Unidirectional Ray Polaritons in Twisted Asymmetric Stacks

TL;DR

This paper demonstrates unidirectional ray polaritons in twisted asymmetric van der Waals stacks, showing their tunable, ray-like propagation for potential applications in nanoimaging, sensing, and thermal management.

Contribution

It introduces a new type of directional polariton propagation in twisted asymmetric stacks, expanding the scope of twistoptics beyond phonon polaritons.

Findings

Unidirectional ray polaritons are observed in twisted hyperbolic stacks.

Their propagation is tunable by twist angle and frequency.

Potential applications include nanoimaging and sensing.

Abstract

The vast repository of van der Waals (vdW) materials supporting polaritons offers numerous possibilities to tailor electromagnetic waves at the nanoscale. The development of twistoptics - the modulation of the optical properties by twisting stacks of vdW materials - enables directional propagation of phonon polaritons (PhPs) along a single spatial direction, known as canalization. Here we demonstrate a complementary type of directional propagation of polaritons by reporting the visualization of unidirectional ray polaritons (URPs). They arise naturally in twisted hyperbolic stacks with very different thicknesses of their constituents, demonstrated for homostructures of $\alpha$-MoO$_3$ and heterostructures of $\alpha$-MoO$_3$ and $\beta$-Ga$_2$O$_3$. Importantly, their ray-like propagation, characterized by large momenta and constant phase, is tunable by both the twist angle and the illumination frequency. Apart from their fundamental importance, our findings introduce twisted asymmetric stacks as efficient platforms for nanoscale directional polariton propagation, opening the door for applications in nanoimaging, (bio)-sensing or polaritonic thermal management.