Spin-dependent edge detection and imaging enabled by optical circularly polarised states

In photonic crystal slab (PCS) structures, the bound states in the continuum (BICs) and circularly polarised states (dubbed C-points) are critical topological polarisation singularities in momentum space that have garnered significant attention owing to their novel topological and optical properties.In this study, we engineered a novel PCS imager featuring two C-points with opposite chirality through symmetry breaking, resulting in maximal asymmetric transmission responses characterised by near-unity circular dichroism (CD) values.By harnessing the chiral selectivity of the C-points, a high-CD PCS imager can provide two sets of optical transfer functions (OTFs) to facilitate both edge detection and bright-field imaging.Notably, one set of OTFs was finely tuned to a Lorentzian line shape to achieve perfect edge detection.We developed a multifunctional imaging system by integrating a PCS imager into a traditional optical system.Both theoretical and experimental demonstrations confirmed that this system provides bright-field and edge-enhanced images with micrometer-scale resolution.Furthermore, these two independent functions can be easily switched by altering the circular polarisation state of the light source.