While the 1890s saw the advent of x-ray technology, the past 12 months have witnessed substantial progress in another powerful imaging innovation—t-ray technology. T-rays are terahertz frequencies, which represent a little-used portion of the electromagnetic spectrum—and like x-rays, they can penetrate most materials. But t-rays are thought to be safer than x-rays. And because compounds react to terahertz radiation in distinctive ways, a t-ray-based imaging system can identify a concealed object's chemical composition.

Because of its capabilities, "terahertz imaging is getting hotter and hotter," says Xi-Cheng Zhang, a terahertz pioneer at Rensselaer Polytechnic Institute. And the technology has potential for a more varied list of applications than that of x-rays, from detecting tumors, to identifying biological warfare agents, to spotting plastic explosives. T-rays can even see through paper and clothing, which means that a terahertz camera could reveal hidden weapons.

And t-ray-producing devices have improved substantially over the past year, thanks to several research projects. These new devices are better at emitting t-rays within a confined frequency band—a prerequisite for accurate chemical sensing and medical imaging—in sharp contrast to the terahertz sources currently on the market, which generally release many frequencies at once. Indeed, it's no easy feat to produce and detect terahertz frequencies, which are higher than microwaves but lower than infrared light. "You're never sure whether to use electronics-based or optics-based" technology, says Martyn Chamberlain of the University of Leeds in England, a prominent terahertz researcher.

But with the past year's innovations, terahertz technology is zeroing in on its most near-term application—medical imaging. In fact, in one advanced project, TeraView, an England-based startup, employed t-rays to detect skin cancers that other imaging systems could not discern. In particular, the t-rays were able to spot tumors that develop invisibly under the skin's surface.

Terahertz imaging can also be used to recognize unfamiliar biological materials, since biomolecules can't help but vibrate at terahertz frequencies, with each doing so in a characteristic manner. This happens because specific proteins soak up certain t-ray frequencies, which alter their molecular arrangement, thus giving them revealing terahertz "fingerprints."

Sensors can pick up these fingerprints and display the protein's identity, making the technology useful in the automated detection of biological warfare agents, such as anthrax. Another possible application for t-rays is in chemical sensing, since other large molecules, such as polymers, also react to terahertz waves distinctively. In fact, QinetiQ of Farnborough, England, has made a terahertz camera that captures highly revealing images of people through their clothing.

But the fact that proteins respond to t-rays may have implications for human exposure. To determine their effect on people, the European Union is funding a program, called Terahertz Bridge, which has already released reassuring preliminary results. Testing t-ray quantities that would be needed for bodily imaging, researchers have found no indication of permanent, x-ray-like tissue damage. "So far, it's safe," says Gian Piero Gallerano, coordinator of Terahertz Bridge.

And in other good news for terahertz advocates, t-ray instruments continue to get more precise. For example, Vermont-based Vermont Photonics has built a device that produces t-rays by transmitting an electron beam across the rippled surface of a conductor, with the beam causing electrons in the conductor to go up and down the surface's grooves and thus shake loose t-rays. The terahertz frequency generated can even be manipulated by changing the energy of the electron beam, says Vermont Photonics cofounder Michael Mross. The company says the device can be used to examine interactions between biomolecules for applications such as drug discovery.

Another innovative t-ray-producing device is the quantum cascade laser, which last year, Qin Hu, an MIT electrical engineer, used to generate a continuous terahertz beam at a narrow frequency. Such lasers are normally used to produce infrared light, and transplanting the technology into the terahertz region demands extremely precise control over materials.

Indeed, while scientists must labor painstakingly to produce t-rays, nature does not have to try so hard. In fact, terahertz radiation is still proliferating throughout space from its starting point in the Big Bang. Notes Chamberlain, "The universe is full of this stuff." And in short order, humans may be able to find uses for it.

Source: Taming the Terahertz
Herb Brody
Technology Review, June 2003
http://www.technologyreview.com/articles/innovation40603.asp