Asymmetric Polaritons with Half-Hyperbolic Shaped Propagation

Traditional hyperbolic phonon polaritons (PhPs) possess intrinsic mirror and rotational symmetry, producing full-hyperbolic fields. Reducing material symmetry enables hyperbolic shear PhPs that break mirror symmetry for asymmetric transmission while maintaining rotational symmetry, yielding shear-hyperbolic fields due to viscous shear effects. Despite advancements, inherent symmetry limits highly directional propagation of polaritons. Here, we introduce a strategy for generating asymmetric polaritons with only a half-hyperbolic shape, thereby enhancing both hyperbolic patterning and unidirectional transmission. By exploiting the asymmetric edge of engineered gold nanopatterns, our method produces highly collimated, directional PhPs while effectively suppressing wave components in the undesired quadrants. We select hyperbolic α-MoO₃ for both theoretical and experimental validation, confirming applicability to the library of natural materials like uniaxial hBN, biaxial V₂O₅, and monoclinic CdWO₄. This technique to manipulate asymmetric directional polaritons not only facilitates planar nanophotonic applications but also broadens the scope for nanoscale optical manipulation.