Speaking at the 223nd national meeting of the American Chemical Society, April 7-11 in Orlando, the researchers will present a discovery that may make faster, cheaper signal conversion possible.
The device used to convert electrical signals to optical signals is called an electro-optic (EO) modulator. Current EO modulators based on inorganic crystalline materials are expensive and are limited to modulation rates of about 10 GHz. EO modulators made with organic materials could be at least 10 times faster and much cheaper. "If EO modulators could be made cheaply enough to be used in the home or office, that could greatly speed up access to the Internet," says Richey M. Davis, associate professor of chemical engineering at Virginia Tech.
Davis and Virginia Tech faculty members Harry Gibson of chemistry, James Heflin of physics, and Kevin Van Cott of chemical engineering are using self assembly techniques to make nano-structured polymer films with selected dye molecules that are all oriented along one direction. When an electric field is applied to such a material, a thin film between one and 10 microns thick, its refractive index can change. The refractive index determines the speed with which light travels through the film. This tunable or switchable refractive index is the basis for changing an electrical signal to an optical signal.
James R. Heflin, associate professor of physics at Virginia Tech, explains that materials used for EO modulators have to have polar order -- the molecules' positively charged ends all have to point in the same direction. "In general, molecules prefer to be in random orientation or to point in the opposite direction of those they are next to."
The researchers are using ionic self assembly multilayer (ISAM) processing to grow films that have polar order built in, dictated by the ionic bonds that make the process possible. ISAM processing is the layering of materials one nanometer at a timebased on electrical attraction or electron sharing by atoms.
However, cancellation can still occur when polymers are used, says Heflin. That is, some molecules will reorient in the opposite direction when the next layer is added. That has been a challenge in the creation of EO modulators from polymers.
The Virginia Tech researchers have developed a technique that uses a monomer layer - a layer of single molecules. One end of the monomer attaches covalently into a layer and the other end attaches ionically into the next layer. "Now we definitely have a directional preference and our EO responses are becoming comparable to those of the best inorganic materials," says Heflin.
The poster, "Second harmonic generation studies of novel polar self-assembled multilayers" (Coll 180), will be presented Monday, April 8, beginning at 6 p.m. in the Rosen Plaza Grand Ballroom A/B, level one. Authors are physics graduate students Patrick J. Neyman and C. Brands, physics professor Heflin, chemical engineering graduate students Matthew T. Guzy and S.M. Shah, chemical engineering professors Davis and Van Cott, chemistry postdoctoral researcher H. Wang, and chemistry professor Gibson.
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