Controlling the propagation asymmetry of hyperbolic shear polaritons in beta-gallium oxide

Joseph Matson, Sören Wasserroth, Xiang Ni, Maximilian Obst, Katja Diaz-Granados, Giulia Carini, Enrico Maria Renzi, Emanuele Galiffi, Thomas G. Folland, Lukas M. Eng, J. Michael Klopf, Stefan Mastel, Sean Armster, Vincent Gambin, Martin Wolf, Susanne C. Kehr, Andrea Alù, Alexander Paarmann and Joshua D. Caldwell

Nature Communications 14, 5240 (2023)
Structural anisotropy in crystals is crucial for controlling light propagation, particularly in the infrared spectral regime where optical frequencies overlap with crystalline lattice resonances, enabling light-matter coupled quasiparticles called phonon polaritons (PhPs). Exploring PhPs in anisotropic materials like hBN and MoO3 has led to advancements in light confinement and manipulation. In a recent study, PhPs in the monoclinic crystal β-Ga2O3 (bGO) were shown to exhibit strongly asymmetric propagation with a frequency dispersive optical axis. Here, using scanning near-field optical microscopy (s-SNOM), we directly image the symmetry-broken propagation of hyperbolic shear polaritons in bGO. Further, we demonstrate the control and enhancement of shear-induced propagation asymmetry by varying the incident laser orientation and polariton momentum using different sizes of nano-antennas. Finally, we observe significant rotation of the hyperbola axis by changing the frequency of incident light. Our findings lay the groundwork for the widespread utilization and implementation of polaritons in low-symmetry crystals.