Surface plasmon polaritons in thin-film Weyl semimetals

Surface plasmon polaritons are electromagnetic waves that travel along the metal-insulator interfaces. Owing to their unique properties, they have shown great potential for developing new plasmonic devices such as optical devices. A good example of such novel innovations is the Weyl semimetals exhibiting unusual optical responses emanating from its topological nature. This has led to theoretical studies of the propagation of the surface plasmon polaritons along with the interface of the Weyl semimetals and insulator.

In actual applications, Weyl semimetals are constructed between two insulators. Thus, the surface plasmon polaritons localized at the interface hybridize to form two new mixed surface plasmon polariton modes i.e. short-range and long-range surface plasmons. In a recently published literature, the authors noted that the surface plasmon polariton dispersion can be controlled by fine-tuning of the various parameters. However, this requires the understanding of the influence of these parameters on their dispersion which have not been fully explored.

To this note, researchers at the University of Tokyo: Dr. Tomohiro Tamaya, Professor Takeo Kato and Kota Tsuchikawa in collaboration with Professor Satoru Konabe from Hosei University and Dr. Shiro Kawabata at Nanoelectronics Research Institute theoretically investigated the propagation of surface plasmon polariton in thin-film Weyl semimetals. In particular, they showed the effects of hybridization between the plasmons localized at the metal-dielectric interface on the properties of the surface plasmon polariton. Their work is currently published in Journal of Physics: Condensed Matter.

Fundamentally, the authors considered three configurations of the axion vector: perpendicular, Voigt, and Faraday configurations. For each of them, the dispersion curves of the mixed modes describing axial anomaly in Weyl semimetals were calculated. For a sufficiently small Weyl semimetal thickness, the localized surface plasmon polaritons at the upper and lower boundaries hybridized to form mixed surface plasmon polariton modes. Even though non-reciprocity could appear in the Voigt configuration without hybridization, it may disappear due to the mutual interference of the surface plasmon polaritons in the symmetric tri-layers. This was attributed to the difference between the dielectric constants.

Just like the external magnetic fields, the disappearance of the surface plasmon polariton modes caused by the chiral magnetic effects in Weyl semimetals was observed. The disappearance magnitude was maximum in Voigt configuration, moderate in perpendicular configuration and minimum in the Faraday configuration. Therefore, the Voigt configuration was picked as the most suitable for the actual applications of Weyl semimetals. On the other hand, the strength of the non-reciprocity and the disappearance interval was controlled by fine-tuning the vital parameters: Weyl semimetals thickness, axion field direction, and the dielectric constants.

According to the authors, the study by Tomohiro Tamaya and colleagues will pave the way for the development of new devices. For instance, it was worth noting that when the dielectric constants of the outer insulators were tuned, stable short-range surface plasmon modes were obtained while long-range surface plasmon mode disappeared. This showed the potential to develop plasmonic waveguides capable of reducing the beam radius to nanometer order beyond that for light diffraction limit. Also, the study enables the exploitation of the topological nature of Weyl semimetals that will further lead to the development of advanced plasmonic devices.