Cavity mode polarisation splitting in organic semiconductor microcavities

Angular-dependent reflectivity techniques are employed to probe the optical structure of microcavities containing a passive spacer layer, and also a fluorescent polymer. For the fluorescent polymer, we used the polymer poly(9,9-dioctylfluorene) (PFO), which was deposited between a dielectric and an aluminium mirror. The reflectivity spectra exhibit a strong polarisation dependence with the cavity mode energy and photon linewidths of the cavity mode different for the TE (transverse electric) and TM (transverse magnetic) polarisations. We find that as the cavity mode is moved away from the centre of the dielectric mirror stop-band, the difference between the energy of the TE and TM modes grows. This results in a significant splitting of up to 110 meV between the TE and TM polarisations detected at large angle. We investigate this effect by varying the energy of the cavity mode with respect to the centre of the mirror stop-band. Results are analysed using a transfer-matrix model. Photoluminescence emission from the PFO containing microcavity was also measured. We find a splitting between TE and TM modes, which is also manifested in the photoluminescence (PL) emission. This allows control of polarisation from an isotropic film, presenting a new method to control exciton emission.