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Magnet-free reconfigurable alterspin photonics at optical frequencies

This paper demonstrates the first magnet-free, reconfigurable alterspin photonic system in the visible-to-near-infrared range by utilizing an all-dielectric photonic crystal with a liquid droplet to dynamically control the alterspin texture without magnetic bias.

Original authors: Cheng-Wei Qiu, Zhe Wang, Shuihua Yang, Jianfeng Chen, Mengqi Liu, Kaixuan Li, Mingjun Chen, Yue Wen, Li Zhang, Xuezhi Ma

Published 2026-10-21

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CC BY 4.0

Original authors: Cheng-Wei Qiu, Zhe Wang, Shuihua Yang, Jianfeng Chen, Mengqi Liu, Kaixuan Li, Mingjun Chen, Yue Wen, Li Zhang, Xuezhi Ma

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Light carries more than just brightness; it carries a hidden twist, a property known as spin. Just as a spinning top can rotate clockwise or counter-clockwise, light waves can twist in two opposite directions, creating what scientists call right-handed or left-handed circular polarization. For decades, researchers have sought ways to control this twist, particularly to create patterns where the direction of the spin flips back and forth in a regular, alternating sequence. This specific arrangement, known as an alterspin texture, is a phenomenon borrowed from the world of magnetism, where magnetic spins align in alternating patterns. In the magnetic world, creating such patterns usually requires strong magnets or magnetic fields. However, when scientists try to move this concept to the speed of light, where frequencies are incredibly high, the usual magnetic tools stop working. The magnetic materials that respond to these fields become ineffective, and the tiny magnets needed to bias the system simply do not exist at the scale of light waves. This has left a gap in our ability to create these alternating spin patterns using light, especially in the visible spectrum where we see the world around us.

A team of researchers at the National University of Singapore has now filled this gap by creating a system that generates these alternating spin patterns without using any magnets at all. Instead of relying on magnetic forces, they built a tiny, suspended sheet of silicon nitride, a transparent material often used in electronics, and etched a precise grid of square holes into it. This grid acts as a photonic crystal, a structure that guides light in specific ways. The key to their discovery lies in how they treated the two sides of this sheet. By placing a single drop of water on one side of the suspended slab while leaving the other side exposed to air, they created an imbalance. This simple difference in the environment above and below the sheet broke a specific symmetry that usually keeps the light's spin neutral. As a result, the light passing through the grid organized itself into a distinct pattern where the spin direction flipped four times as you moved around a circle, creating the desired alternating texture.

The researchers demonstrated that this pattern is not just a static feature of the material but something they could actively control and rewrite. By moving the water droplet from one side of the slab to the other, they could instantly reverse the direction of the spin pattern. If the water was on top, the light twisted in one sequence; when the droplet was moved to the bottom, the entire pattern flipped to the opposite sequence. They could also erase the pattern entirely by removing the water, returning the system to a state where no alternating spin existed. This level of control is significant because previous attempts to create similar effects relied on fixed materials or structures that could not be changed once built. In this new system, the pattern is determined by the environment, allowing it to be written, erased, and reversed simply by manipulating a liquid droplet.

To ensure they were truly observing this phenomenon and not just an artifact of their setup, the team measured the light across a range of wavelengths from 740 to 800 nanometers, which covers the deep red part of the visible spectrum and near-infrared light. They found that the four-period alternating pattern remained consistent across this entire range. They also developed a way to quantify the strength of this pattern, confirming that the effect was robust and reproducible. The study explicitly ruled out the idea that the pattern was caused by the physical shape of the holes or accidental imperfections in the manufacturing, showing instead that the effect was driven entirely by the difference in the materials on either side of the sheet. By proving that an all-dielectric, magnet-free system can generate and control these complex spin textures, the researchers have opened a new path for developing optical devices that can manipulate light in ways previously thought to require magnetic fields. This work suggests that future technologies for processing information with light could be built using simple, reconfigurable structures rather than complex, fixed magnetic components.

Technical Summary: Magnet-free Reconfigurable Alterspin Photonics at Optical Frequencies

Problem Statement
Altermagnetism is a magnetic order characterized by alternating spin polarization between symmetry-related momenta despite zero net magnetization. While recent work demonstrated an altermagnetic photonic crystal by mapping electron spin to light spin, these implementations relied on gyromagnetic materials and magnetic bias. Extending this concept to optical frequencies is challenging because the gyromagnetic response diminishes rapidly at these frequencies, and suitable nanoscale magnetic-bias sources are lacking. Furthermore, existing reconfiguration strategies—such as switching magnetic bias polarity or reversing material chirality—are difficult to achieve dynamically in operation. While dielectric photonic structures allow for easier geometric engineering of chirality, the structural handedness remains fixed once fabricated, limiting reconfigurability.

Methodology
The authors propose and realize a magnet-free, all-dielectric photonic crystal platform to generate and reconfigure "alterspin" textures (optical analogs of altermagnetic spin textures) in the visible-to-near-infrared range (740–800 nm).

Platform Design: The system utilizes a suspended silicon-nitride (SiNx) photonic-crystal slab patterned with a square lattice of rounded-square holes. The lattice constant, hole side length, and slab thickness are defined to support specific optical modes.
Symmetry Engineering: The core mechanism relies on separating the roles of in-plane and out-of-plane symmetries. The in-plane square lattice organizes the alterspin texture into a four-period angular form. The out-of-plane symmetry is selectively broken by introducing a dielectric perturbation (a change in refractive index) on one side of the slab while keeping the other side in air.
Reconfigurable Control: A single water droplet serves as a reversible dielectric perturbation. By placing the droplet above or below the suspended slab, the authors create asymmetric environments ( $n_1 \neq n_2$ ). The surface is treated with a hydrophobic silane to maintain a stable Cassie wetting state, preventing water from penetrating the holes.
Theoretical Framework: The authors employ a Maxwell-Hamiltonian analysis, formulating the Bloch-Maxwell eigenproblem as a two-component Hamiltonian. This analysis identifies that the out-of-plane dielectric perturbation breaks the out-of-plane mirror symmetry ( $M_z$ ), which controls the sign and amplitude of the circular polarization component ( $S_3$ ), while the in-plane mirrors ( $M_x, M_y, M_d$ ) enforce the four-period angular dependence ( $S_3 \propto \sin 4\phi$ ).
Experimental Characterization: The team measures momentum-resolved spectra to identify the fundamental transverse-magnetic-like mode (TM1). They measure the two circular-polarization channels separately to generate intensity maps and normalized difference maps ( $S_3$ ) along the TM1 isofrequency contour.

Key Results

Realization of Alterspin Texture: The experiment successfully demonstrated a magnet-free alterspin texture with a distinct four-period angular form ( $S_3 \propto \sin 4\phi$ ) across the 740–800 nm wavelength range. The texture consists of eight alternating-sign sectors in momentum space.
Reconfigurability: The sign of the alterspin texture was successfully reversed by moving the water droplet from the top to the bottom of the slab (changing the configuration from water-SiNx-air to air-SiNx-water). Removing the droplet entirely (restoring the symmetric air-SiNx-air configuration) suppressed the texture, confirming that the effect is driven by the out-of-plane dielectric asymmetry rather than fabrication defects or fixed chirality.
Quantitative Assessment: The authors established a criterion, the "alterspin-texture asymptotic coefficient" ( $C_{alt}$ $C_{a l t}$ ), to quantify the order and strength of the alterspin texture.
- For the water-induced configurations, $C_{alt}$ showed opposite signs, confirming the reversal of the texture.
- The magnitude of $C_{alt}$ increased with the dielectric contrast ( $|n_1 - n_2|$ ).
- Measured magnitudes approached 0.4, while calculations stabilized above 0.5.
- The symmetric reference (air-SiNx-air) yielded a coefficient near zero, confirming the absence of the texture in the symmetric state.
Comparison: When compared to a recent planar-chiral implementation using staggered structures, the measured alterspin-order magnitude in this work was larger, while retaining the advantage of reversible control.

Significance and Claims
The paper claims to provide a viable route toward visible-to-near-infrared alterspin photonic systems that are both magnet-free and externally reconfigurable. By decoupling the symmetry that organizes the texture (in-plane lattice) from the symmetry that controls its sign (out-of-plane dielectric perturbation), the authors overcome the limitations of gyromagnetic materials and fixed chiral structures. The work demonstrates that a simple, reversible dielectric perturbation (a water droplet) can write, erase, and reverse alterspin textures, offering a new degree of freedom for controlling optical spin at frequencies where magnetic responses are negligible. The introduction of the $C_{alt}$ criterion provides a standardized method for evaluating and comparing alterspin orders across different wavelengths, dielectric contrasts, and photonic implementations.

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