Nanotubes May Make Good Switches

Gary Boas

Ultrafast all-optical switches likely will be integral to future time-division-multiplexing communications and free-space optical computing systems. The nonlinear optical properties of carbon nanotubes suggest that these structures may be useful for such photonic applications. A group at Rensselaer Polytechnic Institute in Troy, N.Y., is investigating the properties of composites of single-wall carbon nanotubes and polymers.

Researchers at Rensselaer Polytechnic Institute suggest that composites of single-wall carbon nanotubes and polymers, seen here under a scanning electron microscope, could find a place in ultrafast optical switching applications. Courtesy of Yuchuan Chen.

"We tried to determine the two main parameters characterizing the optical switching properties of [single-wall nanotubes]: the decay time and the modulation depth, which depends primarily on the third-order nonlinear polarizability," explained researcher Yuchuan Chen. The scientists employed a pump-and-probe technique in the tests, using an IMRA America Inc. FemtoLite laser to produce 1550- and 780-nm radiation with a pulse duration of approximately 150 fs.

The experiments demonstrated that the nanotubes' azimuthal symmetry enables delay times of less than 1 ps and transmission modulation ratios as high as 10²⁴ for a 20-µm-thick composite of single-wall nanotubes and polyimide pumped by 24-µJ/cm² pulses, recommending its use in ultrafast switches. In addition, Chen said, such composites could be easily integrated into fiber optic components.

The researchers conducted the experiments on other materials, including a pure polyimide thin film, graphite powder and a sheet of single-wall nanotubes. They observed almost no signal for the polyimide and graphite. The nanotube sheet behaved similarly to the composite.

Raise the threshold

There are potential applications for subpicosecond switches made from single-wall carbon nanotube/polymer composites in telecommunications and in optical computing. However, more research and development will be necessary before the group considers commercial deployment of the devices.

Specifically, the polyimide composite has a damage threshold of less than 30 µJ/cm², too fragile for use in current systems. "The low damage threshold must be overcome before [single-wall nanotube] composites can be made available," Chen said.

Nanotubes in other polymer matrices also may be suitable in switches. The researchers noted that a nanotube/polyvinyl-alcohol composite displayed a response similar to the polyimide, but that they chose polyimide for the tests because it displays low loss at 1550 nm and is easily patterned.