In search of new sources of terahertz radiation

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Terahertz radiation, invisible to the human eye, can be placed in the electromagnetic spectrum between infrared radiation and microwaves. It is often called the “the forgotten band”, or “the Terahertz gap.” This is because we still have not found the way to use it in practice, although in theoretical terms it demonstrates vast potential for application. One of the most substantial problems that needs to be solved is the structure of the efficient and user-friendly emitters of this radiation. The most recent discovery, in which Polish scientists from the CENTERA centre, operating as part of the International Research Agenda Programme, have contributed their share, provides an opportunity to build new terahertz radiation sources which can be retuned with the use of a magnetic field. The results of their work were published in the prestigious scientific journal Nature Photonics.

Terahertz radiation (also referred to as T-waves, THz, or submillimetre radiation) has numerous unique properties which can be interesting from both the scientific and economic points of view. One of these is the fact that the waves can easily penetrate most non-metallic materials, such as plastic, paper, clothing, and wood, which makes them ideal for analysing the internal structure and composition of these materials.  Unlike X-rays and UV radiation, THz waves are not harmful to humans or animals, so no specific safety measures need be used for them. THz radiation also propagates in the air, and can provide vision under difficult atmospheric conditions, or the possibility to transfer huge amounts of data. Scientists are expecting to find numerous possible applications of submillimetre waves, for example in medical diagnostics and natural sciences (e.g. in terahertz microscopes), in communication (higher transfer speeds), safety (enhanced vision systems for working in difficult atmospheric conditions), and security (to identify hazards in mail packages). For these technologies to become a reality, it is first necessary to develop THz wave sources. “Since the 1940s and 1950s, physicists have been dreaming about building a terahertz radiation source which would allow us to change wavelengths with the use of a magnetic field. Cyclotron resonance could be one of such sources, but a certain effect related to the behaviour of electrons can pose certain problems. In our experimental work, we have managed to demonstrate that this negative effect can be eliminated by applying an appropriate alloy of mercury, cadmium and tellurium as a material. It is a unique matter in which the energy of electrons is proportional to their speed, not to the square of the speed value. This makes electrons behave in a totally different way, and this type of matter could serve as a terahertz radiation source,” said Professor Wojciech Knap and Dmytro But, PhD, from the Centre for Terahertz Research and Applications (CENTERA) in Warsaw, co-authors of a paper published in Nature Photonics.

CENTERA is a new Centre of Excellence financed from EU Funds by the Foundation for Polish Science under the International Research Agendas Programme (IRAP). It was established within the structures of the Institute of High Pressure Physics of the Polish Academy of Science. The centre has participated in studies on new sources of terahertz radiation carried out by several research teams, including teams from the University of Montpellier in France, Helmholtz-Zentrum in Dresden, Germany, and Laboratoire National des Champs Magnétiques Intensesin Grenoble, France. One of the members of the last team was Marek Potemski, PhD, laureate of a TEAM SGOP competition organised by the Foundation for Polish Science. The unique alloy of mercury, cadmium, and tellurium was developed by scientists from the Institute of Semiconductor Physics of the Russian Academy of Sciences in Novosibirsk, as part of the TERAMIR international cooperation programme.

Photo: Prof. Wojciech Knap / by OneHD