317 research outputs found
Ultrafast Spin Dynamics in GaAs/GaSb/InAs Heterostructures Probed by Second Harmonic Generation
We report the first application of pump-probe second harmonic generation
(SHG) measurements to characterize optically-induced magnetization in
non-magnetic multilayer semiconductors. In the experiment, coherent spins are
selectively excited by a pump beam in the GaAs layer of GaAs/GaSb/InAs
structures. However, the resulting net magnetization manifests itself through
the induced SHG probe signal from the GaSb/InAs interface, thus indicating a
coherent spin transport across the heterostructure. We find that the
magnetization dynamics is governed by an interplay between the spin density
evolution at the interfaces and the spin dephasing.Comment: 4 pages + 4 Fig
Ultrafast Dynamics of Interfacial Electric Fields in Semiconductor Heterostructures Monitored by Pump-Probe Second Harmonic Generation
We report first measurements of the ultrafast dynamics of interfacial
electric fields in semiconductor multilayers using pump-probe second harmonic
generation (SHG). A pump beam was tuned to excite carriers in all layers of
GaAs/GaSb and GaAs/GaSb/InAs heterostructures. Further carrier dynamics
manifests itself via electric fields created by by charge separation at
interfaces. The evolution of interfacial fields is monitored by a probe beam
through the eletric-field-induced SHG signal. We distinguish between several
stages of dynamics originating from redistribution of carriers between the
layers. We also find a strong enhancement of the induced electric field caused
by hybridization of the conduction and valence bands at the GaSb/InAs
interface.Comment: 4 pages + 2 fig
Size-dependent Correlation Effects in Ultrafast Optical Dynamics of Metal Nanoparticles
We study the role of collective surface excitations in the electron
relaxation in small metal particles. We show that the dynamically screened
electron-electron interaction in a nanoparticle contains a size-dependent
correction induced by the surface. This leads to new channels of quasiparticle
scattering accompanied by the emission of surface collective excitations. We
calculate the energy and temperature dependence of the corresponding rates,
which depend strongly on the nanoparticle size. We show that the
surface-plasmon-mediated scattering rate of a conduction electron increases
with energy, in contrast to that mediated by a bulk plasmon. In noble-metal
particles, we find that the dipole collective excitations (surface plasmons)
mediate a resonant scattering of d-holes to the conduction band. We study the
role of the latter effect in the ultrafast optical dynamics of small
nanoparticles and show that, with decreasing nanoparticle size, it leads to a
drastic change in the differential absorption lineshape and a strong frequency
dependence of the relaxation near the surface plasmon resonance. The
experimental implications of our results in ultrafast pump-probe spectroscopy
are also discussed.Comment: 29 pages including 6 figure
Ultrafast dynamics of coherences in the quantum Hall system
Using three-pulse four-wave-mixing optical spectroscopy, we study the
ultrafast dynamics of the quantum Hall system. We observe striking differences
as compared to an undoped system, where the 2D electron gas is absent. In
particular, we observe a large off-resonant signal with strong oscillations.
Using a microscopic theory, we show that these are due to many-particle
coherences created by interactions between photoexcited carriers and collective
excitations of the 2D electron gas. We extract quantitative information about
the dephasing and interference of these coherences.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let
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
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
Coherent Magnetization Precession in GaMnAs induced by Ultrafast Optical Excitation
We use femtosecond optical pulses to induce, control and monitor
magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures
below ~40 K we observe coherent oscillations of the local Mn spins, triggered
by an ultrafast photoinduced reorientation of the in-plane easy axis. The
amplitude saturation of the oscillations above a certain pump intensity
indicates that the easy axis remains unchanged above ~TC/2. We find that the
observed magnetization precession damping (Gilbert damping) is strongly
dependent on pump laser intensity, but largely independent on ambient
temperature. We provide a physical interpretation of the observed light-induced
collective Mn-spin relaxation and precession.Comment: 7 pages,3 figure
Size-Dependent Surface Plasmon Dynamics in Metal Nanoparticles
We study the effect of Coulomb correlations on the ultrafast optical dynamics
of small metal particles. We demonstrate that a surface-induced dynamical
screening of the electron-electron interactions leads to quasiparticle
scattering with collective surface excitations. In noble-metal nanoparticles,
it results in an interband resonant scattering of d-holes with surface
plasmons. We show that this size-dependent many-body effect manifests itself in
the differential absorption dynamics for frequencies close to the surface
plasmon resonance. In particular, our self-consistent calculations reveal a
strong frequency dependence of the relaxation, in agreement with recent
femtosecond pump-probe experiments.Comment: 8 pages + 4 figures, final version accepted to PR
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