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 ...

All-organic-based photovoltaic solar cells have attracted considerable attention because of their low-cost processing and short energy payback time. In such systems the primary dissociation of an optical excitation into a pair of photocarriers has been recently shown to be extremely rapid and efficient, but the physical reason for this remains unclear. Here, two-dimensional photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy, is used to probe the ultrafast coherent decay of photoexcitations into charge-producing states in a polymer:fullerene based solar cell. The two-dimensional photocurrent spectra are interpreted by introducing a theoretical model for the description of the coupling of the electronic states of the system to an external environment and to the applied laser fields. The experimental data show no cross-peaks in the twodimensional photocurrent spectra, as predicted by the model for coherence times between the exciton and the photocurrent producing states of 20 fs or less.

We present theoretical and experimental results showing the effects of incoherent population mixing on two-dimensional (2D) coherent excitation spectra that are measured via a time-integrated population and phase-sensitive detection. The technique uses four collinear ultrashort pulses and phase modulation to acquire two-dimensional spectra by isolating specific nonlinear contributions to the photoluminescence or photocurrent excitation signal. We demonstrate that an incoherent contribution to the measured lineshape, arising from nonlinear population dynamics over the entire photoexcitation lifetime, generates a similar lineshape to the expected 2D coherent spectra in condensed-phase systems. In those systems, photoexcitations are mobile such that inter-particle interactions are important on any timescale, including those long compared to the 2D coherent experiment. Measurements on a semicrystalline polymeric semiconductor film at low temperature show that, in some conditions in which multi-exciton interactions are suppressed, the technique predominantly detects coherent signal and can be used, in our example, to extract homogeneous linewidths. The same method used on a lead-halide perovskite photovoltaic cell shows that incoherent population mixing of mobile photocarriers can dominate the measured signal since carrier-carrier bimolecular scattering is active even at low excitation densities, which hides the coherent contribution to the spectral lineshape. In this example, the intensity dependence of the signal matches the theoretical predictions over more than two orders of magnitude, confirming the incoherent nature of the signal. While these effects are typically not significant in dilute solution environments, we demonstrate the necessity to characterize, in condensed-phase materials systems, the extent of nonlinear population dynamics of photoexcitations (excitons, charge carriers, etc.) in the execution of this powerful population-detected coherent spectroscopy technique. a)

Grégoire, P.; Vella, E.; Dyson, M.J.; Bazán, C.M.; Leonelli, R.; Stingelin, N.; Stavrinou, P.N.; Bittner, E.R.; Silva, C.