In a photonic crystal the periodic variation of the dielectric constant results in a range of 'forbidden' frequencies called a photonic bandgap (PBG). Electromagnetic waves with a 'forbidden' frequency cannot propagate through the crystal. This phenomenon can be exploited to create a waveguide. Adding a strip of linear defects to the PBG material destroys the bandgap and creates a 'path' - or waveguide - for the light through the PBG material.

Noda and co-workers started with a virtually two-dimensional waveguide. Light cannot usually escape into the air above or below the waveguide because air has a very different refractive index to the PBG material. The team introduced a single defect into the PBG material close to the waveguide. They found that the defect behaved as an optical resonator with a resonant frequency that depends on its size. Photons travelling through the waveguide with a frequency that matches that resonant frequency are 'trapped' by the defect and re-radiated perpendicular to the surface of the waveguide. "Nobody noticed before that the single defect can act as a coupler from the in-plane to the vertical direction", Noda told PhysicsWeb. This unique geometry is critical: it will enable many such devices to work together in a very compact device. The method also transmits the light with a very high efficiency.

Adding a second defect with a different resonant frequency enabled the device to select two different wavelengths of light. "By changing the defect size, we can easily tune the wavelength of the photons to be trapped and emitted" explains Noda. The team believes that this flexibility - combined with the geometry, efficiency and very small size of the new device - could have an enormous impact in global communications networks.