Shine light on an FePS3 crystal, and it responds with its own glow, like a neon sign, but much fainter and in the infrared range. This glow is not random: it depends on how the crystal is oriented. The scientists took samples from a thick chunk down to a single-atom layer and observed four emission bands — at 1.24 electronvolts, 1.79, 2.3, and 2.56. Each band responds differently to rotation, and this feature persists even in the thinnest layer.

The reason lies in geometry. In FePS3, each iron atom is surrounded by six sulfur atoms, but this "framework" is slightly skewed: the distances between neighboring iron atoms are not equal. As a result, the crystal conducts light differently along different axes, like a wooden board that bends more easily along the grain than across it. Computer calculations confirmed that this skew is exactly what explains why some emission bands are linearly polarized while others are circularly polarized.

This connection between the shape of the crystal lattice and its glow is a ready-made tool for engineers. If you need light with a specific polarization, just take FePS3 of the right thickness and rotate it to the correct angle. No complicated adjustments, just geometry.