Phonon modes which are strongly coupled to the lowest optical transition of the quasi-one-dimensional organic crystal of MePTCDI (N-N′-dimethylperylene-3,4,9,10-dicarboximide) are directly time-resolved as they appear as coherent wavepackets in femtosecond pump–probe experiments. We observe both coherent oscillations of intramolecular vibrational wavepackets (internal phonons) and, for the first time in a quasi-one-dimensional organic crystal, lower-energetic modulations which are related to coherent lattice phonons (external phonons). The phononic wavepacket motion is attributed to the electronic ground-state potential surface. The coherence decay of all observed vibrations is in the range of some picoseconds.

Summary form only given. Thin organic films with semiconducting properties are highly interesting for device applications such as charge transport layers, organic LED, and solar cells. However, the phonon properties of organic crystals, which are important for e.g. exciton self-trapping, are very little understood. Because self-trapping might be critical for the realization of emission devices, a detailed investigation of the exciton-phonon interaction is required.We have directly time resolved coherent phonon oscillations in polycrystalline thin films of MePTCDI (N-N'-dimethylperylene-3,4,9,10dicarboximide) employed as a model system for quasi-one-dimensional organic molecular crystals. The measurements are carried out using a femtosecond pump-probe technique, in which an ultrashort pump pulse excites the sample and a time-delayed probe pulse measures the resulting transmission change as a function of pump-probe delay.

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.

Microscopic photoluminescence (mu-PL) spectroscopy through a photomask with a sub-micron resolution has been performed on In0.15Ga0.85As/GaAs quantum wells (QWs) containing three QWs with the same nominal well width of 30Angstrom but grown at different temperatures. PL from each QW was well resolved demonstrating that the interface abruptness of each QW is different due to the temperature dependence of In surface segregation during epitaxial growth. With decreasing window size down to sub-micron, the PL from QWs grown at higher temperatures showed a more drastic decrease compared with that from QWs grown at lower temperatures. This could be explained by assuming that there are less localized states and/or the in-plane exciton diffusion is enhanced in QWs grown at higher temperatures due to an enhanced smearing of the interfaces in the growth direction and the resultant formation of effectively smooth heterointerfaces.

High-resolution amplitude and phase linear spectroscopy of high-quality bulk GaAs are reported. The detailed structure of the observed full complex transmission is consistently explained by polariton effects on the basis of microscopic calculations. The coupled equations for the excitonic polarization and the light field in the slab configuration are evaluated using appropriate boundary conditions for the electromagnetic field and the excitonic wave function without reference to additional boundary conditions for the macroscopic polarization.

The exciton structure of crystalline PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N-N'-dimethylperylene-3,4,9, 10-dicarboximide) is modeled by a one-dimensional Hamiltonian, which includes interactions between Frenkel excitons with several vibronic levels and charge transfer excitons. With appropriate fitting parameters, which are verified by quantum chemical calculations, 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 peaks. This polarization behavior is explained by the varying contribution of the CT transition dipole, which has a direction different from the Frenkel transition dipole.

The dynamics of the Bloch oscillations in GaAs/Al0.3Ga0.7As superlattices are studied experimentally applying spectrally-resolved four-wave mixing in a self-diffraction geometry. We have found that the interband dephasing rate and the decay rate of the Bloch oscillations have different dependencies on excitation density. We have explored the dynamical features of the Bloch oscillations depending on the number of Wannier-Stark states excited. The conditions for observation of the Bloch oscillations and their harmonics are determined, the destructive role of excitons in the decay of the Bloch oscillations is demonstrated.

Summary form only given. We present applications of phase/amplitude spectroscopy in GaAs-based semiconductor structures, using Fourier Transform Spectral Interferometry. This technique allows one to measure both phase and amplitude of an optical pulse transmitted by the sample and therefore provides simultaneous absorption and refractive index measurement. We have used this approach to study how the optical response is affected by quantum confinement, temperature and external perturbations (e.g., magnetic field, optical excitation). We show here two studies of particular interest. First, the study of the dielectric function of a GaAs/AlGaAs multiple quantum well structure (MQWS) near the bandgap and at different temperatures. We show that it is possible to describe the complex dielectric function across the exciton resonances by analytical formulae assuming that the MQWS has a fractional dimension that translates the fact that real QWs are structures intermediate between the 3D unconfined bulk and ideal 2D systems. Second, the study of magnetically induced Fano-resonances (FR) in bulk GaAs under various excitation densities. They originate from a quantum interference between degenerate 1D continua and magnetoexcitons of different Landau levels coupled via Coulomb interaction. Using new analytical expressions describing this interference, we determine for the first time the collisional broadening of the continuum states.

More than 60 years ago, Zener has predicted that an electron in a periodic potential subject to a static electric field will perform harmonic oscillations (socalled Bloch oscillations) in real space. We present here results which directly prove the existence of these spatial oscillations: By observing the field-induced shift of the wannier-Stark ladder, the macroscopic dipole moment associated with the Block oscillations of the electron wave packets can be traced. The displacement of the electron wave packet can then be derived as a function of time, with the excitation density as the sole parameter. We discuss the dependence of Bloch oscillation dynamics on the optical excitation conditions. In particular, we show that the motion can be continuously tuned between the harmonic spatial motion envisioned by Zener and a symmetric breathing-mode motion where no macroscopic dipole oscillation is present. The results further show that the dynamics of the wave packets can be significantly influenced by excitonic effects due to the photo-excited holes.

Fourier transform spectral interferometry is applied to measure both amplitude and phase of the electric field in different types of semiconductor nanostructures, thus determining the real and imaginary parts of the dielectric function, The importance of measuring the phase is shown and discussed in three studies. First, the phase measurement is used to access directly the refractive index across excitonic resonances in bulk GaAs and AlGaAs-GaAs quantum wells, with unprecedented resolution. Second, we measure the density dependence of the full dielectric function across a Fano resonance in bulk GaAs and show that this allows us to obtain some information on the collisional broadening of the usually hidden linewidth of the coupled esciton/continuum. Third, the phase is studied in a complex heterostructure, a semiconductor microcavity. We investigate and discuss the effect of the cavity detuning and of the excitation density.

We investigate the temporal and spatial displacement dynamics of optically excited wave packets in a semiconductor superlattice. We present an experimental technique which measures directly the displacement of the wave packet center-of-mass: the oscillating Bloch wave packets create a microscopic dipole moment which can be detected using the shift of the Wannier-Stark ladder transition energy as a sensitive field detector. A true spatial oscillation of the photo-excited wave packet identifies quantum beats of the Wannier-Stark states as Bloch oscillations. We show that the Bloch wave packet undergoes harmonic spatial motion in accordance with the predictions of Bloch and Zener. The influence of initial experimental conditions on evolution and composition of the wave packet is discussed.

The absolute spatial displacement of Bloch-oscillating electrons in semiconductor superlattices is measured as a function of time with a few angstrom resolution using a novel experimental technique: The oscillating Bloch wave packet creates a small dipole field which can be determined using the field shift of the Wannier-Stark ladder transitions as a sensitive detector. The total amplitudes and their dependence on the static electric field are in good agreement with a theory including excitonic effects.

We investigate the spatial displacement dynamics of optically excited wave packets in semiconductor superlattices. A short laser pulse exciting semiconductor superlattice induces quantum beats between different excitonic states that in turn leads to formation of a time-varying coherent wave packet. The real space oscillation of the excited wave packet identifies these quantum beats of the Wannier-Stark states as Bloch oscillations: We present an experimental technique which measures directly the displacement of the wave packet center- of-mass. The oscillating Bloch wave packets create a microscopic dipole moment which can be detected using the shift of the Wannier-Stark ladder transition energy as a sensitive field detector. We show that the Bloch wave packet undergoes harmonic spatial motion, proving for the first time the predictions of Bloch and Zener. The influence of an experimental conditions on displacement of the Bloch wave packet is discussed.

We present simultaneous measurement of the refractive index and absorption coefficient in a AlGaAs/GaAs multiple quantum well structure near the band gap by Fourier transform spectral interferometry. Both quantities were measured across heavy- and light-hole excitons for temperatures ranging from liquid helium to room temperature. The experimental results are analyzed using an analytical expression for the complex dielectric function of Wannier excitons in fractional dimension.