Soft spheres yield photonic structures.

A novel method for creating light-manipulating patterns inside photonic crystals-materials that transmit and reflect specific wavelengths of light--could hasten the arrival of a new generation of faster, all-optical telecommunications technologies.

Instead of building photonie crystals out of hard materials such as silica, Andrew Lyon and his colleagues at Georgia Institute of Technology in Atlanta used hydrogel nanoparticles--water-saturated polymer spheres that swell and contract in response to temperature. Roughly the size of large viruses, the spheres measure, on average, 224 nanometers in diameter. The researchers mixed the spheres with even smaller, gold nanopartieles. A heating-and-cooling treatment prompted the spheres to self-assemble into crystal structures with the gold particles between them.

To create light-guiding conduits in the structures, the researchers zap specific portions of the nanoassemblies with a laser. At those places, the gold nanoparticles absorb light and heat up. If this process raises the temperature of the surrounding hydrogel spheres above 31[degrees]C, the spheres expel water and contract. A rapid cooling traps the spheres in the laser-zapped regions into disordered and optically transparent states.

With this technique, researchers could use a laser to draw patterns in the crystal, creating channels that guide light through the material, perhaps even around sharp corners. What's more, Lyon says, the process is reversible until a chemical step locks in a final structure. This means that the researchers can overwrite one light-guiding pathway with a new one.--A.G.