Twisted light-matter systems unlock unusual topological phenomena

Properties that remain unchanged when materials are stretched or bent, which are broadly referred to as topological properties, can contribute to the emergence of unusual physical effects in specific systems.

Over the past few years, many physicists have been investigating the physical effects emerging from the topology of non-Hermitian systems, open systems that exchange energy with their surroundings.

Researchers at Nanyang Technological University and the Australian National University set out to probe non-Hermitian topological phenomena in systems comprised of light and matter particles that strongly interact with each other.

Their paper, published in Nature Physics, reports the emergence of unusual and never before observed behavior in a system made up of quasi-particles known as exciton-polaritons, which are carefully twisted inside an optical cavity.

"Our work was motivated by a long-standing challenge in non-Hermitian physics," Rui Su, senior author of the paper, told Phys.org.

"While non-Hermitian topology has attracted enormous interest in recent years, it remains difficult to realize in an active photonic system in a controllable and reversible way, without relying on magnetic fields or bulky external components."

Exciton-polaritons form when particles of light (i.e., photons) strongly couple with bound electron-hole pairs (i.e., excitons). These hybrid quasiparticles are ideal for studying non-Hermitian physics, as they combine various advantageous properties.

These hybrid quasiparticles can amplify, absorb and lose light, but they also possess other properties that can be experimentally manipulated, such as intrinsic angular momentum (i.e., spin) and light field orientation (i.e., polarization).

"Despite their advantages, achieving non-Hermitian topology and non-reciprocal behavior in polariton systems has remained elusive, largely because it is difficult to precisely engineer the non-Hermitian spectrum," said Su.

"The primary objective of our work was thus to identify a simple mechanism that allows non-Hermitian topology to be engineered, tuned, and even reversed on demand in a realistic experimental system. More broadly, we hoped to open new perspectives on tunable non-Hermitian phenomena and facilitate the development of compact, on-chip polaritonic devices with enhanced functionalities."

A twisted strongly coupled light-matter system

As part of their study, Su and his colleagues built a tiny optical cavity and introduced a perovskite crystal and liquid crystals inside it. They chose a material that strongly interacted with light, as this would facilitate the formation of exciton-polaritons.

The researchers then introduced a controlled geometric twist between the perovskite crystal and the liquid crystal layers inside the cavity.

To probe the effects of this twist on the system's topology, they collected various optical measurements, assessing the energy, momentum, polarization and spatial distribution of exciton-polaritons formed in the cavity.

"We first used angle-resolved photoluminescence spectroscopy to map the polariton dispersion relations," explained Su.

"This technique allows us to resolve polariton energy as a function of in-plane momentum. By performing polarization-resolved measurements, we could further distinguish polariton states with opposite spin polarizations. These measurements allowed us to examine the evolution of polariton dispersions under different experimental conditions."

Su and his colleagues subsequently collected so-called linewidth measurements. These measurements gave them an indication of the gain and loss of light in the system, revealing non-Hermitian effects.

"By fitting the polariton emission spectra at each momentum, we extracted the momentum-dependent linewidths for different spin states," said Su.

"Under a twisted configuration, we observed a clear linewidth asymmetry between polariton states at opposite in-plane momenta (+k and −k).

"As the polariton spin is locked to momentum in our system, this linewidth asymmetry reflects spin-dependent net gain and loss induced by the twist, and it is the key experimental signature of non-reciprocity in the non-Hermitian band structure."

The researchers combined the linewidth measurements they collected with dispersion measurements, which offer an indication of the relationship between the energy of exciton-polaritons and their movement in the system. This allowed them to reconstruct their system's polariton energy spectrum, leading to the discovery of a new twist-induced non-Hermitian topology.

"We next visualized the topological consequences of this complex energy structure using real-space, energy-resolved photoluminescence imaging," explained Su. "This method enabled us to directly observe how polaritons distribute spatially under uniform excitation."

The new twist-induced non-Hermitian topology

The researchers' measurements confirmed that their carefully twisted system exhibited a non-Hermitian topology. Specifically, they found that polaritons in this system moved in opposite directions. Instead of spreading evenly under uniform excitation, as one would expect, they accumulated near one edge of the structure.

This is a hallmark of a non-Hermitian topological effect known as the non-Hermitian skin effect. Interestingly, when the researchers twisted the structure in the opposite direction, they found that polaritons rapidly piled up at the opposite edge. This confirmed that the effect was driven by the twist that they introduced in their system.

"Our work is that it provides experimental access to a previously unexplored regime of non-reciprocity and non-Hermitian topology in a light–matter system," said Su.

"We show that a simple geometric parameter—the twist angle—can act as an effective control knob for non-Hermitian topology, allowing non-reciprocal transport and the non-Hermitian skin effect to be switched and reversed."

The findings gathered by Su and his colleagues highlight the promise of exciton-polaritons for studying non-Hermitian topological effects. In the future, the twist-driven topology realized as part of this study could be leveraged to develop new photonic and non-reciprocal devices, including lasers and optical logic systems.

"Our future research will span both fundamental physics and potential device applications," said Su. "On the fundamental side, we are particularly interested in exploring how the spin-dependent non-Hermitian effects demonstrated in this work interact with the inherently strong nonlinearity in exciton-polariton systems.

"This interplay is expected to give rise to new non-equilibrium macroscopic quantum states, including unconventional forms of polariton Bose-Einstein condensation that have no direct counterpart in equilibrium systems."

As part of their next studies, the researchers also hope to successfully apply the insight they gathered to the development of new technologies. They are particularly keen on trying to develop spin-dependent polariton lasers and compact, ultrafast, non-reciprocal optical components that can be integrated on-chip.

"By bridging fundamental discoveries with device-oriented research, we hope to establish exciton-polaritons as a versatile platform for both exploring non-equilibrium physics and developing future photonic technologies," added Su.

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