Credit: A. BLANCO ET AL.

Photons are the best messengers for really quick information transmission. But creating circuitry that can process light with the versatility of electronic silicon chips is a big technological challenge. The most promising approach relies on fabricated structures known as ‘photonic crystals’. These materials have carefully tailored properties that make them opaque to selected wavelengths — providing a means to control the routing and transmission of optical signals.

Elsewhere in this issue, Alvaro Blanco and colleagues ( Nature 405, 437–440; 2000) describe a simple and cheap method for the large-scale manufacture of three-dimensional silicon photonic crystals. The picture here shows a computer representation of the structures, superimposed on a real image from a scanning electron microscope. Because the crystals are made of silicon, they should be easy to integrate with conventional electronic circuitry.

How is this complex structure produced? The first step is to construct a three-dimensional template that will ultimately be discarded. It consists of a form of artificial opal: uniformly sized silica spheres, up to a micrometre in diameter, grown from seed particles. Application of heat causes small bridges to develop between the spheres, resulting in a connected structure. Next, gases containing silicon are allowed to infiltrate the template. The voids in between the spheres are gradually filled up as the silicon crystallizes. The final step is to dissolve away the silica sphere-and-bridge template using an acid etching process.

Theoretical calculations predict that the resulting structure should block wavelengths around 1.5 μm — perfect for manipulating the infrared signals used in fibre-optic communications. The measured reflectance and transmission spectra of the photonic crystals confirmed this behaviour. Equally importantly, the crystals are unlikely to absorb valuable signal power, as bulk silicon only soaks up higher energies.