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Nanowire technology introduces an interesting level of energy ; Latest fiber-optics lets about half its light energy flow outside

Posted on: Sunday, 29 February 2004, 06:00 CST

A conventional fiber optic cable thattransmitstelephone signals over long distances is a lot like a garden hose. Instead of water, though, light pours through it, contained by its walls.

But a new, extremely tiny light guide is designed quite differently. Instead of holding the light in, this nanoscale fiber lets about half its light energy flow outside in a glowing, evanescent field. It acts "like a rail for the light," said Eric Mazur, a professor of physics at Harvard University who led the research team that developed what are called optical nanowires.

The nanowires, made of glass, are very small -- some are 50 nanometers, or 50 billionths of a meter, in diameter, or about one- thousandth the diameter of a human hair. Because the diameters are smaller than the wavelength of the transmitted light, the nanowires become the path around which the light waves flow.

In one of their experiments, for example, the Harvard team used a nanowire with a diameter of 360 nanometers to guide light with a wavelength of 633 nanometers.

The wires are meant not for the long hauls of conventional fiber optic cables that run between cities and under oceans, Mazur said, but for distances measured at most at an inch or so. For instance, they might be used as practical low-loss interfaces between optical fiber and the devices that process optical and electronic signals, making more compact, faster processors possible.

Right now, a standard fiber optic cable (which is closer to a human hair in diameter) carries thousands of conversations, each at a slightly different wavelength, that are combined through a process called multiplexing. But at journey's end, these light signals must be processed and converted to electronic signals. The new wire may one day be part of this processing, in tiny multiplexers that combine conversations and send them along, or demultiplexers at the other end that separate the signal into its individual components.

The nanowires take advantage of their evanescent fields to couple light from one wire to another.

Instead of having lengths of fiber that must be near one another for the signal to transfer, as in conventional fiber optics, light easily hops from one nanowire to another, a useful property in the future fabrication of multiplexers, demultiplexers and other devices like splitters, Mazur said.

Erich Ippen, a professor of electrical engineering and physics at Massachusetts Institute of Technology, said the small size of the wires produces a larger evanescent field that could be useful in sensors.

The fabrication process for the wires, which was described in the journal Nature, consists of two steps, said Limin Tong, lead author of the paper and a visiting professor at Harvard from Zhejiang University in China. Glass fiber about a micron wide was wound around a sapphire taper and then heated.

The sapphire buffered variations in temperature that could lead to unevenness. Then Tong drew out the fiber. "If you pull fast, it is very thin," he said. Slower pulling produced a thicker fiber.

Others have created silica nanowires, Mazur said. But Tong's are unusual in that they have a constant diameter, rather than uneven diameters or roughness in the sidewalls that has characterized others.

"These wires show surface smoothness at the atomic level, along with uniformity of diameter." Mazur said.

The smoothness will be important when the wires are put to use, said Sidney Yip, a professor of nuclear engineering and materials science engineering at MIT "The roughness of the wire determines its performance."

Mazur was startled by the simplicity of Tong's fabrication technique. Part of this work was done with a $20 Bunsen burner and little pieces of glass.

Tong agreed that the means he used to fabricate the devices were simple, but added that the measurements demonstrating the wires' effectiveness were not. "We needed electron microscopes with high resolution and high-quality optical microscopes to measure the size and optical wave-guiding properties of the new material," he said.

The nanowire research was paid for by the National Science Foundation in the United States and China's National Natural Science Foundation.

The nanowires created by the Harvard team have high tensile strength -- they are two to five times stronger than spider silk -- and can be twisted to make sharp turns that are just a few microns in diameter.

This flexibility might make the wires highly useful, for instance, in the emerging field that puts optical signals onto electronic chips or takes them off, said Richard Osgood Jr., a professor of electrical engineering and applied physics at Columbia University.

"The size is interesting in itself, showing just how small one can make relatively low-loss silica fibers," he said. But even more interesting is the flexibility of the material, which can be tied into tiny knots. "Alignment of optical components is difficult, so a flexible link could be very useful," he said.

Other researchers, too, are working on optical wire as well as related microphotonic devices. Over the past five years, Kerry Vahala, a professor of applied physics at the California Institute of Technology, has created optical wires, typically one to three microns in diameter, as standard elements in complex microdevices that are fabricated by photolithography.

Optical wires are exquisite devices, he said. "They act as beautiful low-loss interfaces between optical fiber and other photonic devices." But Vahala said he doubted that reducing the size of the wires to nanometers would be helpful because this did not significantly increase their waveguide properties. "Making them smaller is questionable," he said. "Reducing the diameter well beyond the wavelength of light yields no improvement in guiding properties."

Osgood said that while the slenderness of the nanowires did not lead to increased light-guiding properties, it did yield other benefits, like the possibility for flexible mechanical connections.

"It's like the old TV's, where we used to have flexible wires to go from one board to another," he said. "You don't have to get everything exactly aligned to close things."

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