  Recently discovered valley photonic crystals (VPCs) mimic many of the unusual
properties of two-dimensional gapped valleytronic materials such as bilayer
graphene or MoS2. Of the utmost interest to optical communications is their
ability to support topologically protected chiral edge (kink) states at the
internal domain wall between two VPCs with spectrally overlapping bandgap zones
and opposite half-integer valley-Chern indices. We experimentally demonstrate
the robustness of the kink states in VPCs that support degenerate
transverse-electric-like (TE) and transverse-magnetic-like (TM) topological
phases, thus enabling polarization multiplexing in a single topological
waveguide. The propagation direction of the kink states is locked to the
valleys of the reverse Brave lattice and, therefore, cannot be reversed in the
absence of inter-valley scattering. At the intersection between the internal
domain wall and the external edge separating the VPCs from free space, the kink
states are shown to exhibit >97% out-coupling efficiency into directional
free-space beams. This constitutes the first experimental demonstration of
meron-like valley-projected topological phases with half-integer valley-Chern
indices.
