  Recent discoveries of polaritons in van der Waals (vdW) crystals with
directional in-plane propagation, ultra-low losses, and broad spectral
tunability have opened the door for unprecedented manipulation of the flow of
light at the nanoscale. However, despite their extraordinary potential for
nano-optics, these unique polaritons also present an important limitation:
their directional propagation is intrinsically determined by the crystal
structure of the host material, which imposes forbidden directions of
propagation and hinders its control. Here, we theoretically predict and
experimentally demonstrate that directional polaritons (in-plane hyperbolic
phonon polaritons) in a vdW biaxial slab (alpha-phase molybdenum trioxide) can
be steered along previously forbidden directions by inducing an optical
topological transition, which naturally emerges when placing the slab on a
substrate with a given negative permittivity (4H-SiC). Importantly, due to the
low-loss nature of this topological transition, we are able to visualize in
real space exotic intermediate polaritonic states between mutually orthogonal
hyperbolic regimes, which permit to unveil the unique topological origin of the
transition. This work provides new insights into the emergence of low-loss
optical topological transitions in vdW crystals, offering a novel route to
efficiently steer the flow of energy at the nanoscale.
