58 research outputs found
Coherent dynamics of interwell excitons in GaAs/AlxGa1-xAs superlattices
Coherent exciton dynamics in a GaAs/AlxGa1âxAs narrow-miniband superlattice is studied by spectrally resolved transient four-wave mixing. Coherent optical properties of the investigated structure are found to be strongly affected by the existence of two different heavy-hole excitonic states. One of them, the 1s heavy-hole exciton, is almost identical to the same state in noninteracting quantum wells, while the other, the heavy-hole interwell exciton, is composed of an electron and a heavy hole in adjacent wells. The interwell exciton leads to a resonant enhancement in the four-wave mixing spectra and exhibits quantum beats with the 1s heavy-hole exciton. The dephasing of the interwell exciton is one order of magnitude faster than that of the heavy-hole exciton and is mostly due to intensity-independent scattering mechanisms
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
Spectrally resolved four-wave mixing in semiconductors: Influence of inhomogeneous broadening
Transient four-wave mixing in T-shaped GaAs quantum wires
The binding energy of excitons and biexcitons and the exciton dephasing in T-shaped GaAs quantum wires is investigated by transient four-wave mixing. The T-shaped structure is fabricated by cleaved-edge overgrowth, and its geometry is engineered to optimize the one-dimensional confinement. In this wire of 6.6Ă24ânm2 size, we find a one-dimensional confinement of more than 20 meV, an inhomogeneous broadening of 3.4 meV, an exciton binding energy of 12 meV, and a biexciton binding energy of 2.0 meV. A dispersion of the homogeneous linewidth within the inhomogeneous broadening due to phonon-assisted relaxation is observed. The exciton acoustic-phonon-scattering coefficient of 6.1±0.5âÎŒeV/K is larger than in comparable quantum-well structures
Welche Assoziationen bestehen zwischen einem Aufnahmeassessment zur sozialen Situation geriatrischer Patienten und dem TÀtigkeitsaufwand eines Krankenhaussozialdienstes in der Geriatrie und inwieweit sind PrÀdiktoren erkennbar
Subcortical Hypoperfusion following Surgery For Aneurysmal Subarachnoid Haemorrhage: Implications For Cognitive Performance?
The incidence and severity of cognitive deficits after surgery for aneurysmal subarachnoid haemorrhage and their relationship to aneurysm site remains controversial. The aim of this study was to investigate the pattern of regional cerebral blood flow which exists in patients one year post-surgery and to identify whether different patterns exist which may be related to the type of cognitive deficit or the location of the aneurysm. 62 patients underwent cognitive assessment and HMPAO SPECT imaging at a mean time of 12 months following surgery. Results were compared to those from healthy control subjects (n = 55 for neuropsychological testing; n = 14 for SPECT imaging). In the patient group, significant stable cognitive deficits occurred in all cognitive domains but no cognitive measure differentiated aneurysm site. On SPECT images, statistical parametric mapping identified a large common area of subcortical hypoperfusion in the patient group as a whole. The findings of this study suggest a possible link between reduced subcortical function and the extent and severity of cognitive deficits
Analysis for possible linkage between the loci for the Waardenburg syndrome and various blood groups and serological traits
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