INTEGRATED circuits that process optical signals are just too large. Now a team at the Massachusetts Institute of Technology have worked out how to miniaturise them.

The team has been working with photonic crystals that work by trapping light in tiny three-dimensional channels carved into their crystal structure. These channels are roughly the same size as the wavelength of light they are designed to carry. But to process signals as well as carry them, the circuits must contain active devices such as lasers and switches that can alter optical signals, and these have proved difficult to create on this scale.

Active devices must be constructed from tiny point defects in the structure of the photonic crystal that resonate at the required frequency of light. But positioning them in a controlled, repeatable way is tricky.

Minghao Qi and his colleagues at MIT got over the problem by using an electron beam to punch defects in the right places in the crystal structure. The team reports that the defects resonate at the infrared wavelengths used in telecommunications fibre-optic networks, opening the possibility that these optical circuits could soon find their way into real devices (Nature, vol 429, p 538).

So far Qi has used the electron beam technique to make tiny samples but it is too slow to make entire devices. The team has begun work on a faster technique and believes it should be possible to create functional devices with it in the near future.