1,720,970 research outputs found
Building a laboratory based XUV microscope
No full textHigh harmonic generation (HHG) is a non-linear interaction between a driving laser pulse and a target gas. The result of this process is the generation of coherent extreme ultraviolet (XUV) and soft X-ray radiation at harmonic frequencies of the driving laser. There are several uses for this source including attosecond time resolved spectroscopy and nanoscale imaging which this thesis will focus on.The work contained within this thesis begins by investigating the development of capillary based HHG by comparison of the experimentally observed propagation of the driving laser pulse to both a linear ionisation based model and a multimode non-linearSchrodinger equation model. Manufacture, mounting and coupling into the capillary are described. A simple linear model of propagation along the capillary is compared to experimental measurements of fluorescence along the capillary showingthe presence of mode beating. The model is extended to a non linear propagation model and validated against the spatio spectral output of the capillary.The second half of the thesis takes the XUV output from a gas cell and uses it for a coherent diffractive imaging experiment (CDI). The development of the imaging setup is described before presenting the results of a preliminary CDI experiment with a binary test sample and a polychromatic beam. Accurate measurement of the XUV beam focal position was determined by recording diffraction through a binary array of apertures. Results of a CDI experiment demonstrating a resolution of better than 200 nm is shown for a polychromatic beam with an algorithm designed for a monochromatic source
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
Full characterisation of a focussed extreme ultraviolet beam using a non-redundant array of apertures
No full textThis paper presents a novel technique for characterising wavefront curvature and M2, by utilising a non-redundant array (NRA) of apertures to define the plane of investigation through an experimental extreme ultraviolet (EUV) focus. Appropriately sampled, far-field EUV scattering from this NRA is captured on a CCD as the NRA is scanned along the beam axis through the focus. By taking the inverse Fourier transform (IFT), it is possible obtain the spatial autocorrelation functions, via the Wiener-Khinchin theorem, of the exit wave field. By observing the position of the first-order peaks in the autocorrelation as a function of grid translation, both the real and imaginary parts of the complex beam parameter can be determined and the M2 calculated, yielding full characterisation of the embedded Gaussian. Since the periodicity of the grid is known, the planar pixel resolution can be calculated, also allowing the translations movement to be confirmed due to the change in angular acceptance of the fixed CCD. This makes the technique self-calibrating. A high impact, easy to use, cross field technique for full profiling of the embedded Gaussian of probe beams using a non-redundant array of apertures is presented. The technique is experimentally verified in the highly absorbing EUV spectral regime, and is expected to play a significant role in other regimes, where experimental issues prevent the use of existing techniques
Nonlinear optical mode coupling by ionization in an Ar-filled capillary with high-power short-pulse excitation
Full text linkHigh-power ultrashort laser pulses at near-infrared wavelengths propagating in gas-filled capillaries can form a compact source of XUV/soft X-ray radiation by high-harmonic generation (HHG) [1]. Maximization of the frequency conversion efficiency requires a detailed understanding of the atomic 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. Here we focus on the numerical simulation of pulse propagation in a parameter regime dominated by plasma effects and by the nonlinear properties of ionization, and compare the results with experimental observations
Temporal Coherence Effects on Coherent Diffractive Imaging of a Binary Sample by a High Harmonic Source
No full textCoherent Diffractive Imaging (CDI) is performed with single and multiple harmonics from an ultrafast HHG source. The effect of HHG source bandwidth on the effectiveness of the reconstruction algorithms is compared. A low quality reconstruction from broadband data is achieved assuming full coherence in the algorithm
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
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