Spontaneous emission of circularly polarized radiation in a chiral cavity

Engineering the helicity of single photons is a central challenge in nanophotonics and quantum optics, with applications ranging from chiral light-matter interactions to quantum communication protocols. We investigate the effects produced by a chiral cavity consisting of two twisted anisotropic photonic crystal mirrors on the spontaneous emission of a quantum system. For a given vacuum wavelength, the considered mirrors can be engineered to concurrently exhibit high helicity-preserving reflectance for one circular polarization and high transmittance for the opposite one, for incidence angles up to 20◦ with respect to the optical axis. Considering an atomic two-level system within thecavity, we evaluate the total dyadic Green function, taking into account both inhomogeneous and scattering electromagnetic contributions. We demonstrate that optimal configurations can achieve a 3% imbalance between the spontaneous emission rates of right- and left-circularly polarized light when averaged over random dipole orientations. Remarkably, this imbalance increases to 20% under suitably tuned dipole orientations. Even larger values are expected for two-dimensional arrays of such dipoles that isolate the normal component of the emitted field, while the remaining contributions interfere destructively. We envision that the proposed system will expand the set of tools availablefor investigating chiral matter and support the development of compact sub-micrometer circularly polarized single-photon sources.