Photonic crystals influence light in the way that semiconductor materials affect electric currents. Typically made out of a regular array of cavities in some refractive medium, a photonic crystal reflects or transmits light depending on the light's wavelength and the interplay of all the tiny wavelets scattered by the holes. In one respect, photonic crystals lag far behind their silicon-based cousins: it is difficult to modulate a photonic crystal's proper-ties--for example, switching one from reflecting to transmitting. Recently research groups have demonstrated a versatile way to make a class of materials consisting of a polymer interspersed with liquid-crystal "droplets" whose optical response can be controlled by applying a voltage.

The fabrication begins with a soup of monomer molecules and liquid-crystal molecules, all sandwiched between two sheets of a substrate, such as glass plated with a thin layer of conducting material. The solution is irradiated with two or more laser beams, which are aligned and polarized to generate a specific interference pattern--the alternating dark and light areas that occur when laser beams overlap. (This is the hologram of the technique.) At the bright points in the pattern, the monomers link up and form a complex network of polymer. As this reaction proceeds, fresh monomers diffuse from the dark regions to the bright regions, causing the liquid crystal to accumulate in the dark regions. The end result is a solid polymer with droplets of liquid crystal embedded in a pattern corresponding to the dark regions of the holographic interference pattern.

This article was originally published with the title Holographic Control.

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