  The Dirac equation is a paradigmatic model that describes a range of
intriguing properties of relativistic spin-1/2 particles, from the existence of
antiparticles to Klein tunneling. However, the Dirac-like equations have found
application far beyond its original scope, and has been used to comprehend the
properties of graphene and topological phases of matter. In the field of
photonics, the opportunity to emulate Dirac physics has also enabled
topological photonic insulators. In this paper, we demonstrate that judiciously
engineered synthetic potentials in photonic Dirac systems can offer physical
properties beyond both the elementary and quasi-particles, and topological
realms. Specifically, we introduce a new class of optical Dirac waveguides,
whose guided electromagnetic modes are endowed with pseudo-spin degree of
freedom. Pseudo-spin coupled with the ability to engineer synthetic gauge
potentials acting on it, enables control over the guided modes which is
unattainable in conventional optical waveguides. In particular, we use a
silicon nanophotonic metasurface that supports pseudo-spin degree of freedom as
a testing platform to predict and experimentally confirm a spin-full nature of
the Dirac waveguides. We also demonstrate that, for suitable trapping
potentials, the guided modes exhibit spin-dependent field distributions, which
gives rise to their distinct transport and radiative properties. Thereby, the
Dirac waveguides manifest spin-dependent radiative lifetimes - the
non-Hermitian spin-Hall effect - and open new avenues for spin-multiplexing,
controlling characteristics of guided optical modes, and tuning light-matter
interactions with photonic pseudo-spins.
