Shape of things to comeT-rays to help in airport screening

There is a fundamental problem with X-ray scanning at airports: Some X-rays technologies are not sensitive or nuanced enough to detect every suspicious object; other technologies, such as back-scatter X-ray, are so sensitive that they offer scanning personnel an anatomically precise image of the scanned pasengers. Also, as we all know from visits to our doctor, X-rays in large — or intense — doses may be harmful.

Now scientists are trying to exploit a barely used portion of the electromagnetic spectrum — terahertz radiation — to scan airline passengers for explosives and illegal drugs. The t-rays offer an attractive alternative to current technologies: They can see through clothing, paper, leather, plastic, wood, and ceramics. Yes, they do not penetrate as well as X-rays, but they also do not damage living human tissue. They can also read spectroscopic signatures, detecting the difference between, say, hair gel and an explosive.

Some commercial systems are already available for limited applications — one Japanese device is in use in scanbing mail for contraband drugs — but it has proven difficult to develop a machine to scan airline passengers becasue of the difficulties in creating the terahertz radiation. Ideally, the scanner would send out a beam of t-rays at passing objects or at people a few meters away, then measure the rays reflected off the subjects and check them against a database of spectroscopic signatures. Most existing sources of t-rays, however, provide only weak beams, which make detection slower and harder.

Qing Hu, a professor in MIT’s Research Laboratory of Electronics, appears close to solving this problem. T-rays lie between infrared light and microwaves on the electromagnetic spectrum, and frequencies between about 0.5 and 4.0 terahertz are of the most interest to researchers. The two typical ways of produce t-rays are impractical for airport applications: The first is to use a laser that produces infrared light and, through optical manipulation, retune it to terahertz frequencies. The problem here is that the resulting output is measured in millionths, or even trillionths, of watts, which means that for the detector to pick up the exceedingly weak signal, the beam would have to be ever-so-slowly scanned over an object from a close distance, building an image one pixel at a time. There is a way to speed up the scanning by using a very large gas laser, but this sheer size of the machinery makes it impractical as well.

Professor Hu has designed pinhead-size lasers which can produce 250 milliwatts at 4.3 terahertz, and slightly less than 100 milliwatts at 1.5 terahertz. This is sufficient power to send a beam over a distance of several meters, bounce it off an object, and use the return signal to create an instantaneous image. Moreover, instead of imaging one pixel at a time, the t-rays may be picked up by a focal plane array, like the detector in a video camera. allowing security personnel to see under coats and into suitcases as people walk by.

In addition to commercial air-travel applications, the technology is of great interest to the military. DARPA, for example, is interested in it to identify suicide bombers. Note that t-rays are not the only technology to identify would-be suicide bombers, and that other systems about to show up in the market use radar and vision-processing software for the purpose.

(See “Walking like a Bomber.”)

Zhang founded a company, Zomega Terahertz that makes a laptop-size T-ray detector that can be attached to a flying drone for remote detection of chemical and biological substances. While the trillionths of a watt produced by the infrared laser in the device is fine for spectroscopic analysis of air samples, it’s not adequate for imaging, and the laser technology is unlikely to improve enough to be used in airport security, Zhang says. He believes that quantum cascade lasers are the future of T-ray detection systems: “They will be the final winner in the market.”

-read more in Neil Savage’s Technology Review report; see the Web site of Zomega Terahertz Corp.; and see more about the technology in Jerome Faist et al., “Quantum Cascade Laser,” Science 264, no. 5158 (22 April 1994): 553-56 (sub. req.)