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”].
