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.
The concept of using a perforated metal film and plasmons to selectively filter light at specific frequencies is not entirely new, but scientists have assumed that the only way to achieve the transmission of radiation through a film has been to use a uniform, or periodic, array of holes. However, what the Utah researchers showed, in the current issue of Nature, was that the perforations did not need to be uniform at all. In fact, in spite of the seemingly haphazard array of holes, nearly all of the terahertz energy was transmitted through the metal. However, the main benefits of discovering that any array of holes can transmit so much energy, says Nahata, is that it gives more freedom to design filters for various frequencies. “We’re not just limited to periodic structures,” he says.
The research could be a boon to terahertz device engineering, which is still in its infancy, says Daniel Mittleman, professor of electrical engineering at Rice University. “There aren’t many devices for manipulating terahertz radiation,” he says. “Any additional knobs that we have to control the terahertz wave are good.” The Utah work is a step toward shaping and selecting terahertz waves, he says.
Discovering that a non-periodic array of holes in a film can transmit terahertz energy is “really something new,” says Martin Koch, professor of electrical engineering and information technology at the Braunschweig Technical University, in Germany. Koch is the director of Braunschweig’s Terahertz Communication Lab, which opened last year, with the goal of building terahertz devices for the next generation of wireless communication. He suspects that terahertz devices are still at least a decade away from being made, and he says that it is currently unclear whether or not the Utah research will be directly applicable to them. However, Koch adds that the work is “nice, fundamental research that I’ll keep in the back of my mind.”
Nahata agrees that terahertz communication devices are many years away, but in the meantime, the work could also help researchers better understand terahertz physics and apply it to applications such as safer replacement for x-rays. (See “Taming the Terahertz.”)