Your skin is silver. Your clothes are transparent, as is the brick wall of your house. Through it, you gaze out at the deep black of the daytime sky. Welcome to the terahertz world, one of the last unexplored regions of the electromagnetic spectrum.
For now, you can only visit it in your imagination. But engineers have high hopes that they will soon have the technology to take us into this alternative reality. And with it will come new ways to tackle a range of tasks, from diagnosing cancer to frisking airline passengers for hidden weapons.
With a frequency of around a trillion waves per second, terahertz radiation spans the gap between light and radio waves in the electromagnetic spectrum. It shares useful characteristics of both its neighbours: like radio waves, terahertz waves pass easily through some solid materials, yet they can also be focused like light to create sharp images. The radiation can also see into your body much like X-rays, but without the dangerous side effects. And its powers of molecular discrimination could see it put to work in settings as diverse as dentists’ surgeries and supermarkets. You may not have heard of these waves already, but take note. You’ll be hearing a lot more about them soon.
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So why haven’t we explored the terahertz world before now? After all, we’ve been exploiting electromagnetic radiation all the way from X-rays to radio waves for years. We have lasers that generate light at frequencies down to about 100 terahertz, and we have electronic circuits that operate at microwave frequencies up to around 100 gigahertz (see Diagram). So why so little in between?
One reason is that tried-and-tested technologies for generating light or radio waves come unstuck in this no-man’s-land between the two. Take radio waves. A radio transmitter relies on electrical circuits in which electrons oscillate back and forth to generate radio waves, typically anywhere between about 100,000 and a billion times each second. However, to create waves at terahertz frequencies, electrons would have to oscillate much more rapidly. To create these oscillations the circuits would have to be very carefully designed so that the oscillations don’t simply die away or the radiation doesn’t leak out in the wrong place.
Similar problems confront engineers hoping to build a terahertz laser- but now it looks like an international effort has cracked it. Lasers rely on the chemical characteristics of the materials they are made from to produce radiation at specific frequencies. Excite these materials with energy from light or an electrical discharge and their electrons gain energy, jumping from one energy level to another. The electrons then lose this extra energy by emitting photons. But the material used to make a terahertz laser relies on much smaller jumps between energy levels than the material used in conventional lasers: the energy difference is about 100 times smaller.
While there are plenty of materials in which the energy transitions produce photons of visible or infrared light, it is hard to find material that can yield terahertz radiation in the same way. That’s why engineers call this unexplored region the terahertz gap.
One solution is to make the material for ourselves. For some time, scientists have known that it is possible to alter the size of the energy jumps an electron makes by trapping it in a thin layer of semiconductor material such as gallium arsenide. The thickness of the layer determines the size of the jump, and this allows engineers to create transitions at energies that would otherwise be impossible or forbidden. Many such layers on top of each other create a “superlattice”, a synthetic crystal that acts as a material with an artificial energy transition. The superlattice can then be used as a laser medium: a voltage applied across it will force electrons to emit light at the forbidden frequency in a “quantum cascade”: as an electron in the first layer emits a photon, then tunnels through to the next layer where it emits another photon, and so on.
In 1994, Federico Capasso at Bell Labs in Murray Hill, New Jersey, and Jerome Faist, now at the University of Neuchâtel in Switzerland, built the first laser based on this notion. Today quantum cascade lasers are available for a range of otherwise unreachable frequencies in the mid-infrared range, but until recently a terahertz laser always eluded scientists who tried to build one.
Self-absorption
One of the biggest problems is that terahertz waves tend to be absorbed by the very materials that create them. It is only too easy to create a superlattice that emits terahertz waves, only to absorb them again before anything useful could be done with them. Earlier this year, however, Alessandro Tredicucci at the National Centre for Nanoscience and Nanotechnology in Pisa, Italy, together with his colleagues at Turin Polytechnic, Italy, and the University of Cambridge came up with a design in which a novel kind of waveguide- a funnel for radiation- is sandwiched into the superlattice at regular intervals. Although some light is reabsorbed, the waveguides channel most of it out of the superlattice before the stuff is lost. The laser operates at 4.4 terahertz, the world’s first at this frequency, and was announced in May in the journal Nature.
There are a few technical challenges ahead. For the moment, the laser only works when cooled to within 30 degrees of absolute zero and there is some debate as to whether it can ever be made to work at room temperature. But that may not be necessary for commercial applications. It’s relatively easy to cool electronic devices to 77 kelvin, the temperature of liquid nitrogen.
“Our next task is to increase the operating temperature to this level,” says Tredicucci. Growing the superlattice- which consists of 1500 separate layers- is also a challenge, but the technique required, known as molecular beam epitaxy, is already widely used to make processor chips for mobile phones. “We don’t foresee any major problems for larger-scale production,” he says.
But while Tredicucci works on improving his laser, others already have plans to bring terahertz waves to market. In a business park on the outskirts of Cambridge, a small start-up company called TeraView is putting the finishing touches to a range of terahertz devices that rely on another technique to produce the radiation. In the 1980s, researchers discovered that zapping certain types of semiconductor crystals with a very short pulse of light from a visible or infrared laser forces them to emit a short burst of radiation in the terahertz region, and this is how the terahertz source in TeraView’s products works.
The company is working on devices that reveal the molecular composition of materials. Terahertz radiation can be beamed at a target and the intensity of the reflected signal used to work out the target’s absorption spectrum. Light-frequency spectroscopy is a long-established tool for chemical analysis, but the terahertz spectrum can reveal some very useful extra information. A terahertz absorption “signature” is the result of both intermolecular bonds – those linking different molecules- and intramolecular bonds, those between different parts of molecules.
This means that it can be used not only to identify large organic molecules but to distinguish between different shapes, or polymorphs, that the same molecule can adopt. And the team at TeraView has discovered that by scanning the radiation across a sample of a material line by line, it is easy to draw a complete “molecular map” of the surface.
For the moment, TeraView’s prototype device is pretty bulky- it would just fit into the back seat of a car. But the biggest component by far is the laser required to zap the semiconductor, and the company plans to replace it with a powerful new-generation compact diode laser that can do the job just as well. “We think we can eventually make the device the size of TV remote control,” says Michael Pepper, a physicist at the University of Cambridge and a director of TeraView.
The potential applications for a hand-held instrument capable of accurately identifying complex molecules and their shapes would be huge. Drugs companies could use it to discover how chemicals change shape as they age – changes that can be critical, as different polymorphs sometimes react with the human body in different ways, but are very difficult to identify using conventional techniques. And supermarkets could monitor the characteristic organic molecules that fruits produce as they age to provide an accurate sell-by date.
The Cambridge team has also shown that it’s possible to use terahertz radiation to distinguish between different kinds of tissue in meat, such as fat and muscle- a result that the food-processing industry should find useful- and even to pick out cavities and decay in teeth.
But the device’s most important role could be the early detection of skin cancer. Don Arnone, TeraView’s chief executive, says that 85 per cent of cancers are thought to originate in the skin. And in the terahertz world, cancerous areas stand out clearly from non-cancerous ones. “You can see tumours growing beneath the skin,” says Arnone.
Exactly why tumours should show up so clearly under terahertz radiation is not entirely clear, but Arnone believes it may be linked with increased blood flow through tumours compared to surrounding parts of the body. The extra fluid this brings to the tumour could make it more visible in the scans. The company is testing its device at a local hospital and indications so far are that terahertz imaging could become an important tool in the early diagnosis of skin cancer. At present there is no accurate way of identifying skin cancers without taking a biopsy, says Arnone.
Meanwhile, another British group is building the gadget that could make terahertz radiation as famous as X-rays: a video camera that will show us the world through terahertz eyes. Just such a device is almost ready at the Rutherford Appleton Laboratory in Oxfordshire. And almost as impressive as the camera itself is its fast-track progress from conception to completion.
As many a frustrated researcher will say, university research is usually hobbled by the red tape of government finance and bogged down in the slow process of peer review. But RAL is pioneering an approach that aims to be as lean and efficient as a successful private start-up business, with its fast decision-making and the ability to create new products at a speed that leaves competitors standing. It has created a small, highly motivated team of scientists and engineers, given them a budget of ¬650,000, a strict deadline and the goal of designing and building the world’s first terahertz video camera. In conventional academic research, a programme like this would be fast if it took three years to bear fruit. “We started designing the system in July and we aim to have built it in October,” says Chris Mann, the engineer in charge of the project which has been named Star Tiger.
Video venture
The camera will have 16 detectors arranged in a square array. Each detector has two parts: a tiny T-shaped antenna a millimetre or so long that is tuned to pick up terahertz radiation and convert it into an electronic signal, and a set of specialised “optics” that can collect and focus terahertz waves onto the antenna.
Unlike the conventional glass lenses and mirrors used in light-imaging cameras or telescopes, the lenses in this terahertz version will consist of photonic crystals- stacks of tiny blocks etched from silicon that can focus or reflect radiation (Â鶹´«Ă˝, 23 March, p 32). The detectors will work at two frequencies- 0.3 and 0.25 terahertz- which the researchers believe will help the camera to distinguish between different materials.
The main challenge is to pack the antennas and their circuitry into the space required: the whole camera is designed to be smaller than a car battery so that it is portable. But Mann believes that advanced design techniques will help his team succeed where others trying to build similar devices have failed. The team will use powerful computers to simulate the design and will then fabricate the antennas and silicon lenses to unforgiving tolerances with state-of-the-art lithographic techniques.
The European Space Agency has put a lot of money into the Star Tiger project as it wants to study terahertz radiation from stars. For instance, astronomers believe they will discover a large number of new galaxies that emit radiation mainly in the terahertz part of the spectrum. Another funder is QinetiQ, the privatised British military research organisation. Clearly, surveillance and anti-terrorism operations would find many uses for Mann’s camera. This is where terahertz rays could cause quite a stir.
Since terahertz waves pass through cardboard, clothes and even walls without harm to body tissue or property, it will be possible to frisk people for hidden weapons at a distance, to read the contents of letters without opening them or to monitor people in their own homes. Plenty of people will object to such invasion of privacy.
In the US, courts have decided that metal detectors at airports do not represent unreasonable intrusion. But according to Jim Dempsey, deputy director of the Center for Democracy and Technology in Washington DC, the latest imaging technology is on a collision course with the US constitution. In July 2001, the US Supreme Court ruled that infrared cameras should not be used to peek inside people’s homes without a court warrant, and it is likely that similar regulations will be demanded for terahertz cameras.
“That rationale, applied to terahertz imaging cameras, would suggest that they cannot be used to see through clothes without a specific suspicion and judicial approval,” says Dempsey. One important factor will be the fineness of their imaging resolution. Will they, for example, be able to show more anatomical detail than infrared cameras, and if so, who will have access to these cameras and for what purpose?
But despite the risks of terahertz peeping Toms and secret police, it will be intriguing to see the world through such a camera. Nobody is quite sure how it will look, but existing cameras that see just a little further down the electromagnetic spectrum, at millimetre wavelengths, give a clue. In the millimetre world, humans appear highly reflective and metallic, like robots with silver skin. “We’ll probably look something like that,” says Mann. But he can’t be certain until he switches on his camera later this month and views the world for the first time through terahertz eyes.