  The low sensitivity of photons to external magnetic fields is one of the
major challenges for the engineering of photonic lattices with broken
time-reversal symmetry. Here we show that time-reversal symmetry can be broken
for microcavity polaritons in the absence of any external magnetic field thanks
to polarization dependent polariton interactions. Circularly polarized
excitation of carriers in a micropillar induces a Zeeman-like energy splitting
between polaritons of opposite polarizations. In combination with optical
spin-orbit coupling inherent to semiconductor microstructures, the interaction
induced Zeeman splitting results in emission of vortical beams with a
well-defined chirality. Our experimental findings can be extended to lattices
of coupled micropillars opening the possibility of controling optically the
topological properties of polariton Chern insulators.
