Photonic crystal

A photonic crystal is a material with a very regular pattern of tiny structures, for example rows of minute holes in a sheet of silicon. These structures are so small, about the size of the wavelength of light, that they determine how light behaves in the material. You can think of it as a kind of traffic control for light.

How does it work?

Where light passes from silicon to air, a little bounces back, like seeing your own reflection in a shop window. One hole does little. If the holes are always exactly the same distance apart, they all bounce back their bit of light in the same rhythm and the reflections add up. The spacing between the holes thus determines which colours of light can and cannot pass through, and how fast they advance.

A well-known example from nature is the shimmer of some butterfly wings and opal: that colour does not come from paint, but from such a regular pattern.

Why it matters

In chips for optical-fibre connections, photonic crystals are used to slow down light. Japanese researchers built a component that converts electrical signals into light. It has a pattern of minute holes in which the light travels more slowly, is 0.3 millimetres long, works at low voltage and transmits up to 200 gigabits per second (link). In the crystal, the light advances five to fifteen times more slowly, so a short switch needs only a little voltage. The next step is to lose less light along the way. That is useful in data centres where AI chips constantly exchange data: smaller and more economical components mean more light connections on one chip.