  Advances of condensed matter physics in exploiting the spin degree of freedom
of electrons led to the emergence of the field of spintronics, which envisions
new and more efficient approaches to data transfer, computing, and storage
[1-3]. These ideas have been inspiring analogous approaches in photonics, where
the manipulation of an artificially engineered pseudo-spin degrees of freedom
is enabled by synthetic gauge fields acting on light [4,5,6]. The ability to
control these additional degrees of freedom can significantly expand the
landscape of available optical responses, which may revolutionize optical
computing and the basic means of controlling light in photonic devices across
the entire electromagnetic spectrum. Here we demonstrate a new class of
photonic systems, described by effective Hamiltonians in which competing
synthetic gauge fields engineered in pseudo-spin, chirality/sublattice and
valley subspaces result in band gap opening at one of the valleys, while the
other valley exhibits Dirac-like conical dispersion. It is shown that such
effective response has dramatic implications on photon transport, among which:
(i) spin-polarized and valley-polarized one-way Klein tunneling and (ii)
topological edge states that coexist within the Dirac continuum for opposite
valley and spin polarizations. These phenomena offer new ways to control light
in photonics, in particular for on-chip optical isolation, filtering and
wave-division multiplexing by selective action on their pseudo-spin and valley
degrees of freedom.
