When light passes through a turbid medium, like a glass of milk or frosted glass, it scatters in all directions. But sometimes, instead of spreading out, it gets trapped in tiny 'pockets' inside the material. Physicists call these pockets localized modes: light falls into them as if into a trap and cannot escape.
Until now, it was difficult to measure how small such a pocket is. The authors of the new study came up with a way: they shape the incoming light so that it perfectly matches the pocket's shape. It's like finding the exact key to a lock — light enters without loss and fills the trap completely. This precise matching allowed them to directly measure the size of the smallest trap for the first time.
The scientists tested the method on two artificial lattices with holes. They found that if the holes are made larger, the pocket shrinks: light gets stuck in an even tighter spot. This discovery could help create random lasers and improve nonlinear optics.
Light localization is a relative of the phenomenon that won the Nobel Prize in 1958: electrons can also get stuck in random 'pits' inside disordered materials.