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Tuesday, February 20, 2007 T-Rays Advance Toward Airport ScreeningA new laser design helps create usable terahertz radiation, which penetrates common materials but doesn't harm tissue. By Neil Savage
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.
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A Compact Chemical Sensor
04/30/2008
Comments
nekote on 02/20/2007 at 7:38 AM
114
How about trying these scanners out in (men's) prisons first, where such issues would be a lesser degree of concern, than necessary security?
Failure to detect "shanks" (home made stabbing / cutting weapons) truly have murderous results.
Detecting contraband would also be extremely valuable.
mkogrady on 02/20/2007 at 7:02 PM
86
One "whole body" scanner - external model
One "inner body" scanner - with replacement doo-hicky's (emphasis on Doo!)
blunney on 02/20/2007 at 9:19 AM
15
The TSA is watching you...
jsessex on 02/20/2007 at 1:18 PM
12
cyberpageman on 02/21/2007 at 12:46 PM
23
When x-rays were first discovered, there was worry about them too. I remember my physics professor quoting:
The Roentgen Rays, the Roentgen Rays,
What is this craze?
The town's ablaze
With the new phase
of X-Ray's ways.
I'm full of daze,
shock and amaze;
For nowadays
I hear they'll gaze
Thro' cloak and gown--and even stays,
Those naughty, naughty Roentgen Rays
No one worries about Superman's x-ray vision, so why worry about terahertz rays, which are a lot less clear?
abcarterjr on 02/21/2007 at 12:30 PM
45
classify a mountain stream as a go-nogo carrier
of micron gold?
ggeek99 on 02/24/2007 at 2:01 AM
2
Why should it be illegal for adults to eat/smoke/drink the compounds of their choosing in a free society? How can society/government maintain a list of approved and nonapproved foods & drugs for adult members of that society?
Why can I not carry a weapon for my protection in a society where the police are armed to the gills and trigger happy and the crooks likewise?
There is no place where people respect each other more than the parking lot of a gunshop. No one cuts each other off, no one takes anothers parking space. Why? Because they know the other maybe packing heat. Perhaps we would all respect each other a little bit more if we knew we could all defend ourselves in the heat of argument, so why argue?
If army exists to keep the peace, then so must guns. So why the hurry to disarm the civilian populace and arm the police & military?
If we implement perfect screening technologies without understanding the implications for our political futures then we deserve the future we get, not matter how how fascist or tyrannical.