Photonic crystals are periodic nanostructures that selectively direct wavelengths of light, similar to semiconductors. Natural materials like gem opal and butterfly wings include photonic crystals. Photonic crystal research has numerous applications in optics and electronics, but the challenge is producing the band gap effect with precision. Mass production may become possible with molecular nanotechnology.
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 pass certain bands of electronic energy. The term “bandgap” simply refers to the gaps in the spectral band of light that shines through. A rainbow, for example, has no band gap, because water is transparent and doesn’t absorb any specific frequencies. A rainbow passing through a photonic crystal would have selective gaps depending on the particular nanostructure within the crystal.
There are a couple of natural materials that approximate the structure of a photonic crystal. One of them is gem opal. Its rainbow iridescence is caused by periodic nanostructures within. The periodicity of the nanostructure determines which wavelengths of light are allowed and which are not. The period of the structure must be half the wavelength of the light being let through. The wavelengths allowed to pass are known as “modes” while the wavelengths forbidden are the photon bands. An opal is not a true photonic crystal because it lacks a full band gap, but it comes close enough to one for the purposes of this article.
Another natural material that includes a photonic crystal are the wings of some butterflies such as the genus Morpho. These give rise to beautiful iridescent blue 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 object 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 matches the photon band gap.
One 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 directions simultaneously. It was quickly realized that such materials would have numerous applications in optics and electronics, such as LEDs, optical fibers, nanoscopic lasers, ultrawhite pigments, radio antennas and reflectors, and even optical computers. Photonic crystal research is ongoing.
One of the major challenges in photonic crystal research is the small size and precision required to produce the band gap effect. Synthesizing crystals with vintage nanostructures is quite difficult with current manufacturing technologies such as photolithography. The 3D photonic crystals were designed but only manufactured on an extremely limited scale. Perhaps with the advent of bottom-up manufacturing, or molecular nanotechnology, mass production of these crystals will become possible.
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