Better than X-rays
In 1895, German physicist Wilhelm Roentgen was messing about in a darkened lab, studying cathode rays using a device called a Crookes tube. As he worked, a screen on the far side of the laboratory began to glow mysteriously. Even more mysterious, when he held his hand in front of the tube, a shadow of his skeleton appeared on the screen. He had discovered X-rays.
By unlocking this unknown part of the electromagnetic spectrum, Roentgen sparked revolutions in science, art, health and (eventually) airport security. Other forms of electromagnetic radiation were already known at this time: William Herschel discovered infrared rays in 1800, and Heinrich Hertz created radio waves in the 1880s. (Gamma rays would be discovered a few years later in France by Paul Villard.) But X-rays changed how we see the world.
Now, there’s a new kid on the radiation block: terahertz rays. And they hold the promise to once again push back the boundaries of our perception. Wedged between infrared and microwave radiation, T-rays have the potential to create new revolutions in, among other things, medical imaging, bioweapons detection and industrial safety.
“One of the interesting things with terahertz rays is they’re capable of passing through certain things, like envelopes and paper and dry leaves, and they’re absorbed by other materials,” says Henry van Driel, dean of
physics at the University of Toronto, whose research involves T-rays. “That’s why security people are interested in this kind of radiation. It allows them to peer inside envelopes, for instance, and peer behind walls of boxes and see what’s inside.”
Terahertz radiation passes through many dry materials, but it is absorbed by water and certain large molecules. These absorption patterns mean that terahertz scanners can do more than identify shapes and sizes; they can
actually decode a substance’s molecular composition. “A lot of molecules, including things like DNA and biomolecules, have strong signatures in the terahertz range,” says Frank Hegmann, a terahertz researcher and associate
professor of physics at the University of Alberta. “Some people are
studying things like detecting anthrax for homeland-security issues.”
Because ink and paper are transparent in the terahertz range, T-rays provide a means of scanning a package or letter for anthrax (or other biological substances, including illicit drugs) without violating the privacy of the correspondent.
Terahertz radiation can also distinguish cancer molecules from healthy tissue, detect particles of many explosives and identify environmental pollutants.
Furthermore, T-rays are low-energy, which means that they carry none of the health risks associated with X-rays and other forms of radiation.
They have applications in the food industry, where it can scan through polypropylene packaging to measure the moisture of the contents, and in landmine detection, where the traces of explosives on the surface can serve as a marker for the mine below.
Terahertz rays can also be used to create a three-dimensional image of an object, by beaming the rays at multiple angles and measuring the resulting absorption, penetration, reflection and scattering patterns. These 3-D images can then be sliced any which way in order to examine the structure within.
Such imaging techniques aren’t unique to T-rays, but the size of objects terahertz radiation can scan is particularly useful. “You can do 3D imaging with a whole range of different wavelengths,” Dr. van Driel says. “The key here is the range of wavelengths — a hundred microns or so. [One-10th of a millimetre.] That wavelength relates to a certain scale size. That allows you to relatively efficiently image particles of that size, or structures that have features of that size.”
As it happens, cavities in human teeth suit the terahertz scale, meaning that dentists can be added to the list of people interested in terahertz scanners.
This broad range of applications recently prompted the Massachusetts Institute of Technology’s Technology Review magazine to name terahertz radiation as one of “10 emerging technologies that will change your world.”
To be fair, every scientist working in the field cautions that terahertz research remains in a highly exploratory phase. The radiation has many limitations, including an inability to penetrate even a thin layer of metal. Anthrax or drugs wrapped in tinfoil could easily elude a terahertz scan.
When it comes to medical imaging, the rays’ absorption by water is both a strength and a weakness. They can’t penetrate deeply through our water-rich muscle tissue, but they can differentiate between the moisture content of healthy tissue and cancer cells. This means that T-rays could replace certain biopsy procedures.
“Ultrasound can detect a tumour,” says Xi-Cheng Zhang, director of the Center for Terahertz Research at Rensselaer Polytechnic Institute in Troy, N.Y. “You can see the shape, but you don’t know if it’s cancerous. It might not be threatening. Terahertz is a way to help you identify what the tumour is. There was a clinical trial in the U.K. where they tested 24 patients, and the terahertz was accurate 100 per cent.
“That’s a small number,” he cautions. “But it’s still impressive.”
T-rays represent the last frontier of exploration in the electromagnetic spectrum. Scientists have known about this small band of wavelengths for decades (they discovered by Heinrich Rubens and Ernest Fox Nichols at the University of Berlin in 1896), but only recently has the technology existed to exploit them meaningfully.
“There’s nothing magical about it. It’s not a new form of energy; it’s not a new physical phenomenon. It just has a particular range of wavelengths, from 100 microns up to about a millimetre,” Dr. van Driel says. “The reason it has attracted interest recently is that people have found ways of making efficient sources of this type of electromagnetic radiation.”
Every area of the electromagnetic spectrum has a unique set of strengths and weaknesses. Infrared allows people to see in the dark. High-frequency (or shortwave) radiation bounces off the ionosphere and the ocean, allowing signals to zigzag far around the curve of the Earth. Ultraviolet rays cause certain chemicals to glow, which is useful for forensic scientists and bouncers.
While some reports have suggested that T-rays could eventually replace X-rays, in reality the two will complement each other at airports and hospitals, each doing a separate job.
Of course, that all depends on T-ray technology becoming more affordable, something that has proved to be a surprisingly stubborn obstacle. While there exist reliable methods to produce and detect radiation in most parts of the electromagnetic spectrum — including wavelengths on both sides of the terahertz range — T-rays have remained elusive.
“The terahertz window just doesn’t have many sources and detectors in it,” Dr. Hegmann says. “From the low-frequency side, you have radio waves and microwaves, and you can have microwave generators that try to go to higher and higher frequencies, toward the terahertz region, but it’s difficult to do that. And then on the visible side, you have visible lasers and infrared lasers. A carbon-dioxide laser is 10 microns so you’re getting closer and closer.”
The first cheap and reliable terahertz generators appeared on the scene a little more than a decade ago with the invention of the “femtosecond laser.” The laser sends out non-terahertz wavelengths in femtosecond pulses (that’s a millionth of a billionth of a second). By training the laser on certain semiconductors, the beam causes electrons to excite and relax in ultrafast cycles that then result in the release of terahertz rays.
This indirect source made T-ray research viable, but widespread commercial use will depend on finding an even cheaper radiation source. It might be the “quantum cascade laser” developed by Bell Labs. This revolutionary device gives scientists greater freedom to control the wavelength of a laser beam. While it works most reliably in the infrared spectrum, it has been tweaked to shoot out energy in the terahertz region.
As always, the scientists speak cautiously. “Quantum cascade lasers have limitations. They generate in a narrow band. You have to know your target, and they don’t operate at room temperature,” Dr. Zhang says, referring to the fact that quantum cascade lasers currently work only at temperatures near absolute zero.
But many people expect that obstacle to be overcome. “They are very cheap, very high in power, and reliable,” Dr. Zhang says. “My guess is they will take a pretty good part of the market in the future.”
At the moment, practical applications are limited to organizations with larger-than-average budgets, such as the U.S. National Aeronautics and Space Administration, which can use terahertz technology to detect flaws in the insulation on the space shuttles.
Researchers say widespread use of terahertz scanning is still at least three to five years away, though even that figure comes with copious caveats. “It’s not an isolated field,” Dr. Hegmann says. “It depends on developments in different fields.”