In a thin layer of tin disulfide, a material just one atom thick, light creates pairs of electrons and "holes" (places where an electron is missing). These pairs are bound together like two dance partners and are called excitons. In ordinary two-dimensional crystals, such dancers move along symmetric paths, but here the paths are curved in a special way: the lowest point for the electron and the highest for the hole are not at the center but at the edge of the band, where the shape resembles a saddle.
Using complex calculations, scientists showed that light with a specific direction of oscillation (polarization) can choose which of the three "valleys"—special points at the band edge — the exciton will be born in. When light is linearly polarized, the symmetry among the three valleys is broken, and three independent states emerge, like three different melodies that can be played on a single instrument.
This sensitivity to light polarization is reminiscent of tuning a radio: by turning the dial, you can pick up different stations. In the future, this could allow information to be written into exciton states, using light as a pen and the material as paper.
A SnS2 monolayer is just three layers of atoms, sulfur-tin-sulfur, but its optical properties are richer than those of many thick crystals.