Crystal Could Improve Optical Communications
Michael D. Wheeler
ALBUQUERQUE, N.M. -- Researchers at Sandia National Laboratories and the Massachusetts Institute of Technology in Cambridge have fabricated a two-dimensional photonic crystal that controls light in three directions. The work raises the prospect of a single chip housing broadband optical communication, optical signal routing and switching.
Constructed of gallium arsenide, the crystal resembles cheesecloth. It effectively blocks most light waves; only those in a select band of wavelengths can navigate through it. By varying the spacing and size of the holes, the researchers can control which wavelengths of light may pass.
The biggest obstacle in designing the crystal was light leakage in the third dimension, because the nature of light is to propagate and scatter in all directions. To overcome this, lead researcher Shawn Y. Lin and colleagues Pierre R. Villeneuve and John D. Joannopoulos focused their early attempts on producing 2-D cylindrical posts that resembled chopsticks. However, fabricating long posts at dimensions as small as 100 to 200 nm proved a formidable task.
Lin and his team concluded that the solution lay in index guiding, similar to the way that the core of optical fiber guides light within the cladding. As part of its work on vertical-cavity surface-emitting lasers, the lab had developed a technique called wet oxidation that turns high-index layers into low-index ones, perfect for tailoring the crystal. "By combining a new theoretical concept with a new technology breakthrough, I then started to work on this 2-D photonic crystal slab structure," Lin said.
The researchers used direct electron-beam lithography to define the patterns on the structure and anisotropic etching to create the 2-D holes and posts. The structure controlled the light in one plane with photonic bandgaps and in the third direction with index guiding.
Now that the team has demonstrated the design, Lin is working on building the structure so that it will not crack and on testing it for potential commercial applications, including increasing the number of optical channels to satisfy the demands of the Internet or enabling more efficient optical signal routing and processing.
The crystal may help advance optical computers, but Lin noted challenges linked to integrating nonlinear optical material into the 2-D crystal.
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