  We design, in a most simple way, Fabry-Perot cavities with longitudinal
chiral modes by sandwiching between two smooth metallic silver mirrors a layer
of polystyrene made planar chiral by torsional shear stress. We demonstrate
that the helicity-preserving features of our cavities stem from a spin-orbit
coupling mechanism seeded inside the cavities by the specific chiroptical
features of planar chirality. Planar chirality gives rise to an extrinsic
source of three-dimensional chirality under oblique illumination that endows
the cavities with enantiomorphic signatures measured experimentally and
simulated with excellent agreement. The simplicity of our scheme is
particularly promising in the context of chiral cavity QED and polaritonic
asymmetric chemistry.
