  We experimentally demonstrate topological edge states arising from the
valley-Hall effect in twodimensional honeycomb photonic lattices with broken
inversion symmetry. We break inversion symmetry by detuning the refractive
indices of the two honeycomb sublattices, giving rise to a boron nitride-like
band structure. The edge states therefore exist along the domain walls between
regions of opposite valley Chern numbers. We probe both the armchair and
zig-zag domain walls and show that the former become gapped for any detuning,
whereas the latter remain ungapped until a cutoff is reached. The valley-Hall
effect provides a new mechanism for the realization of time-reversal invariant
photonic topological insulators.
