Source-configured symmetry-broken hyperbolic polaritons

Polaritons are quasi-particles that combine light with matter, enabling precise control of light at deep subwavelength scales. The excitation and propagation of polaritons are closely linked to the structural symmetries of the host materials, resulting in symmetrical polariton propagation in high-symmetry materials. However, in low-symmetry crystals, symmetry-broken polaritons exist, exhibiting enhanced directionality of polariton propagation for nanoscale light manipulation and steering. Here, we theoretically propose and experimentally demonstrate the existence of symmetry-broken polaritons, with hyperbolic dispersion, in a high-symmetry crystal. We show that an optical disk-antenna positioned on the crystal surface can act as an in-plane polarized excitation source, enabling dynamic tailoring of the asymmetry of hyperbolic polariton propagation in the high-symmetry crystal over a broad frequency range. Additionally, we provide an intuitive analysis model that predicts the condition under which the asymmetric polaritonic behavior is maximized, which is corroborated by our simulations and experiments. Our results demonstrate that the directionality of polariton propagation can be conveniently configured, independent of the structure symmetry of crystals, providing a tuning knob for the polaritonic response and in-plane anisotropy in nanophotonic applications.