hermitian topology in systems with strong light-matter interactions opens new opportunities to control these effects and design compact, chip-based devices capable of manipulating light. However, experimentally demonstrating non-Hermitian topology remains challenging because it requires breaking reciprocity, meaning the system behaves differently on the direction of energy flow. Although non-Hermitian topology has attracted significant interest in recent years, it remains difficult to realize in active photonic systems in a controllable and reversible manner, without relying on magnetic fields or bulky external components.
Now, researchers from Nanyang Technological University, led by Nanyang Assistant Professor Su Rui have successfully demonstrated twist-induced non-Hermitian topology of exciton-polaritons in a liquid-crystal filled CsPbBr3 perovskite microcavity at room temperature. They showed that twisting the liquid-crystal layer provides a new way to control how light and matter interact. As a result, they observed light-matter quasiparticles with unusual loop-shaped energy patterns that move asymmetrically and accumulate at the edges, revealing a new form of topological behaviour known as the non-Hermitian skin effect.
The main breakthrough of this research is the experimental demonstration that non-Hermitian effects in exciton-polaritons can be created, tuned, and reversed through a geometric twist between perovskites and liquid crystals in a microcavity. Beyond fundamental physics, the twist-based control opens new opportunities for small, on-chip polaritonic devices with enhanced functionality, including unidirectional light flow, non-reciprocal optical components, and reconfigurable circuits. This study was published in Nature Physics (Twist-induced non-Hermitian topology of exciton–polaritons | Nature Physics).
“What makes our approach unique is that we realize non-Hermitian topology using a simple and intuitive geometric control — a twist between anisotropic materials in a tiny microcavity, rather than relying on complex structure fabrications, magnetic fields, or fixed material properties,” says Prof. Su Rui. “This twist acts as a reversible and highly tuneable degree of freedom, allowing us to dynamically control non-reciprocity and topological behaviour in a strongly coupled light–matter system. As a result, our platform is both experimentally accessible and broadly applicable to other polaritonic and photonic systems.”
Controlling Polaritons with Twisted Perovskite Layers
In order to realize the non-Hermitian skin effect, Prof. Su Rui and the team explored how light-matter quasiparticles behave in a specially designed system consisting of a layered microcavity (a small structure where light and matter interact), a perovskite material and a liquid-crystal layer. In this system, light and matter interact strongly, forming new hybrid states called polaritons. Importantly, the liquid crystal enables the linking between the particle’s orbit motion and its spin, a phenomenon known as spin-orbit coupling, which allows the control of particles motion through manipulation of their spin.
Twisting materials to control how light is absorbed
Inspired by twistronics, where twisting stacked layers provides a powerful way to tailor material properties, the researchers twisted the perovskite layer relative to the liquid-crystal layer. This twist modifies how the system interacts with circularly polarized light. With a +30° twist, spin-polarized polariton states experience stronger gain and absorb more light, while reversing the twist leads to the opposite behaviour.
Figure: Observation of the non-Hermitian skin effect with exciton-polaritons, revealed by energy-resolved spatial intensity distributions for microcavities twisted by +30° (left) and -30° (right).
In the presence of non-Hermitian topology, twisting the structure causes polaritons to move primarily in one direction, further accumulating at one edge instead of spreading evenly. This edge accumulation is known as the non-Hermitian exciton-polariton skin effect. By tuning the applied electrical voltage, the team could electrically switch the effect, transforming the light distribution from uniform to strongly asymmetric. This study demonstrates that twisting, voltage control, and spin-orbit coupling can be used to direct polaritons to move in specific directions.
Paving the Way for Next-Generation Photonic Devices
“We believe that this work would open new opportunities to explore the interplay between non-Hermiticity, spin–orbit coupling, and nonlinearity in polariton systems,” says Prof. Su Rui. “More broadly, it demonstrates that a simple geometric twist can serve as a powerful and intuitive control knob for non-reciprocal and non-Hermitian effects, pointing toward a new generation of compact, reconfigurable photonic devices.”
Their future research will focus on both fundamental physics and potential device applications. They are particularly interested in exploring how the spin-dependent non-Hermitian effects demonstrated in this work interact with inherently strong non-linearity in exciton-polariton systems. This interplay is expected to give rise to new non-equilibrium macroscopic quantum states, such as unconventional forms of polariton Bose-Einstein condensate. At the same time, they aim to translate these physical insights into practical technologies includes spin-dependent polariton lasers and compact, ultrafast, non-reciprocal optical components that can be integrated on-chip.