Origin of giant polarization splitting in high quality organic microcavities

We study the emission properties of a high quality monolithic microcavity with an embedded ultrathin organic tetrakis(4-methoxyphenyl)porphyrin layer. Spectral and angle-resolved photoluminescence measurements show a well defined polarized doublet at detection angles larger than 15 degrees. The splitting energy ranges up to 70 meV. The usual employed mechanism, based on the mismatch between the center of the mirror stop band and the wavelength of the microcavity, accounts only for a small fraction of the observed splitting. A transfer matrix model taking into account the optical anisotropy of the organic layer, has been employed to reproduce the observed experimental results. The anisotropy of the organic layer, confirmed by optical absorption spectroscopy, origins from a local molecular order as inferred by atomic force microscopy measurements.