A class of molecules whose size, structure and chemical composition have been optimized for photonic use could provide the demanding combination of properties needed to serve as the foundation for low-power, high-speed, all-optical signal processing.

All-optical processing could allow dramatic speed increases in telecommunications by eliminating the need to convert photonic signals to electronic signals – and back – for switching. All-optical processing could also facilitate photonic computers with similar speed advances.

Details of these materials – and the design approach behind them – were reported in the March 19, 2010, issue of the journal Science. The research was funded by the National Science Foundation (NSF), the Defense Advanced Research Projects Agency (DARPA) and the Office of Naval Research (ONR).

“This work provides proof that at least from a molecular point of view we can identify and produce materials that have the right properties for all-optical processing,” said Seth Marder, a professor in the Georgia Tech School of Chemistry and Biochemistry and co-author of the paper. “This opens the door for looking at this issue in an entirely different way.”

The Materials

The polymethine organic dye materials developed by the Georgia Tech team combine large nonlinear properties, low nonlinear optical losses, and low linear losses. Materials with these properties are essential for optical engineers to develop a new generation of devices for low-power and high-contrast optical switching of signals at telecommunications wavelengths. Keeping data all-optical would greatly facilitate the rapid transmission of detailed medical images, development of new telepresence applications, high-speed image recognition – and even the fast download of high-definition movies.

But the favorable optical properties of these new materials have only been demonstrated in solution. For their materials to have practical value, the researchers will have to incorporate them in a solid phase for use in optical waveguides – and address a long list of other challenges.

“We have developed high-performing molecules by starting with highly polarizable molecules and getting the combination of needed molecular properties right,” said co-author Joseph Perry, a professor in the Georgia Tech School of Chemistry and Biochemistry. “Now we have to figure out how to pack them together so they have a high concentration and useful physical forms that would be stable under operation.”

Marder, Perry and collaborators in Georgia Tech’s Center for Organic Photonics and Electronics have been working on the molecules for several years, refining their properties and adding atoms to maximize their length without inducing symmetry breaking, a phenomenon in which the molecules become less polarizable and the nonlinear properties are reduced. This molecular design effort, which builds on earlier research with smaller molecules, included both experimental work and theoretical studies done in collaboration with Jean-Luc Brédas, also a professor in the School of Chemistry and Biochemistry.

Georgia Tech professor Seth Marder (center) is part of the team that developed a new photonic material that could facilitate all-optical signal processing. (Photo by Rob Felt, Georgia Tech)

Design Strategies

The design strategies identified by the research team – which also included Joel Hales, Jonathan Matichak, Stephen Barlow, Shino Ohira and Kada Yesudas – could be applied to development of even more active molecules, though Marder believes the existing materials could be modified to meet the needs of all-optical processing

“For this class of molecules, we can with a high degree of reliability predict where the molecules will have both large optical nonlinearities and low two-photon absorption,” said Marder. “Not only can we predict that, but using well-established chemical principles, we can tune the structure of the molecules to have the properties optimized so that people can work at telecommunications wavelengths.”

Switching of optical signals carried in telecommunications networks currently requires conversion to electrical signals, which must be switched and then converted back to optical format. Existing electro-optical technology may ultimately be able to provide transmission speeds of up to 100 gigabits-per-second. However, all-optical processing could theoretically transmit data at speeds as high as 2,000 gigabits-per-second, allowing download of high-definition movies in minutes rather than hours.

Researcher Joe Perry also was part of the Georgia Tech photonic research team. Its findings already have been published.

“While we have not made all-optical switches, what we have done is provide a fundamental understanding of what the systems are that could have the combined set of properties that would make this possible,” Marder said. “Conceptually, we have probably made it over the hump with this class of molecules. The next part of this work will be difficult, but it will not require a fundamental new understanding of the molecular structure.”

(Editor’s note: This article, which first appeared in Georgia Tech’s Spring 2010 Research Horizons magazine, is based on work supported in part by the STC program of the National Science Foundation under agreement DMR-0120967, the DARPA MORPH Program and ONR (N00014-04-0095 and N00014-06-1-0897) and the DARPA ZOE Program (W31P4Q-09-1-0012). The comments and opinions expressed are those of the researchers and do not necessarily represent the views of the NSF, DARPA or ONR. For more information, contact Jean-Luc Bredas at 404/385-4986 or jean-luc.bredas@chemistry.gatech.edu; Seth Marder 404/385-6048 or seth.marder@chemistry.gatech.edu; or Joseph Perry at 404/385-6046 or joe.perry@chemistry.gatech.edu.)

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