  We demonstrate theoretically and experimentally a topological transition of
classical light in "photonic graphene": an array of waveguides arranged in the
honeycomb geometry. As the system is uniaxially strained (compressed), the two
unique Dirac points (present in the spectrum of conventional graphene) merge
and annihilate each other, and a band gap forms. As a result, edge states are
created on the zig-zag edge and destroyed on the bearded edge. These results
are applicable for any 2D honeycomb-type structure, from carbon-based graphene
to photonic lattices and crystals.
