Enabling propagation of anisotropic polaritons along forbidden directions via a topological transition

TL;DR

This paper demonstrates how to control and steer anisotropic polaritons in van der Waals crystals along forbidden directions by inducing a topological transition, enabling new ways to manipulate light at the nanoscale.

Contribution

It introduces a method to induce and visualize topological transitions in polaritons in vdW materials, allowing control over their propagation directions.

Findings

Directional polaritons can be steered along forbidden directions.

Optical topological transition occurs when placing the slab on a negative permittivity substrate.

Real-space visualization of exotic intermediate polaritonic states.

Abstract

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.