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.