J. Matson, S. Wasserroth, X. Ni, M. Obst, K. Diaz-Granados, G. Carini, E. Renzi, E. Galiffi, T. G. Folland, L. Eng, J. Klopf, S. Mastel, S. Armster, V. Gambin, M. Wolf, S. C. Kehr, A. Alu, A. Paarmann and J. Caldwell
Research Square (2023)
Structural anisotropy in a crystal is one of the key tools for controlling light propagation. The correlation between crystalline structure and the interaction with light is strongest in the infrared spectral regime, where optical frequencies overlap with anisotropic lattice resonances of materials, thereby enabling light-matter coupling through quasiparticles called phonon polaritons (PhPs). In recent years, the exploration of PhPs in anisotropic materials has yielded new levels of confinement and manipulation of light. The strongly anisotropic bonds in materials such as hBN and MoO3 lead to hyperbolic phonon polaritons that exhibit even stronger light confinement and propagation directionality than conventional polaritons. Recently, we showed that the enhanced anisotropy and non-orthogonal structure of the monoclinic crystal β-Ga2O3 (bGO) induces a remarkable rotation of the optical axis with frequency ("axial dispersion") associated to nano- to microscopic shear phenomena, resulting in hyperbolic PhPs that exhibit dramatic propagation asymmetry along the hyperbolic polaritonic branches. Here, we employ a Free-Electron Laser (FEL) coupled to a scattering-type scanning near-field optical microscope (s-SNOM) to enable direct imaging of the symmetry-broken propagation patterns of hyperbolic shear polaritons in bGO within the far-infrared. Further, we demonstrate how we can control and enhance the shear-induced propagation asymmetry of nano-antenna-launched shear polaritons, by varying the incidence direction with respect to the crystal orientation, as well as by momentum selection using different sizes of nano-antennas. Finally, we also observe significant rotation of the hyperbola axis as we change the incident light frequency. Our work thereby paves a foundation for widespread utilization and device implementation of polaritons in low-symmetry crystals.