Parametric Four-Photon Generation of Picosecond Light at High ConversionEfficiency

Parametric four-photon interaction in isotropic media was studied in the saturation range. Up to 10% of input laser energy could be converted into a broad frequency spectrum ranging from the ultraviolet to the infrared. Parameters which influence the conversion efficiency are discussed

Laser spectroscopy of localized quantum dot states interacting with electron reservoirs

Self-assembled InGaAs quantum dots are nano-objects embedded in the solid-state matrix of GaAs. They act as natural potential traps for charge carriers and feature a number of quantized states due to the quantum confinement. When incorporated in a field effect structure the quantum dot states can be conveniently manipulated with an electric field and probed by resonant laser spectroscopy. In this thesis self-assembled quantum dots were investigated with an emphasis on the study of interactions between localized quantum dot states and charge or spin reservoirs in the environment. Experimentally the quantum dots were addressed in distinct regimes where the quantum dot spectrum was sensitive to individual charge fluctuations or mesoscopic reservoirs. The fundamental transition of a neutral quantum dot was found to exhibit a number of discontinuities in the usually linear dispersion of the exciton energy in external electrostatic fields. The discontinuities were identified to arise from charge fluctuations in the surrounding crystalline matrix in which impurity atoms can capture or release electrons. At characteristic conditions charging and discharging events lead to discrete changes of the electrostatic environment which in turn gives rise to an energy shift of the optical resonance condition. An electrostatic model was developed for a quantitative analysis of charging events and their signatures. On the basis of the model a comprehensive study of nearby quantum dots allowed to map out the relative spatial positions of quantum dots and impurities. In contrast to previous reports our results provide evidence for bulk impurities as the main source of charge fluctuations. By means of resonant laser spectroscopy in the energy dispersion of the neutral exciton a kink with a continuous energy shift has been observed which only occurs close to the regime where an electron is tunneling between the quantum dot and a 2D electron reservoir. The tunneling induces a weak coupling between the localized electron state of the quantum dot and the continuum of states in the reservoir. The tunnel coupling between the interacting states leads to hybridization into a new superposition state. In consequence the energy of the transition is renormalized which explains the kink in the energy dispersion. The hybridization model based on an Anderson-Fano approach quantitatively agrees with the experimental data and allows to extract the coupling strength between the reservoir and the localized state. In addition to the neutral exciton hybridization effects were also ob-served on the charged exciton. To study optical signatures of many-body effects sub-K laser spectroscopy was established and the setup performance was characterized with optical studies of a quantum dot in the Pauli-blockade regime. The electron bath temperature was determined using experimental and calculated electron spin populations as a function of magnetic field and temperature. The experiment provided quantitative access to all parameters except the electron bath temperature. With the optical Bloch equations the electron spin populations were modeled taking into account all relevant external parameters. An analysis of the evolution of the spin population in magnetic fields with the electron bath temperature as the only free fitting parameter was performed. An electron bath temperature of 380 mK was derived being slightly offset to the nominal base temperature of 250 mK. This proves the successful implementation of the sub-K laser spectroscopy setup

Generation of frequency shifted picosecond pulses with low temporal jitter

Transient stimulated Raman scattering is used for the generation of a frequency shifted picosecond light pulse; part of this Raman shifted pulse is subsequently coherently scattered at a material excitation of a second Raman cell. Starting with the second harmonic pulse (tp = 4 ps) of a mode-locked Nd : glass laser system, both the stimulated and the coherently produced pulses have durations of 2.3 ps at different wavelengths. By the appropriate choice of the Raman medium pulses between 13 000 and 21 000 cm-1 can be generated. The coherent generation process minimizes the temporal jitter between the two pulses and allows to obtain a high time resolution of better than 0.3 ps in excite and probe experiments

Forming and confining of dipolar excitons by quantizing magnetic fields

We show that a magnetic field perpendicular to an AlGaAs/GaAs coupled quantum well efficiently traps dipolar excitons and leads to the stabilization of the excitonic formation and confinement in the illumination area. Hereby, the density of dipolar excitons is remarkably enhanced up to $\sim 10^{11} cm^{-2}$. By means of Landau level spectroscopy we study the density of excess holes in the illuminated region. Depending on the excitation power and the applied electric field, the hole density can be tuned over one order of magnitude up to $\sim 2.5$ $10^{11} cm^{-2}$ - a value comparable with typical carrier densities in modulation-doped structures.Comment: 4.3 Pages, 4 Figure

Excited-state reaction dynamics of bacteriorhodopsin studied by femtosecond spectroscopy

The photodynamics of bacteriorhodopsin were studied by transient absorption and gain measurements after excitation with femtosecond pulses at 620 nm. With probing pulses at longer wavelengths (λ > 770 nm) the previously reported formation of the J intermediate (with a time constant of 500±100 fs) was confirmed. With probing pulses around 700 nm, a faster process with a relaxation time of 200±70 fs was observed. The data analysis strongly suggests that this kinetic constant describes the reactive motion of the polyatomic molecule on its excited-state potential energy surface, i.e. one observes directly the incipient isomerization of the retinal molecule. The minimum of the S1 potential energy surface reached in 200 fs lies approximately 13300 cm−1 above the ground state of bacteriorhodopsin and from this minimum the intermediate J is formed with a time constant of 500 fs

The wheat ω-gliadin genes: structure and EST analysis

A survey and analysis is made of all available ω-gliadin DNA sequences including ω-gliadin genes within a large genomic clone, previously reported gene sequences, and ESTs identified from the large wheat EST collection. A contiguous portion of the Gli-B3 locus is shown to contain two apparently active ω-gliadin genes, two pseudogenes, and four fragments of the 3′ portion of ω-gliadin sequences. Comparison of ω-gliadin sequences allows a phylogenetic picture of their relationships and genomes of origin. Results show three groupings of ω-gliadin active gene sequences assigned to each of the three hexaploid wheat genomes, and a fourth group thus far consisting of pseudogenes assigned to the A-genome. Analysis of ω-gliadin ESTs allows reconstruction of two full-length model sequences encoding the AREL- and ARQL-type proteins from the Gli-A3 and Gli-D3 loci, respectively. There is no DNA evidence of multiple active genes from these two loci. In contrast, ESTs allow identification of at least three to four distinct active genes at the Gli-B3 locus of some cultivars. Additional results include more information on the position of cysteines in some ω-gliadin genes and discussion of problems in studying the ω-gliadin gene family