U of T research holds promise for optical chip

April 28, 2003

University of Toronto researchers have developed a hybrid plastic that can produce light at wavelengths used for fibre-optic communication, paving the way for an optical computer chip.

The material, developed by a joint team of engineers and chemists, is a plastic embedded with quantum dots - crystals just five billionths of a metre in size - that convert electrons into photons. The findings hold promise for directly linking high-speed computers with networks that transmit information using light - the largest capacity carrier of information available.

"While others have worked in quantum dots before," says investigator Ted Sargent, a professor in the Edward S. Rogers Sr. Department of Electrical and Computer Engineering, "we have shown how quantum dots can be tuned and incorporated into the right materials to address the whole set of communication wavelengths.

"Our study is the first to demonstrate experimentally that we can convert electrical current into light using a particularly promising class of nanocrystals," says Sargent, who holds the Nortel Networks-Canada Research Chair in Emerging Technologies. The study appears in the April 28 issue of the journal Applied Physics Letters.

"Our research is based on nanotechnology: engineering based on the length of a nanometer - one billionth of a metre," he says. "We are building custom materials from the ground up." Working with colleagues in Professor Gregory Scholes' group from U of T's Department of Chemistry, the team created nanocrystals of lead sulphide using a cost-effective technique that allowed them to work at room pressure and at temperatures of less than 150 degrees Celsius. Traditionally, creating the crystals used in generating light for fibre-optic communications means working in a vacuum at temperatures approaching 600 to 800 degrees Celsius.

Despite the precise way in which quantum dot nanocrystals are created, the surfaces of the crystals are unstable, Scholes explains. To stabilize them, the team placed a special layer of molecules around the nanocrystals. These crystals were combined with a semiconducting polymer material to create a thin, smooth film of the hybrid polymer.

Sargent explains that when electrons cross the conductive polymer, they encounter what are essentially "canyons," with a quantum dot located at the bottom. Electrons must fall over the edge of the "canyon" and reach the bottom before producing light. The team tailored the stabilizing molecules so they would hold special electrical properties, ensuring a flow of electrons into the light-producing "canyons."

The colours of light the researchers generated, ranging from 1.3 microns to 1.6 microns in wavelength, spanned the full range of colours used to communicate information using light.

"Our work represents a step towards the integration of many fibre-optic communications devices on one chip," says Sargent. "We've shown that our hybrid plastic can convert electric current into light, with promising efficiency and with a defined path towards further improvement. With this light source combined with fast electronic transistors, light modulators, light guides and detectors, the optical chip is in view."

The research team included Ludmila Bakueva, Sergei Musikhin, Margaret Hines, Tung-Wah Frederick Chang and Marian Tzolov from the departments of chemistry and electrical and computer engineering. The research was supported by Nortel Networks, the Natural Sciences and Engineering Research Council of Canada, Materials and Manufacturing Ontario, the Canada Foundation for Innovation, the Ontario Innovation Trust and the Canada Research Chairs Program.

Ted Sargent
Edward S. Rogers Department of Electrical and Computer Engineering

Nicolle Wahl
U of T Public Affairs

University of Toronto

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