Researchers around the world are trying to tap a barely used portion of the electromagnetic spectrum–terahertz radiation–to scan airline passengers for explosives and illegal drugs. The rays are particularly attractive: they can see through clothing, paper, leather, plastic, wood, and ceramics. They don’t penetrate as well as x-rays, but they also don’t damage living tissue. And they can read spectroscopic signatures, detecting the difference between, say, hair gel and an explosive.
While some commercial systems are already available for limited applications–one Japanese device scans mail for contraband drugs–a machine to scan airline passengers has been slow to evolve, mainly due to the difficulty of creating the terahertz radiation. The ideal 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. But most existing sources of t-rays only provide weak beams, which make detection slower and harder.
Now one MIT professor may be on the verge of solving this problem with a new type of laser.
A typical method of producing t-rays–which lie between infrared light and microwaves on the electromagnetic spectrum; frequencies between about 0.5 and 4.0 terahertz are of the most interest–is to use a laser that produces infrared light and, through optical manipulation, retune it to terahertz frequencies. The resulting output is measured in millionths, or even trillionths, of watts. For the detector to pick up that kind of very weak signal, the beam would have to be slowly scanned over an object from a close distance, building an image one pixel at a time. The alternative source is a huge gas laser that takes up an entire lab bench top. Neither is practical for quickly processing thousands of air travelers.
But Qing Hu, a professor in MIT’s Research Laboratory of Electronics, has designed pinhead-size lasers that can produce 250 milliwatts at 4.3 terahertz, and slightly less than 100 milliwatts at 1.5 terahertz. That’s enough 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. Instead of imaging one pixel at a time, the t-rays could be picked up by a focal plane array, like the detector in a video camera. This would allow security personnel to see under coats and into suitcases as people walk by. “We are able to make a movie in t-rays,” Hu says, meaning that his technology can provide real-time imaging.
The key to Hu’s technology is a quantum cascade laser, tiny semiconductor with nanometer-scale indentations called quantum wells etched into it. In standard lasers, an electron in a high-energy state drops into a low-energy state, releasing the excess energy as a photon of light. In quantum cascade lasers, the electron drops into a quantum well, emits a photon, and then moves through a thin barrier to the next well, where it emits another photon, and so on–“just like a ping-pong ball going downstairs,” Hu says. The result is many more photons, and thus more-powerful t-rays.
Hu’s lasers are a “key component” of a terahertz security device that Sandia National Laboratory is developing, says Sandia principal investigator Mike Wanke. The three-year project at the lab, now in its second year, aims to integrate a laser source and a detector into the same device. That eliminates complex optical setups and improves detector sensitivity by orders of magnitude, Wanke says. He envisions a module that can be used to make compact, commercially viable t-ray systems for use in airports. “We’re trying to make this so it’s a turnkey, drop-into-place system,” he says. He adds that once Sandia has a successful prototype, businesses can tackle the challenge of product development.
The lasers need to reach lower frequencies to do a better job of penetrating material–the lower, the better, says Hu. But lower frequencies mean smaller quantum wells, which are harder to build accurately. Hu won’t predict when commercial systems could be available.
But Xi-Cheng Zhang, director of Rensselaer Polytechnic Institute’s Center for Terahertz Research says Hu “always breaks the record he sets for himself.” Zhang says that either improvements in the engineering or use of different semiconductor materials is likely to make even better quantum cascade lasers. He expects that most of the problems will be solved in a year or two. One such problem is that the lasers operate at cryogenic temperatures and require bulky cooling equipment; Hu holds the record for highest operating temperature. After such problems are solved, market forces rather than technical issues will determine how long it takes for a commercial scanner to show up in an airport, Zhang says.
Hu says that the technology is of particular interest to, besides commercial air-travel applications, the military. “DARPA [the Defense Advanced Research Projects Agency] is very interested in this to identify suicide bombers,” he says. T-rays aren’t the only way to do this; other systems reaching market use radar and vision-processing software. (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.”
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