Photoactive Molecules Enable Optical Logic

Hank Hogan

For high-speed telecommunications, electronics are a drag. The optoelectronic devices that control today's networks threaten to choke the terabit-per-second data rates that soon will be available. In hopes of avoiding such logjams, a research team at the University of Miami in Coral Gables, Fla., has demonstrated an all-optical technique that routes signals using solutions of photochromic molecules.

Researchers at the University of Miami have demonstrated optical logic gates based on photochromic molecules for switching applications. Stimulation by ultraviolet radiation transforms a solution of the molecules into a state that absorbs the 563-nm signal. Attaching three cells of the solution in series yields an NOR gate. Courtesy of Françisco Raymo. Reprinted with permission of Proceedings of the National Academy of Sciences, © 2002 National Academy of Sciences USA.

The molecules are derivatives of spiropyran, which is colorless in one state but which can be transformed into purple merocyanine or yellow-green protonated merocyanine. Specifically, spiropyran converts into merocyanine within picoseconds upon exposure to ultraviolet radiation and transforms back when exposed to visible light or when left in the dark. Both molecules change into protonated merocyanine in the presence of an acid; visible light or a base changes them back.

The researchers constructed routing elements using this light-controlled behavior. They sent a monochromatic 563-nm signal through a quartz cell filled with the photochromic molecules. At a right angle to the cell, they placed an ultraviolet source. With the molecule in the spiropyran state, the visible signal passed through the cell and to a detector. When the researchers turned on the ultraviolet source, however, the molecules transformed into merocyanine, which has an absorption peak at 563 nm, and the intensity of the signal at the detector dropped by a factor of 30.

From this binary switching element, the researchers built more complex structures, including NOT, NAND and NOR logic gates using up to three inputs.

Although the demonstration establishes the feasibility of molecular switching, more work remains to be done before such an approach finds application in an all-optical network, said Françisco Raymo, an assistant professor of chemistry at the university and head of the team. Current investigations seek to improve the speed and stability of the photochromic switches.

But Raymo said that other approaches also are being investigated. "We are now trying to reproduce the same switching operations with solid-state materials."