Observing tunable evolutions of optical singularities by C2 symmetry breaking

Optical singularities of polarizations have received much interest owing to their extraordinary performances. Recently, it was reported that originating from high-order singularities, a variety of controllable developments of singularities could be achieved by reducing symmetries, providing a feasible way to produce and manipulate optical singularities in momentum space. However, there are rare investigations on evolutions of polarization singularities in the system with ${C}_{2}$ symmetry transiting into no in-plane rotation symmetry. In this work, we realized adjustable evolutions of singularities through breaking ${C}_{2}$ symmetry. The singularities including bound states in the continuum (BICs) and degenerate states occur in a ${C}_{2}$ symmetry photonic crystal slab. The evolutions of singularities were attained during the process of symmetry transition by introducing a symmetry breaking factor. A process of off-$\mathrm{\ensuremath{\Gamma}}\phantom{\rule{0.16em}{0ex}}C$ points spawned from off-$\mathrm{\ensuremath{\Gamma}}$ BICs and merged to be quasi-BICs eventually were observed by increasing the symmetry breaking factor. In addition, an at-$\mathrm{\ensuremath{\Gamma}}$ quasi-BIC accompanied by a pair of $C$ points and axis conversions of singularities were observed. Our observations could enrich the understanding of the evolution of optical singularities and supply a meaningful way to engineer optical singularities.