"These novel fiber structures offer a unique possibility for constructing an optoelectronic functional fabric because the fibers are both flexible and mechanically tough, and can thus be woven," write the researchers in the October 14 issue of Nature. "Interesting device applications follow not only from the ability to engineer the single-fiber properties, but also from the specifics of fiber arrangements into larger assemblies."
The team's leader, Yoel Fink, notes that "the technique we developed allows us to bring together two disparate technologies: those involved in creating optical fibers and those for electronic components." The work "challenges the traditional barrier between semiconductor devices and fiber-optic processing," said Fink, the Thomas B. King Assistant Professor of Materials Science and Engineering.
The result? The team can create devices which marry the ease of fabrication, length and flexibility associated with optical fibers with the many integrated functions associated with semiconductor devices.
"Being able, for the first time, to precisely control the behavior of electrons, photons and their interactions within a fiber framework leads naturally to the exciting possibility of eventually creating intrinsically smart fabrics," said co-author John D. Joannopoulos, the Francis Wright Davis Professor of Physics.
Already the team has created two different prototype fibers with the new technology. The first is a fiber that simultaneously conducts two types of information carriers: electrons and photons. The photons are guided in a hollow core lined by a highly confining reflective surface dubbed "the perfect mirror" when Fink invented it in 1998 as an MIT graduate student. The electrons are conducted through metal microwires that surround the fiber core. The photons and electrons do not interact as they are confined to different spatial locations within the fiber.
A second fiber utilizes an interaction between photons and electrons. This fiber photodetector was designed to be sensitive to external illumination at specific wavelengths of light. It is made of a cylindrical semiconductor core contacted by four metal microwires that are surrounded by an optical cavity structure. The electrical conductance of this fiber was found to increase dramatically upon illumination with light at the wavelength it was designed to detect.
Some of the most exciting and novel potential applications stem from assembling the fibers into woven structures. As the authors point out, "Öit is the assembly of such fibers into 2-D grids or webs that enables the identification of the location of an illumination point on a surface," and does so with a very small number of fibers.
Embedding these grids in computer screens or onto projection boards could therefore provide a new type of interface, said Fink. "Instead of having a mechanical mouse, you could just use a light beam, like a laser pointer, to communicate with the computer because the screen would know where it was being hit."
The researchers included as supplementary material to their publication (archived on Nature's website) a short video demonstrating a novel optical system developed using the fiber photodetector. Go to http://www.nature.com/nature/journal/v431/n7010/suppinfo/nature02937.html to view the video.
The paper's lead author is Mehmet Bayindir, a postdoctoral associate in the Research Laboratory for Electronics (RLE). Additional authors are RLE Postdoctoral Associate Ayman Abouraddy, Graduate Students Fabien Sorin and Jeff Viens of the Department of Materials Science and Engineering, and Shandon Hart (MIT Ph.D. 2004, now at 3M). All of the authors are also affiliated with the Center for Materials Science and Engineering and RLE.
HOW THEY DID IT
"Just as in the movie 'Honey, I Shrunk the Kids,' wouldn't it be wonderful if you could fabricate something on the macroscale, then shrink it to a microscopic size?" said Fink. "That's what we did. But the magic shrink-down apparatus we used is not a 'shrinking beam' from science fiction ... it's a furnace."
The team first created a macroscopic cylinder, or preform, some 20 centimeters long by 35 millimeters in diameter containing a low-melting-temperature conductor, an amorphous semiconductor, and a high-glass transition thermoplastic insulator. The preform shares the final geometry of the fiber, but lacks functionality due to the absence of intimate contact between its constituents and proper element dimensions.
The preform is subsequently fed into a tube furnace where it is heated and drawn into a fiber that does exhibit both electrical and optical functionalities. "These follow from the excellent contact, appropriate element dimensions, and the fact that the resulting fiber retains the same structure as the macroscopic preform cylinder throughout the drawing process," said Bayindir, who designed and synthesized the semiconducting glasses, assembled the preforms and drew the fibers presented in the paper.
The authors conclude that their new ability to interface materials with widely disparate electrical and optical properties in a fiber, achieve submicrometer features, and realize arbitrary geometries over extended fiber lengths presents "the opportunity to deliver novel semiconductor device functionalities at fiber-optic length scales and cost."
This work is funded by the Defense Advanced Research Projects Agency, the Army Research Office, the Office of Naval Research, the Air Force Office of Scientific Research, the Department of Energy, MIT's Institute for Soldier Nanotechnology, and the Materials Research Science and Engineering Center (MRSEC) program of the National Science Foundation.
The authors would like to express their deep gratitude to Esra Bayindir, C. Bruce, A. McGurl, J. Connolly, B. Smith, M. Young and the entire staff of MIT's Research Laboratory for Electronics and Center for Materials Science and Engineering (part of the NSF's MRSEC program).
For more information go to the Fink group's website at http://mit-pbg.mit.edu/.
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