Nanoscale Guiding of Infrared Light with Hyperbolic Volume and Surface Polaritons in van der Waals Material Ribbons

• Published in: Advanced materials (Weinheim) 2020-03, Vol.32 (9), p.e1906530-n/a
• Main Authors: Dolado, Irene, Alfaro‐Mozaz, Francisco Javier, Li, Peining, Nikulina, Elizaveta, Bylinkin, Andrei, Liu, Song, Edgar, James H., Casanova, Felix, Hueso, Luis E., Alonso‐González, Pablo, Vélez, Saül, Nikitin, Alexey Y., Hillenbrand, Rainer
• Format: Article
• Language: English
• Subjects: Boron nitride; Flakes; hexagonal boron nitride; hyperbolic phonon polaritons; Infrared radiation; linear waveguides; Materials science; mid‐infrared; Photonics; Polaritons; s‐SNOM; Waveguides
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.201906530

Van der Waals (vdW) materials host a variety of polaritons, which make them an emerging material platform for manipulating light at the nanoscale. Due to the layered structure of vdW materials, the polaritons can exhibit a hyperbolic dispersion and propagate as nanoscale‐confined volume modes in thin flakes. On the other hand, surface‐confined modes can be found at the flake edges. Surprisingly, the guiding of these modes in ribbons—representing typical linear waveguide structures—is widely unexplored. Here, a detailed study of hyperbolic phonon polaritons propagating in hexagonal boron nitride ribbons is reported. Employing infrared nanoimaging, a variety of modes are observed. Particularly, the fundamental volume waveguide mode that exhibits a cutoff width is identified, which, interestingly, can be lowered by reducing the waveguide thickness. Further, hybridization of the surface modes and their evolution with varying frequency and waveguide width are observed. Most importantly, it is demonstrated that the symmetrically hybridized surface mode does not exhibit a cutoff width, and thus enables linear waveguiding of the polaritons in arbitrarily narrow ribbons. The experimental data, supported by simulations, establish a solid basis for the understanding of hyperbolic polaritons in linear waveguides, which is of critical importance for their application in future photonic devices. Infrared nanoimaging and theoretical simulations are applied to study phonon polariton waveguide modes in nanoscale hexagonal boron nitride ribbons. Fundamental volume and hybridized surface modes are identified. Most importantly, the symmetrically hybridized surface mode does not exhibit a cutoff width, and thus allows for linear waveguiding of infrared energy in the narrowest ribbons that can be fabricated.