Dynamics of diverse polarization singularities in momentum space with far-field interference

Abstract

Polarization singularities in momentum space, including bound states in the continuum (BICs) and circularly polarized states (CPSs), render great potential in singular and chiral optics. As far-field topological signatures of Bloch modes in photonic crystal slab, the dynamic evolution of the polarization singularities is observed by tuning the geometric parameters of the slab itself. Here, we combine the photonic crystal slab with a reflected multilayer substrate in which interference between the reflected optical field and the upward radiation of the Bloch mode brings us an extra degree of freedom to manipulate the polarization singularities without breaking the structural symmetry. As the polarization singularities evolve with the spacing between the photonic crystal slab and the substrate, the CPSs and extrinsic BICs are both generated from charge reversal of the at-\ensuremath{\Gamma} intrinsic BIC and then collide with the off-\ensuremath{\Gamma} intrinsic BICs. Importantly, all BICs are topologically protected from splitting throughout the whole dynamic process owing to the preservation of structural symmetry. Our findings suggest an alternative route to pursue ultrahigh-$Q$ CPS resonances and merge diverse BICs, which is potentially applicable in chiral optics, topological photonics, and beam modulation.