E. O. Göbel

Nonlinear dynamics of semiconductor laser emission under variable feedback conditions

A study was made of the behavior of external cavity semiconductor lasers with tilted external cavity mirrors providing variable feedback. The appearance of intermittent breakdowns of the light intensity induced by the tilting of the external mirror is demonstrated. The rate of the breakdowns of the light intensity is directly related to the relaxation oscillation frequency of the external cavity semiconductor laser. The authors describe the experimental observations theoretically by the coupled rate equations for the electric field density and the carrier density. >

Coherence properties of gain- and index-guided semiconductor lasers

The spectral linewidth of the longitudinal laser modes has been measured as a function of mode power for different types of gain-guided as well as index-guided (GaAl)As double heterostructure laser diodes. Various effects contributing to line broadening, such as spontaneous emission, carrier density fluctuations, changes of the resonator parameters, and mode competition are discussed.

Energy transfer in molecularly doped conjugated polymers

Abstract We present a detailed investigation of energy transfer of optical excitations in a molecularly doped conjugated polymer. We have incorporated dye molecules into a polymer blend system consisting of poly (phenylphenylene vinylene) (PPPV) and polycarbonate. Using time-resolved luminescence spectroscopy we observe efficient picosecond energy transfer from the PPPV chromophores to the dye 4-dicyanomethylene-2-methyl-6- p -dimethylaminostyryl-4H-pyran (DCM) molecules. Our results are interpreted in terms of Forster-type energy transfer.

Picosecond luminescence of excitons localized by disorder in CdSxSe1−x

Abstract We have investigated in CdS x Se 1− x and in pure CdSe the temporal evolution of the excitonic luminescence with 20 ps time resolution. In CdSe and in alloys with x x > 0.15, which is attributed to the relaxation of excitons localized by compositional disorder. A simplified hopping model is presented which accounts for the experimental findings.

Dynamics of photoexcitations in electric fields in poly(p-phenylenevinylene) diodes

Abstract We report on a study of the dissociation of neutral photoexcitations in electric fields in ITO/poly( p -phenylenevinylene) (PPV)/Al diodes. This process leads simultaneously to photoconductivity and field-induced quenching of the photoluminescence. By illuminating the samples through the different contacts we show that the high electric fields are restricted to a region close to the Al layer as characteristic for a Schottky diode. Time-resolved luminescence measurements show that the dissociation takes place on a picosecond time scale. The experimental data are compared with a Monte-Carlo simulation of the dynamics of photoexcitations created in a disorder-broadened density of states.

Dissipative dynamics of an electronic wavepacket in a semiconductor double well potential

The authors present a detailed study of coherent tunneling oscillations of an optically excited electronic wavepacket in a semiconductor double quantum-well structure. The oscillation of the wavepacket between the two wells is traced by time-resolved pump-probe spectroscopy as well as time-resolved degenerate four-wave mixing. The experimental observations are compared with a simple theoretical model. The motion of the wave packet is studied as a function of both an external electric field and the lattice temperature. While the oscillation frequency shows the expected dependence on electric field, it increases with temperature at fixed electric field when perfectly delocalized electronic states exist at low temperature. >

Absorption at the dipole-forbidden optical gap of crystalline C60☆

Abstract We report detailed optical studies of crystalline C60 applying several optical spectroscopic techniques such as absorption, photoluminescence, and one-photon and two-photon photoluminescence excitation spectroscopy. The energetically lowest purely electronic 0′-0 transition is found at 1.846 eV. At low temperatures, the absorption spectrum systematically exhibits doublet features with the same energetic splitting as observed for the low-temperature fluorescence spectrum. These transition pairs either represent two distinct vibration-induced transitions or only one-vibration-induced transition and the splitting is then due to the two distinct alignments of neighbouring C60 molecules (merohedral disorder). Temperature-dependent absorption measurements show that the two molecular alignments might indeed be responsible for the double resonances.

Exciton lifetimes in CdTe/CdMnTe single quantum wells

The lifetime of excitons in CdTe/Cd0.75Mn0.25Te single quantum wells (SQW) has been measured by time‐resolved photoluminescence for quantum wells with thicknesses from 34 to 300 A and in the temperature range from 1.6 to 300 K. The exciton lifetimes are of the order of 150 ps at the lowest temperatures, where radiative recombination dominates. The lifetimes are only weakly dependent on well thickness and decrease from 170 to 140 ps for quantum wells with thickness decreasing from 300 to 34 A. A comparison with a high quality GaAs/AlGaAs SQW yields comparable luminescence efficiencies at low temperatures, yet the lifetimes are considerably shorter in the CdTe/Cd0.75Mn0.25Te quantum wells due to the higher exciton oscillator strength.

Picosecond photoinduced index changes in a-Si:H and related alloys measured by transient grating experiments

Abstract We report on transient grating experiments on amorphous films as a sensitive technique to study refractive index changes due to carrier relaxation processes. The experiments are performed on a-Si:H and a-SiC:H samples at room temperature with a time resolution of 10 ps. The observed fast component of the transient grating signal is attributed to the capture of carriers from extended states into localized states within 10 ps. The slow decay component reflects carrier relaxation into deeper tail states or traps and recombination as verified by experiments on a-SiC:H films with different defect densities.

Model calculations of diffusion limited trapping dynamics in quantum well laser structures

We present a phenomenological theoretical model to treat the trapping of carriers into quantum wells of semiconductor laser structures. We consider explicitely the transport within the barrier layers by solving the continuity equation with the appropriate boundary conditions taking into account surface recombination, radiative and nonradiative recombination in the barrier layers and trapping of carriers into the quantum wells. The experimental findings for the trapping dynamics in GaAs/AlGaAs quantum well structures can be consistently interpreted by the model calculations.