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Femtosecond Pump-Probe Diagnostics Of Preformed Plasma Channels
We report on recent ultrafast pump-probe experiments 28 in He plasma waveguides using 800 nm, 80 fs pump pulses of 0.2 x 1018 W/cm2 peak guided intensity, and single orthogonally-polarized 800 nm probe pulses with similar to0.1% of pump intensity. The main results are: (1) We observe frequency-domain interference between the probe and a weak, depolarized component of the pump that differs substantially in mode shape from the injected pump pulse; (2) we observe spectral blue-shifts in the transmitted probe that are not evident in the transmitted pump. The evidence indicates that pump depolarization and probe blue-shifts both originate near the channel entrance.Physic
Spin dynamics and magnetic-field-induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment
The exciton spin dynamics are investigated both experimentally and
theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect
band gap and a type-II band alignment. The magnetic-field-induced circular
polarization of photoluminescence, , is studied as function of the
magnetic field strength and direction as well as sample temperature. The
observed nonmonotonic behaviour of these functions is provided by the interplay
of bright and dark exciton states contributing to the emission. To interpret
the experiment, we have developed a kinetic master equation model which
accounts for the dynamics of the spin states in this exciton quartet, radiative
and nonradiative recombination processes, and redistribution of excitons
between these states as result of spin relaxation. The model offers
quantitative agreement with experiment and allows us to evaluate, for the
studied structure, the heavy-hole factor, , and the spin
relaxation times of electron, s, and hole, s, bound in the exciton.Comment: 17 pages, 16 figure
the role of firms
Der Standortwechsel von Firmen in Länder mit niedrigen Sozialstandards wird in
der Regel als treibende Kraft des abwärtsgerichteten regulativen Wettbewerbs
zwischen Staaten angesehen. Es gibt allerdings eine Vielzahl von Fällen in
denen genau das Gegenteil passiert: Firmen halten sich freiwillig an
Sozialstandards (CSR) und üben sogar Druck auf Regierungen aus, um striktere
Regulierung zu erwirken. In diesem Arbeitspapier zeigen wir, unter welchen
Bedingungen Firmen zur Verwirklichung anspruchsvollerer Sozialstandards in
Ländern mit geringer regulativer Kapazität beitragen. Zunächst stellen wir
Hypothesen aus der bereits existierenden Literatur vor und arbeiten ihre
Erklärungskraft für die hier diskutierte Problematik heraus. Das Arbeitspapier
untersucht die Reaktion der südafrikanischen Textil- und Automobilindustrie
auf die HIV Pandemie. Die südafrikanische Regierung hat nur begrenzte
Kapazitäten aufgebracht, um sich gegen die Verbreitung des Virus zur Wehr zu
setzen. Unter welchen Bedingungen versuchen Firmen den Staat im Kampf gegen
HIV/AIDS zu unterstützen?Firms relocating production to countries with lower social standards are
regarded as driving force behind the regulatory ‘race to the bottom’. However,
there are numerous instances in which the behavior of firms reveals just the
opposite: They adhere to self-regulatory standards (CSR) and even pressure
governments to issue stricter public regulations. We intend to identify the
conditions under which firms contribute to higher regulatory standards in
states with weak regulatory capacities, thereby following a ‘race to the top’
rather than a ‘race to the bottom’- logic. Theoretically, we set out to test
in how far the existing literature can be utilized to answer this question.
Empirically, the assessment concentrates on the textile and automotive
industries in South Africa and HIV/AIDS abatement. Only limited state
capacities have been involved in fighting HIV/AIDS in South Africa. Under
which conditions do firms try to foster state capacities for the fight against
the disease
Restoring betatron phase coherence in a beam-loaded laser-wakefield accelerator
Matched beam loading in laser wakefield acceleration (LWFA), characterizing
the state of flattening of the acceleration electric field along the bunch,
leads to the minimization of energy spread at high bunch charges. Here, we
demonstrate by independently controlling injected charge and acceleration
gradients, using the self-truncated ionization injection scheme, that minimal
energy spread coincides with a reduction of the normalized beam divergence.
With the simultaneous confirmation of a constant beam radius at the plasma
exit, deduced from betatron radiation spectroscopy, we attribute this effect to
the reduction of chromatic betatron decoherence. Thus, beam loaded LWFA enables
highest longitudinal and transverse phase space densities
Demonstration of a beam loaded nanocoulomb-class laser wakefield accelerator.
Laser-plasma wakefield accelerators have seen tremendous progress, now capable of producing quasi-monoenergetic electron beams in the GeV energy range with few-femtoseconds bunch duration. Scaling these accelerators to the nanocoulomb range would yield hundreds of kiloamperes peak current and stimulate the next generation of radiation sources covering high-field THz, high-brightness X-ray and γ-ray sources, compact free-electron lasers and laboratory-size beam-driven plasma accelerators. However, accelerators generating such currents operate in the beam loading regime where the accelerating field is strongly modified by the self-fields of the injected bunch, potentially deteriorating key beam parameters. Here we demonstrate that, if appropriately controlled, the beam loading effect can be employed to improve the accelerator's performance. Self-truncated ionization injection enables loading of unprecedented charges of ∼0.5 nC within a mono-energetic peak. As the energy balance is reached, we show that the accelerator operates at the theoretically predicted optimal loading condition and the final energy spread is minimized.Higher beam quality and stability are desired in laser-plasma accelerators for their applications in compact light sources. Here the authors demonstrate in laser plasma wakefield electron acceleration that the beam loading effect can be employed to improve beam quality by controlling the beam charge
Influence of the heterointerface sharpness on exciton recombination dynamics in an ensemble of (In,Al)As/AlAs quantum dots with indirect band-gap
The dynamics of exciton recombination in an ensemble of indirect band-gap
(In,Al)As/AlAs quantum dots with type-I band alignment is studied. The lifetime
of confined excitons which are indirect in momentum-space is mainly influenced
by the sharpness of the heterointerface between the (In,Al)As quantum dot and
the AlAs barrier matrix. Time-resolved photoluminescence experiments and
theoretical model calculations reveal a strong dependence of the exciton
lifetime on the thickness of the interface diffusion layer. The lifetime of
excitons with a particular optical transition energy varies because this energy
is obtained for quantum dots differing in size, shape and composition. The
different exciton lifetimes, which result in photoluminescence with
non-exponential decay obeying a power-law function, can be described by a
phenomenological distribution function, which allows one to explain the
photoluminescence decay with one fitting parameter only.Comment: 10 pages, 7 figure
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