Catching T-waves

Terahertz radiation is gentle but piercing

Call it Karnak for the age of anthrax. In one envelope is a letter from Grandma; in the other is a life-threatening dose of a biological weapon. How can the post office tell the difference before you open your mail? The answer could lie in a mostly untapped band of the electromagnetic spectrum, the terahertz range.

Stuck in a no man's land between microwaves and visible light, forms of radiation harnessed as easily as flicking a switch, terahertz waves for decades have dodged the yoke. That's changing, as recent breakthroughs — combined with a range of potential applications stretching from diagnosing cancer to detecting dangerous flaws in space shuttle components — are moving T-rays out of the world of academic curiosity toward the mainstream. "It's matured from a field of engineers just trying to outgun each other to one where they are trying to find the killer application that will make this a viable technology," says Daniel van der Weide, a terahertz researcher at the University of Wisconsin-Madison.

For decades, the region of the spectrum at about a trillion cycles per second — a terahertz — frustrated scientists, who had trouble producing and capturing these rays. Technologies that generate microwaves and radio couldn't reach terahertz frequencies, while lasers work best at frequencies far above the terahertz range. Detectors were cumbersome and slow. Yet T-rays beckoned engineers with the promise of X-ray-like penetration without the harmful side effects but with unique sensitivity to biological materials and substances like plastics. "Just because it's no man's land doesn't mean it's not a land of opportunity," says van der Weide.

Medical tool. Now the terahertz realm is opening up, thanks to sensitive, miniaturized detectors and clever schemes for extracting T-rays from lasers and other energy sources. Experiments at the Jefferson Lab in Newport News, Va., for example, used controlled magnetic fields to coax high-speed electrons into emitting terahertz beams. TeraView, a British company, assembled semiconductor materials into a filter that converts laser pulses into more sedate terahertz waves.

Such techniques are letting scientists and companies test out many of the applications they have long envisioned. Chief among them is medical imaging. While terahertz waves can penetrate many solids, such as clothes or walls, they are quickly absorbed by water and similar fluids. That makes them adept at picking healthy cells from cancerous ones, because blood flows differently in the two tissue types. At Addenbrooke's Hospital in Cambridge, England, TeraView is testing a prototype cancer diagnostic machine, which bounces terahertz pulses off cells and distinguishes skin cancer from healthy tissue based on the absorption rate.

T-ray technology could save lives in other settings, too. When anthrax-laced letters began passing through the mail in late 2001, van der Weide tried adapting the terahertz device he was already developing to see if it could distinguish between living material — a potential biological agent — and plain old talcum powder in an envelope. It worked, offering an almost instantaneous readout.

Van der Weide also says the technology could someday help airport personnel determine whether passengers or luggage are carrying bombs. X-rays can see metal components, but a terahertz scan could detect the explosives themselves. And researchers at Rensselaer Polytechnic Institute found that T-rays could find weak spots in the foam insulation on a space shuttle's fuel tank. That might avert another tragedy like the destruction of the Columbia, triggered when its tank shed a chunk of foam.

Terahertz rays could even make those trips to the dentist's office a little less painful. Unlike X-rays, which can detect tooth decay only after it has progressed so far that "drill and fill" is the sole option, T-rays — sensitive even to the first stages of demineralization — can spot erosion early enough that it can be remedied with less invasive methods such as fluoride rinses. Could that be T-rays' killer app? Anthrax may never show up in your mailbox, but the dentist's chair awaits you every six months.