# Mode conversion of hyperbolic phonon polaritons in van der Waals terraces

## TL;DR

This paper shows how to convert hyperbolic polaritons between different modes using step-shaped van der Waals materials, enabling new nanophotonics applications.

## Contribution

The first demonstration of polariton mode conversion across dispersion orders in van der Waals terraces.

## Key findings

- Mode conversion from fundamental to high-order hyperbolic polaritons was imaged in hBN and α-MoO3.
- Step size in vdW terraces alters polariton mode conversion, confirmed by s-SNOM and simulations.
- The technique enables integration of different-order polaritons for nano-optical applications.

## Abstract

Electromagnetic hyperbolicity has driven key functionalities in nanophotonics, including super-resolution imaging, efficient energy control, and extreme light manipulation. Central to these advances are hyperbolic polaritons—nanometer-scale light-matter waves—spanning multiple energy-momentum dispersion orders with distinct mode profiles and incrementally high optical momenta. In this work, we report the mode conversion of hyperbolic polaritons across different dispersion orders by breaking the structure symmetry in engineered step-shape van der Waals (vdW) terraces. The mode conversion from the fundamental to high-order hyperbolic polaritons is imaged using scattering-type scanning near-field optical microscopy (s-SNOM) on both hexagonal boron nitride (hBN) and alpha-phase molybdenum trioxide (α-MoO3) vdW terraces. Our s-SNOM data, augmented with electromagnetics simulations, further demonstrate the alteration of polariton mode conversion by varying the step size of vdW terraces. The mode conversion reported here offers a practical approach toward integrating previously independent different-order hyperbolic polaritons with ultra-high momenta, paving the way for promising applications in nano-optical circuits, sensing, computation, information processing, and super-resolution imaging.

High-order modes of hyperbolic polaritons – hybrid light-matter interactions – in anisotropic van der Waals materials hold potential for nanophotonics applications. Here, the authors report the observation of polariton mode conversion in step-shaped terraces of hexagonal boron nitride and α-MoO3.