See through it all with terahertz rays
ETH researchers are developing a novel organic crystal. This crystal and new lasers are used to generate pulsed terahertz radiation that enables interesting insights and views through objects, and is therefore attractive for security checks at airports or in the postal service.
We perceive our surroundings through our eyes and can distinguish between transparent and opaque materials, for example. Now whether something is opaque depends greatly on the wavelength or frequency of the light. For example things that are opaque in the visible region can be viewed by passing X-rays through them, such as a doctor examining broken limbs or a security check on a suitcase at an airport. However, X-rays have the disadvantage that they are ionising and are therefore harmful to humans and the environment.
Great potential – difficult to detect
What are known as terahertz rays allow interesting visibility through materials, like X-rays, but are non-ionising and therefore harmless. The majority of packaging materials such as paper, cardboard or plastics are transparent to this radiation, whereas metals and substances containing water are not. The main reason why terahertz cameras and scanners are not already in widespread use lies in the difficulty of detecting the radiation. Up to now it was necessary to make a very non-specific measurement of the weak thermal power of the radiation, which is only possible at temperatures below -200°C.
Terahertz pulses enable easy detection
Researchers at the Nonlinear Optics Laboratory of ETH Zurich rely on terahertz pulses. In this case the pulse waveform can be observed directly. The researchers pass a short laser pulse through what is known as an electro-optical crystal. The polarisation state of the laser pulse is modified by the terahertz wave passing through it at the same time and can be measured relatively easily using conventional measurement methods, and this can be done at room temperature.
ETH Zurich has great know-how in crystal growing
In the last few years ETH researchers have worked on the development of a novel ionic organic crystal with almost ideal properties for uses in photonics. The result is the DAST crystal (4-N,N-dimethylamino-4′-N’-methylstilbazolium tosylate) which is manufactured by ETH Zurich and its spin-off company Rainbow Photonics. The terahertz pulse is formed in the DAST from a laser pulse with a duration of about a hundred femtoseconds (10-13 seconds) by what is known as optical rectification, a non-linear process.
Identifying a substance by its absorption pattern
Another advantage of the terahertz pulse arises directly from its short pulse duration. If the pulse consists of only one to two oscillation cycles of the electrical light field, this corresponds to a very broad frequency spectrum that can cover five or more octaves. Therefore spectroscopic measurements can be made with single pulses. In this way, many materials can be identified through their characteristic absorption pattern in the terahertz region in a similar way to a fingerprint. Because most packaging materials are transparent to terahertz radiation, this identification works even for samples in envelopes or plastic bags sealed by welding. Thanks to the ETH crystal and new lasers, there is nothing more to stop the further development of terahertz scanners to produce compact devices that can be used in airports or in the postal service, for example.
Contact:
Dr. Arno Schneider
Laboratory for Non-linear Optics
Tel. +41 44 633 23 29
E-Mail arno@phys.ethz.ch
Prof. Peter Günter
Tel. +41 44 633 22 90
E-Mail gunter@phys.ethz.ch
Source: ETH Zurich