It is not easy to keep track of what’s happening in the installation measuring several square metres in size and consisting of mirrors, lenses, measuring electronics and other devices. However it is soon clear to the observer that, in spite of its complexity, the purpose of the apparatus here in the cellar of the HPF Physics Building on the Hönggerberg is simply to see through things. As soon as ETH researchers Arno Schneider and Marcel Stillhart start a measurement, the ETH logo slowly appears on the monitor even though to the naked eye the sample looks like a uniform film.

But how were the scientists from the ETH Laboratory for Non-linear Optics able to show (1) that the film is doubled and the logo was cut out of one layer? The measuring technique used was pulsed terahertz radiation. The equipment now standing there in the cellar is a test apparatus for a new way to generate and measure pulsed terahertz radiation.

Penetrating radiation, hard to detect

Terahertz rays are in the electromagnetic radiation spectrum in the frequency region of one trillion oscillations per second, i.e. between infra-red radiation and microwaves. They allow entirely new insights and views through things. For example the majority of packaging materials such as paper, cardboard, plastics or cloth are transparent to this radiation, whereas metals or substances containing water absorb it.

However this ability to see through things, which is easy to obtain and gentle – terahertz radiation is non-ionising and low-energy – is still little used because the rays are difficult to detect. For example the usual electronic instruments used for infra-red or visible light are ineffective. Detection via their thermal power is in turn very laborious because it is only possible without interference below –200° Centigrade.

To avoid this expensive cooling, researchers hit on the idea of using terahertz radiation in pulsed form. Its implementation was not possible until the development of laser and materials research in recent years. Pulsed terahertz radiation is generated when a short laser pulse is passed through crystals with a particular asymmetry. What can then actually be measured is the polarisation state of a second laser beam that is variously modified by the terahertz radiation passing through the specimen. The advantage of the method is that the detectors do not need cooling and the pulse waveform can be observed. Conclusions about the material being examined can then be drawn from the characteristic change in the waveform.

ETH becomes visible: Almost nothing is seen in ordinary optical photography (left) of the two superimposed films with the ETH logo punched out, whereas there is no problem recognising it thanks to the new terahertz spectroscopy (right) . The visualisation would also function through an envelope. (Photo: Arno Schneider) large

A new measurement crystallises out

ETH professor Peter Günter’s group, to which Schneider and Stillhart belong, has now developed and studied special crystals for a measuring system of this kind. A novel ionic organic crystal with the abbreviation DAST (which stands for 4-N,N-dimethylamino-4’-N’-methylstilbazolium tosylate) and which can now also be manufactured by ETH spin-off company Rainbow Photonics proved almost ideal for this. Arno Schneider then combined this crystal with a femtosecond laser in his dissertation and established the entire measuring system. The ETH scientists give a brief description of the measuring system in a scientific journal (2).

The new instrument now enables the absorption properties of various substances to be determined over a wide spectrum of terahertz radiation. If the substance is suitable for the system, this creates a characteristic pattern, a kind of absorption fingerprint. Alternatively a sample is scanned and its thickness or density is determined based on the signals.

Anthrax can be detected even inside a letter

What this could now actually mean is that for example anthrax or certain drugs can be recognised through notepaper on the basis of their specific absorption pattern, or a hole is revealed beneath a surface. Arno Schneider draws attention to the fact that basically very many things that are packaged or concealed can now be rendered visible. The system might even be used for skin cancer, because the response of healthy tissue to terahertz pulses is different to that of diseased tissue.

Schneider is convinced that their system has the potential for widespread use. In addition to the efficiency achieved, one reason he sees is that it uses laser pulses with a wavelength that is optimum for data transmission in glass fibre cables. This means that the telecommunications industry will offer increasingly more convenient and more reliable lasers for their system. By using a laser of this kind the ETH scientist wants to achieve his next goal, namely optimising and thus miniaturising the terahertz measuring system. In future there would no longer be any need to go down into the cellar of a physics building to see through things, and instead Schneider would be able to bring the measuring instrument with him and check a suspicious letter.