9 research outputs found
Ultrafast light-induced magnetization dynamics in ferromagnetic semiconductors
We develop a theory of the magnetization dynamics triggered by ultrafast
optical excitation of ferromagnetic semiconductors. We describe the effects of
the strong carrier spin relaxation on the nonlinear optical response by using
the Lindblad semigroup method. We demonstrate magnetization control during
femtosecond timescales via the interplay between circularly polarized optical
excitation, hole-spin damping, polarization dephasing, and the Mn-hole spin
interactions. Our results show a light-induced magnetization precession and
relaxation for the duration of the optical pulse.Comment: 4 pages, 2 figure
Signatures of spin blockade in the optical response of a charged quantum dot
We model spin blockade for optically excited electrons and holes in a charged
semiconductor quantum dot. We study the case where the quantum dot is initially
charged with a single electron and is then filled with an additional, optically
excited electron-hole pair, thus forming a charged exciton (trion). To make
contact with recent experiments, we model an optical pump-probe setup, in which
the two lowest quantum dot levels (s and p shells) are photo excited. Using the
Lindblad master equation, we calculate the differential transmission spectrum
as a function of the pump-probe time delay. Taking into account both spin
conserving and spin-flip intraband relaxation processes, we find that the
presence of the ground-state electron spin leads to an optical spin blockade at
short delay times which is visible as a crossover between two exponential
decays of the differential transmission. To make predictions for future
experiments, we also study the dependence of the spin-blockade on an external
magnetic field.Comment: 8 pages, 8 figure
Strong Electronic Correlation Effects in Coherent Multidimensional Nonlinear Optical Spectroscopy
We discuss a manyâbody theory of the coherent ultrafast nonlinear optical response of systems with a strongly correlated electronic ground state that responds unadiabatically to photoexcitation. We introduce a truncation of quantum kinetic density matrix equations of motion that does not rely on an expansion in terms of the interactions and thus applies to strongly correlated systems. For this we expand in terms of the optical field, separate out contributions to the timeâevolved manyâbody state due to correlated and uncorrelated multiple optical transitions, and use âHubbard operatorâ density matrices to describe the exact dynamics of the individual contributions within a subspace of strongly coupled states, including âpure dephasingâ. Our purpose is to develop a quantum mechanical tool capable of exploring how, by coherently photoexciting selected modes, one can trigger nonlinear dynamics of strongly coupled degrees of freedom. Such dynamics could lead to photoinduced phase transitions. We apply our theory to the nonlinear response of a twoâdimensional electron gas (2DEG) in a magnetic field. We coherently photoexcite the two lowest Landau level (LL) excitations using three timeâdelayed optical pulses. We identify some striking temporal and spectral features due to dynamical coupling of the two LLs facilitated by interâLandauâlevel magnetoplasmon and magnetoroton excitations and compare to threeâpulse fourâwaveâmixing (FWM) experiments. We show that these features depend sensitively on the dynamics of fourâparticle correlations between an electronâhole pair and a magnetoplasmon/magnetoroton, reminiscent of excitonâexciton correlations in undoped semiconductors. Our results shed light into unexplored coherent dynamics and relaxation of the quantum Hall system (QHS) and can provide new insight into nonâequilibrium coâoperative phenomena in strongly correlated systems
Observation of inter-Landau-level quantum coherence in semiconductor quantum wells
Using three-pulse four-wave-mixing femtosecond spectroscopy, we excite a
non-radiative coherence between the discrete Landau levels of an undoped
quantum well and study its dynamics. We observe quantum beats that reflect the
time evolution of the coherence between the two lowest Landau level
magnetoexcitons. We interpret our observations using a many-body theory and
find that the inter Landau level coherence decays with a new time constant,
substantially longer than the corresponding interband magnetoexciton dephasing
times. Our results indicate a new intraband excitation dynamics that cannot be
described in terms of uncorrelated interband excitations.Comment: 5 pages, 5 figures, to appear in Phys. Rev. B Rapid Communication
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
Tyrosine kinase Flt3/Flt3-ligand signaling in the modulation of immune responses in experimental arthritis
Rheumatoid arthritis (RA) is an autoimmune, chronic systemic inflammatory disorder that
primarily affects flexible joints resulting in severe joint destruction and disability if left untreated.
Today, advances in treatment have significantly improved the outcome for patients, although the
pathogenesis of RA remains relatively unknown. Signaling through the tyrosine kinase receptor
fms-like tyrosine kinase 3 (Flt3) has been suggested to play a part in the RA pathogenesis. Flt3 is
primarily expressed on hematopoietic stem cells and lymphoid progenitors in the bone marrow
and has an important role in early B-cell development and formation of dendritic cells (DC).
Furthermore, the ligand for Flt3 (Flt3L) serves as a regulator of regulatory T-cell (Treg)
homeostasis and has been suggested to support differentiation of bone-resorbing osteoclasts.
This thesis aimed to investigate the effect of Flt3/Flt3L signaling on the immune system
during development of arthritis using an experimental animal model of human RA. Our study
shows that Flt3 signaling supports formation of DCs and Treg cells during arthritis development.
Treg expansion associated with Flt3L treatment resulted in a reduced production of inflammatory
cytokines, reduced levels of antigen-specific antibodies and reduced bone destruction. On the
contrary, lack of Flt3L was associated with reduced Treg formation resulting in loss of control over
T-cell proliferation, and bone destruction during arthritis. Flt3L was found to positively influence
the transcription of the osteoclast-regulating factor IRF8, and could by this mechanism influence
osteoclast formation. Impaired signaling through Flt3 resulted in low IRF8 expression,
accumulation of osteoclasts in the arthritic joint and an increased loss of femoral trabecular bone.
Conversely, Flt3L treatment was associated with increased IRF8 expression, reduced osteoclast
formation and restoration of trabecular bone formation in mice lacking Flt3L (Flt3LKO). Finally,
we could identify a previously unacknowledged role for Flt3 in peripheral B-cell responses. We
demonstrated that Flt3 was re-expressed on activated B-cells following LPS stimulation in vitro and
on a population of germinal center B-cells in vivo. By using Flt3LKO mice we could identify an
important role for Flt3L in class switch recombination (CSR) to IgG1. B-cells from Flt3LKO mice
were found have reduced activation of Stat6 after IL-4 stimulation, resulting in impaired initiation
of CSR to IgG1 and highly reduced formation of IgG1+ B-cells and IgG1 production.
In summary this thesis shows that Flt3L has an important function in regulating DC and
Treg homeostasis and function during arthritis. Furthermore, Flt3L has a regulatory role on
osteoclast development and on trabecular bone formation. Finally, signaling through the Flt3
receptor on activated B-cells has an important role in the CSR process and deficiency of Flt3L leads
to a skewed antibody response towards the more potent IgG subclasses IgG2b and IgG2c.
Together, these results suggest that Flt3L might play a protective role during arthritis by reduction
of bone destruction, induction of regulatory T-cells and regulation of antibody effector functions.
The conclusion of this thesis is that signaling through the tyrosine kinase Flt3 plays an important
role in modulating immune responses during experimental arthritis