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 phase. Materials featuring extreme optical anisotropy can support a so-called hyperbolic response, enabling coupled light-matter interactions, also known as polaritons, with highly directional propagation and com¬pression of light to deeply sub-wavelength scales [1].
Here, we show that monoclinic crystals, such as β-gallium oxide (bGO), can support hyperbolic shear polaritons [2], 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 response [3].
We demonstrate hyperbolic shear polaritons in bGO experimentally using Otto-type prism coupling experiments [4], 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 different azimuth angles. This allowed us to reveal the key novel features of HShPs, i.e., the continuous rotation of their propagation direction with frequency and asymmetric responses.
Further, we observe the associated sub-diffractional propagation patterns of shear polaritons directly in real space using scattering near-field optical microscopy (s-SNOM). These data experimentally verify how the energy flow is tilted away from the major propagation direction, and confirm the asymmetric response in real space.
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] J.D. Caldwell et al., Nat. Commun. 5, 5221 (2014).
[2] N.C. Passler, X. Ni, G. Hu, J.R. Matson, G. Carini, M. Wolf, M. Schubert, A. Alù,
J.D. Caldwell, T.G. Folland, and A. Paarmann, Nature 602, 595-602 (2022).
[3] M. Schubert et al., Phys. Rev. B 93, 125209 (2016).
[4] N.C. Passler, I. Razdolski, S. Gewinner, W. Schöllkopf, M. Wolf, and A. Paarmann,
ACS Photonics 4, 1048–1053 (2017).
| Abstract Number (department-wise) | PC 19 |
|---|---|
| Department | PC (Wolf) |
