Asymmetric Propagation of Hyperbolic Polaritons in MoO₃ and β-Ga₂O₃ Heterostructures

Opposite signs of the dielectric permittivity along different principal directions of optical materials (hyperbolic anisotropy) allow for exotic quasiparticles to be formed due to the hybridization of photons and phonons (phonon polariton) in the mid-infrared spectral region. Phonon-polaritons in such hyperbolic materials enable the confinement of electromagnetic waves at the nanoscale, which offers mechanisms to induce large enhancements of the optical response and reduce the footprint of photonic devices such as molecule sensors, polarizers, waveplates, optical modulators, IR sources and detectors, and many others. To design nanophotonic devices, it is necessary to engineer the characteristic features of phonon polaritons, like propagation direction and distances. It has been demonstrated that characteristic features of hyperbolic phonon polaritons can be engineered via the choice of crystal symmetry. Hyperbolic polaritons in crystals with varying symmetries from hexagonal, such as boron nitride (h-BN), exhibit isotropic propagation in plane, whereas reducing to orthorhombic structures like MoO₃ and V₂O₅, results in directional propagation. Monoclinic systems, such as β-Ga₂O₃ or CdWO₄, with further reduced symmetry exhibit exotic phenomena in the form of shear, whereby the polariton wavelength and direction of propagation disperse with changing frequency. This provides an additional degree of freedom to manipulate polariton propagation. By employing twisted low symmetry structures, such as twisted slabs of MoO₃, highly directional propagation with minimal divergence can be observed (canalization) at specific ‘magic’ angles. Yet, it remains unclear if such canalization can be observed in twisted structures of different Bravais lattices. Here, we investigate heterostructures of MoO₃ orthorhombic) and Ga₂O₃ (monoclinic) to determine the role of twisting with non-degenerate polariton resonances, as well as the potential influence of shear. By investigating a series of MoO₃ slabs twisted with respect to the a-axis of β-Ga₂O₃ we observed the change in the direction of propagation along with asymmetric dispersion via a scattering-type SNOM coupled to a free-electron laser. It is determined that the high asymmetry in the propagation in the twisted structure is driven principally by the twist angle, rather than the inherent shear in the underlying β-Ga₂O₃, indicating that the propagation direction can be dictated via twist angle, making such heterostructure platforms designable for future nanophotonic devices such as sensors, chiral sources, IR imaging components, and many others.