What if light itself could be bent, switched, and reshaped at will? MIT researchers have now developed a new platform for manipulating light on the nanoscale, unveiling ultracompact optical devices that are efficient, reprogrammable, and adaptive.
Built from the layered quantum material chromium sulfide bromide (CrSBr), these devices exploit magnetic fields to dynamically reconfigure how light flows, a long-sought milestone in nanophotonics. The findings, published in Nature Photonics, mark a breakthrough in both material science and optics that could reshape future quantum technologies.
Why Traditional Nanophotonics Hit a Wall
For decades, silicon, titanium dioxide, and similar materials have formed the foundation of nanophotonic devices. These platforms can bend, confine, and guide light, but they suffer two big limitations. Their refractive indices are modest, restricting how tightly light can be confined. And once fabricated, their optical behavior is fixed, offering little room for reconfiguration.
“Tunability is essential for many next-gen photonics applications, enabling adaptive imaging, precision sensing, reconfigurable light sources, and trainable optical neural networks,” said MIT postdoc Sachin Vaidya.
In other words, the optical devices that power emerging technologies remain static in a world that demands flexibility. That is where CrSBr enters the story.
CrSBr: The Material That Bends the Rules
Chromium sulfide bromide is no ordinary crystal. It is a van der Waals antiferromagnet, meaning its atomic spins align in alternating directions, and it hosts powerful optical excitations known as excitons. These electron-hole pairs form when light excites the material, and in CrSBr, they are unusually strong and highly sensitive to magnetic fields.
This unique combination grants CrSBr an exceptionally large refractive index, allowing researchers to build photonic structures that are up to an order of magnitude thinner than those made from silicon. Devices as thin as 6 nanometers—just seven layers of atoms—have now been fabricated at MIT’s MIT.nano facility.
By applying a modest magnetic field, the researchers demonstrated that they could reversibly switch the way light traveled through the structure. Instead of relying on moving parts or thermal control, the material itself responded instantly to the external field.
When Light and Matter Hybridize
The interaction between light and excitons in CrSBr is so intense that it creates polaritons, hybrid light-matter particles that blur the line between energy and matter. These polaritons open new regimes of photonic behavior, such as enhanced nonlinear effects and unusual forms of light transport. Unlike many other systems, CrSBr supports these polaritons intrinsically, without needing bulky external cavities.
- Optical structures as thin as λ/150 of the operating wavelength
- High-quality (Q) resonances exceeding 1,200
- Magnetically tunable modes across wide spectral ranges
- Potential switching speeds as fast as 30 gigahertz
The Cold Catch
There is one caveat: CrSBr’s tunable properties only emerge at cryogenic temperatures below 132 kelvins (-222 °F). Yet history offers a reason for optimism. Superconductors, once dismissed for the same reason, went on to transform medical imaging, power grids, and high-energy physics. MIT physicist Ahmet Kemal Demir suggests the payoff may also be worth it here. “CrSBr is so unique with respect to other common materials that even going down to cryogenic temperatures will be worth the trouble, hopefully,” he said.
The team is already exploring related materials that may operate at higher, more practical temperatures, potentially broadening the reach of this technology.
From Lab to Future Technologies
The implications of reprogrammable nanophotonics span diverse fields. Adaptive imaging could allow microscopes to dynamically change resolution. Quantum simulators might harness tunable polaritons for modeling complex systems. Optical neural networks could be trained on the fly, mimicking the plasticity of biological brains. And integrated photonic circuits, already central to data communications, could gain switchable layers of CrSBr for enhanced performance.
As Riccardo Comin, MIT’s Class of 1947 Career Development Associate Professor of Physics, put it, “The marriage of emerging quantum materials and established nanophotonics architectures will surely bring advances to both fields.”
A New Chapter in Light Control
By uniting the quirky magnetism of CrSBr with the precision of nanophotonics, MIT researchers have revealed a new way to sculpt light itself. It may not yet be ready for consumer gadgets, but in fields like quantum optics and next-generation computing, the stage is set for a material once hidden in obscurity to shine at the heart of future technologies.
Journal: Nature Photonics. DOI: 10.1038/s41566-025-01712-2
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