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Crystal Simulation Shows Promise in High-Efficiency Light Bending

Photonics Spectra
Feb 1997
Kathleen G. Tatterson

CAMBRIDGE, Mass. -- A photonic crystal specially designed to bend light sharply with near-perfect transmission could be the key to further miniaturization of optical computer chips and lasers, according to the scientists at the Massachusetts Institute of Technology who are developing it.
The research team simulated a two-dimensional GaAs photonic crystal that acts as a waveguide, steering light around a slightly rounded corner at 100 percent transmission and around a sharp 90° bend at 98 percent efficiency. Within the same volume, the best yield the traditional means of controlling light, total internal reflection, has achieved is 30 percent transmission efficiency, said John Joannopoulos, Frank Wright Davis professor of physics at MIT.
The ability to make light bend in smaller spaces would allow engineers to place a greater number of bends in a circuit, shrinking processors and the instruments that use them.
MIT's photonic crystal is made up of a series of evenly spaced dielectric rods. The spacing and size of the rods determine the crystal's opacity. To create waveguides, researchers remove rows of rods -- in essence, carving a tunnel into the crystal.
"If you have a defect in the crystal, you can create states in the gap where photons can exist at certain frequencies," Joannopoulos said. Thus, when light hits a wall, photons are not lost in scattering.
In total internal reflection, photons are guided through a waveguide of highly dielectric material surrounded by a border material with a lower index of refraction. When making a turn, photons of infrared or visible light hit the wall and escape.

A matter of space
Although it is possible to achieve 100 percent transmission through 90° bends using the traditional technique, the radius of the curvature is much larger -- on the order of millimeters -- limiting its usefulness for microcircuit construction.
The research team hopes to demonstrate the three-dimensional photonic crystal within the next couple of years, if the crystal can be grown. Joannopoulos cited the lack of ability to fabricate on the nanometer scale as the major barrier to the realization of this technology.
"Things need to be dielectrically periodic on that scale using nanolithographic techniques," he said. "No one has been successful in doing that yet, but when they do, all hell will break loose in the industry.

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