Nanowire networks route light

By Eric Smalley, Technology Research News

Computer chips that use light rather than electricity to dramatically speed computing are probably a long way off, but research efforts aimed at producing nanoscale optics could make chip-scale communications devices and scientific equipment practical within a decade.

An increasing amount of this research effort is focused on using photonic crystal chips to channel light, but photonic crystal channels are generally larger than a thousandth of a millimeter, or 1,000 nanometers. Another approach is to use nanowires to guide lightwaves.

Researchers from the University of California at Berkeley, the Lawrence Berkeley National Laboratory and the National Aeronautics and Space Administration (NASA) have devised nanoscale optical networks made from nanoscale wires and ribbons and have routed light signals through the networks. They have also used nanoribbons to guide light into microscopic droplets as a way of identifying molecules.

The proof-of-concept work could lead to electronic computer chips that sport high-speed optical links, miniaturized communications devices, and chip-scale scientific and medical spectroscopy. Practical applications include on-chip information processing that uses photons to represent information, and chemical and biological sensing devices that could be integrated into biochips, said Peidong Yang, an associate professor of chemistry at the University of California at Berkeley and faculty scientist at the Lawrence Berkeley National Laboratory.

The researchers showed previously that nanowires and ribbons are efficient lightwave guides. The nanowires and ribbons are narrower than the wavelengths of light they transmit, and unlike traditional optical fiber, some light is transmitted along the outside of the nanowires and ribbons.

The researchers expanded the work by showing that it is possible to transmit nanosecond light pulses from gallium nitride and zinc oxide nanowire lasers through connected tin oxide nanoribbon waveguides.

They connected a 130- by 65,000-nanometer gallium nitride nanowire to a 240- by 260- by 460,000-nanometer tin oxide nanoribbon and used a laser to stimulate the nanowire into lasing, which sent 8-nanosecond light pulses into the nanoribbon. About 50 percent of the light made it across the connection. The wire and ribbon overlapped by two microns and were held together by natural electrostatic forces. By comparison, human hair is about 75,000 nanometers thick.

The researchers also built simple nanoribbon networks that filter white light into different colors and route the separate colors. Different wavelengths of light can be used to send information along the same channel at the same time, increasing the bandwidth of the channel. This is a widely-used technique in today's communations networks.

The researchers attached three successively smaller nanoribbons to a larger nanoribbon and transmitted white light through the larger nanoribbon. The structure caused green light to flow into the first branch nanoribbon, aqua light into the second, and blue into the third. When they transmitted red light the light remained within the larger nanoribbon, while green light flowed into the first branch nanoribbon and blue light flowed through all four nanoribbons.

The researchers also demonstrated that the nano networks are capable of relatively complicated optical routing. They configured four nanoribbons into a grid and routed light signals around 90 degree bends in the grid. Though only about 5 percent of the light was transmitted at the nanoribbon junctions, the researchers found that light pumped into the end of one nanoribbon was transmitted to the seven other nanoribbon ends, showing that some of the light made it through a pair of 90 degree turns.

The researchers also showed that the nanowires and ribbons are capable of sensing molecules.

They inserted a nanoribbon into a 3-picoliter dye-containing droplet and sent blue light through the nanoribbon. This caused the dye to fluoresce, and some of the fluorescent light traveled back through the nanoribbon. They were able to determine the substance that emitted the light by spectroscopy -- analyzing the mix of lightwaves in the fluorescent light. A picoliter is one trillionth of a liter, or about one 20,000th of a drop of water.

They also placed a 1-picoliter dye-containing droplet on the middle of a nanoribbon and transmitted white light through the ribbon. The dye absorbed a color of light that indicated the substance that makes that makes up the dye. Measuring the spectrum of the light at the end of the nanoribbon shows the missing color.

These spectroscopy techniques demonstrate that nanoribbons could be used in biochips for sensing chemicals and biological molecules, according to Yang.

Making nanowire and ribbon waveguides practical requires methods of manufacturing the devices in parallel, said Yang. The significant bottleneck to making the devices practical is developing parallel processes for integrating, or constructing them, he said. Although the integration process used in the current work is serial, hence slow and inefficient, it "represents the first step towards integrating nanowire-based photonic elements [into chips]," he said.

Networks of nanowires and ribbons that carry out optical routing and nanowire and ribbon spectroscopy devices could be used practically in five to ten years, said Yang.

Yang's research colleagues were Donald J. Sirbuly and Matt Law of the University of California at Berkeley and Lawrence Berkeley National Laboratory, Peter Pauzauskie, Haoquan Yan, Kelly Knutsen and Richard J. Saykally of the University of California at Berkeley, and Alex V. Maslov and Cun-Zheng Ning of NASA. They published the research in the May 20, 2005 issue of the Proceedings of the National Academy Of Sciences. The research was funded by the Department of Energy, the Camille and Henry Dreyfus Foundation and the Beckman Foundation.

Timeline:   5-10 years
Funding:   Government, Private
TRN Categories:  Optical Computing, Optoelectronics and Photonics; Nanotechnology; Materials Science and Engineering
Story Type:   News
Related Elements:  Technical paper, "Optical Routing and Sensing with Nanowire Assemblies," Proceedings of the National Academy of Sciences, May 20, 2005


June 29/July 6, 2005

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