Recently, it was predicted that polaritons can propagate “anisotropically” along the surface of 2D materials, in which the electronic or structural properties are different along different directions. In this case, the velocity and wavelength of the polaritons strongly depend on the direction in which they propagate. This property can lead to highly directional polariton propagation in the form of nanoscale confined rays, which could find future applications in the fields of sensing, heat management or maybe even quantum computing.
Recently, we have discovered ultra-confined infrared polaritons that propagate only in specific directions along thin slabs of the natural 2D material molybdenum trioxide (alfa-MoO3) . Apart of directional propagation, polaritons on alfa-MoO3 show an extraordinarily long lifetime of 20 picoseconds, which is 40 times larger than the best-possible polariton lifetime in high-quality graphene at room temperature.
This work was just the beginning of a series of studies  focused on directional control and manipulation of light with the help of ultra-low-loss polaritons at the nanoscale, which can benefit the development of more efficient nanophotonic devices for optical sensing and signal processing or heat management.
 W. Ma, P. Alonso-González*, S. Li, A.Y Nikitin, J. Yuan, J. Martín-Sánchez, J. Taboada-Gutiérrez, I. Amenabar, P. Li, S. Vélez, C. Tollan, Z. Dai, Y. Zhang, S. Sriram, K. Kalantar-Zadeh, S-T Lee, R. Hillenbrand*, and Q. Bao* "In-plane anisotropic and ultra-low-loss polaritons in a natural van der Waals crystal"
Nature 562, pp 557-562, (2018).
. Javier Taboada-Gutiérrez, Gonzalo Álvarez-Pérez, Jiahua Duan, Weiliang Ma, Kyle Crowley, Iván Prieto, Andrei Bylinkin, Marta Autore, Halyna Volkova, Kenta Kimura, Tsuyoshi Kimura, M-H Berger, Shaojuan Li, Qiaoliang Bao, Xuan PA Gao, Ion Errea, Alexey Y Nikitin, Rainer Hillenbrand, Javier Martín-Sánchez*, Pablo Alonso-González* "Broad spectral tuning of ultra-low-loss polaritons in a van der Waals crystal by intercalation"
Nature Materials 19, 964-968 (2020).