  For photonic crystal slab (PCS) structures, bound states in the continuum
(BICs) and circularly polarized states (dubbed C-points) are important
topological polarization singularities in momentum-space and have attracted
burgeoning attention due to their novel topological and optical properties. In
our work, the evolution of polarization singularities from BICs to C-points is
achieved by breaking the in-plane C2 symmetry of a PCS structure of a square
lattice with C4v symmetry. Correspondingly, a BIC is split into two C-points
with opposite chirality, incurring distinct optical transmission responses with
the incidence of right or left circular polarization (RCP or LCP). Harnessing
such chirality selectivity of the C-points, we propose a multifunctional
imaging system by integrating the designed PCS into a conventional 4-f imaging
system, to realize both the edge imaging and conventional bright-field imaging,
determined by the circular polarization state of the light source. In addition
to multifunctional imaging, our system also provides a vivid picture about the
evolution of the PCS platforms' singularities.
