Reducing decoherence of the confined exciton state in a quantum dot by pulse-sequence control

We study the phonon-induced dephasing of the exciton state in a quantum dot excited by a sequence of ultra-short pulses. We show that the multiple-pulse control leads to a considerable improvement of the coherence of the optically excited state. For a fixed control time window, the optimized pulsed control often leads to a higher degree of coherence than the control by a smooth single Gaussian pulse. The reduction of dephasing is considerable already for 2-3 pulses.Comment: Final version (moderate changes

Compensation device of breakaway torque of reaction wheel assembly

Статья посвящена разработке устройства компенсации момента трогания в управляющихдвигателях-маховиках. В статье анализируются проблемы, связанные с возникновением явления момента трогания в двигателях. Автором предлагаются пути решения данной проблемы, а так же рассматриваются преимущества и недостатки каждого из них. В результате было предложено устройство, которое позволяет снизить момент трогания управляющих двигателей-маховиков и исключить зону нечувствительности к малым управляющим сигналам, описана его структура и представлен краткий алгоритм работы такого устройства. Оценка результатов была произведена на испытательном стенде, представляющем собой имитационную модель управляющего двигателя-маховика.The article is devoted to the development of compensation device of breakaway torque of reaction wheel assembly. The article analyses the problems associated with the emergence of phenomenon of breakaway torque in reaction wheel assembly. The author suggests ways of solving this problem, as well as discusses the advantages anddisadvantages of each of them. As a result, it was suggested the device which reduces breakaway torque of reaction wheel assembly and exclude the dead zone in range of small signals, described its structure and provides an algorithm for such a device. Evaluation of the results was carried out on a test bench, which represent a simulationmodel of reaction wheel assembly

Exciton-LO-phonon dynamics in InAs/GaAs quantum dots: Effects of zone-edge phonon damping

The dynamics of an exciton-LO-phonon system after an ultrafast optical excitation in an InAs/GaAs quantum dot is studied theoretically. Influence of anharmonic phonon damping and its interplay with the phonon dispersion is analyzed. The signatures of the zone-edge decay process in the absorption spectrum and time evolution are highlighted, providing a possible way of experimental investigation on phonon anharmonicity effects.Comment: 10 pages, 2 figure

Theory of exciton-exciton correlation in nonlinear optical response

We present a systematic theory of Coulomb interaction effects in the nonlinear optical processes in semiconductors using a perturbation series in the exciting laser field. The third-order dynamical response consists of phase-space filling correction, mean-field exciton-exciton interaction, and two-exciton correlation effects expressed as a force-force correlation function. The theory provides a unified description of effects of bound and unbound biexcitons, including memory-effects beyond the Markovian approximation. Approximations for the correlation function are presented.Comment: RevTex, 35 pages, 10 PostScript figs, shorter version submitted to Physical Review

Coherent Propagation of Polaritons in Semiconductor Heterostructures: Nonlinear Pulse Transmission in Theory and Experiment

The influence of coherent optical nonlinearities on polariton propagation effects is studied within a theory-experiment comparison. A novel approach that combines a microscopic treatment of the boundary problem in a sample of finite thickness with excitonic and biexcitonic nonlinearities is introduced. Light-polarization dependent spectral changes are analyzed for single-pulse transmission and pump-probe excitation

Геолого-геофизическая и фациально-петрофизическая характеристика пласта Ю[1]{3} Казанского нефтегазоконденсатного месторождения

Выделение границ высокопродуктивной зоны, выделение карбонатных прослоев и граница их распространения, построение карты фаций, расчёт петрофизической модели на основе геофизических исследований скважины, оценка влияния распространения карбонатных тел на разработку.Identification of high-productive zone boundaries, identification of carbonate interlayers and boundary of their distribution, construction of facies map, calculation of petrophysical model on the basis of geophysical studies of the well, assessment of the influence of carbonate bodies distribution on the development

Two-band superconductors: Extended Ginzburg-Landau formalism by a systematic expansion in small deviation from the critical temperature

We derive the extended Ginzburg-Landau (GL) formalism for a clean s-wave two-band superconductor by employing a systematic expansion of the free-energy functional and the corresponding matrix gap equation in powers of the small deviation from the critical temperature tau = 1-T/T_c. The two lowest orders of this expansion produce the equation for T_c and the GL theory. It is shown that in agreement with previous studies, the two-band GL theory maps onto the single-band GL model and thus fails to describe the difference in the spatial profiles of the two band condensates. We prove that except for some very special cases, this difference appears already in the leading correction to the GL theory, which constitutes the extended GL formalism. We derive linear differential equations that determine the leading corrections to the band order parameters and magnetic field, discuss the validity of these equations, and consider examples of an important interplay between the band condensates. Finally, we present numerical results for the thermodynamic critical magnetic field and temperature-dependent band gaps (at zero field), which are in a very good agreement with those obtained from the full BCS approach in a wide temperature range. To this end, we emphasize the advantages of our extended GL theory in comparison with the often used two-component GL-like model based on an unreconstructed two-band generalization of the Gor'kov derivation

Ultrafast Coulomb-induced dynamics of 2D magnetoexcitons

We study theoretically the ultrafast nonlinear optical response of quantum well excitons in a perpendicular magnetic field. We show that for magnetoexcitons confined to the lowest Landau levels, the third-order four-wave-mixing (FWM) polarization is dominated by the exciton-exciton interaction effects. For repulsive interactions, we identify two regimes in the time-evolution of the optical polarization characterized by exponential and {\em power law} decay of the FWM signal. We describe these regimes by deriving an analytical solution for the memory kernel of the two-exciton wave-function in strong magnetic field. For strong exciton-exciton interactions, the decay of the FWM signal is governed by an antibound resonance with an interaction-dependent decay rate. For weak interactions, the continuum of exciton-exciton scattering states leads to a long tail of the time-integrated FWM signal for negative time delays, which is described by the product of a power law and a logarithmic factor. By combining this analytic solution with numerical calculations, we study the crossover between the exponential and non-exponential regimes as a function of magnetic field. For attractive exciton-exciton interaction, we show that the time-evolution of the FWM signal is dominated by the biexcitonic effects.Comment: 41 pages with 11 fig