High-energy laser-pulse self-compression in short gas-filled fibers

We examine the spatiotemporal compression of energetic femtosecond laser pulses within short gas-filled fibers. The study is undertaken using an advanced nonlinear pulse propagation model based on a multimode generalized nonlinear Schrödinger equation that has been modified to include plasma effects. Plasma defocusing and linear propagation effects are shown to be the dominant processes within a highly dynamical mechanism that enables 100-fs pulses to be compressed into the few-cycle regime after <50 mm of propagation. Once the mechanism has been introduced, parameter spaces are explored and compressor designs suitable for performing high-field experiments in situ are presented. We finish by showing how these designs may be extended to novel wavelengths and driving pulses delivered by state-of-the-art high-repetition-rate lasers

Experimental demonstration of a high-flux capillary based XUV source in the high ionisation regime

High harmonic generation (HHG) has proven to be a fascinating and incredibly useful nonlinear optical phenomenon and has led to the realisation of tabletop sources of coherent extreme ultraviolet (XUV) radiation. Capillary based geometries in particular have attracted a great deal of attention due to their lengthy interaction regions and the potential to phase-match the HHG process leading to a large increase in XUV flux. Unfortunately due to plasma induced nonlinear and dispersive effects the simple phase-matching mechanism detailed in [1] cannot be scaled to high energy pump pulses and high gas pressures. In this work we have used a computational model [2] to design a capillary that can support a broad interaction region well-suited to quasi-phase-matching (QPM) while simultaneously reducing the effect that XUV reabsorption has on the output flux of the source. This modelling work has involved adjusting both the capillary length and gas density profile (figure 1a) in order to produce rapid oscillations in the radially integrated ionization fraction (figure 1b) coupled with a rapid decrease in gas pressure at the capillary exit. Our theory suggests that these oscillations are driven by a nonlinear self-compression process modulating the intensity of the pump pulse as it propagates through the plasma-filled waveguide [3]. Subsequent experimental work has shown an increase in XUV flux of almost 50 times over our previous capillary-based source (see figure 1c), and preliminary estimates suggest a photon flux of 1012 photons s-1 harmonic-1 in the 45 eV spectral region

Statistical analysis of pump-pulse propagation in gas-filled capillaries for high-harmonic generation

Driving high-harmonic generation (HHG) with ultrashort pulses confined to gas-filled capillaries is an efficient method of generating extreme ultraviolet and x-ray radiation. In-situ pulse compression can significantly enhance HHG efficiency [1] but requires operation in the high-ionisation limit, leading to high sensitivity to initial conditions and causing the Gaussian driving pulse to break up into a train of subpulses as it propagates. Our previous studies [1,2] have focused on the most intense subpulse, which can be very short (<10 fs). Here, we perform statistical analysis of all pulse components predicted by numerical simulation, including the contribution of the weaker subpulses, with the aim of predicting generated HHG profiles

Single exposure wavefront curvature estimation of high harmonic radiation by diffraction from a regular array

We present a novel technique for estimating the radius of curvature from a single exposure of EUV light from a high harmonic source diffracted by a grating of square apertures

Charmless Three-Body Baryonic B Decays

Motivated by recent data on B-> p pbar K decay, we study various charmless three-body baryonic B decay modes, including Lambda pbar pi, Sigma0 pbar pi, p pbar pi, p pbar Kbar0, in a factorization approach. These modes have rates of order 10^{-6}. There are two mechanisms for the baryon pair production, current-produced and transition. The behavior of decay spectra from these baryon production mechanisms can be understood by using QCD counting rules. Predictions on rates and decay spectra can be checked in the near future.Comment: 26 pages, 9 figures; version to appear in Phys. Rev.

Technological Devices in the Archives: A Policy Analysis

Doing research in the archive is the cornerstone of humanities scholarship. Various archives institute policies regarding the use of technological devices, such as mobile phones, laptops, and cameras in their reading rooms. Such policies directly affect the scholars as the devices mediate the nature of their interaction with the source materials in terms of capturing, organizing, note taking, and record keeping for future use of found materials. In this paper, we present our analysis of the policies of thirty archives regarding the use of technology in their reading rooms. This policy analysis, along with data from interviews of scholars and archivists, is intended to serve as a basis for developing mobile applications for assisting scholars in their research activities. In this paper we introduce an early prototype of such a mobile application— AMTracker.Informatio

Minimal model for aeolian sand dunes

We present a minimal model for the formation and migration of aeolian sand dunes. It combines a perturbative description of the turbulent wind velocity field above the dune with a continuum saltation model that allows for saturation transients in the sand flux. The latter are shown to provide the characteristic length scale. The model can explain the origin of important features of dunes, such as the formation of a slip face, the broken scale invariance, and the existence of a minimum dune size. It also predicts the longitudinal shape and aspect ratio of dunes and heaps, their migration velocity and shape relaxation dynamics. Although the minimal model employs non-local expressions for the wind shear stress as well as for the sand flux, it is simple enough to serve as a very efficient tool for analytical and numerical investigations and to open up the way to simulations of large scale desert topographies.Comment: 19 pages, 22 figure

Effect of Dopant on the Nanostructured Morphology of Poly (1-naphthylamine) Synthesized by Template Free Method

The study reports some preliminary investigations on the template free synthesis of ascantlyinvestigated polyaniline (PANI) derivative—poly (1-naphthylamine) (PNA) by template free method in presence as well as absence of hydrochloric acid (HCl) (dopant), using ferric chloride as oxidant. The polymerization was carried out in alcoholic medium. Polymerization of 1-naphthylamine (NPA) was confirmed by the FT-IR as well as UV–visible studies. The morphology and size of PNA particles was strongly influenced by the presence and absence of acid which was confirmed by transmission electron microscopy (TEM) studies

An ARPES view on the high-Tc problem: phonons vs spin-fluctuations

We review the search for a mediator of high-Tc superconductivity focusing on ARPES experiment. In case of HTSC cuprates, we summarize and discuss a consistent view of electronic interactions that provides natural explanation of both the origin of the pseudogap state and the mechanism for high temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. On the other hand, a similar analysis being applied to the iron pnictides reveals especially strong electron-phonon coupling that suggests important role of phonons for high-Tc superconductivity in pnictides.Comment: A summary of the ARPES part of the Research Unit FOR538, http://for538.wmi.badw.d

Charmless Two-body Baryonic B Decays

We study charmless two-body baryonic B decays in a diagramatic approach. Relations on decay amplitudes are obtained. In general there are more than one tree and more than one penguin amplitudes. The number of independent amplitudes can be reduced in the large m_B limit. It leads to more predictive results. Some prominent modes for experimental searches are pointed out.Comment: 15 pages, 2 figures. To appear in Phys. Rev.