Space Camera Blazes New Terahertz Trails

"When we started last June we set an ambitious goal: to build in four months the first compact submillimetre-wave imager with near real time image capturing using state-of-the-art micro-machining technology," said Peter de Maagt, ESA's StarTiger Project Manager, "we reached this goal when the first terahertz images were taken in September." This breakthrough in terahertz imaging opens up the possibility for a new generation of applications, not only related to space but also in many non-space fields, including medicine, pharmaceuticals, security and aeronautics.

StarTiger is a new approach for conducting research and development (R&D) launched last year by ESA. The concept is to bring together a small group of highly motivated researchers, grant them full access to laboratory and production facilities, remove all administrative distractions, and let them work for an intense period of four to six months. The goal is to achieve a quantum increase in a promising and important technology within a short period of time.

The first project was started at CCLRC Rutherford Appleton Laboratory (RAL) in June 2002 and was scheduled for four months. RAL was chosen as the best location for this particular pilot project with its advanced laboratories and all support technology required, and its specific expertise in relevant fields.

What is Terahertz?

Terahertz waves occupy a portion of the spectrum between infrared and microwaves, from 10¹¹ to 10¹³ Hertz.

Until now, this has been an unexplored part of the electromagnetic spectrum. However, terahertz waves are very interesting as they possess characteristics of both their neighbours: terahertz waves can pass easily through some solid materials, like walls and clothes, yet can be focused as light to create images of objects.

The imager built by the StarTiger team takes pictures at two frequencies, 0.25 and 0.3 THz, to create a two-colour picture to create a contrast between materials with different transmission and reflection properties.

The main advantage of a terahertz imager is that it does not emit any radiation; it is a passive camera capturing pictures of the natural terahertz rays emitted by almost everything, including people, rocks, water, trees and stars.

Space Applications

The imager could be applied in several areas of space science, including astronomy, atmospheric physics, and Earth and environmental monitoring by satellites. In the field of planetary, cometary and atmospheric sensing, it could have a major impact on instrumentation for monitoring issues.

In space astronomy, observing terahertz frequencies could provide answers to some key questions on how galaxies were formed in the early universe, and how stars form, and have been forming, throughout the history of the universe.

For environmental monitoring, a terahertz imager could be used to obtain data for studies on ozone depletion mechanisms. The frequencies can be selected to focus on exchanges between the troposphere and the stratosphere, adding information useful for studies on global climate changes.

"Observations from space may be on the verge of a revolution with the possibility of looking into the terahertz frequency range," said de Maagt. He emphasized that the wide array of potential applications for such an imager must not be underestimated.

"Apart from its use on space missions, our everyday lives could soon be reaping the benefits of this innovative technology," he added.

Non-space applications

If possible space applications are numerous, many more have been identified in non-space fields for use on Earth. A terahertz imager could open up a whole new range of systems in a variety of fields on Earth.

For instance, the imager could have various uses in the medical, dermatology and cosmetic sectors. Terahertz imaging is rapidly becoming recognised as a totally new diagnostic technique. By observing these types of waves, it is possible to see through many optically opaque materials. Terahertz waves could provide an image that has X-ray-like properties without the use of potentially harmful radiation. A terahertz imager is a passive instrument and, since the source of the signals for such an imager occurs naturally, is completely safe.

X-rays are important tools for dentists to evaluate patients' teeth and to pinpoint cavities and other signs of disease that might not be detected through only a visual examination. The terahertz imager could complement X-ray examinations without adding further health risks. No single type of sensor can provide all the information required for most tasks and combining the use of different sensors could become a valuable tool in many other medical fields.

Terahertz waves are also able to penetrate the uppermost layers of skin, making the early detection of skin cancers an interesting possibility. Skin cancer is usually curable if detected quickly enough. Looking into terahertz waves could provide earlier detection than is possible today.

And what about looking behind a dressing to see if a wound is healing correctly? This again should be possible using terahertz instrument.

An application of a different nature could be the detection of chemical and biological threats. As all materials emit terahertz waves, each having it own frequency pattern as a kind of 'fingerprint'. It could be possible to identify not only the existence of powder in envelopes and postal parcels, but also which kind of material is enclosed.

In airports, it could be possible to see through clothes and identify weapons, but not based upon the metal-detection techniques used today. Even non-metal explosives could be possible to spot since they may have their own terahertz 'fingerprint'.

"By exploiting operations at two frequencies 250 and 300 GHz, it should be possible to discriminate between materials of different types based on their optical properties e.g. reflectivity," said Roger Appleby, Technical Leader for the Passive Millimetre Wave Imaging Group at QinetiQ, a UK company. "When imaging the body, reflectivity falls and emissivity increases as the frequency is increased. These properties could be used to reduce false alarms in images of people collected for security scanning."

In aviation, terahertz frequencies could penetrate fog. When the technology is more developed, it is conceivable to build a monitor that would give a pilot a clear view ahead. A higher resolution imager than currently developed would be needed.

Another potential application came to light when a zoo asked the StarTiger team if a terahertz imager could look behind fur from a distance to diagnose an animal's health. Examining certain animals with thick fur, such as lions and bears, is not always easy. The team thought this was possible in future versions of an imager.

"We have recognised the huge potential in non-space applications, and in parallel to exploiting the use of terahertz waves and the StarTiger technology in space, we have kicked-off a commercialisation study to identify the best way of transferring it into terrestrial systems," said Pierre Brisson, Head of ESA's Technology Transfer and Promotion Office.

StarTiger team reaching the Terahertz Imager

The significant milestone of developing a working prototype was reached last September by the 11-member StarTiger team.

"At the end of July we had a prototype to test the various elements," explained Chris Mann, StarTiger Project Manager at RAL. "We had the scanning mechanism in place and we managed to demonstrate the first passive terahertz image in September at one frequency."

However, the resolution was low, 8-by-8 pixels, and the time needed to acquire the image was too long. The team then took the techniques further and pushed the development of a lithographically and micro-machined detector array.

"The final version was an enhanced imaging system incorporating a two-colour 16-pixel detector array of the size of a postage stamp. This advanced system incorporated revolutionary silicon micro-electrical-mechanical systems (MEMs) technology," continued Chris Mann. "The enhanced system delivered images that confirmed the mysterious nature of terahertz waves. An imager can show details of features under the skin, confirming the potential of this technique."

The team also tried to scan through a book, and the terahertz imager acquired pictures through different materials. Knives and even non-metallic items hidden in pockets or newspapers were clearly seen.

To reach the results in so short time was a tribute to the StarTiger R&D approach. In addition, several recent technology developments made it possible to build the StarTiger terahertz imager in its relatively small size.

Attempts to construct a camera operating in the submillimetre wave range have so far resulted in very bulky solutions. Such cameras have primarily been based on waveguide-based technology and usually assembled from discrete elements.

The recent advances in lithographically and micro-machining offered the potential for the realisation of the same performance with much smaller physical dimensions.

"The StarTiger imager fits within a briefcase, is easy transportable. The core of the instruments is the size of a cigarette package," said Peter de Maagt. "Next generation instruments will go for another magnitude smaller size, by using electronic scanning."

What's next? "With StarTiger we want to dramatically reduce the turnaround time for state-of-the art technology developments. This we have demonstrated as possible with this first StarTiger project," said Niels Jensen, ESA's Head of Technology Programmes Department.

Niels Jensen continued, "Putting together a highly motivated team in the same laboratory for an intense period with everything they can possibly require, we can create a synergy not attainable to the same extent in conventional R&D. This provides a real chance to advance a well-defined key technology and reach a scientific breakthrough within a relative short period."

"We intend to use this approach for selected key technologies in the future. The location for projects will of course change from project to project. The objective is to select the best European laboratory for the each specific technology, to provide the best support for the teams," concluded Niels Jensen.

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StarTiger is an acronym for 'Space Technology Advancements by Resourceful, Targeted and Innovative Groups of Experts and Researchers'. It is a pioneering ESA initiative designed to facilitate innovative research, launched last year under its Basic Technology Research Programme (TRP). The aim is to reach a quantum increase in a promising technology within a short period of time.