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Spintronics is the idea of encoding and processing information in the spins of electrons, rather than in their electrical charges. Currently, such devices only work at ultralow cryogenic temperatures, and for this reason, researchers have long hoped to develop an optical counterpart—so-called “spinoptronics.” This is because optical excitations interact far less strongly with the forces that rapidly destroy electron-spin coherence in conventional devices, possibly allowing device operation at room temperature. |
One appealing route to spinoptronics involves exciton-polaritons – hybrid light–matter particles created when excitons (electron–hole pairs bound by Coulomb attraction) inside a semiconductor couple strongly to photons trapped inside an optical microcavity. These light–matter particles can carry spin information, but researchers have so far struggled to engineer the interactions between polaritons required to carry out logical operations.
Now, Presidential Postdoctoral Fellow Jiaxin Zhao, Associate Professor Timothy Liew of Nanyang Technological University, and colleagues have demonstrated one pathway to doing so by stacking ultrathin layers of the semiconductor WS₂ inside a microcavity. As they show in experiments, this layered design appears to create a natural anisotropy in the interactions between exciton-polaritons in different spin states, a key element allowing the controlled switching of spin states. They hope the work provides an essential stepping stone toward realizing the full suite of components—spin switches, memories, and logic gates—required to make spin-optronics practical at room temperature.
This research was published in the journal Nature Photonics in November 2025 (https://www.nature.com/articles/s41566-025-01786-y).
For over a decade, Prof. Timothy Liew and other researchers have been developing spinoptronics using in GaAs semiconductor microcavities, providing some of the first evidence that spin-based optical logic is possible. But the excitons in GaAs are too weakly bound to survive at room temperature, meaning all such devices required ultracold conditions.
“I actually worked on the spin switch in GaAs back in 2010 with collaborators,” says Prof. Timothy Liew. “It worked, but the big limitation was the need for cryogenic temperature.”
Pursuing spintronics at room temperature
A more promising route to room-temperature spin-optronics, he says, is by exploiting exciton-polaritons in another class of semiconductors called transition-metal dichalcogenides (TMD). Excitons are electron–hole pairs bound by the Coulomb attraction. In contrast to semiconductors such as GaAs, the atomically thin TMD materials bind electrons and holes with binding energies hundreds of millielectronvolts higher, allowing polaritons to form and interact strongly even at room temperatures.
“We’ve been developing exciton-polaritons in TMDs at NTU for some time,” says Prof. Timothy Liew, “beginning with the work of Prof. Qihua Xiong and Jiaxin Zhao, the first author of our present paper.
Yet TMD-based polaritons have also faced a stubborn obstacle: their interactions do not seem to depend on spin in the way that logic devices require. In conventional polaritonic systems, particles with the same spin interact more strongly than those with opposite spins. This kind of anisotropy is essential because logic elements rely on asymmetric interactions: one spin state must be able to suppress, amplify, or redirect the other to perform switching or logic operations. But in TMD monolayers at room temperature, experiments show that spin-selective interactions are largely washed out, most likely because polaritons couple strongly to lattice vibrations that obscure the underlying spin physics. Without a clear difference between the two spin states, the essential building blocks of spin-optronic logic remain out of reach.
In the new work, Prof Timothy Liew and colleagues show a pathway around this issue by putting several monolayers close together.
“It just so happened,” says Professor Timothy Liew, “that in our previous work our favourite sample was one with three TMD layers, as this sample showed a stronger coupling between light and excitons. Surprisingly, it also showed spin anisotropic interactions.” Hence, following this clue, the researchers decided to explore multiple TMD layer systems more explicitly in their new research.
A pathway to spin anisotropy through multiple TMD layers
In experiments, the team fabricated microcavity structures containing either a single monolayer of WS₂ or two monolayers separated by a thin layer of SiO₂. When such structures are excited with light, two different branches of exciton-polaritons can form. Polaritons in the lowest -energy branch—known as the lower polariton state—shift their energy slightly when large numbers of polaritons are created. By shining laser pulses of increasing intensity onto the samples and measuring how the energy of this lower state changed, the researchers could assess how strongly polaritons interact under different spin configurations.
They found that in the single-layer samples, the energy shifts produced by circularly polarised light—which creates polaritons all in the same spin state—were almost identical to those produced by linearly polarised light, which creates an equal mixture of the two spin states. Because any difference between same-spin and opposite-spin interactions would have shown up as a difference between these two shifts, the near equality of the signals indicates that the interactions are effectively spin-independent. In contrast, in the multi-layer structures, circularly polarized excitation consistently produced a noticeably larger shift than the linear case, signalling that polaritons of the same spin were now repelling each other more strongly than polaritons of opposite spin.
This restored anisotropy, says Prof. Timothy Liew, is precisely the behaviour needed for spin-based switching and logic, demonstrating that the layered design revives the spin selectivity that monolayer TMD polaritons typically lack.
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:Energy shift of the lower polariton state as a function of the polariton density in two distinct cases: for linearly (red dots) and circularly (light blue and dark blue dots) polarized excitations. The larger shift observed for the circularly polarized excitations indicates that polaritons of the same spin repel one another more strongly than polaritons of opposite spin, demonstrating the anisotropy in interactions required for carrying out spintronic logic operations. |
Prof Timothy Liew and colleagues have presented an explanation for the observed anisotropy, which is consistent with all existing measurements, but will require further experiments for direct confirmation. In principle, this achievement could pave the way to build spinoptronics devices able to operate at room temperature, because the excitons in TMD materials bind much more strongly than those in GaAs materials and can therefore persist even in the face of the strong thermal fluctuations present at room temperatures.
“Our working hypothesis is that the presence of more than one layer greatly restricts exciton-phonon coupling,” says Prof Timothy Liew, “however, we don’t have a precise theory for why this affects interactions, only hints from previous works from other groups. Our next steps should be to study more directly phonon interactions in our system.”
Overall, says Prof Timothy Liew, the new findings open up promising new avenues for the manipulation of coherent spin states in two-dimensional materials held within cavity-engineered photonic systems. The understanding it provides should be particularly useful in the development of polaritonic devices, where several theoretical designs for polaritonic information platforms have been completed and await the right materials for their implementation. These include optical circuits, cellular automata, quantum neural networks, entanglement networks, and spin simulators.