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 investigations of optical excitations in polycrystalline organic molecular crystals with quasi-1D-stacked crystal structure and negative exciton dispersion. As model system, we choose thin films of the perylene derivative 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). Using pump–probe spectroscopy, we show how the relaxation from the absorbing state towards the border of the Brillouin zone occurs on a 120fs timescale. Time-resolved luminescence anisotropy gives evidence that as a result of the coherent coupling between adjacent stacks, populations of the Davydov-split states that are prepared during photo-excitation relax into the emitting states in less than 5ps. The behavior of the luminescence anisotropy can be explained by the orientation of the two PTCDA molecules in the unit cell. However, a full understanding of the ultrafast pump–probe anisotropy requires novel explanations beyond current models.

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 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 report the formation of doughnut-shaped focal intensity distributions with hole diameters of λ/3.3=232 nm full-width-at-half-maximum. The doughnut shape is created by illuminating a high-numerical-aperture lens with the output of a Mach–Zehnder interferometer, in which half of the wavefront in each arm is phase retarded by π. The focal intensities are probed with a point-like scatterer and compared with the predictions of a vectorial focusing theory. The orientation of the phase-discontinuity line with respect to the electric field determines whether a strong longitudinal or a vanishing electric field is produced at the focal point. Conditions are given for creating high-contrast focal holes at the sub-micron scale.

We report the formation of doughnut-shaped focal intensity distributions with hole diameters of lambda/3.3 = 232 nm full-width-at-half-maximum. The doughnut shape is created by illuminating a high-numerical-aperture lens with the output of a Mach-Zehnder interferometer, in which half of the wavefront in each arm is phase retarded by pi. The focal intensities are probed with a point-like scatterer and compared with the predictions of a vectorial focusing theory. The orientation of the phase-discontinuity line with respect to the electric field determines whether a strong longitudinal or a vanishing electric field is produced at the focal point. Conditions are given for creating high-contrast focal holes at the sub-micron scale.

We report on the generation of various hole-centered beams in the focal region of a lens and investigate their effectiveness to break the diffraction barrier in fluorescence microscopy by stimulated emission. Patterning of the phase of the stimulating beam across the entrance pupil of the objective lens produces point-spread-functions with twofold, fourfold, and circular symmetry, which narrow down the focal spot to 65-100 nm. Comparison with high-resolution confocal images exhibits a resolution much beyond the diffraction barrier. Particles that are only 65-nm apart are resolved with focused light.