  The intriguing properties of graphene, a two-dimensional material composed of
a honeycomb lattice of carbon atoms, have attracted a great deal of interest in
recent years. Specifically, the fact that electrons in graphene behave as
massless relativistic particles gives rise to unconventional phenomena such as
Klein tunneling, the anomalous quantum Hall effect, and strain-induced
pseudo-magnetic fields. Graphene edge states play a crucial role in the
understanding and use of these electronic properties. However, the coarse or
impure nature of the edges hampers the ability to directly probe the edge
states and their band structure. Perhaps the best example is the edge states on
the bearded edge (also called the Klein edge) that have thus far never been
observed - because such an edge is unstable in graphene. Here, we use the
optical equivalent of graphene - a photonic honeycomb lattice - to
experimentally and theoretically study edge states and their properties. We
directly image the edge states on both the zig-zag and bearded edges of this
photonic graphene, measure their dispersion properties, and most importantly,
find a new type of edge state: one residing on the bearded edge which was
unknown and cannot be explained through conventional tight-binding theory. Such
a new edge state lies near the van-Hove singularity in the edge band structure
and can be classified as a Tamm state lacking any surface defect. Our photonic
system offers the opportunity to probe new graphene-related phenomena that are
difficult or impossible to access in conventional carbon-based graphene. Edge
states in graphene-type structures play the central role in achieving photonic
topological insulation, in which light can propagate along the edges of
photonic structures without any parasitic scattering whatsoever.
