Giant, Room Temperature Rabi Splitting in Solid-State Organic Microcavities Prof. Donal Bradley Imperial College, UK Organic semiconductors are attracting increasing interest for a variety of applications and as novel materials in which to explore solid state physics. Their molecular nature leads to important differences in comparison with conventional inorganic semiconductors. Localisation of wavefunctions within molecular sub units combined with a low dielectric constant causes strong electron interactions and leads to large Coulomb and exchange energies. Consequently their photophysics is dominated by exciton states of both singlet and triplet multiplicity. Another consequence of their low dimensional character is that the oscillator strengths associated with their optical transitions are very large. This combination of strongly bound excitons (room temperature stable) and large oscillator strengths is attractive for studies of solid-state microcavity polariton physics. Localisation of wavefunctions can however have a negative effect on linewidths due to coupling of the electronic transitions with local molecular vibrations. This leads to typical emission and absorption bandwidths of order 0.5 eV and suggests that strong coupling between such states and cavity photons is unlikely. There are however ways in which the linewidth can be reduced to of order 0.05 - 0.10 eV and we have investigated such materials as candidates for cavity polariton formation. Both planar macrocycles (porphyrins) and J-aggregated cyanine dyes have been studied. The resulting cavities show giant Rabi splitting of order 0.1 - 0.2 eV at room temperature. Solid-state microcavity polariton emission is also detected for the first time at room temperature. Double resonance Raman experiments can also be undertaken in which both the incident and scattered photons are resonant with the cavity polariton states. This yields resonance enhancements of up to 300 relative to non-cavity films. Experiments on two exciton/ one photon polaritons have also been undertaken and demonstrate exciton coupling over distances of order 100 nm, much in excess of the = 10 nm Forster radius for dipole-dipole coupling. These experiments will be described during the talk and prospects for future experiments will be outlined. References: ¡§Strong Exciton-Photon Coupling in an Organic Semiconductor Microcavity¡¨ D.G. Lidzey, D.D.C. Bradley, M.S. Skolnick, T. Virgili, S. Walker and D.M. Whittaker, Nature 395 (1998), 53-55. ¡§Room Temperature Polariton Emission from Strongly-Coupled Microcavities based on Cyanine-Dye J-Aggregates¡¨ D.G. Lidzey, D.D.C. Bradley, T. Virgili, A. Armitage, M.S. Skolnick and S. Walker, Phys.Rev.Lett. 82 (1999), 3316-3319 ¡§Photon-Mediated Hybridisation of Frenkel Excitons in Organic Semiconductor Microcavities¡¨ D.G. Lidzey, D.D.C. Bradley, A. Armitage, S. Walker and M.S. Skolnick, Science 288 (2000), 1620-1623. ¡§Raman Scattering in Strongly Coupled Organic Semiconductor Microcavities¡¨ A.I. Tartakovskii, A. Emam-Ismail, D.G. Lidzey, M.S. Skolnick, D.D.C. Bradley, S. Walker, V.M. Agranovich Phys.Rev.B 63 (2001), 121302(R).