Abstract
The lattice symmetry of a crystal is one of the most important factors in determining its physical properties. Particularly, low-symmetry crystals offer powerful opportunities to control light propagation, polarization and phase1–4. 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 scales5. Here we show that monoclinic crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid-infrared to far-infrared due to shear phenomena in the dielectric response. This feature emerges in materials in which the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals in which several oscillators with non-orthogonal relative orientations contribute to the optical response6,7. Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic1,3,4 and hexagonal8,9 crystal systems, unveiling new features, such as the continuous evolution of their propagation direction with frequency, tilted wavefronts and asymmetric responses. 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 minerals10, many common oxides11 and organic crystals12, greatly expanding the material base and extending design opportunities for compact photonic devices.
Highlights
Monoclinic crystals make up the largest crystal system, with around one-third of the minerals of Earth belonging to one of its three classes[18]
As an exemplary material to demonstrate this phenomenon, we study beta-phase Ga2O3, which has gained a large amount of research and industrial attention for its high breakdown field[21] and applications in photovoltaics[22], optical displays[23] and gas sensors[24]
Its low symmetry has two consequences on the polariton propagation when compared with more conventional hyperbolic materials with a diagonal permittivity tensor, such as hBN, aQ and α-MoO3
Summary
Materials featuring extreme optical anisotropy can support a hyperbolic response, enabling coupled light–matter interactions, known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales[5]. The interaction of light with low-symmetry materials is important, as it allows fine control over the phase, propagation direction and polarization[1,2,3,4] This control can be especially pronounced for sub-diffractional surface waves, for instance, surface phonon polaritons (SPhPs)[15] and surface plasmon polaritons (SPPs), supported at the surface of polar crystals and conductors, respectively. More exotic polaritonic responses may be expected in crystals with further reduced symmetry, such as monoclinic and triclinic lattices
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