Tuesday, April 03, 2007

The Ultrafast Future of Wireless

A new metal film could help control terahertz radiation and lead to wireless devices that are thousands of times faster than today's Wi-Fi.

By Kate Greene

T-rays: University of Utah researchers have found a new way to control terahertz radiation that could lead to ultra-fast wireless communication. The researchers shined terahertz radiation on stainless steel metal foils (above) that are perforated with a pattern of holes. The spacing between the holes determines the specific frequency of light that passes through. Credit: Tatsunosuke Matsui, University of Utah

Researchers at the University of Utah have found a way to control terahertz radiation with more precision than ever before, potentially laying the foundation for a new breed of wireless devices that can take advantage of the previously untapped frequencies. Although still years from commercialization, routers and receivers that use terahertz radiation--which technically ranges from about 100 gigahertz to 10 terahertz--could eventually pack more data onto airwaves, speeding up wireless Internet links a thousand times, says Ajay Nahata, a professor of electrical and computer engineering who led the research.

Nahata and his team designed a perforated stainless steel film that is able to selectively allow certain terahertz frequencies to pass through and cancel out others. In effect, the researchers have built a simple terahertz filter, a potential precursor to terahertz communication devices.

Most wireless gadgets use radiation in the microwave frequency; Wi-Fi, for instance, operates at 2.4 gigahertz. At this frequency, technologies such as radiation sources, detectors, and modulators (devices that encode data on the waves) are well established. But currently, efficient terahertz sources and detectors are still being developed, and "there are effectively no real devices to manipulate those frequencies," says Nahata. "Because of this, terahertz is the gap in the electromagnetic spectrum. We're making new devices so terahertz can be useful."

The benefits of terahertz communication could be great. A typical modulator for a 2.4-gigahertz signal can only encode information at far lower frequencies--at about 50 megahertz. But a 2.4-terahertz wave oscillates a thousand times faster than a 2.4 gigahertz signal, and correspondingly, if terahertz modulators could be made, the modulated signal would also be a thousand times faster, says Nahata. These terahertz waves would be most useful for relatively short-range communication such as within a room, he says, because over greater distances, the signal dies off.

The researchers' new device is essentially a stainless steel metal film with arrays of holes in it. When a terahertz source shines on the film, the radiation gets trapped on its surface. In effect, the energy from the terahertz radiation is converted from a three-dimensional electromagnetic wave to a two-dimensional surface wave, called a plasmon. Nahata explains that as these surface waves move about the film, they can bump into structures on the surface such as troughs and holes. At the holes, he says, the waves constructively interfere, meaning that there is a buildup of light; the energy of the plasmons passes through the holes and is essentially converted back into three-dimensional terahertz radiation, once on the other side of the film. The specific frequency of light that is emitted depends on the spacing of the holes.