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  1. Free, publicly-accessible full text available April 20, 2023
  2. 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 1–4 . 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 scales 5 . 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 response 6,7 . Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic 1,3,4 and hexagonal 8,9 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 resultsmore »will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals 10 , many common oxides 11 and organic crystals 12 , greatly expanding the material base and extending design opportunities for compact photonic devices.« less
    Free, publicly-accessible full text available February 24, 2023
  3. Abstract As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices 1–9 . However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity 10,11 , are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries 12,13 , the physical mechanisms and length scales through which interfacial effects influence materials remain unclear. Here we demonstrate the localized vibrational response of interfaces in strontium titanate–calcium titanate superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy, density functional theory calculations and ultrafast optical spectroscopy. Structurally diffuse interfaces that bridge the bounding materials are observed and this local structure creates phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. Our results provide direct visualization of the progression of the local atomic structure and interface vibrations as they come to determine the vibrational response of an entire superlattice. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantifiedmore »to understand macroscopic behaviour. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids with emergent infrared and thermal responses.« less
    Free, publicly-accessible full text available January 27, 2023
  4. Free, publicly-accessible full text available September 6, 2023