Opening up the last part of the spectrum

Terahertz (THz) waves occupy the part of the spectrum between light and radio between infrared and millimetre waves.

With wavelengths of 0.1 to 1 mm, terahertz waves can be used like light or X-rays to create detailed images of solid objects. They can pass through packaging and clothes and since they are non-ionising they are safer than X-rays.

The waves can probe the content of objects as well as their shapes, because of their ability to respond to chemical properties. This is because the frequency range of 0.3 to 3 THz matches the natural molecular vibrations of many common substances and biological materials.

As a result, you have a scanner that can not only detect a hidden package, but also show what is inside.

European research on terahertz waves could enable applications that include detecting tumours beneath the skin, a new and powerful kind of microscope for biological research and quality control in semiconductor and pharmaceutical factories, as well as smart security scanners.

Some terahertz instruments are already available commercially — though the 3D scanners found in some airports mostly use the adjacent millimetre waveband.

So why the need for more terahertz research?

According to terahertz expert Martyn Chamberlain, terahertz radiation is hard to generate, lying as it does in the ‘no-man’s land’ between electronics and optics.

Electronic generators cannot yet operate at frequencies above 0.3 THz, Chamberlain explains, while traditional terahertz lasers are too bulky for most practical applications.

This is set to change following the results of an EU-funded research project called TeraNova.

The four-year project is in its final stage and its partners have made several important developments in generating, using and detecting terahertz waves. For example, quantum cascade lasers (QCLs) are semiconductor devices that take advantage of quantum effects to operate at frequencies in the terahertz range.

The researchers were able to extend the range of operating frequencies down to 850 GHz and are on the brink of producing QCLs that operate with simple cooling instead of the liquid nitrogen previously required.

The project also developed lasers that produce intense pulses of near-infrared light lasting as little as one femtosecond. When these short pulses hit a special semiconductor target they produce ‘broadband’ terahertz radiation that has the potential for a range of research tools in chemistry, biology and basic physics.

To complement the improved terahertz sources, the researchers developed amplifiers and detectors.

The TeraNova partners used their new terahertz sources and detectors to create prototypes of two terahertz devices — one aimed at electronics manufacturers and the other at animal breeders.

The first device relies on the ability of terahertz waves to measure the concentration and mobility of charge carriers in semiconductor wafers, especially those created using a technique known as molecular beam epitaxy (MBE).

Traditional measurements of these properties require wafers to be sacrificed, so the non-destructive nature of the terahertz scanner is a step forward.

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