Tunable Photonic Crystals

Photonic crystals affect the flow of photons in much the same way that electronic devices affect the flow of electrons. Most photonic crystals, however, have specific properties that cannot be varied once the crystals are made. A few types of photonic crystals, such as fluid suspensions of colloidal silica, can be modified on the fly, but the time required to change configurations is inconveniently long. Researchers at Brown University have now made photonic crystals that can be modified in milliseconds. The tunable photonic crystals consist of a class of materials known as holographic-polymer dispersed liquid crystals (H-PDLCs). Complex structures are defined in the material by exposing it to an interference pattern produced by a set of four laser beams. Liquid crystal droplets form in regions where the laser light interferes coherently; these droplets constitute a photonic crystal. An electric field applied to the suspension of liquid crystals modifies the refraction index of the droplets, which changes the spectrum of light that the photonic crystals transmits. The new photonic crystals are easily constructed on a wide range of scales, which allows them to affect a wide spectrum of light, and can replicate sophisticated structures including diamond lattices as well as anisotropic lattices that affect light differently depending on the direction of propagation through the crystal. Potential applications of the tunable photonic crystals include filters to selectively block certain light frequencies. With further improvement, they may also lead other optical devices such as to novel lasers and optical waveguides. Jun Qi of Brown University (401-863-3078) described the tunable photonic crystals in a paper he presented recently at the Optical Fiber and Communication Conference in Atlanta.