  At photonic Dirac points, electromagnetic waves are governed by the same
equations as two-component massless relativistic fermions. However, photonic
Dirac points are known to occur in pairs in "photonic graphene" and other
similar photonic crystals, which necessitates special precautions to excite
only states near one of the Dirac points. Systems hosting unpaired photonic
Dirac points are significantly harder to realize, as they require broken
time-reversal symmetry. Here, we report on the first observation of an unpaired
Dirac point in a planar two-dimensional photonic crystal. The structure
incorporates gyromagnetic materials, which break time-reversal symmetry; the
unpaired Dirac point occurs when a parity-breaking parameter is fine-tuned to a
topological transition between a photonic Chern insulator and a conventional
photonic insulator phase. Evidence for the unpaired Dirac point is provided by
transmission and field-mapping experiments, including a demonstration of
strongly non-reciprocal reflection. This photonic crystal is suitable for
investigating the unique features of two-dimensional Dirac states, such as
one-way Klein tunneling.
