On the threshold of success

摘要

Tiny crystals that interact strongly with certain frequencies of light can be used to make semiconductor lasers. They can now be stimulated electrically, and thus be integrated with conventional electronic devices. Photonic crystals occur naturally as semiprecious opal and the microscopic structures on the wings of some tropical butterflies. They are repeating structures that inhibit the propagation of some frequencies of light. They often produce brilliant colour effects by selecting narrow parts of the spectrum of white light falling on them. If the selection effect is strong enough, the crystal can completely exclude photons of a particular wavelength, creating a three-dimensional 'photonic bandgap'. Any defect in the structure will bottle up the forbidden light—like a tiny optical cavity— by blocking the propagation of light in all directions. To be useful in optoelectronics, the light has to be allowed to escape from this cavity.