Visualization of topological shear polaritons in gypsum thin films

Low-symmetry crystals have emerged as a platform for exploring unique light-matter interactions in the form of hyperbolic shear polaritons. These excitations exhibit unique properties such as frequency-dispersive optical axes and asymmetric light propagation and energy dissipation. However, only non-vdW materials have been demonstrated to support hyperbolic shear polaritons, limiting their exotic properties and potential applications. Here, we introduce shear phenomena in low symmetry crystals by demonstrating 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. We observe a notable slowdown of group velocity, reaching values as low as 0.0005c. 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.