Harnessing New Frequencies for Wireless and Anti-Terrorism
Modern technology uses many frequencies of electromagnetic radiation for communication, including radio waves, TV signals, microwaves and visible light. Now, a University of Utah study shows how far-infrared light – the last unexploited part of the electromagnetic spectrum – could be harnessed to build much faster wireless communications and to detect concealed explosives and biological weapons.
“We found a way to manipulate a form of infrared radiation that is not now used for communications so that, in the future, it may be possible to use it for high-speed, short-range communication between computers and other devices,” says Ajay Nahata, an associate professor of electrical and computer engineering.
The study in the March 29, 2007, issue of the journal Nature also shows the feasibility of building devices that emit and detect specific frequencies of far-infrared light – also known as terahertz radiation – to spot chemical or biological warfare agents such as anthrax bacteria and to make images of packages or people to find concealed weapons and plastic explosives, Nahata adds.
The new study was conducted by Nahata and principal author Z. Valy Vardeny, a distinguished professor of physics at the University of Utah, along with Tatsunosuke Matsui, a postdoctoral researcher in physics, and Amit Agrawal, a doctoral student in electrical and computer engineering.
To visualize their discovery, imagine shining a flashlight through a kitchen colander, and that holes make up 20 percent of the colander’s surface. Only 20 percent of the light will pass through the colander. But when the Utah researchers shined far-infrared radiation through holes punched in a thin steel foil or film, almost all of the radiation passed through the film if the holes were arranged in semi-regular patterns known as “quasicrystals” or “quasicrystal approximates.”
(Crystals have repeating patterns over a short distance, such as the ordered pattern of carbon atoms in diamond. Quasicrystals have less structure, but display a pattern over a larger area. Quasicrystal approximates – a term coined by Vardeny and Nahata – also have patterns, but less so than quasicrystals. Crystals, quasicrystals and approximates all can bend or break up light or other electromagnetic waves.)
Until now, such efficient transmission of far-infrared light was achieved only when crystal patterns were used, but unwanted frequencies also were transmitted. In the new study, the researchers could select the wavelength of far-infrared light transmitted through the holes and, by tilting the films, they could switch the transmission on and off.
That shows high-frequency terahertz signals can be switched on and off to carry data in the digital code of ones and zeroes, and that it someday may be possible to build superfast switches to carry terahertz data at terahertz speeds. That is 1,000 times faster than gigahertz fiber optic lines that carry data as near-infrared and visible light, and 10,000 times faster than microwaves that carry cordless and cell phone conversations.
Talking with Terahertz: An Unexploited Part of the Spectrum
The spectrum of electromagnetic radiation ranges from short to long wavelengths (or from high to low frequency): gamma rays, X-rays, ultraviolet rays, visible light (violet, blue, green, yellow, orange and red), infrared rays (including radiant heat), microwaves, FM radio waves, television, short wave and AM radio.
Near-infrared radiation and some visible light now are used for fiber optic phone and data lines. But terahertz or far-infrared radiation – on the spectrum between microwaves and mid-infrared radiation – is not now used for communication.
“Terahertz is a new region of the spectrum for communications” because the rest of the spectrum is crowded with communication and broadcasting signals, says Nahata.
Vardeny adds: “Industry is starving for more electromagnetic frequencies,” yet terahertz frequencies are unexplored. They are too high for electronics and there are technical obstacles in generating, manipulating and detecting terahertz radiation.
For electromagnetic radiation to transmit data, the signal must be turned on and off to rapidly create the binary code of ones and zeroes. Modern optical and electronic switches cannot do that fast enough to handle signals with terahertz frequencies (1,000 billion waves per second), but can handle gigahertz signals (1 billion waves per second).
No one has built terahertz switches, but Nahata says the new study shows it is possible to use terahertz radiation to carry data and thus may be possible to create terahertz-speed switches for superfast wireless communication over short distances, such as between a cellular phone and headsets, a wireless mouse and a computer, and a PDA (personal digital assistant) and a computer.
Terahertz against Terror
The study was funded as part of a three-year, $250,000 grant from the U.S. Army Research Office and by $100,000 from the Synergy program, operated by the University of Utah’s vice president for research to promote interdisciplinary research.
Nahata says terahertz technology has two main uses for homeland security:
— “Vibrational spectroscopy” uses emitters and detectors of terahertz radiation to detect materials – such as anthrax or other biological or chemical weapons – that resonate at a terahertz frequency when exposed to far-infrared light. Early terahertz devices emit numerous frequencies. The new study shows perforated films can serve as filters so future terahertz devices can use desired frequencies to zero in on specific chemical or biological weapons or concealed guns and explosives.
— Another method uses a terahertz emitter and a camera. “Since plastics and clothing are transparent to terahertz wavelengths, metal reflects terahertz, and certain chemicals – such as plastic explosives – strongly absorb terahertz radiation at specific frequencies, this approach is being pursued for package inspection and whole-body imaging to look for concealed weapons or explosives,” Nahata says. (Recently publicized scanners use X-rays or microwaves. But scanners using terahertz radiation should lack the risk of X-rays and be more precise than microwave scanners, he adds.)
How the Study was Conducted
Normally, any frequency of electromagnetic radiation or light cannot pass through holes smaller than the radiation’s wavelength. A cook can see food in a microwave oven because visible light waves are smaller than the holes in the oven door’s grating. But microwaves, with wavelengths larger than the holes, cannot escape to injure the cook.
The study used stainless steel film about three-quarters the thickness of a human hair. Different patterns of holes were punched in the film. The holes were one-quarter to one-half millimeter in diameter (about one-hundredth to one-fiftieth of an inch). That is smaller than the roughly 1-millimeter wavelength of far-infrared light.
Study co-author Agrawal used a computer to design patterns of holes that he expected would allow “resonance” or “anomalous transmission,” meaning all the far-infrared light passes through the holes in the metal films. The researchers projected terahertz or far-infrared light onto the metal films with punched patterns. They found certain frequencies of the far-infrared radiation were completely transmitted through the films with crystal, quasicrystal and quasicrystal-approximate patterns – even though the terahertz radiation has wavelengths larger than the holes.
Vardeny says such efficient transmission occurs because the far-infrared light not only goes through the holes, but also moves electrons in the metal film, generating “surface plasma waves” that launch all the far-infrared radiation through the holes.
Source: University of Utah
Light seems to pass through solid metal
Researchers directing a special type of light at metal poked with holes in irregular patterns recently discovered that all the light behaved like a liquid and fell across the metal to find its way through the escape holes.
That means the light was acting pretty weird. Picture shining a flashlight at your kitchen colander. While some of the light from the flashlight will travel through its holes, the solid part of the colander will keep much of the light from shining through.
In contrast, experiments described in the March 28 issue of the journal Nature demonstrated that terahertz radiation — a low-frequency light on the electromagnetic spectrum located between microwaves and mid-infrared regions — traveled around a thin sheet of metal, through patterned holes, and all of it came out the other side. Experts sometimes refer to this radiation as T-rays.
“You can get 100 percent transmission of light, even if holes only make up 20 percent of the area,” University of Utah physicist Ajay Nahata told LiveScience. Nahata is one of the experimenters.
A ’surprising’ earlier finding
Although it sounds simple, understanding how so much light can move around to fit through holes is a relatively new idea. An explanation started when Thomas Ebbesen illustrated in research published in 1998 that the amount of visible light that traveled through a single hole was more than scientists expected.
“It was surprising, because a hole is the simplest thing you could imagine,” said electrical engineer Daniel Mittleman, who works in Rice University’s T-ray lab but is not affiliated with the new study.
Since Ebbesen’s findings, researchers have assumed that the theory only applied to light traveling through holes in periodic patterns, such as squares. But Nahata and physicist Z. Valy Vardeny found in the new experiments that light moved across the metal surface and passed through holes in a number of different irregular designs.
Nahata and Vardeny are also the first researchers to observe how terahertz radiation reacts with the metal and around the holes. While visible light oscillates so quickly that it’s difficult to measure, scientists can accurately gauge the low frequency of terahertz radiation.
“By using terahertz, you can really see how and when light comes out of the holes,” Mittleman told LiveScience. “Once you illuminate the hole, some light goes through and some comes out a little later.”
T-rays and other light
Since all light waves tend to act similarly, Mittleman said, researchers can assume that the behavior they observe of the terahertz radiation also occurs across the electromagnetic spectrum.
The University of Utah researchers have high hopes for applications of terahertz radiation in wireless communication and homeland security operations.
Today, much of the low-frequency electromagnetic spectrum is crowded with communication and broadcasting signals. Terahertz is unchartered, promising territory, Nahata said, to open up more space for transmitting data at high speeds.
Also, since many everyday materials, such as clothing, plastics and wood look transparent under terahertz imaging, the technology could be used to spot concealed bombs and other explosive devices. In addition, materials absorb T-rays at varying frequencies, depending on the type of material. Anthrax, for example, can be detected with terahertz imaging by its frequency fingerprint.
“We’re trying to make building block devices so we can go after a broad range of applications,” said Nahata.
Source: MSNBC.