  Pseudospin plays a very important role in understanding various interesting
physical phenomena associated with 2D materials such as graphene. It has been
proposed that pseudospin is directly related to angular momentum, and it was
recently experimentally demonstrated that orbit angular momentum is an
intrinsic property of pseudospin in a photonic honeycomb lattice. However, in
photonics, the interaction between spin and pseudospin for light has never been
investigated. In this Letter, we propose that, in an optical analogue of
staggered graphene, i.e. a photonic honeycomb lattice waveguide with in-plane
inversion symmetry breaking, the pseudospin mode can strongly couple to the
spin of an optical beam incident along certain directions. The spin-pseudospin
coupling, caused by the spin-orbit conversion in the scattering process,
induces a strong optical chiral effect for the transmitted optical beam.
Spin-pseudospin coupling of light opens door to the design of
pseudospin-mediated spin or valley selective photonic devices.
