Engineering shear polaritons in 2D twisted heterostructures.

Materials hosting polaritons with extreme optical anisotropy enable nanoscale light manipulation, crucial for nanophotonic applications. In particular, hyperbolic shear polaritons (HShPs), featuring asymmetric propagation, axial dispersion, and loss redistribution, arise in low-symmetry materials (e.g., β-GaO, CdWO) through the intricate interplay of photons and non-orthogonal detuned resonant excitations supported by crystals with broken spatial symmetries. Versatile control over HShPs is still challenging to achieve, due to the properties of such bulk natural materials. Here, we unveil engineering and control over HShPs in two-dimensional materials by manipulating twisted bilayers of α-MoO, which does not feature broken lattice symmetry at the material level. Infrared nanoimaging reveals precise control over HShP asymmetry in propagation, loss redistribution and confinement, achieved by adjusting the thickness and twist angle of the bilayer. Integration of a graphene electrostatic gate further enhances this control, enabling dynamic manipulation of HShPs. Our work expands the HShP platform for customizable polaritonics, advancing on-chip photonic applications.