The authors report the observation of an intense conical emission from an organic microcavity. The emission is characterized experimentally and modeled as a higher order Fabry-Perot mode predicted 45years ago by Kastler [Appl. Opt. 1, 17 (1962)]. A distinct cone is only visible above the normal mode laser threshold which implies a stimulated emission nature of the off-axial light. The input/output characteristics of normal and cone emissions hint that both originate from one and the same lasing mode.

We report on the dynamics of laser emission from an anisotropic organic microcavity filled with the guest-host composite of tris-(8-hydroxy quinoline) aluminium (Alq 3 ) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). In a single microcavity, a linesplitting of 0.18 THz between two perpendicularly polarized laser modes is observed. We ascribe this effect to an optical anisotropy in the distributed Bragg reflectors surrounding the organic layer. The temporal behavior of the electromagnetic field is studied by an up-conversion technique and shows an optical beating of 0.18 THz. Two modeling approaches are used to gain insight in the temporal evolution and phase behavior of the two modes. Both point towards the presence of a phase-coupling mechanism in this system.

We investigate triplet-triplet annihilation in molecular host-guest systems where triplets are localized on spatially separated guest molecules. Our results indicate that the dominant mechanism of annihilation is single-step long-range (Förster-type) energy transfer between two excited guests. This mechanism leads to a fundamental limit for the efficiency of phosphorescent organic light emitting diodes at high luminance. Our model is confirmed by photoluminescence decay experiments on 2,3,7,8,12,13,17,18-octaethylporphine platinum as guest in a host matrix of 4,4'-N,N'-dicarbazole-biphenyl.

Femtosecond pump–probe spectroscopy is applied to thin films of the quasi-one-dimensional organic semiconductor 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). We present transient absorption spectra over a broad spectral range. Ultrafast intraband relaxation in the S1 manifold towards the border of the Brillouin zone is shown to depend on temperature and excitation density. The intraband relaxation time is of the order of 100 fs. At high excitation densities (>1019 cm−3), the major de-excitation mechanism for the relaxed excitons is exciton–exciton annihilation. The experimental decay dynamics can be explained very well by two alternative annihilation models: one-dimensional diffusion limited bimolecular recombination or single-step long range Förster-type annihilation. In contrast, a three-dimensional diffusion limited annihilation model is significantly inferior. For all three models, we extract annihilation rates, diffusion constants, diffusion lengths, and Förster radii for room and liquid Helium temperature.

We present a comprehensive experimental and theoretical study of the optical properties of matrix-isolated molecules of the two perylene derivatives N,N-'-dimethylperylene-3,4,9,10-dicarboximide (MePTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). A solid solution of the dyes in an SiO2 matrix exhibits monomer-like behavior. Transient absorption pump-probe spectroscopy in the range 1.2-2.6 eV has been performed on an ultrafast time scale. The differential transmittance reveals contributions from ground-state bleaching, stimulated emission, and excited-state absorption. Both systems exhibit broad excited-state absorption features below 2.0 eV with a clear peak around 1.8 eV. The spectra can be consistently explained by the results of quantum-chemical calculations. We have applied both the coupled cluster singles and doubles (CCSD) model and the multireference-determinant single and double configuration-interaction (MRD-CI) technique on the basis of the intermediate neglect of differential overlap (INDO) Hamiltonian. The results are insensitive to whether the geometry is optimized for the electronic ground state or first excited state. The experimental polarization anisotropies for the two major transitions are in agreement with the calculated polarizations.

We present a comprehensive study of ultrafast relaxation properties of optical excitations in thin films of quasi-1D stacked organic materials PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N'-dimethylperylene-3,4,9,10-dicarboximide) over five decades of time. Pump-probe experiments reveal excitonic intraband relaxation time constants of 65 fs for MePTCDI and 100 fs for PTCDA. The initial time-resolved luminescence anisotropy is consistent with the exciton model of Davydov-split states. The subsequent decay of the anisotropy can be explained with a thermally activated exciton hopping process. A full understanding of the pump-probe experiments calls for an explanation beyond the models presently available.

We report on experimental and theoretical investigations of matrix-isolated molecules of MePTCDI, presenting transient absorption spectra obtained with ultrafast pump-probe spectroscopy. The experimental transient absorption spectrum is consistently explained by highly-correlated quantum-chemical calculation methods.

We investigate the dynamics of the organic laser guest-host composite of tris-(8-hydroxy quinoline) aluminium and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran embedded in a high-Q (Q≈4500) double distributed Bragg reflector microcavity using subpicosecond up-conversion techniques. Lasing is observed at a threshold of 0.4nJ∕20μJcm−2 with a linewidth of 0.05 nm (resolution limit). We observe a strongly nonlinear intensity-dependent delay of the emitted radiation burst. All experimental results are successfully modeled by a set of nonlinear rate equations, emphasizing the importance of a feedback mechanism for lasing.

The optical properties of microcavities (MCs) are strongly dependent on both polarization of incident and emitted light and its angle of observation. Here we report the measurements of cw- and time-resolved photoluminescence (PL) observed at negative detuning and at resonance for s- and p-polarization in the strong coupling regime of a planar MC containing J-aggregates of a cyanine dye. Following non-resonant excitation, the emission spectra consist of three types of features: direct J-aggregate exciton emission, polariton emission, and uncoupled monomer emission through the transmission maxima of the distributed Bragg reflector beyond the stop-band. We compare our experimental results with a transfer-matrix calculation of the transmission for s- and p-polarization and explain the different positions of the polariton branches, the stop-band width, and the high- and low energy transmission maxima of the MC. Time-resolved PL experiments show an increase in the decay lifetime of the exciton-like mode when it is positioned far from the cavity mode. Close to resonance, the lower polariton branch decays with the natural lifetime of the J-aggregates.

In this chapter we present the results of the photoluminescent and optical investigations of the influence of cation vacancy-related defects on CdSe/ZnSe quantum dot organization. Self-assembling growth was achieved under molecular beam epitaxy with subsequent annealing step. Number of cation vacancies was controlled by the intensity of the emission band connected with complex that includes cation vacancy and shallow donor. For the first time it is shown that increase of number of cation vacancy related defects results in the reduction of potential fluctuations in the QD layer. In this case a relatively uniform dense array of QDs with shallow localization potential is organized. It is proposed that generation of cation vacancies during the growth suppresses both Cd segregation and Cd surface diffusion as well as facilitates Cd/Zn interdiffusion. Interdiffusion process is proved by the changes in the photoluminescence and optical reflection spectra of ZnSe layers. It is showned that Cd/Zn interdiffusion can play an important role in CdSe/ZnSe intermixing during the QD formation at least under such growth conditions which can stimulate generation of cation vacancies.

We present luminescence quenching experiments and determine the exciton diffusion length in polycrystalline thin films of PTCDA (perylene-3,4,9,10-tetracarboxylic-dianhydride). From an analysis of time-resolved experiments, we can distinguish between exciton transport during an ultra-fast initial relaxation phase and transport in the long-living emitting states. The temperature dependence of the exciton diffusion constant in the emitting states indicates thermally activated hopping.

We observe a sub-30-fs intra-band relaxation in MePTCDI by pump-probe spectroscopy of the excited state absorption. Furthermore, fluorescence and polarization anisotropies are determined to understand energy transfer processes between different molecules

We investigate the optical absorption spectrum in a Holstein model for a molecular chain with Frenkel excitons and linear coupling to one internal vibration. The model is extended for nearest-neighbor charge-transfer excitons that mix with the Frenkel excitons. We represent the Hamiltonian in a displaced oscillator (Lang-Firsov) basis and employ a problem-adapted scheme for the truncation of the phonon basis. For weak and intermediate electronic coupling, the complete absorption spectrum and the structure of the relevant eigenstates become accessible by direct numerical diagonalization. We discuss the structure of the phonon clouds and the applicability of the molecular vibron model, in which only joint exciton-phonon configurations are included. As examples, we model absorption spectra of PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI $(N\ensuremath{-}{N}^{\ensuremath{'}}$-dimethylperylene-3,4,9,10-dicarboximide).

The exciton structure of crystalline MePTCDI (N-N'-dimethylperylene-3,4,9,10-dicarboximide) is modeled by a one-dimensional Hamiltonian, which includes the interactions between Frenkel excitons; with several vibronic levels and charge-transfer excitons. Using appropriate fitting parameters, which are verified by quantum chemical calculations, this model can explain the main features of the low temperature absorption spectrum. Polarized absorption spectra show different polarization ratios for the various peaks. This polarization behavior is explained by the varying contribution of the charge-transfer transition dipole, which has a direction different from the Frenkel transition dipole. Our model for the exciton band structure is supported by transient emission measurements.

Summary form only given. Recently, there has been large interest in organic materials due to promising device applications. Particular attractive are organic molecules which form quasi-1D crystals, with close stacking in one direction: The resulting strong inter-molecular interaction favors generation and transport of free carriers. The optical properties of such organic semiconducting materials, which are dominated by excitonic excitations, are much less understood than those of their inorganic counterparts. Therefore, a detailed investigation of the initial exciton relaxation and of the exciton-phonon coupling is needed. Here, we use pump-probe spectroscopy to study the energy relaxation of free excitons in the model 1D-system MePTCDI (N-N'-dimethylperylene-3,4,9,10-dicarboximide). The linear absorption spectrum of the crystalline layer is given.

We have directly time resolved coherent phonon oscillations in quasi-one-dimensional organic crystals of MePTCDI ( N-N'-dimethylperylene-3,4,9,10-dicarboximide), using femtosecond pump-probe experiments. We observe both higher-energy oscillations caused by intramolecular vibrations (internal phonons) and, for the first time in a quasi-one-dimensional organic system, lower-energy modulations which are related to coherent lattice phonons (external phonons). For internal A(g) vibrations, the coherence decay time of about 2 ps is almost independent of the mode. In contrast, the damping time of the external phonons increases strongly with decreasing energy.

We consider the exciton states in quasi-one-dimensional organic crystals with strong orbital overlap between neighboring molecules. In such crystals, the energy difference between the lowest Frenkel exciton and the nearest-neighbor charge-transfer excitons becomes small and their strong mixing determines the nature of the lowest energy states. We discuss these effects for crystalline N,N′-dimethylperylene-3,4,9,10-dicarboximide (MePTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). To model the exciton states, we use a Hamiltonian which includes the mixing of Frenkel excitons with several vibronic levels and charge-transfer excitons. With appropriate fitting parameters, we demonstrate that this model can explain the main features of the low temperature absorption spectra. Polarized absorption spectra of MePTCDI show different polarization ratios for the various absorption peaks. This polarization behavior is discussed as a qualitative proof for the varying contribution of the charge-transfer excitons, which have a transition dipole direction different from that of the Frenkel excitons.

The dissociation processes of excited states in vapor-deposited thin films of N,N′-dimethylperylene-3,4,9,10-dicarboximide (Me-PTCDI) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) are investigated in detail by means of electric field induced fluorescence quenching and photoresponse. The decay mechanisms of the two materials are quite different: Time resolved field induced fluorescence quenching on Me-PTCDI shows rate type quenching. Two spectral ranges with different field dependencies and decay times were observed. An efficient dissociation requires external fields of up to 2×106 V/cm. In PTCDA films, the quenching is of both rate and amplitude type. For this material, two spectral ranges with different field dependencies are recorded. A field induced luminescence enhancement is measured in the range between 520 and 600 nm. The combination of the fluorescence quenching measurement with the relative photoresponse and absolute luminescence quantum yield of Me-PTCDI allows to estimate the decay rate constants of the visible states of Me-PTCDI.

In this article, we present a simple method for the determination of the absolute internal quantum efficiency of thin organic dye layers. The basic idea is the comparison of the luminescence of the film with the reflection of a white diffusive reflectance standard measured at one angle with a simple spectrofluorometer. The method is compared to the procedure of de Mello [J. C. de Mello, H. F. Wittmann and R. H. Friend, Adv. Mater. 9, 230 (1997)], which uses an integrating sphere. As examples, the quantum yields of films of the two perylene derivatives N,N′-dimethylperylene-3,4:9,10-dicarboximide and 3,4,9,10-perylenetetracarboxylic dianhydride, are determined.

Fluorescence spectroscopy is used to investigate energy transfer processes in evaporated layers consisting of several different dyes. In this study films ofN,N′-dimethylperylene-3,4∶9,10-bis-dicarboximide (methylperylene pigment, MPP), coevaporated with copper phthalocyanine (CuPc) at varying ratios, and double layers of MPP and CuPc with different thicknesses are investigated. It is shown that energy transfer from MPP to CuPc occurs in both mixed and double layers. The energy transfer leads to a strong quenching of the MPP fluorescence and sensitized CuPc emission in the NIR region. The concentration dependence of the fluorescence quenching in mixed layers can be described by a Stern-Volmer plot. A simple model based on exciton diffusion between MPP molecules toward active quenching centers is used to determine the diffusion length.