Exciton-polaritons are quasiparticles with mixed photon and exciton character that demonstrate rich quantum phenomena, novel optoelectronic devices and the potential to modify chemical properties of materials. Organic semiconductors are of current interest for their room-temperature polariton formation. However, within organic optoelectronic devices, it is often the 'dark' spin-1 triplet excitons that dominate operation. These triplets have been largely ignored in treatments of polariton physics. Here we demonstrate polariton population from the triplet manifold via triplettriplet annihilation, leading to polariton emission that is longer-lived (>μs) even than exciton emission in bare films. This enhancement arises from spin-2 triplet-pair states, formed by singlet fission or triplet-triplet annihilation, feeding the polariton. This is possible due to state mixing, which -in the strong coupling regime-leads to sharing of photonic character with states that are formally non-emissive. Such 'photonic sharing' offers the enticing possibility of harvesting or manipulating even states that are formally dark.

We observe polariton condensation in the yellow part of the visible spectrum from a planar organic semiconductor microcavity containing the molecular dye BODIPY-Br. We provide experimental fingerprint of polariton condensation under non-resonant optical excitation, including the non-linear dependence of the emission intensity and wavelength blueshift with increasing excitation density, single excitation pulse dispersion imaging and real space interferometry. The latter two allow us to visualise the collapse of the energy distribution and the long-range coherence of the polariton condensate.

Abstract:We have explored the optical properties of a series of strongly-coupled microcavities containing the fluorescent molecular dye BODIPY-Br (bromine-substituted boron-dipyrromethene) dispersed into a transparent dielectric matrix with each cavity having a different exciton-photon detuning. Using temperature dependent emission, time-resolved spectroscopy, white-light reflectivity and measurements of fluorescence quantum yield, we explore the population of polaritons along the lower polariton branch. We find that both the cavity fluorescence quantum efficiency and the distribution of polariton states along the lower polariton branch is a function of exciton-photon detuning. Importantly, we show that in the most negatively detuned cavities, the emission quantum efficiency approaches that of a control (non-cavity) film. We develop a simple fitting model based upon direct radiative pumping of polariton states along the LPB and use it to obtain an excellent agreement with measured photoluminescence as a function of temperature and excitonphoton detuning, and qualitative agreement with the measured photoluminescence quantum efficiency. The radiative pumping mechanism that we identify indicates that to facilitate the formation of a non-equilibrium polariton condensate in an organic-semiconductor microcavity, it is important to utilize materials having high fluorescent quantum efficiency and fast radiative rates.A semiconductor-microcavity is an optical structure that can be used to control interactions between light and matter [1] . A typical cavity structure is composed of two mirrors separated by a layer of semiconducting material having a thickness commensurate with the wavelength of light (~100 nm). Such structures confine the local electromagnetic field, and if the energy of the confined photon and excitonic transition are degenerate, interactions can occur in the strong-coupling regime [2][3][4][5] . Here exchange of energy between excitons and photons is faster than the photon damping or exciton-photon dephasing, with the eigenstates of the system being cavity polaritons (a coherent superposition between light and matter). Polaritons are observed through an anticrossing 3 around the resonant energy of the exciton and photon modes in optical reflectivity or photoluminescence (PL) emission measurements [6] . Polaritons are bosonic quasi-particles that exhibit properties of both their excitonic and photonic components, namely the ability undergo scattering through their matter component, to form a coherent polariton condensate [7] . The ability to create and manipulate such condensates offers significant opportunities to create low threshold lasers that operate without the need for a population inversion and devices for quantumsimulations [8][9][10][11] .Most studies of strong-coupling have been performed using cavities containing inorganic-based semiconductors such as GaAs [12] , CdTe [13] , ZnO [14] and GaN [15] , either using a bulk semiconductor layer or in more sophisticated quantum well-based structures ...

Hybridization of two fluorescent BODIPY dyes in a microcavity is achieved by coupling different exciton transitions to the same cavity mode. We characterise the luminescence of such hybrid system following non-resonant laser excitation and show that the relative population along the different polariton branches can be controlled by changing cavity detuning. This effect is used to enhance exciton energy-transfer to states along the lower polariton branch in negatively detuned cavities. We compare the efficiency of energy transfer via exciton hybridisation with that achieved by dipole-dipole coupling.