Nov. 13, 1997 In laboratories at Columbia University, scientists are bonding light and electricity.
They have taken the first important step toward creating a microchip that combines electronics and its optical equivalent, photonics. The technology could simplify fiber optic communications and lead to the development of such miniaturized optical devices as tiny lasers and implantable medical sensors.
The Columbia scientists, working with colleagues from the State University of New York at Albany, have bonded an ultra-thin sheet of magnetic garnet, a photonic material that transmits light in only one direction, to a semiconductor, a component of microelectric circuitry.
"Ultimately, manufacturers will be able to combine optical and electronic capacity on the same silicon crystals, which are superior electronics platforms," said Richard M. Osgood, Higgins Professor of Electrical Engineering and professor of applied physics and co-author of the research.
A crucial step was slicing an ultra-thin sheet - 9 microns, or millionths of a meter, thick - from the magnetic garnet crystal, the subject of a scientific paper published in the Nov. 3 issue of Applied Physics Letters. Columbia has applied for a patent on the new technology.
Professor Osgood and the co-inventor of the new technology, Miguel Levy, senior research scientist at Columbia, have already begun to receive requests for single-crystal magnetic garnet films from other laboratories around the world, for such diverse research applications as microwave electronics and optical isolators. Columbia is the only institution that can produce the thin films.
The work took place at Columbia's Microelectronics Sciences Laboratory and at the Columbia Radiation Laboratory, both in the Fu Foundation School of Engineering and Applied Science. The research group included two materials scientists at the State University of New York at Albany, Hassaram Bahkru and Atul Kumar, who assisted in processing the garnet used in the experiments at SUNY Albany's ion accelerator.
"I'm excited that this technology can be used to build a whole new range of miniaturized systems, from medical sensors to ultra-small, powerful laser systems," Professor Osgood said.
Miniaturized optical processors for fiber optic telecommunications are also possible. Currently, optic messages travel by laser light to an isolator that prevents destabilization of the laser by outside interference, then to a modulator that imprints a signal, then to a multiplexer that combines signals of different wavelengths, each of which can carry a different message. A similar system is required at the receiving end to decode the light message into sound or picture.
"Right now, these are all very bulky devices," Dr. Levy said. "If you could put all these optic circuits on a chip, it would be cheaper, more efficient and sturdier, and there has been a lot of research geared towards integrating these components. Our work is an important step in this direction."
Such integration between photonics and electronics had not been possible because garnet and other magnetic crystals cannot be grown on a semiconductor substrate. Magnetic isolators cannot be made efficiently on any material other than magnetic garnets. Thus the need to place garnet crystals on semiconductors, providing a bridge to an already mature technology, the researchers said.
The Columbia research team fired high-energy beams of helium ions at a planar region that is just below the surface of the crystalline material, yttrium iron garnet (YIG), to loosen it from its substrate, gadolinium gallium garnet. They then applied chemicals to the region to cut the bonds entirely, slicing off an ultra-thin sheet of magnetic material from a single crystal. The sample was then lifted off and bonded to a high-quality semiconductor.
The goal of this effort is to make devices that allow light to go in only one direction on a fiber optic microchip, Professor Osgood said. Light guides etched into the magnetic crystal, when exposed to a magnetic field, allow the light to travel in one direction only, making the light guide an effective routing device in an optic fiber network.
The work is the result of a collaboration between Columbia and the University of Minnesota to create integrated photonic devices for use in fiber optic communications systems. The collaboration is funded by the federal Advanced Research Projects Agency.
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