  Optical activity (OA), also called circular birefringence, is known for two
hundred years, but its applications for topological photonics remain
unexplored. Unlike the Faraday effect, OA provokes rotation of the linear
polarization of light without magnetic effects, thus preserving the
time-reversal symmetry. Here, we report a direct measurement of the Berry
curvature and quantum metric of the photonic modes of a planar cavity
containing an optically active organic microcrystal (perylene). Photonic
spin-orbit-coupling induced by the cavity results in the action of a
non-Abelian gauge field on photons. The addition of high OA makes emerge
geometrically non-trivial bands containing two gapped Dirac cones with opposite
topological charges. This experiment performed at room temperature and at
visible wavelength establishes the potential of optically active organic
materials for implementing non-magnetic and low-cost topological photonic
devices.
