Why is terahertz radiation blocked by water?

Full view with terahertz rays

We perceive our surroundings through our eyes and can distinguish between transparent and opaque materials, for example. What is now transparent depends heavily on the wavelength and frequency of the light. Things that are opaque in the visible range can be x-rayed, for example. That could be broken limbs at the doctor's or suitcases at the security check in airports. However, X-rays have the disadvantage that they are ionizing and therefore harmful to humans and the environment.

So-called terahertz rays, like X-rays, allow interesting views through materials. However, they are not ionizing and are therefore harmless, according to the Swiss Federal Institute of Technology in Zurich. Most packaging materials such as paper, cardboard or plastic are transparent to these rays, but metals and water-containing substances are not. The main reason why terahertz cameras and scanners are not already being used across the board is the difficulty in detecting the rays. So far, the weak heat output of the radiation has to be measured very unspecifically, which is only possible at temperatures below -200 ° C.

Researchers at the Laboratory for Nonlinear Optics at ETH Zurich rely on terahertzPulse. With these, the waveform can be observed directly. The researchers run a short laser pulse through what is known as an electro-optical crystal. The polarization state of the laser pulse is modified by the traveling terahertz wave. This can be measured relatively easily using conventional methods at room temperature.

In recent years, ETH researchers have been working on the development of a new type of ionic organic crystal. According to the ETH, it has almost ideal properties for applications in photonics. The crystal bears the cryptic designation "4-N, N-dimethylamino-4'-N'-methylstilbazolium tosylate", abbreviated DAST. From the laser pulse of around one hundred femtoseconds (10-13 Seconds) duration, the terahertz pulse in DAST is created by the so-called optical rectification, a non-linear process.

Another advantage of terahertz pulses results directly from the short pulse duration. If the pulse consists of only one or two oscillation cycles of the electric light field, this corresponds to a very broad frequency spectrum. It can span five or more octaves. Spectroscopic measurements can thus be carried out with individual pulses. Many materials can therefore be identified by their characteristic absorption patterns in the terahertz range, like with a fingerprint. Because most packaging materials are transparent to terahertz radiation, this identification also works for samples in envelopes or sealed plastic bags.