Light passing through a window pane refracts the same in all directions. But in thin layered crystals, it's a different story: light traveling along the layers sees the material differently than light traveling across them. It's as if a sheet of paper were transparent but transmitted light differently along and across its fibers.

This behavior is called optical anisotropy, and it is especially pronounced in two-dimensional materials — crystals just a few atomic layers thick. Within a layer, atoms are held together by strong covalent bonds, like tightly interlocked planks, while between layers, weak van der Waals forces act, like sheets of paper stacked on top of each other. Because of this difference, light 'feels' the structure differently depending on direction.

The scientists measured the refractive indices for two such materials: γ-InSe (indium selenide) and ε-GaSe (gallium selenide). They found that both materials exhibit moderate optical anisotropy, and their special excited states, excitons oriented across the layers, noticeably affect light refraction in that direction.

Knowing the precise refractive index values allows accurate modeling of light behavior in such materials. This paves the way for creating ultra-compact optical devices: polarizers, polarization sensors, and sources of polarized light. Such devices could be thinner than a human hair and find applications in telecommunications, displays, and sensors.