Enabling Propagation of Hyperbolic Polaritons Along Forbidden Directions

As has been shown so far in this thesis, polaritons with in-plane hyperbolicity and ultralow losses in van der Waals (vdW) crystals promise unprecedented manipulation of light at the nanoscale. However, these polaritons present a crucial limitation: the directions along which they are allowed or not to propagate are intrinsically determined by the crystal structure of the host material. Therefore, there are forbidden directions of propagation which are rooted on the arrangement of the atoms of the host material, in principle hard to modify. In this chapter, we demonstrate that in-plane hyperbolic phonon polaritons in vdW crystals—in particular \(\alpha \) -phase molybdenum trioxide—can be directed along forbidden directions by inducing an optical topological transition, which emerges when the slab is placed on a substrate with a given negative permittivity—in particular, we choose 4H-silicon carbide. By visualizing the transition in real space, we observe exotic polaritonic states between mutually orthogonal hyperbolic regimes, which unveil the topological origin of the transition: a gap opening in the dispersion. This work provides insights into optical topological transitions in vdW crystals, which introduce a route to direct light at the nanoscale. The near-field experiments reported in this chapter were carried out in collaboration with Dr. Jiahua Duan (University of Oviedo, Spain). Sample fabrication was performed by Dr. Jiahua Duan (University of Oviedo, Spain) and Dr. Iván Prieto (Institute of Science and Technology, Austria). Some of the calculations were performed in collaboration with Kirill Voronin (Moscow Institute of Physics and Technology, Russia). The results were published in “Enabling propagation of anisotropic polaritons along forbidden directions via a topological transition” by Jiahua Duan, Gonzalo Álvarez-Pérez et al. Science Advances 7(14), eabf2690 (2021).