Using electric fields for pulse compression and group velocity control

In this article, we experimentally demonstrate a new way of controlling the group velocity of an optical pulse by using a combination of spectral hole burning, slow light effect and linear Stark effect in a rare-earth-ion-doped crystal. The group velocity can be changed continuously by a factor of 20 without significant pulse distortion or absorption of the pulse energy. With a similar technique, an optical pulse can also be compressed in time. Theoretical simulations were developed to simulate the group velocity control and the pulse compression processes. The group velocity as well as the pulse reshaping are solely controlled by external voltages which makes it promising in quantum information and quantum communication processes. It is also proposed that the group velocity can be changed even more in an Er doped crystal while at the same time having a transmission band matching the telecommunication wavelength.Comment: 8 pages, 7 figure

Cavity enhanced storage - preparing for high efficiency quantum memories

Cavity assisted quantum memory storage has been proposed [PRA 82, 022310 (2010), PRA 82, 022311 (2010)] for creating efficient (close to unity) quantum memories using weakly absorbing materials. Using this approach we experimentally demonstrate a significant (about 20-fold) enhancement in quantum memory efficiency compared to the no cavity case. A strong dispersion originating from absorption engineering inside the cavity was observed, which directly affect the cavity line-width. A more than 3 orders of magnitude reduction of cavity mode spacing and cavity line-width from GHz to MHz was observed. We are not aware of any previous observation of several orders of magnitudes cavity mode spacing and cavity line-width reduction due to slow light effects.Comment: 13 pages, 5 figure

Swelling and Microstructure of Nanoplatelet Systems

Many clay minerals consist of charged nanoplatelets that swell in an aqueous solution. The swelling and the microstructure depend on the type of clay mineral but are also dependent on, for example, the ionic composition and the temperature of the solution. The synthetic clay mineral Laponite and the natural clay mineral montmorillonite have been studied experimentally and theoretically. The swelling has been studied by swelling pressure measurements in a test cell and the microstructure has been studied by small angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryo-TEM). Theoretically, Metropolis Monte Carlo (MC) simulations and molecular dynamics (MD) simulations were used to study the electrostatic interactions between the platelets and an adsorbing polymer was also included. Coarse-grained models have been used to represent the platelets, the ions, and the polymers. This thesis can be divided into three parts:In the first part, the tactoid formation (platelets aggregating face-to-face with an equidistant separation) and the microstructure of negatively charged platelets were investigated by simulations. Tactoid formation was promoted by increasing the platelet surface charge density, the platelet size, the ion valency, and the salt concentration. With enough added salt, an isodesmic model was suggested that gives a monotonically decaying distribution of aggregation numbers.In the second part, the tactoid formation and the microstructure were investigated with respect to the platelet size for flocculated Laponite and montmorillonite at elevated salt concentrations, with and without the addition of the polymer polyethylene glycol (PEG). The smaller Laponite platelets give rise to a more disordered microstructure compared to the larger montmorillonite platelets. The number of platelets per tactoid increased with the addition of PEG. The simulations suggested that the role of the salt is to screen the repulsive interactions between the platelets, and the role of the polymer is to bridge between the platelets.In the third part, the temperature response of Na/Ca montmorillonite was investigated. It was found that the swelling pressure increased with increased temperature if sodium is the dominating counterion whereas the opposite was found if calcium is the dominating counterion. The simulations predicted this behavior and it was shown that the results could be explained by a single equation

Skolidrottens omklädningsrum : Undersökning av klimat elever emellan och kränkande handlingar ur elevperspektiv

Skolidrottens omklädningsrum är ett intressant område där eleverna ofta vistas utan vuxennärvaro. Den första frågan som ställs i arbetet är hur elever upplever klimatet i omklädningsrummet. Den andra frågan handlar om elever erfarenheter av kränkande handlingar i anslutning till skolidrottens omklädningsrum. Metoden som används är kvalitativ och frågorna är av semistrukturerad karaktär. Resultaten visar att majoriteten av våra intervjupersoner inte alls önskar en ökad vuxennärvaro i skolidrottens omklädningsrum. Eleverna uppskattar att vara ensamma i omklädningsrummen då det ger dem möjligheten att tala om vad de vill. Kränkande handlingar består mestadels av psykologisk natur. Fysisk misshandel är mer sällsynt

Skolidrottens omklädningsrum : Undersökning av klimat elever emellan och kränkande handlingar ur elevperspektiv

Skolidrottens omklädningsrum är ett intressant område där eleverna ofta vistas utan vuxennärvaro. Den första frågan som ställs i arbetet är hur elever upplever klimatet i omklädningsrummet. Den andra frågan handlar om elever erfarenheter av kränkande handlingar i anslutning till skolidrottens omklädningsrum. Metoden som används är kvalitativ och frågorna är av semistrukturerad karaktär. Resultaten visar att majoriteten av våra intervjupersoner inte alls önskar en ökad vuxennärvaro i skolidrottens omklädningsrum. Eleverna uppskattar att vara ensamma i omklädningsrummen då det ger dem möjligheten att tala om vad de vill. Kränkande handlingar består mestadels av psykologisk natur. Fysisk misshandel är mer sällsynt

Dielectric response from lattices of dipoles with fixed orientation.

The properties of dipolar cubic lattices are studied and the paradox of how to obtain a long range polarization in such lattices is resolved by choosing a proper shape of the total system. It has been shown that large but finite number of aligned dipoles prefer to exist as needle shaped macroscopic particles [M. Yoon and D. Tománek, J. Phys.: Condens. Matter 22, 455105 (2010)]. The total energy for a particle in such a system has one short range contribution depending on the packing (the chosen lattice) and one long range term depending on the dipole density of the system. We show that the latter term corresponds exactly to the polarization term from a dielectric medium embedding a sphere of the considered system. There is no need to include a dielectric medium in this modeling and the "dielectric stabilization" is generated solely by the dipoles of the system

Flocculated Laponite-PEG/PEO dispersions with monovalent salt, a SAXS and simulation study.

It is well-known that clay can form lamellar structures i.e. tactoids, and recently it has been shown that the tactoid formation is dependent on the platelet diameter. To the authors knowledge, no tactoid formation has been observed for montmorillonite platelets with a diameter less than 60nm. In this study, small angle X-ray scattering in combination with coarse-grained modeling and molecular dynamics simulations have been utilized to study the sediment of Laponite-polyethylene glycol/polyethylene oxide (PEG/PEO) at elevated salt concentrations (150mM-1M). Laponite consists of platelets with a diameter of 25nm and it is known to have a relatively monodisperse size-distribution. At pH 10, the face of the platelets has a strong negative charge, whereas the rim is slightly positive. Here we show that it is possible to induce tactoids for Laponite if two constraints are fulfilled: (1) addition of high amount of salt such as NaCl, and (2) addition of a neutral polymer such as PEG. The role of the salt is to screen the repulsive interactions between the platelets and the role of the polymer is to bridge the platelets together: hence the loss in configurational entropy of the polymer is counteracted by the gain in attractive polymer-platelet interaction. As the concentration of NaCl and/or PEG increases, the Bragg peak becomes sharper, which is an indication of that larger tactoids are formed. Comparison between Laponite and montmorillonite shows that the interlayer distance between the platelets increases linearly with an increased Debye screening length for both type of clays, whereas the structure peaks of Laponite are broader compared to the montmorillonite. We argue that the main reason to the latter is due to the size of the platelets: (i) smaller platelets are less rotationally restricted and (ii) the effect of positive edge charges is larger when the platelets are smaller, which results in more irregular aggregates. In absence of the polymer, montmorillonite form tactoids above ∼0.3MNaCl whereas Laponite does not. Even though the model used is simple, we find qualitative agreement between experiments and simulations, which verifies that the underlying physics for tactoid formation is captured

Flocculated Laponite-PEG/PEO Dispersions with Multivalent Salt : A SAXS, Cryo-TEM, and Computer Simulation Study

The aim of this study is to scrutinize the mechanism behind aggregation, i.e., tactoid formation of nanostructures with the shape of a platelet. For that purpose, the clay minerals Laponite and montmorillonite have been used as model systems. More specifically, we are interested in the role of: the platelet size, the electrostatic interactions, and adsorbing polymers. Our hypothesis is that the presence of PEG is crucial for tactoid formation if the system is constituted by small nanometric platelets. For this purpose, SAXS, USAXS, Cryo-TEM, and coarse-grained molecular dynamics simulations have been used to study how the formation and the morphology of the tactoids are affected by the platelet size. The simulations indicate that ion-ion correlations are not enough to induce large tactoids solely if the platelets are small and the absolute charge is too low, i.e., in the size and charge range of Laponite. When a polymer is introduced into the system, the tactoid size grows, and the results can be explained by weak attractive electrostatic correlation forces and polymer bridging. It is shown that when the salt concentration increases the long-ranged electrostatic repulsion is screened, and a free energy minimum appears at short distances due to the ion-ion correlation effects. When a strongly adsorbing polymer is introduced into the system, a second free energy minimum appears at a slightly larger separation. The latter dominates if the polymer is relatively long and/or the polymer concentration is high enough. (Graph Presented)

Monte Carlo Simulations of Parallel Charged Platelets as an Approach to Tactoid Formation in Clay

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