Visualization of topological shear polaritons in gypsum thin films

TL;DR

This paper demonstrates the existence of topological shear polaritons in gypsum thin films, revealing a transition from hyperbolic to elliptical shear polaritons, with potential for slow light and advanced photonic applications.

Contribution

It introduces the first observation of shear phonon polaritons in a low-symmetry, exfoliable crystal, expanding the understanding of light-matter interactions in non-vdW materials.

Findings

Existence of elliptical and canalized shear phonon polaritons in gypsum.

Topological transition from hyperbolic to elliptical shear polaritons.

Group velocity as low as 0.0005c, enabling slow light applications.

Abstract

Low symmetry crystals have recently emerged as a platform for exploring novel light-matter interactions in the form of hyperbolic shear polaritons. These excitations exhibit unique optical properties such as frequency-dispersive optical axes and asymmetric light propagation and energy dissipation, which arise from the presence of non-orthogonal resonances. However, only non-vdW materials have been demonstrated to support hyperbolic shear polaritons, limiting their exotic properties and potential applications. Here we introduce for the first time novel shear phenomena in low symmetry crystal thin films by demonstrating the existence of elliptical and canalized shear phonon polaritons in gypsum, an exfoliable monoclinic sulphate mineral. Our results unveil a topological transition from hyperbolic shear to elliptical shear polaritons, passing through a canalization regime with strong field confinement. Importantly, we observe a significant slowdown of group velocity, reaching values as low as 0.0005c, highlighting the potential of gypsum for "slow light" applications and extreme light-matter interaction control. These findings expand the application scope of low-symmetry crystals with the benefits that an exfoliable material provides, such as stronger field confinement, tunability, and versatility for its incorporation in complex photonic devices that might unlock new optical phenomena at the nanoscale.