175 research outputs found
Signatures of the excitonic memory effects in four-wave mixing processes in cavity polaritons
We report the signatures of the exciton correlation effects with finite
memory time in frequency domain degenerate four-wave mixing (DFWM) in
semiconductor microcavity. By utilizing the polarization selection rules, we
discriminate instantaneous, mean field interactions between excitons with the
same spins, long-living correlation due to the formation of biexciton state by
excitons with opposite spins, and short-memory correlation effects in the
continuum of unbound two-exciton states. The DFWM spectra give us the relative
contributions of these effects and the upper limit for the time of the
exciton-exciton correlation in the unbound two-exciton continuum. The obtained
results reveal the basis of the cavity polariton scattering model for the DFWM
processes in high-Q GaAs microcavity.Comment: 11 pages, 1 figur
Ultrafast Spin Dynamics in Nickel
The spin dynamics in Ni is studied by an exact diagonalization method on the
ultrafast time scale. It is shown that the femtosecond relaxation of the
magneto-optical response results from exchange interaction and spin-orbit
coupling. Each of the two mechanisms affects the relaxation process
differently. We find that the intrinsic spin dynamics occurs during about 10 fs
while extrinsic effects such as laser-pulse duration and spectral width can
slow down the observed dynamics considerably. Thus, our theory indicates that
there is still room to accelerate the spin dynamics in experiments.Comment: 4 pages, Latex, 4 postscript figure
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Photocarrier relaxation in the quantum kinetics regime
We present a study of carrier relaxation in GaAs in the quantum- kinetics regime. The results cannot be explained by Bolzmann kinetics. They are in satisfactory agreement with quantum-kinetic theory calculation of the occupation number in k-space
Collective oscillations driven by correlation in the nonlinear optical regime
We present an analytical and numerical study of the coherent exciton
polarization including exciton-exciton correlation. The time evolution after
excitation with ultrashort optical pulses can be divided into a slowly varying
polarization component and novel ultrafast collective modes. The frequency and
damping of the collective modes are determined by the high-frequency properties
of the retarded two-exciton correlation function, which includes Coulomb
effects beyond the mean-field approximation. The overall time evolution depends
on the low-frequency spectral behavior. The collective mode, well separated
from the slower coherent density evolution, manifests itself in the coherent
emission of a resonantly excited excitonic system, as demonstrated numerically.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
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
Renormalized Bosonic Interaction of Excitons
An effective bosonic Hamiltonian of excitons with ``spin'' degrees of
freedom in two dimension is obtained through a projection procedure, starting
from a conventional electron-hole Hamiltonian . We first
demonstrate that a straightforward transformation of into a
Hamiltonian of bosonic excitons does not give the two-body interaction between
an ``up-spin'' exciton and a ``down-spin'' exciton, which are created by the
left- and right-circularly polarized light beams, respectively. We then show
that this interaction is generated through a projection procedure onto the
subspace spanned by excitons, as a renormalization effect coming from
higher exciton states. The projection also renormalizes the interaction between
excitons with the same spins by a large amount. These renormalization
effects are crucial for the polarization dependence of the optical responses
from semiconductors. The present theory gives the microscopic foundation of the
phenomenology that was successfully applied to the analysis of four-wave mixing
experiments in GaAs quantum wells strongly coupled to the radiation field in a
high-Q micro cavity.Comment: 2 figure
Ultrafast Nonlinear Optical Response of Strongly Correlated Systems: Dynamics in the Quantum Hall Effect Regime
We present a theoretical formulation of the coherent ultrafast nonlinear
optical response of a strongly correlated system and discuss an example where
the Coulomb correlations dominate. We separate out the correlated contributions
to the third-order nonlinear polarization, and identify non-Markovian dephasing
effects coming from the non-instantaneous interactions and propagation in time
of the collective excitations of the many-body system. We discuss the
signatures, in the time and frequency dependence of the four-wave-mixing (FWM)
spectrum, of the inter-Landau level magnetoplasmon (MP) excitations of the
two-dimensional electron gas (2DEG) in a perpendicular magnetic field. We
predict a resonant enhancement of the lowest Landau level (LL) FWM signal, a
strong non-Markovian dephasing of the next LL magnetoexciton (X), a symmetric
FWM temporal profile, and strong oscillations as function of time delay, of
quantum kinetic origin. We show that the correlation effects can be controlled
experimentally by tuning the central frequency of the optical excitation
between the two lowest LLs.Comment: 21 pages, 10 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
Two new species of the Hypostomus cochliodon group (Siluriformes: Loricariidae) from the rio Paraguay basin, with a redescription of Hypostomus cochliodonKner, 1854
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