Analyzing and quantifying symmetry breaking of anisotropic shear polaritons in monoclinic crystal slabs

Anisotropic phonon polaritons in materials with high symmetry including hexagonal, trigonal and orthorhombic crystals, enable nanoscale light confinement and manipulation, crucial for nanophotonic on-chip technologies. With reduced material symmetry, monoclinic crystals have been found to endow more intriguing polaritonic phenomena, including axial dispersion and asymmetric propagation patterns, offering new freedoms of nanoscale field manipulations. However, such symmetry-broken anisotropic phonon polaritons, stemming from the intrinsic broken mirror symmetry of dispersion, so far have been observed only in bulk natural materials. Here, we unveil shear polaritons in monoclinic crystal slabs with finite thickness through dispersion analysis and field simulation, and quantify the associated symmetry breaking by introducing a new shear factor based on the k-space integral of the rate of energy dissipation. We demonstrate axial dispersion, mirror symmetry-broken dispersion and asymmetric propagation of shear polaritons. By identifying surface-and volume-confined shear polaritons, we reveal their opposite intensity distribution. In addition, the thickness of monoclinic crystal slab can induce the asymmetry transition, which the surface shear polaritons dominate. Our findings expand the shear polariton platform and provide valuable strategies for manipulating light at the nanoscale through symmetry breaking.