Meron Spin Textures in Momentum Space.

We show that a momentum-space meron spin texture for electromagnetic fields in free space can be generated by controlling the interaction of light with a photonic crystal slab having a nonzero Berry curvature. These spin textures in momentum space have not been previously noted either in electronic or photonic systems. Breaking the inversion symmetry of a honeycomb photonic crystal gaps out the Dirac cones at the corners of Brillouin zone. The pseudospin textures of photonic bands near the gaps exhibit a meron or antimeron. Unlike the electronic systems, the pseudospin texture of the photonic modes manifests directly in the spin (polarization) texture of the leakage radiation, as the Dirac points can be above the light line. Such a spin texture provides a direct approach to visualize the local Berry curvature. Our work highlights the significant opportunities of using photonic structures for the exploration of topological spin textures, with potential applications towards topologically robust ways to manipulate polarizations and other modal characteristics of light.