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Intelligent Machines

Guiding Light

Constructing optical waveguides in photonic crystals–cheap and easy.

To Paul Braun, the future of optical computing is crystal clear. Braun and his colleagues at the University of Illinois at Urbana-Champaign report that they’ve found a cheaper and simpler way to construct tiny optical “waveguides” inside photonic crystals. These waveguides have the potential to behave like the microscopic wires on a conventional microchip, except that they would transport photons rather than electrons around tangles of sub-micrometer-scale circuitry. And that could help make photonic crystals the basis for a new generation of far faster telecommunications and computing devices.

Photonic crystals are intricate microscopic structures pocked with regularly spaced holes, like an orderly Swiss cheese. The holes create a barrier against light of certain wavelengths, and in the right arrangement, they can force photons along prescribed paths. Unlike optical fibers, which leak light when bent too far, these waveguides can hurl photons around sharp corners, making them ideal components for optical switches, microlasers, light-emitting diodes, even all-optical integrated circuits.

While companies like Agilent Technologies and a number of academic and government labs are developing photonic crystals, creating pathways that snake through them with the required micrometer-level precision is a major challenge. Several research groups, including one at Sandia National Laboratories in Albuquerque, NM, have built and tested photonic-crystal waveguides on silicon wafers, but their fabrication technique is the same complex, repetitive and expensive lithographic process used to pattern today’s microchips. “It’s a wonderful technique-if you don’t care what it costs,” Braun says.

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Braun’s technique starts with tiny silica spheres that assemble themselves in solution into a three-dimensional, crystal-like structure. Braun’s real achievement was finding a way to create precisely shaped pathways through these crystals after they assemble: his group fills the space between the spheres with a photosensitive polymer, then uses a microscope to focus a laser on specific points, causing the polymer to harden. Drain the surrounding, unhardened polymer, and the result is a “defect” in the crystal: a perfectly sculpted pathway through the spheres, built with only one pass of the laser.

“A lot of people have been thinking about how to add defects to these self-assembling materials,” says David Norris, a photonics researcher in the Chemical Engineering and Materials Science department at the University of Minnesota. “Paul’s group has shown the first example of how that might be done.” While Braun says it could take three years or more for self-assembling photonic crystals to find their way into commercial devices, he expects to demonstrate a working prototype-perhaps made from a material such as silicon that transmits light more reliably than the polymer-within the next six months.

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