Terahertz Waves: No Silver Bullet
By Glenn Zorpette
Among the more exotic technologies being pursued to detect improvised explosive devices, or IEDs, from a distance are terahertz-frequency waves.
Why terahertz radiation? Three reasons. It penetrates most nonmetals, such as dirt, plastic, and wood, as well as other materials that contain or conceal IEDs. It does not ionize atoms or molecules in the body, so it does not cause cancer. And it also triggers unique and detectable responses in certain molecules. “The molecules in explosives, mostly nitrogen based, have rotational and vibrational characteristics that show up in the terahertz region,” says Michael Shlesinger, a division director with the Office of Naval Research (ONR), which together with the U.S. Naval Research Laboratory is spending US $30 million a year on counter-IED R&D.
Terahertz waves are odd, not quite radio and not quite light, and they are a vast and largely unused swath of the electromagnetic spectrum [see “T-Rays vs. Terrorists,” IEEE Spectrum, July 2007]. As you ascend in frequency, first you have millimeter waves (30 gigahertz to about 400 GHz) and then the terahertz region, from roughly 400 GHz to 10 THz, and then the far infrared.
To detect an explosive sample using terahertz radiation, researchers fire pulses from terahertz-wave lasers at an explosive sample, which absorbs specific bands of frequencies. The reflected radiation is omnidirectional, so the signal is weak, Shlesinger notes. To detect it, researchers are working with very sensitive antennas.
On the streets of a war zone, soldiers don’t know where the explosives are, of course. So a practical system would have to scan a wide area, and do it fast, because it would be in a moving vehicle. “The unsolved problem is the scan rate, when the explosives are well-hidden,” Shlesinger says.
If researchers can solve the scan-rate problem and build a terahertz-wave-based detection system, “there will be circumstances where it is useful, but it won’t be a general answer,” says Captain Mark Stoffel, a program manager in the ONR’s Expeditionary Maneuver, Warfare, and Combating Terrorism Department.
Another intriguing feature of terahertz waves is their ability to form images of objects hidden by nonmetals. This capability is attractive to military researchers because it suggests—in theory, anyway—the possibility of a system that can detect explosives hidden in a suicide bomber’s clothing from enough of a distance for it to be useful. A few commercial imaging systems are already available, but they are slow and must be very close to their target. A practical system is years, maybe decades away.