  We reveal the meron and antimeron spin textures in momentum space in a
photonic crystal slab. 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 spin textures of photonic bands near the
gaps exhibit a meron or antimeron. Unlike the electronic systems, the spin
texture of the photonic modes manifests directly in the polarization of the
leakage radiation, as the Dirac points can be above the light line. The 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.
