A crystal of germanium with tin is like a mosaic where each tile is an atom. If you lay the tiles randomly, the mosaic has one color, but if you arrange them in a pattern, it has another. Physicists measured how the atomic pattern changes a semiconductor's most important property: its band gap, that is, the energy needed to get an electron moving and creating current.

Until now, this step was adjusted in two ways: by changing the germanium-to-tin ratio or by stretching the crystal. The authors added a third way: they heated the crystal after growth, and the atoms rearranged themselves into a more ordered pattern. Using X-ray radiation and machine learning, they calculated how strongly the order affects the glow.

It turned out that ordering shifts the color of the glow — the crystal starts emitting light with a different energy. It's as if the same mosaic began to glow green or yellow depending on how the tiles are arranged. Now engineers have a new tool for fine-tuning semiconductor lasers and LEDs.