Record-breaking Terahertz Laser Beam

Claudia Gollner and her team developed an extremely efficient source of terahertz radiation. (Source: TU Wien)

Terahertz radiation is used for security checks at airports, for medical exa­minations and also for quality checks in industry. However, radiation in the terahertz range is extremely difficult to generate. Scientists at TU Wien have now succeeded in developing a terahertz radiation source that breaks several records: it is extremely efficient, and its spectrum is very broad – it generates different wave­lengths from the entire terahertz range. This opens up the possi­bility of creating short radiation pulses with extremely high radiation intensity.

“Terahertz radiation has very useful pro­perties,” says Claudia Gollner from the Institute of Photonics at TU Wien. “It can easily penetrate many materials, but unlike X-rays, it is harmless because it is not ionising radia­tion”. From a technical point of view, however, terahertz radiation is located in a frequency region which is very hard to access: Radiation with higher frequencies can be generated by ordinary solid-state lasers. Low-frequency radiation, on the other hand, as it is used in mobile communi­cations, is emitted by antennas. The greatest challenges lie exactly in between, in the terahertz range.

In the laser laboratories of TU Wien, a great deal of effort must therefore be put into generating the desired high-intensity terahertz radiation pulses. “Our starting point is the radiation of an infrared laser system. It was deve­loped at our Institute and it is unique in the world,” says Claudia Gollner. First, the laser light is sent through a non-linear medium. In this material, the infrared radiation is modified, part of it is converted into radiation with twice the frequency. “So now we have two different types of infrared radiation. These two kinds of radiation are then super­imposed. This creates a wave with an electric field with a very specific asymmetric shape,” says Gollner.

This electro­magnetic wave is intense enough to rip electrons out of the molecules in the air. The air turns into a glowing plasma. Then, the special shape of the wave’s electric field acce­lerates the electrons in such a way that they produce the desired terahertz radiaton. “Our method is extremely efficient: 2.3% of the supplied energy is converted into terahertz radiation – that is orders of magnitude more than can be achieved with other methods. This results in excep­tionally high THz energies of almost 200 μJ,” says Claudia Gollner. Another important advantage of the new method is that a very broad spectrum of terahertz radiation is generated. Very different wave­lengths throughout the terahertz range are emitted simul­taneously. This produces extremely intense short radiation pulses. The larger the spectrum of different terahertz wavelengths, the shorter and more intense pulses can be generated.

“This means that for the first time a terahertz source for extremely high intensity radiation is now available”, says Andrius Baltuska, the head of the research group. “Initial experi­ments with zinc-telluride crystals already show that terahertz radiation is excel­lently suited to answer important questions from material science in a completely new way. We are convinced that this method has a great future.” (Source: TU Vienna)

Reference: A. D. Koulouklidis et al.: Observation of extremely efficient terahertz generation from mid-infrared two-color laser filaments, Nat. Commun. 11, 292 (2020); DOI: 10.1038/s41467-019-14206-x

Link: Photonics Institute, Technical University Wien, Vienna, Austria

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