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 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.

Anisotropy and Modal Hybridization in Infrared Nanophotonics Using Low-Symmetry Materials

“…Under such conditions the material simultaneously behaves as an optical metal (negative real part of the permittivity tensor, Re­(ε) < 0) and a dielectric (Re­(ε) > 0), a phenomenon known as hyperbolicity. , This form of anisotropy has seen intense interest in recent years following the first observation of naturally hyperbolic materials, − as it offers significantly improved compression of light and does so within the volume, rather than the surface of the medium, as well as enabling the manipulation of properties not inherent to isotropic materials. Notably, the role that strong material anisotropy plays in controlling polariton propagation has led to exciting new developments, such as in-plane hyperbolicity − and so-called “ghost” and “shear” polaritons in low symmetry off-cut crystals and monoclinic and triclinic crystals, respectively. However, in addition to hyperbolic behavior, advances in artificial stacking and structuring of materials (both isotropic and anisotropic in nature) through heterostructure and superlattice fabrication have given rise to hybrid materials, such as the electromagnetic − or crystalline hybrid materials, − as well as the field of twist-optics. , Such discoveries have enabled advances in our understanding of light–matter interactions at nanoscale dimensions, including realizing the ability to restrict light propagation to specific directions, providing the subdiffractional equivalent of Fabry–Perot cavities and offering frequency-dependent polariton propagation rotation.…”

Temporal interaction of hybrid signals in various phases of Eu3+: BiPO4 through photon–phonon dressing

“…Phase transition or lattice symmetry of a functional crystal is one of the important factors in determining its physical properties [5,6]. Present theoretical and experimental studies make available an in-depth understanding of the morphological, structural, electronic, and optical properties of different phases of bismuth phosphate BiPO 4 [7][8][9][10].…”

In-plane hyperbolic polariton tuners in terahertz and long-wave infrared regimes

“…It is noted that in the current study, as a demonstration of principle, we only employ the simplest form of patterns, and only demonstrate with one type of material. In fact, patterns can be chosen depending on desirable applications, and also materials as long as supporting HPhPs in the THz and LWIR regimes 7,68 . The tuner allows us to modulate the wavefront of the incident light and control their power flow in an engineered space 22−29 , which therefore enables a variety of interesting applications, such as negative refraction, beam steering, holography, metalens, polarization conversion, and among others, in visible and near-infrared ranges to be expanded into the THz and LWIR spectral regimes.…”