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, $P_c$, 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 $g$ factor, $g_{hh}=+3.5$, and the spin relaxation times of electron, $\tau_{se} = 33~\mu$s, and hole, $\tau_{sh} = 3~\mu$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