Janus bound states in the continuum and robust unidirectional guided resonances induced by shear

Polarization singularities (PSs) in low-symmetry photonic crystal (PhC) slabs underpin a range of intriguing phenomena, including topology, unidirectional radiation, chirality, nonlinear optics, etc. However, the reported symmetry-broken schemes rely on complicated mirror-asymmetric geometries that demand challenging nanofabrication processes. Here, we report the symmetry management of PhC slabs through the shear induced by the misalignment between the crystal axes of the macroscopic PhC slabs and anisotropic bulk crystals rather than asymmetrical geometries. Such a shear effect can induce compound low symmetry even in nanostructures and materials with inherently high symmetry. We demonstrate that such shear can induce Friedrich-Wintgen bound states in the continuum (BICs) with inherently decoupled topological charges in the upward and downward radiation channels, leading to the formation of Janus BICs. Besides, we discover shear-induced unidirectional guided resonances carrying zero topological charge with symmetry-protected robustness. Our findings harness the shear rather than low-symmetry geometries to achieve the asymmetric far-field radiation, enrich the understanding of the topological nature of PSs, and significantly simplify fabrication. These findings offer promising pathways for broad applications in unidirectional emission, optical interconnect, light detection, thermal radiation control, chiral optics, and others.