TY - JOUR A1 - Hartmann, Michael A1 - Zimmermann, R. A1 - Stolz, Heinrich T1 - Carrier Kinetics in a Semiconductor with Light-Induced Gaps JF - Physica Status Solidi (B) N2 - Due to the influence of a strong electromagnetic field an induced gap is expected to occur in the band structure of a semiconductor. In this paper the influence of the damping by electron–phonon collisions and by recombination on the new band structure is investigated. Taking into account damping kinetic equations for the electrons are derived and discussed with the help of calculations. Unter dem Einfluß eines starken elektromagnetischen Feldes kommt es in der Bandstruktur eines Halbleiters zur Ausbildung eines Zusatzgaps. In dieser Arbeit wird der Einfluß der Dämpfung durch Elektron–Phononstöße und Rekombination auf die neue Bandstruktur untersucht. Weiterhin werden kinetische Gleichungen unter Berücksichtigung der Dämpfung abgeleitet und an Hand von numerischen Berechnungen diskutiert. KW - Physics KW - Kinetics KW - Semiconductors Y1 - 1988 N1 - Volltext aus urheberrechtlichen Gründen nicht verfügbar. Fulltext due to copyright not available. VL - 146 IS - 1 SP - 357 EP - 369 ER - TY - JOUR A1 - Zimmermann, R. A1 - Hartmann, Michael T1 - Resonant and Off-Resonant Light–Matter Interaction in Semiconductors Kinetics and Transients JF - Physica Status Solidi (B) N2 - Kinetic problems for off-resonance excitation (dynamical Stark effect) and resonant excitation (laser-induced gaps) are studied. Within the Hartree-Fock theory, Stark shift and bleaching of the exciton line are evaluated. The influence of photon statistics on light-induced gaps is investigated. Open questions concerning the transient response are clarified. Es werden kinetische Probleme für nichtresonante Anregung (dynamischer Stark-Effekt) und resonante Anregung (laserinduzierte Lücken) betrachtet. Im Rahmen der Hartree-Fock-Theorie werden Stark-Verschiebung und Oszillatorstärkenänderung für die Exzitonlinie berechnet. Der Einfluß der Photon-Statistik auf die lichtinduzierten Lücken wird untersucht. Einige offene Fragen des Kurzzeit-Response werden geklärt. KW - Physics KW - Kinetics KW - Semiconductors Y1 - 1988 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:0298-opus4-6048 VL - 150 IS - 2 SP - 365 EP - 378 ER - TY - JOUR A1 - Zimmermann, R. A1 - Hartmann, Michael T1 - Theory of the dynamical stark effect of excitons in CdS JF - Journal of Crystal Growth N2 - The dynamical Stark effect of excitons in CdS bulk crystals has been measured recently using differential transmission spectroscopy (DTS). A general DTS formula is derived taking into account exciton-exciton interaction. A perculiar influence of the pump and test polarization on its magnitude is predicted. Concerning the dephasing (transverse relaxation), coupling to optical phonons is treated on a microscopic level. A density-matrix approach leads to the imaginary part of the exciton-phonon self energy. Its strong frequency dependence which shows up as a non-Markoff relaxation strongly suppresses the relaxation for the pump pulse at large detuning. A more sophisticated approach using the non-equilibrium Green function gives deeper insight into the underlying scattering processes. KW - Physics Y1 - 1990 N1 - Volltext aus urheberrechtlichen Gründen nicht verfügbar. Full text due to copyright not available. VL - 101 IS - 1-4 SP - 341 EP - 345 ER - TY - JOUR A1 - Hartmann, Michael A1 - Zimmermann, R. A1 - Stolz, Heinrich T1 - Kinetics of Screening in Optically Excited Semiconductors JF - Physica Status Solidi (B) N2 - Kinetic equations for the density correlation and density-density response function are derived. Solving these equations numerically and comparing its solution with the adiabatic approximation the change in the transmitted spectrum of a single electronic transition is calculated where a red-shift can be observed. Es werden kinetische Gleichungen für die Di Dichtekorrelationsfunktion und die Dichte-Dichte-Responsefunktion abgeleitet. Die numerische Lösung dieser Gleichungen wird mit der adiabatischen Approximation verglichen. Die Berechnung der Änderung im Transmissionsspektrum für einen einzelnen elektronischen Übergang ergibt eine Rotverschiebung. KW - Physics KW - Kinetics Y1 - 1990 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:0298-opus4-6028 VL - 159 IS - 1 SP - 35 EP - 42 ER - TY - CHAP A1 - Hartmann, Michael A1 - Schäfer, Wilfried A1 - Schmitt-Rink, Stefan T1 - Time-resolved and time-integrated four-wave-mixing signals of two-dimensional magneto-excitons T2 - International Meeting on Optics of Excitons in Confined Systems, Giardini Naxos N2 - Keine Angabe - No details KW - Physics KW - Optics Y1 - 1991 N1 - Volltext aus urheberrechtlichen Gründen nicht verfügbar. Full text due to copyright not available. ER - TY - JOUR A1 - Hartmann, Michael A1 - Schäfer, Wilfried T1 - Real Time Approach to Relaxation and Dephasing Processes in Semiconductors JF - Physica Status Solidi (B) N2 - Starting from the kinetic equations for the nonequilibrium Green's functions, a practical method is proposed which allows to solve the set of equations for the two-time propagators numerically. In contrast to the usual semiconductor Bloch equations with phenomenological damping relaxation and dephasing processes are treated microscopically for the case of coupling to a bath of LO-phonons. Beside a standard approximation for the treatment of LO-phonon scattering, no further approximations are performed. For the low-density limit we compare exact results with approximate treatments, as the Marcovian and non-Marcovian approximations resulting from a density matrix approach. First results concerning nonlinear optical properties are presented. KW - Physics KW - Real time approach KW - Semiconductors Y1 - 1992 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:0298-opus4-5980 VL - 173 IS - 1 SP - 165 EP - 176 ER - TY - JOUR A1 - Kim, Dai-Sik A1 - Shah, Jagdeep A1 - Cunningham, J. E. A1 - Damen, T. C. A1 - Schäfer, Wilfried A1 - Hartmann, Michael A1 - Schmitt-Rink, Stefan T1 - Giant excitonic resonance in time-resolved four-wave mixing in quantum wells JF - Physical Review Letters N2 - We report a strong (300:1) excitonic resonance in time-resolved (100 fs) four-wave mixing in GaAs quantum wells as a function of the incident photon energy and show that it results from the large difference in the dephasing rates of excitons and free carriers. We conclude that excitons dominate even when they make up only a small fraction of the excited states. These results, in conjuction with new results in exciton-phonon scattering, provide new insights into previous studies performed with spectrally broad ultrashort pulses. KW - Physics Y1 - 1992 N1 - Volltext aus urheberrechtlichen Gründen nicht verfügbar. Fulltext due to copyright not available. VL - 68 SP - 1006 EP - Keine Angabe - No details ER - TY - JOUR A1 - Hartmann, Michael A1 - Pittner, Jiřı́ A1 - Dam, Huub van A1 - Veyret, Vincent A1 - Bonacic-Koutecký, Vlasta T1 - Theoretical exploration of stationary and of ultrafast spectroscopy of small clusters JF - Applied Physics B N2 - Stationary spectra offer information on the interplay between the structures and the nature of electronic excitations reflecting bonding properties, as shown by comparing Sin with Agn (n=4-6) clusters. In order to study the dynamical properties, simulations and analysis of femtosecond (fs) time-resolved pump–probe or pump–dump signals have been carried out, which allows us to determine the timescales and the nature of configurational changes versus internal vibrational relaxation (IVR) in electronic ground or excited states. For this purpose we have developed a multi-state ab initio molecular dynamics (involving ground as well as adiabatic or non-adiabatic excited electronic states) on the timescale of the nuclear motion, using the time evolution of a thermal ensemble in the Wigner representation. The combination of ab initio quantum-chemical methods used for the molecular dynamics ‘on the fly’ and the Wigner-distribution approach for the description of the motion of the nuclei also allowed the accurate determination of pump–probe and pump–dump signals under temperature-dependent initial conditions. We use this novel combination of methods to investigate the dynamics in excited states of non-stoichiometric NanFn-1 clusters with a single excess electron. The timescales of the structural relaxation in excited states versus intramolecular vibrational relaxation processes have been determined, as illustrated for the example of Na4F3. This is the first study of the system with 15 degrees of freedom for which the dynamics in the excited states has been carried out without the precalculation of the energy surfaces. KW - Physics KW - Laser technology KW - Engineering Y1 - 2000 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:0298-opus4-5693 VL - 71 IS - 3 SP - 343 EP - 349 ER - TY - JOUR A1 - Hartmann, Michael A1 - Pittner, Jiřı́ A1 - Dam, Huub van A1 - Bonačić-Koutecký, Vlasta T1 - Theoretical study of femtosecond pump–probe signals of nonstoichiometric alkali halide clusters JF - The European Physical Journal D N2 - We present the outline of the ab–initio molecular dynamics method for adiabatic excited states which is suitable to treat the involved excited electronic states simultaneously with the motion of the nuclei. The method allows the simulation of time–resolved pump–probe signals and the analysis of the signals in terms of the underlying nuclear dynamics. Moreover, the timescales of the structural relaxation in excited electronic states vs. intramolecular vibrational relaxation processes can be estimated and the influence of the initial temperature of the cluster can be determined. The method is applied to the analysis of the timescales as well as of the dynamics of localized/delocalized electronic excitations in the Na2F molecule which represents a test example for nonstoichiometric alkali–halides with one excess electron. The aim is to develop a method for the investigation of time–dependent laser induced processes in clusters which is capable to extract the leading physical effects responsible for the controlled excitation dynamics and for the evolution of the non–equilibrium states. KW - Physics KW - Optical physics KW - Molecular physics Y1 - 1999 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:0298-opus4-5640 VL - 9 IS - 1 SP - 393 EP - 397 ER -