Photonic crystals: making a cage for light

Photonic crystals are optical materials with an intricate three-dimensional structure that manipulates light in unusual ways thanks to multiple Bragg diffraction. The structure has the length scale of the order of the wavelength of light. In future, such crystals could steer light beams around tiny optical chips or be the heart of a new breed of efficient light emitters. An example of a photonic crystal is the gem opal, which consists of a regular array of tiny silicate spheres, ordered like the atoms in a crystal lattice, but on a scale thousand times larger. If the structure has a large variation in refractive index a "photonic bandgap" occurs. Under these special circumstances, Bragg diffraction prevents a certain range of wavelengths from propagating in any direction inside the crystal. As an important quantumoptical consequence, spontaneous emission of excited atoms or molecules inside the crystal is completely inhibited. Moreover, controlled defects in a photonic crystal will result in localized states in the band gap, as if light is trapped in a cage (see Figure 1).