Slow light

Light is extremely fast in air, but in a specially made structure you can slow it down considerably. That is called slow light. The light does not change colour in the process: it just takes longer to cover a given stretch.

How does it work?

A common way is a regular pattern of tiny holes in silicon. At each hole, a little light bounces back. As a result, the light keeps going a bit back and forth, and on balance advances much more slowly. In a recent Japanese study, the light went five to fifteen times slower this way than in an ordinary silicon light guide.

Compare it to walking through a rain shower: someone who walks slowly gets much wetter over the same distance than someone who runs. Light that goes slowly stays longer in the section where you want to influence it.

Why it matters

That is handy for components that have to steer light with an electrical voltage. The longer the light stays in such a component, the more effect a small voltage has, and so the component can be shorter and more economical. Japanese researchers built just such a component, which converts electrical signals into light and is only 0.3 millimetres long, six to twelve times shorter than the standard one, yet sends up to 200 gigabits per second (link). In data centres where thousands of AI chips exchange data, that means more light connections on one chip using less power. The team's next step is to lose less light along the way.