NCCR MUST - Terahertz generation

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The terahertz world:
From security to the structure of water


To realize a laser emitting in the terahertz region, a radiation known to be only very weakly harmful to health and situated between the microwave and infrared spectrum, this is the challenge taken up by Thomas Südmeyer in the frame of the National Centre of Competence in Research (NCCR) MUST (Molecular Ultrafast Science and Technology). This interdisciplinary project – coordinated by the ETH Zurich and the University of Bern – unites physics and chemistry for studying ultrafast processes in both natural and artificial materials.


Waves for radio broadcasting, microwave ovens, or mobile phones, but also infrared signals of TV remote controls – all these types of invisible radiation are part of our everyday life. They all have in common to belong to the wide family of electromagnetic waves. There exists however a region in the electromagnetic spectrum where radiation is not so easy to generate: these are the terahertz frequencies that are situated between the microwaves (domain of electrical radio signals) and the infrared (in the optical domain). An interesting property of terahertz radiation is that they are strongly absorbed by water molecules, which makes them the ideal tool for studying interactions of water (H2O) with other substances. By using extremely short terahertz pulses it will be possible to analyse ultrafast chemical processes, for example when a solvent binds to water molecules. “It is like a stroboscope effect”, explains Thomas Südmeyer. “The ultrafast pulses allow capturing a series of images showing the dynamics of interactions between atoms or molecules. Such information is currently inaccessible by other methods.”


At the doors of our airports

In addition to their potential for fundamental research, one property of terahertz radiation has already made its way to commercial exploitation: in the security scanners installed in airports, enabled by recent progress in semiconductor technology. In fact, many electrically isolating materials are transparent for terahertz waves, for instance human skin, paper, wood, cardboard, plastics, and textiles including clothes. Using these properties for detecting and visualizing hidden metallic objects is thus an evident application. And – contrary to x-rays – terahertz radiation is only weakly ionising and therefore in principle inoffensive to our health. Still, the scanners’ ability to show a person undressed on a control screen makes visible intimate parts of the body. Furthermore, to preserve personal privacy, the software used must include specific features such as anonymization of the face.


But terahertz radiation is not only of interest for security specialists. Also all domains of physics and chemistry can benefit, by clearly revealing the microscopic structure, at the level of the distance between atoms, of any material be it solid or liquid. Today’s technological limitations push us to realize sources of terahertz radiation that at the same time should be simple and practical to use, as well as financially interesting. Compact and ultrafast terahertz sources with high average power output are still very rare, a gap that the LTF aims to fill. “The new instruments we are developing will enable us to make measurements 100 times faster that possible with existing sources. This will result in an important gain of time, for instance when observing the dynamics of the water structure” smiles Thomas Südmeyer.


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