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

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

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

Spin diffusion in the Mn2+ ion system of II-VI diluted magnetic semiconductor heterostructures

The magnetization dynamics in diluted magnetic semiconductor heterostructures based on (Zn,Mn)Se and (Cd,Mn)Te has been studied experimentally by optical methods and simulated numerically. In the samples with nonhomogeneous magnetic ion distribution this dynamics is contributed by spin-lattice relaxation and spin diffusion in the Mn spin system. The spin diffusion coefficient of 7x10^(-8) cm^2/s has been evaluated for Zn(0.99)Mn(0.01)Se from comparison of experimental and numerical results. Calculations of the giant Zeeman splitting of the exciton states and the magnetization dynamics in the ordered alloys and parabolic quantum wells fabricated by the digital growth technique show perfect agreement with the experimental data. In both structure types the spin diffusion has an essential contribution to the magnetization dynamics.Comment: 12 pages, 11 figure

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

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

Genetic analyses of archival specimens of the Atlantic sturgeon Acipenser sturio L., 1758

Se analizó la variabilidad genética en Acipenser sturio L., 1758 y Acipenser oxyrinchus oxyrinchus Mitchill, 1815 usando la variación en la región D-loop del ADN mitocondrial y en un número de microsatélites (marcadores nucleares). El material estudiado incluyó muestras de tejidos de: (1) 38 ejemplares almacenados de A. sturio colectados en diferentes museos de historia natural alemanes, suecos, daneses y franceses; (2) 27 A. sturio vivos correspondientes al stock de cría para recuperación de esta especie en aguas alemanas (Instituto de Ecología Dulceacuícola y Pescas Interiores, Berlín, Alemania); (3) 30 A. o. oxyrinchus silvestres capturados en el océano Atlántico cerca de la costa de Nueva Jersey y en el río Delaware (USA); y (4) 60 individuos de A. o. oxyrinchus de un stock reproducido artificialmente obtenido originalmente de diversos esturiones silvestres capturados en el río San Juan (Canadá). Se clonó y secuenció un fragmento de 250 pares de bases de la región D-loop del ADN mitocondrial. La longitud de una unidad repetida fue de 80 pares de bases en A. sturio y de 79 en A. o. oxyrinchus. Las unidades repetidas de A. sturio y A. o. oxyrinchus difirieron por 11 sustituciones y una delección o inserción, respectivamente. No se encontró heteroplasmia. Se observaron tres diferentes haplotipos de ADN mitocondrial en ambas especies. Cinco microsatélites presentaron patrones polimórficos de bandas. En A. sturio los análisis de microsatélites mostraron una disminución en números alélicos entre los años 1823 y 1992. Este declive tuvo como consecuencia la fijación de varios alelos. Para A. sturio, se observaron un haplotipo de ADN mitocondrial y siete alelos sólo en los ejemplares almacenados. Los cálculos de distancia genética mostraron una gran similitud genética entre las poblaciones del Gironda y del Mar del Norte, y una posición basal de las poblaciones de A. sturio del Mediterráneo y del Adriático. En A. o. oxyrinchus el número de alelos de microsatélite varió entre 14 (ríos Hudson y San Juan) y 22 (río Delaware). Los cálculos de distancia genética mostraron una gran similitud genética entre las subpoblaciones de A. o. oxyrinchus.Genetic variability was analysed in Acipenser sturio L., 1758 and Acipenser oxyrinchus oxyrinchus Mitchill, 1815 using variation in the D-loop region of mtDNA and a number of microsatellites (nuclear markers). The studied material included tissue samples from: (1) 38 A. sturio archival specimens collected in different German, Swedish, Danish, and French museums of natural history; (2) 27 live A. sturio representing a broodstock for restoration of this species in German waters (Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany); (3) 30 wild A. o. oxyrinchus caught in the Atlantic Ocean near the coast of New Jersey and in the Delaware River (USA); and (4) 60 individuals of A. o. oxyrinchus from an artificially reproduced stock originally obtained from several wild sturgeons captured in the St. John River (Canada). A 250-bp fragment of the D-loop region of mtDNA was cloned and sequenced. The length of a repeated unit was 80 bp in A. sturio and 79 bp in A. o. oxyrinchus. The repeated units of A. sturio and A. o. oxyrinchus differed by 11 substitutions and one deletion or insertion, respectively. No heteroplasmy was found. Three different haplotypes of mtDNA were observed in both species. Five microsatellites had polymorphic band patterns. In A. sturio, analyses of microsatellites showed a decrease in allelic numbers between the years 1823 and 1992. This decline resulted in a fixation of several alleles. For A. sturio, one mtDNA haplotype and seven alleles were observed only in archival samples. Genetic distance calculations showed a great genetic similarity between A. sturio populations in the Gironde River and the North Sea, and a basal position of the Mediterranean and Adriatic Sea A. sturio populations. In A. o. oxyrinchus, the number of microsatellite alleles ranged between 14 (Hudson and St. John rivers) and 22 (Delaware River). Genetic distance calculations showed a high genetic similarity between subpopulations of A. o. oxyrinchus.Instituto Español de Oceanografí