Diverse manipulation of polarization singularities in momentum space

Polarization singularities in momentum space, encompassing bound states in the continuum (BICs) and circularly polarized states, offer significant potential for light manipulation. Prior researches have focused on the dynamic manipulation of these singularities, specifically merging BICs or spawning BICs into a pair of $C$ points. However, adjusting the geometric parameters or the tunable material can achieve just one of these outcomes. Here, we present a hybrid photonic system combining bilayer photonic crystals with tunable negative permittivity plasma, enabling the diverse manipulation of polarization singularities in momentum space, which includes generation, evolution, and annihilation of singularities by manipulating plasma permittivity without structural symmetry perturbations. We demonstrate that a symmetry-protected BIC can not only split into multiple accidental BICs and remerge with an ultrahigh $Q$ factor, but also split into a pair of $C$ points and remerge again by continuously decreasing permittivity. Furthermore, by breaking ${\ensuremath{\delta}}_z$ symmetry, multiple accidental BICs merge into a symmetry-protected BIC with controllable arbitrary ellipticity of polarization states. Importantly, the evolution of the singularities in our system covers the full Poincar\'e sphere, suggesting potentially applications in beam modulation and chiral optics.