Speaker
Description
The lattice symmetry of a crystal is one of the most important factors in determining its optical properties. Particularly, low-symmetry crystals offer powerful opportunities to control light propagation, polarization and phase1. Materials featuring extreme optical anisotropy can support a hyperbolic response, enabling coupled light-matter interactions, also known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales2,3.
Here, we show that monoclinic crystals, such as β-gallium oxide (bGO), can support hyperbolic shear polaritons, a new polariton class arising in the mid- to far-infrared due to shear phenomena in the dielectric response. This feature emerges in low-symmetry monoclinic (and triclinic) crystals, that is, in materials whose dielectric tensor cannot be diagonalized, due to the presence of multiple oscillators with non-orthogonal relative orientations contributing to the optical response4. We demonstrate hyperbolic shear polaritons in bGO experimentally using the Otto-type prism coupling experiments, by mapping out the azimuthal polariton dispersion. Further, we determine the full in-plane hyperbolic dispersion of a HShP mode, by following the frequency momentum dispersion at many azimuth angles. This allowed us to reveal the main novel features of HShPs, that is the continuous evolution of its propagation direction with frequency, tilted wave fronts and asymmetric responses.
Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic1 and hexagonal2 crystal systems. The interplay between diagonal loss and off-diagonal shear phenomena in the dielectric response of these materials has implications for new forms of non-Hermitian and topological photonic states. We anticipate that our results will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals, many common oxides and organic crystals, significantly expanding the material base and extending design opportunities for compact photonic devices.
References
[1] W. Ma et al., Nature 562, 557-562 (2018).
[2] J. D. Caldwell et al., Nat. Commun. 5, 5221 (2014).
[3] T. Low et al., Nat. Mat. 562, 557-562 (2018).
[4] M. Schubert et al., Phys. Rev. B 93, 125209 (2016).
