Scientists in Japan have demonstrated a new device that could lead to the development of all-optical circuits. This verifies predictions that photons can be manipulated inside photonic crystals (see May p9) by introducing particular defects in the photon bandgap structure.

In a photonic crystal, the dielectric constant of the material varies periodically, so certain frequencies of electromagnetic radiation cannot propagate. Making a strip of defects in the crystal forms a path for "forbidden-frequency" waves, and the crystal becomes a waveguide.

On a thin slab of just such a waveguide (a 0.25 mm InGaAsP layer on an InP substrate), the researchers introduced another crystal defect close to the forbidden-frequency path. Usually photons cannot escape the two-dimensional waveguide because the refractive index of the air above and below the plane is very different to the refractive index of the material. However, the extra defect acts as a coupler from the in-plane to the vertical direction - photons propagating through the waveguide are trapped by the defect, which then efficiently emits them into free space. The defect acts as an optical resonator, with a resonant frequency determined by the size of the defect and the lattice constant of the photonic crystal.

The researchers expect that, thanks to its small size and flexibility, their new device will have an enormous impact on global communications networks.