Fire opals may hold the key to photonic communications.
When I'm not playing or writing about electronic music, I pursue several hobbies, one of which is mineralogy. Among my favorite types of minerals are those that exhibit interesting optical properties, such as fire opals, which glint intense red, green, or blue depending on the angle at which light hits them.
That phenomenon, commonly called opalescence but known technically as Bragg reflection, offers promising potential in the field of photonics. The goal of photonics is to manipulate photons in much the same way that electronic components manipulate electrons. If the fledgling technology is successfully developed, it could increase digital communication and processing speeds by several orders of magnitude.
When light strikes a boundary between two transparent materials, part of it is reflected and part is transmitted through the boundary. If the transmitted part encounters another boundary after the first one, the same thing happens again. If the spacing between the boundaries is exactly half the wavelength of the incoming light, the part reflected from the second boundary constructively interferes with the part reflected from the first boundary, which intensifies the light that finally reaches your eyes (see Fig. 1). If there are many equally spaced boundaries, all the incident light at the critical wavelength is reflected; that is Bragg reflection.
Opals exhibit Bragg reflection because they consist of many tiny spheres of silicate glass. Those spheres are tightly packed, much like oranges in a crate, and they measure several hundred nanometers in diameter, which is on the same scale as visible-light wavelengths from 400 to 700 nm. As a result, when white light strikes an opal from certain directions, the gap between the spheres is exactly half the wavelength of certain components of the light, and the eye sees dazzling green, red, or blue reflected from the stone.
If a material could be made that exhibited Bragg reflection from all directions at a particular wavelength, no light at that wavelength could enter from the outside and any light at that wavelength originating within the material would be trapped by endless internal reflections. Then, by introducing imperfections in the crystal lattice, the flow of photons into and out of the material could be controlled, much like the way in which impurities in semiconductor material allow electron flow to be controlled.
Many scientists are trying to create such materials. For example, Willem Vos at the University of Amsterdam is experimenting with suspending tiny polystyrene spheres in a liquid and letting them naturally settle into a crystal. After drying the crystal, he fills the air between the spheres with a highly refractive material, such as gallium arsenide, and heats the crystal to evaporate the polystyrene, which leaves a latticework of gallium arsenide surrounding spheres of air — a sort of inverse opal.
The key to unlocking the technology's potential is placing a microscopic light source within an inverse opal. That would trap the light until it could be released in a controlled manner through imperfections in the lattice. It hasn't yet been accomplished, but according to one pioneer in the field, physicist Sajeev John, “We are tantalizingly close.”
If photonic circuitry can be developed to keep data in the form of light pulses rather than convert it from photons to electrons and back again, as it is in current communication systems, the pace of data handling will increase dramatically. The Internet will operate at World Wide Warp speed, and all sorts of processing, including musical applications, will benefit greatly. So the next time you admire an opal, remember how it might inspire the future of communications and computing.