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We have investigated the photophysics of the luminescent polymer poly(1,3-phenylenevinylene-alt-2,5-dioctyloxy-1,4-phenylenevinylene) by a variety of optical spectroscopies. The incorporation of alternating meta-phenylene and para-phenylene rings restricts conjugation with respect to poly(para-phenylenevinylene) derivatives. As a consequence, excited states are blueshifted and the nonlinear susceptibility is reduced. Absorption by aggregates, which can be obscured by scattering and interference, is clearly revealed by electroabsorption spectroscopy. The reduced symmetry of 1,3-PPV modifies optical selection rules, resulting in strong overlap of the fluorescence spectrum with excited state absorption by charged polarons.

We use angular-resolved photoluminescence excitation spectroscopy to probe energy relaxation towards the lowest lying states (${k}_{\ensuremath{\parallel}}=0$) on the lower polariton branch (LPB) of two different strong-coupled $J$-aggregate organic semiconductor microcavities. We find that states along the upper polariton branch (UPB) are able to undergo efficient relaxation to populate states around ${k}_{\ensuremath{\parallel}}=0$ on the LPB; a process that occurs through the rapid relaxation of UPB polaritons to the exciton reservoir. In contrast, LPB states having a photon fraction $g$0.5 are not apparently able to undergo further relaxation towards ${k}_{\ensuremath{\parallel}}=0$, constituting an effective relaxation bottleneck.

We explore the distribution of polaritons along the upper polariton branch of a strongly coupled organic-semiconductor microcavity as a function of temperature following nonresonant optical excitation. Measurements of polariton emission from a high-finesse cavity containing a thin film of a $J$-aggregated cyanine dye were performed as a function of external detection angle and temperature and compared with the results of detailed numerical simulations. We show that a full description of temperature-dependent upper-branch polariton emission can only be obtained by accounting for the interplay between two mechanisms that populate polariton states, namely, thermally assisted exciton scattering and direct radiative pumping of the photonic component of polariton states via the radiative decay of weakly coupled ``reservoir'' excitons. Our measurements provide a full description of the basic mechanisms at play in an organic microcavity, and may help guide the development of organic polariton-based devices.

We characterize strongly coupled microcavities in which optical hybridization occurs between the organic-dye zinc tetraphenyl-porphrin (ZnTPP) and a thin film of a self-assembled perovskite. Position-resolved reflectivity measurements demonstrate the formation of hybrid organic-inorganic exciton polaritons corresponding to a three-way admixture of the cavity photon with the electronic transitions of the ZnTPP and the perovskite. We find that such hybrid-polariton states can undergo radiative emission, despite the existence of a competing decay channel to lower-lying $Q$-band states. We argue that such hybrid polaritons can in principle act as an intermediate energy-transfer state between the organic and inorganic excitations.

Time and angle-resolved measurements reveal ultrafast dynamics of excitations in organic microcavities leading to dramatic modulation of probe transmission (~30%). We find that the induced changes have both fast and long-lived components. Fast response times are defined by vibronic relaxation and intersystem crossing (S1 ? T1), whereas long-lived changes are attributed to a build up of carriers in the nonradiative triplet state whose lifetime is longer than the repetition rate of the laser pulses, resulting thus in incomplete recovery of the ground state. Blueshifts of the lower polariton branch in the presence of the pump pulse indicate the presence of nonlinear interactions in the sample. However, there is no evidence yet for the pair-type scattering processes in porphyrin microcavities.

We have studied the dynamics of optically generated excitations in spin-coated glassy films of poly(9,9-dioctylfluorene) (PFO) and in $\ensuremath{\beta}$-phase PFO films using picosecond time resolved photoluminescence (PL) spectroscopy, performed both at room temperature (RT) and at 5 K. We also present measurements of the PL emission of PFO and $\ensuremath{\beta}$-phase PFO at RT and 5 K following continuous wave (cw) excitation. We show that the cw emission from $\ensuremath{\beta}$-phase PFO at 5 K is very highly resolved, permitting us to make an assignment of the different vibrational modes of the molecule that couple to the ${S}_{1}\ensuremath{\rightarrow}{S}_{0}$ transition. Via time-dependent spectroscopy measurements performed at 5 K, we are able to follow exciton diffusion and relaxation through an energetically broadened density of states to polymer chains having a longer conjugation length and lower energy gap. By comparing the relative emission intensity of the different vibronic transitions as a function of time, we are able to directly demonstrate that the lower energy emissive states are associated with longer conjugation length polymeric chains that have enhanced rigidity. At room temperature, we find that these relaxation processes occur faster than the resolution of our detector due to thermally assisted energy migration.

Many organic materials (disordered and crystalline as well) have rather broad and dispersionless electronic resonances. The excitations in these materials are localized, and thus for these excitations the wave vector is not a ``good'' quantum number. We analyze the optical properties of exciton polaritons in a microcavity, which utilize such organic materials as the optically active semiconductor. We show that as a result of strong light-matter coupling two polariton branches appear in the microcavity, which are analogous to the cavity-polariton branches observed in inorganic semiconductor structures. However, in contrast to the case of polaritons in inorganic semiconductors, the lower polariton branch exists only in a certain restricted intervals of wave vector values. We also show that in such materials the majority of the electronic excited states do not strongly couple to the cavity photon, and these states can be regarded as essentially incoherent. Applying this physical picture for the microcavities containing disordered cyanine dye J aggregates, we examine the decay of the upper cavity polaritons accompanied by the emission of an intramolecular phonon. We show that the main contribution to the upper polariton nonradiative decay rate arises from the transition to incoherent states, and the transition is very fast (of the order of 50 fs). From the results of our calculations we discuss the dynamics of excitations in a microcavity containing organic materials

We present a study of the photon emission from strongly coupled organic microcavities, following both cw and ultrafast nonresonant excitation. A thin film of molecular J aggregates is used as the active semiconductor material in the cavity. Due to the very large oscillator strength of the organic semiconductor, a Rabi splitting of 80 meV is observed in emission between the upper and lower polariton branches. We find that at all exciton-photon detunings, the emission from the cavity comes mainly from the lower polariton branch. We show that the photon emission rate from the cavity is not significantly different to that from J aggregates outside the cavity, indicating that the overall exciton decay dynamics are not significantly modified by strong coupling. We compare our cw emission measurements with the predictions of a two-level polariton model, in which we allow the transfer of polaritons between the upper and lower polariton branches. It is found that this model provides a very satisfactory description of the observed emission. Our model indicates that at resonance, population transfer between the branches occurs over time scales of approximately 30 fs, and is significantly faster than normal radiative decay of polaritons. We argue that such interbranch transitions might be used in advanced optoelectronic devices, to transfer energy via a cavity photon between different coherently coupled excitonic states.

The origin of electrophosphorescence from a doped polymer light emitting diode (LED) has been investigated. A luminescent polymer host, poly(9,9-dioctylfluorene) (PFO), was doped with a red phosphorescent dye, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (PtOEP). The maximum external quantum efficiency of 3.5% was obtained at a concentration of 4% PtOEP by weight. Energy transfer mechanisms between PFO and PtOEP were studied by absorption, photoluminescence, and photoinduced absorption spectroscopy. Even though electroluminescence spectra were dominated by PtOEP at a concentration of only 0.2 wt% PtOEP, Forster transfer of singlet excitons was weak and there was no evidence for Dexter transfer of triplet excitons. We conclude that the dominant emission mechanism in doped LED{close_quote}s is charge trapping followed by recombination on PtOEP molecules.

We report the direct observation of 30-fs period Rabi oscillations between excitons and cavity photons in a strongly coupled $J$-aggregate microcavity by means of time-resolved up-conversion and spectral interferometry measurements. The time structure of the transmitted electric field, measured by linear spectral interferometry, shows pronounced ultrafast beats. Its spectral phase reveals a distinct signature caused by destructive interference between the coherent drive and the field radiated by the exciton. This antiresonance selectively probes the uncoupled exciton excitation, and its observation uncovers the coherent and ultrafast exchange of energy between the optically excited cavity and the $J$-aggregate excitons, as confirmed by transfer matrix calculations.