Polarized mode splitting in high quality monolithic organic microcavity fron angle-resolved photoluminescence experiments

A high quality monolithic microcavity structure, containing a thin film of TMPP: tetrakis-(3,4,5-trimethoxyphenyl)porphyrin), has been designed and realized by thermal evaporation. Angle and polarization-resolved photoluminescence (PL) spectroscopy measurements [ R. Houndré, C. Weisbuch, R.P. Stanley, U. Oesterle, P. Pellandini and M. Ilegems, Phys. Rev. Lett. 73 (1994), 2043] have been carried out to characterize the device performances and to study the light – matter interaction in the so called “weak coupling regime”. Porphyrins are a well known class of materials whose unique characteristics make them very promising materials for optical applications [M.O. Senge, M. Fazekas, E.G.A. Notaras, W.J. Blau, M. Zawadzka, O.B. Locos, E.M. Ni Mhuircheartaigh, Adv. Mater. 19 (2007) 2737-2774]. Their ground state absorption is mostly confined to a few narrow regions (Soret and Q bands) allowing high transmission in the spectral window between these bands. Their sharp absorption bands in the visible and NIR can be used for resonance enhancement of χ 3 as well. A doublet, with a splitting energy ranging between 3 and 70 meV, is observed in the photoluminescence PL angle-resolved measurements of our microcavity at detection angles larger than 15°. From polarization measurements, we have found that the peak on the high energy side is TM polarized, while the peak on the low energy side is TE polarized [ T. Virgili, D.G. Lidzey, D.D.C. Bradley, S. Walkerb, Synthetic Metals 116 (2001) 497±500; C.Y. Hu, H.Z. Zheng, J.D. Zhang, H. Zhang, and F.H. Yang, Y.P. Zeng, App. Phys. Lett. 82 (2003) 665]. The little amount of deposited organic material, the high Q factor of the monolithic structure, and the mismatch between the centre of the DBR stop-band and the position of the cavity mode (85 meV) make the split between TE and TM cavity modes more discernible. At higher angles, the difference in energy between the two peaks increases and a significant splitting between the TE and TM polarisations is clearly observed. A rigorous calculation of the TE-TM splitting can be made using the transfer matrix method. The splitting observed between the two polarized mode is lower respect to the experimental results. We believe that this mismatch is due to a local order of thin organic film that leads to a partial polarization of the PL.