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.