As the discovery of X-rays changed the world, terahertz rays pose the same potential, with implications for fields from medicine to the military.
T-ray technology, the waves of the future
The science fiction writer Sir Arthur C Clarke was famed for his ability to predict future breakthroughs - such as the human-like computer HAL in 2001: A Space Odyssey. Clarke ascribed his prescience to the application of some simple principles, among them being the rule that: "Any sufficiently advanced technology is indistinguishable from magic."
By that criterion, it is a wonder that most of Clarke's works were not replete with references to terahertz or "T-ray" technology, which is now beginning to emerge as one of the most amazing scientific developments of the 21st century. A form of electromagnetic radiation lying between microwaves and the infra-red waves on the spectrum, T-rays are so-named because of their frequency - around a million million cycles per second, or one terahertz. Emitted by virtually everything in the universe, their existence was first demonstrated over a century ago.
Yet only now are scientists learning how to exploit them. Last week, a team of researchers in the US showed how T-rays can be used to peer inside objects from tens of metres away - with implications for everything from medicine and defence to airport security. The technology has been slow to take off largely because of the nature of T-rays, whose characteristic wavelength makes them harder to generate than light or X-rays, and also leads to their rapid absorption by water molecules.
Now that these problems are being solved, the abilities of T-rays are starting to be exploited. And chief among them is being able to see inside otherwise opaque material. Until now, that has been considered the preserve of X-rays, whose far shorter wavelength and much higher energy can penetrate even solids like bone and steel. But such powers come at a cost: X-rays are so energetic that they can knock electrons out of atoms, creating so-called ions which can cause potentially lethal damage to living cells.
T-ray imaging works in an entirely different way. For a start, like infra-red radiation, T-rays are emitted by all objects at everyday temperatures - so there is no need to expose targets to a radiation source, as with X-rays. All that is required is some way of detecting the T-rays being naturally emitted. Taking advantage of this has taken a long time, however. A key breakthrough occurred in the mid-1990s, with the emergence of lab-top lasers capable of the ultra-short pulses needed to create T-rays. The physicists Bin Bin Hu and Martin Nuss at AT&T Bell Labs in New Jersey were using T-rays to examine microchips when they had an idea - and turned them on to a slice of bacon. They found that T-rays revealed the internal composition of the tissue in an image that looked like a conventional X-ray, the brightness of the image depending on the amount of water present.
It is this dependence on water that makes T-ray imaging possible. But it was also long considered to be its Achilles heel, as normal air is packed with water vapour, which rapidly mops up T-rays emitted by objects. And on the face of it, that rules out any real sci-fi applications, like being able to peer inside objects from far away. Now that barrier has been overcome with a neat bit of lateral thinking by a team led by Professor Xi-Cheng Zhang, at Rensselaer Polytechnic Institute in Troy, New York. They found a way of remotely detecting the T-rays emitted by objects before they are absorbed by water vapour. The trick lies in using two laser beams to heat the air close to the object until it turns into a so-called plasma. The T-rays emitted by the object then strike this plasma, triggering a fluorescence effect which - unlike the original T-rays - can be detected far from the object.
In the current issue of Nature Photonics, Prof Zhang and his colleagues report how they have been able to use this trick to work out the internal composition of targets from around 20 metres away. They believe, however, that it should be possible to increase the range to hundreds of metres or even kilometres. While T-rays can be blocked by liquids or a sheet of metal, the breakthrough opens the way to the remote inspection of anything from chemical spills to terrorist suspects and suspicious-looking roadside devices.
Meanwhile, other applications for T-ray technology are starting to emerge. For example, one of the biggest challenges in cancer medicine is targeting the malignant cells while leaving their healthy counterparts alone. This had led medical scientists to hunt for distinguishing characteristics in cancer cells. Among them is higher than normal water content. This results in their absorbing relatively high levels of T-ray energy, which may be capable of stopping them multiplying. Researchers in Europe and the US are investigating this approach for treating skin cancer.
More conventional forms of cancer therapy are also likely to benefit from T-ray technology. One of the key problems in cancer surgery is ensuring that all the cancerous cells are removed. If even a few are left, the speed at which they multiply can lead to a fatal relapse. T-ray scanning could be used by surgeons to identify where healthy tissue stops and cancerous tissue starts - and thus improve the chances of removing all the cancer, while minimising the amount of surgery needed.
Researchers at TeraView, a terahertz technology company based in Cambridge, in the UK, have carried out a study of tissue samples taken from women with breast cancer, and showed that T-rays are effective at detecting the cancerous cells. When X-rays were discovered over a century ago, they led to a host of life-saving advances. It is not too much to hope that with the emergence of T-ray technology, history may be about to repeat itself.
Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England