TY - JOUR AB - Polaritons with directional in-plane propagation and ultralow losses in van der Waals (vdW) crystals promise unprecedented manipulation of light at the nanoscale. However, these polaritons present a crucial limitation: their directional propagation is intrinsically determined by the crystal structure of the host material, imposing forbidden directions of propagation. Here, we demonstrate that directional polaritons (in-plane hyperbolic phonon polaritons) in a vdW crystal (α-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 (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. AU - Duan, J. AU - Álvarez-Pérez, G. AU - Voronin, K. V. AU - Prieto Gonzalez, Ivan AU - Taboada-Gutiérrez, J. AU - Volkov, V. S. AU - Martín-Sánchez, J. AU - Nikitin, A. Y. AU - Alonso-González, P. ID - 9334 IS - 14 JF - Science Advances TI - Enabling propagation of anisotropic polaritons along forbidden directions via a topological transition VL - 7 ER -