Photonic crystals, also known as photonic bandgap materials, are periodic nanostructures that can selectively direct wavelengths of light in much the same way as semiconductors on a computer chip selectively let through certain electronic energy bands. The term “bandgap” merely refers to gaps in the spectral band of light shining through. A rainbow, for instance, lacks band gaps, because water is transparent and does not absorb any specific frequency. A rainbow going through a photonic crystal would have selective gaps depending on the particular nanostructure within the crystal.
There are a couple of natural materials which approximate the structure of a photonic crystal. One of them is the gemstone opal. Its rainbow-like iridescence is caused by periodic nanostructures within. The periodicity of the nanostructure determines which wavelengths of light are permitted through and which aren’t. The period of the structure must be half the wavelength of the light that is allowed through. The wavelengths permitted passage are known as “modes” whereas the forbidden wavelengths are the photonic band gaps. An opal is not a true photonic crystal because it lacks a complete band gap, but it approximates one closely enough for the purposes of this article.
Another naturally occurring material that includes a photonic crystal is the wings of some butterflies such as the genus Morpho. These give rise to beautiful blue iridescent wings.
Photonic crystals were first studied by the famous British scientist Lord Raleigh in 1887. A synthetic one-dimensional photonic crystal called a Bragg mirror was the subject of his studies. Although the Bragg mirror itself is a two dimensional surface, it only produces the band gap effect in one dimension. These have been used to produce reflective coatings where the reflection band corresponds to the photonic band gap.
A hundred years later, in 1987, Eli Yablonovitch and Sajeev John suggested the possibility of two- or three-dimensional photonic crystals, which would produce band gaps in several different directions at once. It was quickly realized that such materials would have numerous applications in optics and electronics, such as LEDs, optical fiber, nanoscopic lasers, ultrawhite pigment, radio antennas and reflectors, and even optical computers. Research into photonic crystals is ongoing.
One of the largest challenges in photonic crystal research is the tiny size and precision required to produce the band gap effect. Synthesizing crystals with period nanostructures is quite difficult with present-day manufacturing technologies such as photolithography. 3-D photonic crystals have been designed but only fabricated on an extremely limited scale. Perhaps with the advent of bottom-up manufacturing, or molecular nanotechnology, will the mass-production of these crystals become possible.
