Observing vortex polarization singularities at optical band degeneracies

In the momentum space of two-dimensional (2D) periodic photonic systems, the polarization states of radiating Bloch modes on a single band can form vortex polarization singularities. Here, we study vortex polarization singularities at the band degeneracies. We design a quadratic degeneracy in a plasmonic crystal with C-4v symmetry which, by symmetry breaking, splits into a pair of Dirac points with half vortices. The integer vortex singularity at the quadratic degeneracy and the half one at the Dirac degeneracy are measured using polarization-resolved momentum-space imaging spectroscopy. The complete evolution of the splitting from the integer vortex to a pair of half vortices is observed in experiments.