Hyperbolic Shear Polaritons in Low-Symmetry Crystals

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 phase. 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 scales. Here we show that monoclinic crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid- to far-infrared due to shear dissipation in the dielectric response. This feature emerges in materials where the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals where multiple oscillators with non-orthogonal relative orientations contribute to the optical response. Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic and hexagonal 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 dissipation 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.

Manipulating polaritons at the extreme scale in van der Waals materials

“…Coupling vdW polaritons into mechanical vibrations generated by either optomechanical crystals 202 or surface acoustic wave resonators 203 appears to be promising future directions in for the quantum control of polaritons. Engineering polariton symmetries, including dispersion topological symmetry (or crystallographic symmetry) 204,205 and time-reversal symmetry 206,207 , is of fundamental importance for the study of surface electromagnetic waves in non-Hermitian states and non-equilibrium states.…”

Active Tuning of Highly Anisotropic Phonon Polaritons in Van der Waals Crystal Slabs by Gated Graphene

“…Particularly, HPhPs in some vdW crystals, such as alpha-phase molybdenum trioxide , (α-MoO 3 ) or alpha-phase vanadium pentaoxide (α-V 2 O 5 ), have their hyperbolic IFCs in the plane parallel to the faces of the vdW crystal slab (which we denote ( k x , k y ) plane), thus showing in-plane ray-like propagation. This unique characteristic can result in exciting optical phenomena, such as in-plane canalization, − or subdiffractional planar hyper-lensing and hyper-focusing. , A series of efforts have been recently carried out in order to control HPhPs using phase change materials and low-symmetry crystals. , However, the dynamical control of the propagation properties of in-plane HPhPs, crucial for their potential implementation in optical and optoelectronic applications, such as tunable photodetection or sensing, has so far remained elusive.…”