T-Rays vs. Terrorists
Terahertz radiation lets security screeners find bombs and weapons wherever they’re hidden
If you’re of a certain age, you may remember those miraculous-sounding “X-ray specs” advertised in comic books. They’d let you see through walls, boxes, and—best of all, for a teenager, anyway—clothing. They were bogus, of course. But technology is finally on the verge of giving us all those capabilities, and more, albeit in a package too big to perch on the bridge of your nose.
The key advances are devices and circuitry that emit and sense radiation in the terahertz band of the electromagnetic spectrum, which extends from the upper edge of microwaves to the near infrared. The rays are reflected by metal but go through most other materials. Water soaks up the radiation, so human tissue, which is mostly water, absorbs it. But unlike X-rays, terahertz rays are thought to be harmless. Terahertz radiation (“T-rays”) can’t penetrate much past your skin, and it lacks the energy to ionize molecules in human tissue the way X-rays do, so it cannot cause cancers by smashing up your DNA. What’s more, the power levels most T-ray imagers produce are lower than that of the infrared LED in your TV remote control.
T-ray technology will probably find its first big uses in security-related applications, now an enormously fast-growing business because of recent high-profile terrorist attacks. The technology’s appeal here is undeniable: in a terahertz image, a gun or a knife shines through whatever clothing it’s concealed in—even a plastic knife shows up, because of the way its sharp edges scatter the radiation. And yet, unlike X-rays, T-ray screeners could be used routinely on people, because the radiation is harmless.
But some terahertz imagers have another ability, one not even claimed by the comic-book specs: not only can they see hidden objects, but they can tell what those objects are made of. Many explosives, including all the plastic explosives popular with terrorist groups, reflect and transmit a characteristic combination of terahertz waves that make them distinguishable from other materials, even those that might seem identical to the eye and hand. That same chemical-discriminating capability—spectroscopy—also applies to pharmaceuticals and drugs. In essence, different materials appear as different colors to the terahertz imaging system. So future screening devices should be able to tell whether that’s plastique in your pocket or just Play-Doh, a package of sugar or an envelope of methamphetamines.
Best of all, T-ray scanners have “standoff capability,” meaning they can see a few meters away, a very desirable feature in the security business. The first commercially available products are being tested now, and although they can make out images from several meters away, they cannot yet make use of their chemical analysis capabilities from those distances. Nevertheless, in just two or three years, versions that can see at a distance of tens of meters should be available, which would be a great safety boon to security personnel. One project under U.S. Department of Defense sponsorship is studying the ability of T-rays to detect improvised explosive devices (IEDs). This challenge is one of the most urgent and highest-funded research thrusts at the moment, because these bombs have killed about 2000 people in Iraq alone. Some short-range imagers available now can also do spectroscopy, but the imaging rate is currently too slow for use in a walk-through scanner. But as the literally hundreds of engineers and scientists working on new terahertz sources and devices push the technology’s limits, we expect to see a machine over the next five years that can do both imaging and spectroscopy at 50 meters or more.
There are lots of uses outside of the security arena for T-rays, too. Drug companies are buying T-ray imagers for their ability to distinguish good pills from bad by their spectral signatures. T-rays can distinguish normal skin tissue from tumors even when a trained dermatologist cannot. Manufacturers can do the mundane job of checking the contents of a box without opening it, or they can perform such crucial tasks as finding the invisible defects in the protective coatings on an aircraft’s wings. NASA recently commissioned a T-ray imaging company, Picometrix, in Ann Arbor, Mich., to build a scanner to look for tiny holes and other structural failings inside the foam that lines the external fuel tanks in the space shuttle fleet. A chunk of that foam fell away in 2003 and led to the deaths of seven astronauts and the fiery destruction of the Columbia.
Picometrix is one of a growing group of companies pushing the limits of T-ray technology. Others include TeraView, in Cambridge, England, and ThruVision, in Abingdon, England, both spin-offs of British national labs, as well as companies like Spire Corp., in Bedford, Mass., and Advanced Energy Systems, in Princeton, N.J., established firms whose mix of technologies happened to lend itself to terahertz research. National, corporate, and academic laboratories are spearheading much of the new technology development. Sandia National Laboratories, Jefferson Laboratory, and Bell Laboratories—to name a few—have been key to creating ever-brighter sources of T-rays. Meanwhile academic groups at Rensselaer Polytechnic Institute, MIT, the New Jersey Institute of Technology, Rice University, and elsewhere have made strides in terahertz imaging systems. Brian Schulkin, a student from Xi-Cheng Zhang’s lab at RPI recently produced the first handheld T-ray imager—weighing just 2 kilograms.
T-rays are odd: they’re not quite what we think of as radio and not quite what we expect from light. They can radiate from metal antennas as radio waves do, but they also bounce off ordinary mirrors as light does. They can be focused with silicon lenses but are typically sensed in a circuit by their electric field.
They make up one of the least-used chunks of the electromagnetic rainbow, comprising an absolutely vast swath of relatively virgin territory. It has long been a gap in our otherwise extensive mastery of electromagnetic waves. On the one side are radio waves, which emanate from and are received by antennas and are manipulated with electronics. On the other there’s light, which we’ve become quite adept at bounding, bending, and steering with mirrors, lenses, and optical fibers.
Where the terahertz band begins and ends depends to a degree on whom you ask. We put it between 500 gigahertz and 10 terahertz, for a few reasons. That region is largely beyond the reach of pure radio frequency technology such as microwave circuits, requiring combinations of electronics and optics instead. Also, many interesting materials such as plastic explosives have distinctive colors in that region. On the downside, most terahertz radiation is absorbed by the atmosphere. And the technology that is needed to see in that band is much less mature than, say, the technology for the region near 100 GHz, whose fundamental components have been around for half a century.
Others choose to define the terahertz band beginning at a lower frequency, 10 GHz in some cases, where light has a wavelength measured in millimeters. Like higher-frequency T-rays, millimeter waves can pass through clothing, a property applied in scanners built by companies such as Millivision, Quinetiq, and Safeview, which the companies have tested in airports and other locations. The scanners made by the former two companies rely on the small amount of millimeter-wave radiation emitted by all warm bodies. They find hidden weapons beneath people’s clothing by noting the difference in the amount of radiation between the warm body and the cooler objects.
Known as passive imagers, these devices can see through many of the same materials as T-rays, but they can’t determine an object’s chemical makeup the way T-rays can. Also, their resolution is naturally not as good as terahertz imagers, because as the imaging radiation’s wavelength gets shorter, an imager’s resolution improves. These scanners are capable of discovering that someone is hiding something, but that something—a cellphone, a knife, a bomb—usually looks like a blob on the millimeter-wave imager.
Only in the last decade have scientists and engineers found ways, exotic though they are, to break into the true terahertz band. The most extreme of these—using a particle accelerator—is also the most powerful. The accelerators work well for this purpose, but they typically take up a hectare or more and cost tens of millions of dollars. Commercial systems, from Picometrix or TeraView, for example, generate T-rays much more economically and compactly: they zap semiconductors with femtosecond-long laser pulses or mix together a pair of infrared laser beams. And researchers are looking into other promising T-ray sources, ones that use semiconductor lasers cooled by liquid helium.
Scientists are also working on new ways to form an image from T-rays. The ideal terahertz camera would be just like any digital camera—a dense array of millions of detectors arranged as pixels on an integrated circuit. Unfortunately, most terahertz detectors lack the combination of compactness, cheapness, and sensitivity to allow for that. Instead, terahertz researchers have come up with a number of alternatives that use one or only a few detectors. Two of the leading approaches are to reconstruct a terahertz image from the way T-rays interfere with one another or to convert the otherwise invisible rays into something a digital camera can see.
But before you can make a picture, you need to be able to produce the radiation. In the last 10 years or so, researchers have come up with a number of ways of generating terahertz waves, each with their own distinct disadvantages—cost, complexity, the need for cryogenic cooling, size, or some combination of all four. Synchrotrons, which accelerate bunches of electrons along an enormous track to nearly the speed of light, are the brightest sources, but they typically occupy an entire building, and a rather large one at that. To produce T-rays, the synchrotron forces the fast-moving electrons to make either a sharp bend or to wriggle through a gauntlet of magnets, both resulting in a shower of Tâ’’rays, though of different bandwidths. The latter, a specialized portion of a synchrotron known as a free-electron laser, is in use at a new facility in Novosibirsk, Russia. Last August scientists there reported the production of a terahertz laser beam of up to a record 400 watts.
The other synchrotron version, the sharp bend, is in use by Gwyn P. William and colleagues at the Jefferson Lab, in Newport News, Va. Forcing a fast-moving electron to make a sharp turn produces a broad spectrum of T-rays instead of the single frequency of a laser beam. At many tens of watts, the Jefferson machine is still orders of magnitude more powerful than most other sources. In fact, it may be powerful enough to penetrate some distance into the ground and discover land mines and IEDs at a distance; because of this the U.S. military has contracted Advanced Energy Systems to design an electron accelerator and T-ray generator compact enough to fit in a Humvee and capable of producing 1 W of radiation. The portable version would have a design more akin to a free-electron laser, but it would produce a broader spectrum of T-rays than a laser can.
There are many types of T-ray sources that have smaller footprints than the enormous electron racetracks in Newport News and Novosibirsk. These depend on combining electronics with lasers, befitting the radiation’s straddling of the two worlds.
Gigahertz-frequency oscillation is no big deal—the inexpensive circuits in your cellphone are a testament to that fact. But it’s quite another thing to build a circuit that oscillates trillions of times per second at terahertz rates. Even the 2006 record holder for the fastest-switching transistor in the world, the 845-GHz device made by Milton Feng’s group at the University of Illinois at Urbana-Champaign, is barely in the terahertz range. However, for over a decade, scientists have been able to generate laser pulses so short that 10 trillion or more could fit in a single second. So the most common commercial method of making T-rays is to drive an electronic circuit with a picosecond pulse of laser light. Such a T-ray generator is basically a photosensitive semiconductor with a pair of antennas etched onto its surface. A voltage on the antennas sets up a strong electric field across the semiconductor between them. When the laser pulse strikes the semiconductor it creates pairs of charge carriers: electrons and holes. These accelerate across the semiconductor and through the antennas. For a femtosecond-long pulse, the rush of current lasts about a picosecond, about the period of one cycle of 1-THz radiation.
The resulting T-ray pulse is weak, with an average power only somewhere around a microwatt, but it’s still bright enough to produce still images. And the pulses have a couple of interesting side benefits. First, as with radar, timing the pulse’s echo as it bounces off an object gives the range to that object. Range is useful in processing multilevel T-ray images, such as a scan of a suitcase that might be difficult to interpret unless it had been scrutinized layer by layer. Second, pulses let you perform spectroscopy, the identification of a substance by the wavelengths of light it reflects. This capability comes from the fact that a single pulse actually comprises a broad swath of T-ray frequencies. You need only analyze the shape of the pulse’s echo to calculate which frequencies were absorbed and then look up what substances produce that absorption pattern.
The problem with pulses is that they are quickly absorbed and “smeared” in air, particularly humid air. After only a few meters in moist air, a 1-ps pulse lasts 30 ps, and the resolution of an image it forms degrades, as does its spectroscopic signal. Fortunately, the terahertz spectrum has a few transmission windows at frequencies that aren’t strongly absorbed in air. So one solution is to generate a continuous wave at one or more of those frequencies.
Researchers are already making such continuous-wave sources, basically with the same setup of a laser shining on the surface of an antenna-equipped semiconductor, but with the femtosecond laser replaced with a continuous one whose amplitude is oscillating at a terahertz frequency [see illustration " T-Ray Scanner”].
You start by focusing two infrared lasers in a device called a photomixer, with the lasers tuned so that the difference between their frequencies is a frequency corresponding to one of the terahertz transmission windows. The photomixer combines the lasers so that the resulting light “beats” at this terahertz- frequency difference. The beating laser drives a similar photoconductor-antenna structure to the one used to generate pulses, causing current to flow through it at the terahertz-beat frequency, thereby generating many microwatts of T-rays.
The method was demonstrated over a decade ago but became practical only a few years ago, thanks to pioneering work by researchers at the imaging start-up TeraView. The key was in a new type of photomixer, made of indium-gallium-arsenide, which could efficiently mix lasers of a wavelength easily carried on optical fibers. Channeling the lasers on optical fibers instead of having to carefully align laser beams with expensive optics has greatly simplified terahertz imagers and has also had the added benefit of driving down their cost.
These optoelectronic methods work well enough, but they are of limited brightness and are still quite cumbersome. What terahertz researchers really want is to replace these technologies with a bright, completely solid-state terahertz laser. It’s their best hope of getting imagers smaller, lighter, and cheap enough to mass-produce, not only because the light source is smaller but also because its higher brightness would allow for less expensive and more compact detector arrays. Unfortunately, the wavelength of semiconductor lasers is largely determined by the materials that are used to make them, and none naturally produce T-rays.
A device called a quantum cascade laser, invented in 1994 by Federico Capasso, among others, at Bell Labs, could be a big part of the answer. Unlike other semiconductor lasers, QC lasers can be engineered to emit any of a range of micrometer-wavelength light, including terahertz wavelengths. The secret to the QC laser is that its wavelength is determined by the thicknesses of the layers of semiconductor that make it up—something that can be carefully controlled.
Here’s how it works: lasers emit light when electrons that have been excited to a particular energy level fall to a lower energy level. A key difference between a laser and an ordinary light emitter is that there are always more electrons in the excited state than in the lower energy level. In a QC laser that aspect is guaranteed by sweeping fallen electrons from the lower, unexcited state into a third state, at a still lower energy level. In the QC laser these energy levels exist in three layers called quantum wells, each nanometers thick . Quantum wells are structures so thin that, from an electron’s perspective, they are two-dimensional. Confinement in a quantum well makes the electrons behave as though they were bound to an atom, with their energy constrained to certain specific levels.
An electron injected into the highest energy level falls to the lower one, emitting radiation (photons) of a wavelength that is determined by the thicknesses of the quantum wells. The electron then immediately falls into the still lower third state, emitting a quantum of heat called a phonon. What’s really remarkable is that this same three-layer structure can be repeated more than two dozen times. At each structure the electron goes through exactly the same dance, emitting the same color photon. So a single electron can emit 24 or more photons on its journey through the QC laser, as if it were falling down a set of stairs and emitting a photon at each step.
Last year, researchers at Sandia National Laboratories, in Albuquerque, used QC lasers to produce 138 milliwatts of terahertz laser power—a record. The one catch, and it’s a pretty big one, is that QC lasers must be cooled to within 10 degrees of absolute zero to perform at that level. At liquid nitrogen temperature, 77 K, QC lasers can’t even crack 10 mW. So the aim of much of the research into terahertz QC lasers is in improving the power output at higher and higher temperatures. The dream is a terahertz QC laser that operates at room temperature, or at least at 250 K, which is in the range of compact, inexpensive thermoelectric coolers.
T-rays can be detected in a number of ways. But one of the more common detector types is merely an extension of T-ray generation technology. Recall the picosecond-pulse generators and the continuous-wave generators. You can easily take the laser beam, split it, and feed it to another photoconductive antenna structure. But instead of applying a voltage across the antennas to push current through them and generate terahertz radiation, you measure the current through the antennas. As in the pulse-generation scheme, when the laser pulse hits a photoconductor, it creates short-lived pairs of electrons and holes. These then flow through the antenna under the influence of the electric field of incoming terahertz waves. So the current in the antenna, which is amplified, acts as a measure of terahertz radiation.
Because the detector is sensing T-rays only during the picosecond or so that the laser pulse allows, it takes several pulses to get the full waveform of the incoming T-rays. To get the full waveform, small increments of delay in the form of a longer path for the laser are added to the detector’s optical fiber line. Measuring the electric field at a number of increments produces slices of the terahertz wave that can be pasted together in a computer.
The same scheme works when pairs of tuned lasers are used instead of pulses. Recall that the lasers are tuned so that the difference in their frequencies is equal to a terahertz frequency. When T-rays hit the antenna, they mix with the terahertz frequency of the combined lasers to produce a dc signal. These two schemes are how detectors work in systems built by TeraView, Picometrix, and others.
Of course, detection is only the start of image making. The simplest way of producing an image is to scan a single transmitter and detector over an object and record the phase and amplitude of the T-rays that reflect back at each point. State-of-the-art terahertz-imaging security systems are capable of such raster scanning at a rate of 100 pixels per second, certainly not fast enough for video and only marginal for scanning a bag on a conveyor belt. A briefcase containing a gun, a glass bottle, and a knife would take half an hour to scan at a resolution of 1.5 millimeters per pixel using a T-ray pulse-based imager from Picometrix.
Although there are no terahertz camera chips, there are infrared camera chips, and you can tweak those so they can pick up T-rays. Such chips detect infrared radiation at each pixel because the radiation reduces the resistance of a minuscule patch of semiconductor there. By themselves, some of these chips are slightly sensitive to terahertz radiation, but to get a decent image you need a bright source such as the QC lasers under development.
Another infrared detector concept is electro-optic terahertz imaging. In this scheme, T-rays striking certain types of crystal—such as zinc telluride—will cause the crystal’s index of refraction to change. The result is that the polarization of infrared light passing through the crystal will rotate. Place a polarization filter between the crystal and a camera chip so that it blocks out any infrared light that hasn’t been rotated and you get an infrared facsimile of the terahertz image. In a sense, the terahertz radiation has been shifted up the electromagnetic spectrum into the infrared. Such cameras can produce pictures in less than one-sixtieth of a second, far quicker than raster scanners and fast enough to produce video. Unfortunately, they also erase the spectral information that lets you chemically fingerprint objects. All that information is wrapped up in the mix of T-ray wavelengths that strike the crystal, but the crystal’s change in refractive index, which produces the image, is relatively insensitive to color.
In an effort to get both speed and spectroscopy at a reasonable price, our team at New Jersey Institute of Technology, in Newark, has been developing an imager that, with only a dozen detectors, can produce complete images quickly enough for video and at a resolution comparable to what you’d expect from a kilopixel camera chip. The method, called interferometric imaging, relies on a common mathematical concept used in image processing, the spatial Fourier transform. According to Fourier theory, any signal can be broken down into the sum of many sine waves of different frequencies, phases, and amplitudes. Though it is less intuitive, the same can be said of any image. To get a grasp of spatial frequency in an image, imagine a photo of an American football referee in the traditional black-and-white vertical-stripe jersey. The spatial Fourier transform of that image would be dominated by the frequency that matches the jersey’s stripes.
Instead of obtaining the image itself, an interferometric imager derives a picture’s spatial Fourier transform from a measure of the electric field when a terahertz wave reaches a pair of detectors. Then a computer reverses the transformation to give the real image. In an interferometric imager, spatial frequencies are represented by the distance between two detectors on an array [see illustration, "T-Ray Scanner"]. The imager takes advantage of the fact that when you increase the number of detectors, the number of spatial frequencies you can measure increases by a greater amount. For example, imagine three detectors spaced unevenly along a line. There are three different distances between pairs of detectors—1 to 2, 2 to 3, and 1 to 3—and thus three spatial frequencies. Now, add a fourth detector farther out on the line. The pairs are now 1 to 2, 2 to 3, 3 to 4, 1 to 3, 1 to 4, and 2 to 4, or six spatial frequencies. In general, for n detectors, the number of spatial frequencies is n(n–1)/2.
TeraherTz and the spectral fingerprint
The Fourier image is acquired almost instantly, and the main limit on the camera’s speed is the time it takes the computer to digitize the data from the detectors and perform the needed calculations. The resolution of a reverse-transformed image comes from the number and arrangement of spatial frequencies represented in the transform. Imagine trying to represent the picture of the football referee in a Fourier transform, restricting yourself only to the jersey’s frequency and one or two others. When you reverse the transformation you might be able to make out the bars in the jersey, but the rest of the picture would be a blur. But a transform having dozens, or—better yet—hundreds of spatial frequencies represented would reconstruct the picture reasonably well.
One of the imagers we’re developing is made up of 12 detectors arranged in a spiral pattern on a 1-meter disk; it can measure 66 spatial frequencies. The imager is good enough to resolve a 2.5-centimeter square of RDX plastic explosive at 50 meters.
Powerful as terahertz imaging is, no imaging or detection technology can reliably find every threat to security. Each technology has its own strengths and weaknesses. But when several are used as part of a sensor suite, their collective strengths are integrated. As an example, consider how you would screen trucks at a port or other checkpoint. There are detection systems for monitoring nuclear radiation, and X-ray systems could penetrate through the truck’s metal walls to probe the contents inside. But how would the driver himself be screened? The driver may be carrying concealed weapons, a bomb, a trigger mechanism for a weapon hidden in the truck, or other threats that would be overlooked if only the vehicle were screened. You can’t X-ray the driver, but you can subject him to a suite of safer imagers including a T-ray imager.
That said, security screening is a more challenging application than it might seem at first. It has to work in real-world conditions, has to be small enough to fit in already crowded spaces such as an airport security checkpoint, and has to have a very low rate of false alarms. Otherwise it would become a bottleneck in the flow of people or goods.
T-ray imagers are close to meeting all those requirements for short-range scanners, as long as the additional feature of spectroscopy is excluded. That feature may take a few more years of work, requiring smaller, brighter T-ray sources and more sensitive detectors.
As for imagers that can see a suspicious object in your shopping bag from 50 meters, those are more like five years away at our current rate of progress. Adding the ability to tell whether that object is a taco or some TNT is an additional five years away, at least. And getting them small enough to wear like spectacles? We won’t even hazard a guess. So if you see them advertised in a comic book, trust us: they’re fake.
About the Authors
John F. Federici and Dale Gary are professors of physics at New Jersey Institute of Technology, in Newark. Robert Barat is a professor of chemical engineering and Zoi-Heleni Michalopoulu is a professor of mathematics, also at NJIT.
To Probe Further
The U.S. National Research Council commissioned a report on the use of millimeter-wave and terahertz technologies in security. See Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons (National Academies Press, 2007).