An international team of scientists from Spain, China, Germany and the Czech Republic has published the results of a unique experiment. In the process, they have been able to image and analyse terahertz waves that propagate in the form of so-called plasmon polaritons along thin plates of a hessite crystal. Understanding this principle opens the way to the development of new communication technologies. The domestic representative in the team was researcher Andrea Konečná from Brno University of Technology. The work was published by the prestigious journal from the Nature "family", namely Nature Materials.

The researchers worked with a material called hessite (Ag2Te, silver telluride). Although the mineral is also found in the wild, for the purposes of the experiment it was prepared in the form of nanocrystals by experts from Fudan University in Shanghai, China. The samples used were less than 100 nanometres thin – thousands of times thinner than a human hair.

"Hessite has very interesting properties. We measured its optical properties, but at very low frequencies that humans cannot normally see. Using a special microscope, we have been able to build up so-called plasmons in the material, which are formed by the response of conduction electrons to light. In our case, we can imagine plasmons as waves that are localised very closely on the surface of the nanocrystal, which allows light at very low frequencies to be focused into this special material," says Andrea Konečná, a scientist from Brno University of Technology.

Hessite is also interesting because it behaves like a "bad metal". "Ordinary metals behave the same way in all respects; for example, they conduct an electric current in the same way, or just light in the form of surface plasmons. Hessite, on the other hand, behaves a little differently in each direction. To put it in hyperbolic terms, the light in it can be 'warped' depending on how you rotate the material, for example, or how you shine light on it," adds Konečná.

For the first time ever, scientists have been able to experimentally demonstrate the so-called anisotropy in a metal, i.e. the aforementioned potential to change the properties of a certain quantity depending on the choice of direction. From a research point of view, this is a very interesting possibility. "Imagine throwing a stone into water – the waves will then propagate evenly in all directions and form concentric circles on the surface. But the material we studied behaved in such a way that we observed ellipses: in one direction the surface plasmons propagated at a smaller wavelength, in the next at a larger one. And one could even get cases where they would not propagate at all in one direction. If we know this mechanism perfectly, we can work with it further," says Konečná.

Andrea Konečná works at BUT in the CEITEC centre and also at the Faculty of Mechanical Engineering, where she is building her own research group under the Department of Physical Engineering. Her expertise is in nanophotonics, a field that focuses on the study of light in nanostructures, as well as the development of new techniques in electron microscopy. In her published research, she has complemented the experiment with a theoretical model and calculations. Her collaboration was also helped by the many years of international experience she gathered, including at the Basque research centre CIC nanoGUNE BRTA, which led the research.

The results, in which the University of Shanghai, the University of the Basque Country, the International Physics Centre in Donostia and the Max Planck Institute for Chemical Solid State Physics in Dresden were involved, were published in the prestigious journal Nature Materials. "As far as professional journals in the field of materials are concerned, this is the absolute top," confirms Konečná. The research was also quite complex, she says, and would not have been possible without significant international cooperation. "It was a big challenge to make the measurements using a specially adapted microscope, because we were working with radiation that is not normally visible and required a not-so-ordinary laser to create it. Then, to be able to visualise the surface plasmons at all, we came up with a trick where we put the material – very figuratively speaking – over a gold mirror, which was able to prolong their propagation," Konečná adds.

The results offer the promise of practical applications, although the scientists are still working on basic research. "Terahertz waves in particular are promising, a technologically important area of the spectrum of interest for the development of communication technologies. When we talk about terahertz, it is three orders of magnitude faster than the gigahertz waves used in today's technology, for example in computers. The waves could also be used to investigate the fundamental properties of materials at the nanoscale, so there is potential for applications." However, as Konečná points out, “we are still at the beginning and are trying to understand what properties the material has and how to improve it.”

Reference: Real-space observation of ultra-confined in-plane anisotropic acoustic THz plasmon polaritons S. Chen*, P. L. Leng*, A. Konečná, E. Modin, M. Gutierrez-Amigo, E. Vicentini, B. Martín-García, M. Barra-Burillo, I. Niehues, C. Maciel Escudero, X. Y. Xie, L. E. Hueso, E. Artacho, J. Aizpurua, I. Errea, M. G. Vergniory, A. Chuvilin, F. X. Xiu, R. Hillenbrand Nature Materials, DOI: 10.1038/s41563-023-01547-8