Terahertz spectroscopy has the unique ability to identify hidden explosives and hazardous materials, but a key limitation is that the detection must be done at close range, possibly jeopardizing a device’s operator(s).
The technology has taken a leap backward as the result of recent research at Rensselaer Polytechnic Institute, Troy, N.Y., where work by a graduate student has moved the standoff distance back at least 30 meters (125 feet), a huge step in the direction of safe, remote terahertz (THz) sensing. That standoff distance might increase; 30 meters is only the size of the lab where the device was tested.
Until now, remote THz sensing was impossible due to fundamental limitations caused by water vapor in the air, which disturbs THz radiation. THz sensors are desirable to homeland security and military personnel because THz rays can penetrate packaging or clothing and identify the unique chemical fingerprints of hidden materials.
Terahertz waves, or T-rays, occupy the electromagnetic spectrum between the far-infrared and the high-frequency microwave bands. Frequencies ranges from 300 to 3,000 gigahertz, and the corresponding wavelengths from 0.1 mm (or 100 μm) infrared to 1 mm.
Unlike X-rays and microwaves, T-rays pose no known health threat to humans.
“Many first responders and soldiers are faced with dangerous situations that involve close proximity to potential explosive threats,” said the technology’s inventor, Benjamin Clough, a researcher and Ph.D. candidate in Rensselaer’s Department of Electrical, Computer, and Systems Engineering.
Clough believes this technology will help responders determine what a threat is earlier, from a safe distance.
The technology uses sound waves from a plasma laser to boost the effective distance of THz spectroscopy from a few feet to many meters.
Clough told Homeland1 that what’s exciting is that the laser plasma is being used both as a THz emitter and detector. “This means the device uses sound waves to remotely ‘listen’ to THz signals.”
It works this way. By focusing two laser beams into the air, small bursts of plasma are created, which in turn creates THz pulses. Another pair of lasers is aimed near the target to create a second plasma for detecting THz pulses after they have interacted with the suspect material. The detection plasma produces acoustic waves as it ionizes the air.
Clough discovered that by using a sensitive microphone to “listen” to the plasma, he could detect THz wave information embedded in these sound waves. This audio information can then be converted into digital data and instantly checked against a library of known THz “fingerprints,” to determine the chemical composition of the unknown material.
Clough said that although the technology is extremely new and remains relatively unexplored, “with the fast-paced evolution of laser technology, and the mature acoustic technology already available, this sensing method has potential for commercialization and implementation at some point in the future.”