Light usually tends to scatter in all directions, like splashes from a thrown stone. But it can be caught in an invisible cage where it swirls in place, losing almost no energy. Physicists have built just such a cage using the magnetic material CrSBr — a layered crystal that can be split into flakes just a few atoms thick.
On this flake, they carved a pattern of tiny ridges, creating a metasurface. It acts like a maze for light waves: certain oscillations of light get trapped and cannot escape. Such states are called "bound states in the continuum"—light remains confined within the structure even though free space surrounds it. In ordinary materials, this is rare, but here the trap proved especially robust: the quality factor (a measure of how long light is held) exceeded 1500.
The big surprise: the trap works over a wide range of incidence angles. Usually, such states exist only at a strictly defined angle, but here there's a "flat band"—light remains trapped even when directed at different tilts. This resembles a pool with a flat bottom: wherever you jump, the depth is the same.
When the scientists changed the polarization of the light (the direction of wave oscillation), the trap disappeared, but something else emerged: surface hyperbolic polaritons. Light began to propagate along the surface in a special way, like ripples running across water in only one direction. This opens the way to controlling light at the nanoscale — for example, for creating ultra-compact optical chips.
Thus, a thin magnetic film became an arena where light can be caught, held, and directed at will. Perhaps such materials will form the basis of future quantum computers, where information is transmitted not by electrons but by particles of light.
CrSBr is a layered magnet: it can be peeled into films just one atom thick using ordinary Scotch tape, much like graphene from graphite.
In the series "The Expanse," an alien protomolecule creates structures that manipulate light at the nanoscale — somewhat similar to the described metasurfaces, but with fantastical complexity.