1,720,967 research outputs found
High-power laser pulse compression for optimized high-harmonic generation in short hollow fibers
No full textHigh-harmonic generation (HHG) driven by high-power femtosecond lasers is a promising route towards tabletop sources of coherent radiation at extreme ultraviolet (XUV) and soft Xray wavelengths. In order to maximize conversion efficiency and to obtain attosecond pulse lengths of the harmonics, pump pulse lengths down to the few-cycle regime are required. Traditionally, this has been achieved by nonlinear spectral broadening in gases and subsequent dispersion compression or by self-compression in laser-induced filaments. Here we investigate an alternative method that is simpler to implement experimentally, based on multimode non-linear propagation effects in short, centimeter-length, gas-filled hollow fibers operating in the high-ionization regime
Few-cycle self-compression via multimode nonlinear optics in gas filled waveguides
No full textMultimode simulations predict dramatic ionization-induced self-compression of high energy ultrashort pulses within short gas filled capillaries. The mechanism observed allows for the temporal compression of 53 fs pulses into the few-cycle regime
Experimental demonstration of a high-flux capillary based XUV source in the high ionisation regime
No full textHigh 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
Influence of gas jet structure on high harmonic generation
No full textSpatially coherent X-rays are a potentially useful source for imaging biological and crystalline material at nanometre length scales [1]. Such radiation can be produced via high harmonic generation (HHG), commonly achieved by focussing a high-energy ultrashort laser pulse into ajet of noble gas [2]. However, gas jets have a complex three dimensional density and velocity profile [3], and since the generation efficiency depends on both the density of nonlinear material present and the degree of phase matching, the profile of the gas jet can have a significant influence on the generation process
Spatio-spectral technique to verify pump-pulse propagation model in an Ar-filled capillary in the presence of high harmonic generation
No full textHigh-power ultrashort laser pulses propagating in gas-filled capillaries can form a compact source of extreme ultraviolet (XUV) and soft X-ray radiation by high-harmonic generation (HHG) [1]. Maximisation of the frequency conversion requires a detailed understanding of the atom-light interaction mechanism as well as the propagation properties of both the near-infrared pump in the presence of a partially ionized gas, and of the generated XUV. Previous theoretical studies of capillary based HHG by Christov et al. [2] have used numerical solutions of the 3-dimensional version of the scalar wave equation
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Single exposure wavefront curvature estimation of high harmonic radiation by diffraction from a regular array
No full textWe 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
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