1,721,048 research outputs found
Numerical and analytical models to study the laser-driven plasma perturbation in a dielectric gas-filled capillary waveguide
No full textAn alternative fast approach to study the propagation of an intense laser pulse through a dielectric capillary waveguide filled with plasma is presented. The numerical model com- putes the evolution of the capillary mode coupling coeffi- cients and from these the properties of the laser and plasma response are retrieved. Moreover, an analytical description of the process is presented and compared to the numerical model
Selection rules for the orbital angular momentum of optically-produced THz radiation
Full text linkIn this work we theoretically study the transduction ofthe Orbital Angular Momentum (OAM)lfor infraredpump lasers into the THz domain. In the case of opti-cal rectification, the transduction of OAM occurs onlythrough a spin-orbit interaction, with the selection ruleon the OAMl=0valid for any kind of polarizationof the pump, which means that there is no transfer ofOAM along the propagation axis. In the difference fre-quency generation the selection rule for the difference∆lbetween the OAM of the pump fields with linear orcircular polarization isl=∆l, whereaslranges from∆l−2to∆l+2in both the cases of radial and azimuthalpolarization. Moreover, for THz generation in the lat-ter case, high diffraction obtained with tightly focusedpumps yieldsltending to∆l±2, whileltends to zeroin the opposite case of large pump beam
Measurements of fluence profiles in femtosecond laser filaments in air
Full text linkWe introduce a technique to measure fluence distributions in femtosecond laser beams with peak intensity of up to several hundred terawatts per square centimeter. Our ap- proach is based on the dependence of single-shot laser abla- tion threshold for gold on the angle of incidence of the laser beam on the gold sample. We apply this technique to the profiling of fluence distributions in femtosecond laser fila- ments at a wavelength of 800 nm in air. The peak intensity is found to be clamped at a level that depends on the ex- ternal beam focusing. The limiting value of the peak inten- sity attainable in long-range 800 nm air filaments, under very loose focusing conditions (f -number above ∼500), is about 55 TW∕cm2
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
High-power third-harmonic flat pulse laser generation
No full textThe generation of a high-power laser pulse at 266 mn that is longitudinally shaped according to a prefixed intensity profile is reported. The main features of the pulse shape modifications due to second- and third-harmonic conversions are measured, and the results are in good agreement with the theory. The UV temporal shape depends on the chirp of the fundamental pulse and on the crystal phase-matching angle. Exploiting the large stretching imposed on the third-harmonic signal, we show that the pulse intensity profile can be obtained by spectral single-shot measurements
Simple scheme for ultraviolet time-pulse shaping
No full textWe present a method to generate high-energy flat-top UV laser pulses such as the ones needed to optimally drive high-brightness radio-frequency photoinjectors. In this scheme we believe to be novel, the longitudinal profile of a laser pulse from a Ti:sapphire master oscillator power amplifier system is controlled using a mechanical mask in the Fourier plane of a 4f stretcher located after the harmonic conversion crystals. Such a scheme allows us to overcome many of the difficulties faced by current state-of-the-art pulse-shaping designs. These are in fact based on various versions of preamplifier infrared shapers and hence suffer from the limitations set by the nonlinearities of chirped-pulse amplification and harmonic conversion. Beyond the clear advantages of simplicity and robustness, the proposed solution offers the possibility to deliver a pulse with very short rise and fall times and to freely change the output pulse length. We also note that, after proper calibration between spectral and temporal profiles, the shaper optical setup offers the possibility to retrieve the longitudinal profile of the laser pulse on a shot-to-shot basis
Simultaneous elliptically and radially polarized THz from one-colour laser-induced plasma filament
Full text linkTHz-based technologies and research applications have seen a rapid increment in recent period together with the development of novel radiation sources based both on relativistic electrons and laser techniques. In this framework, laser-induced plasma filament plays an important role in generating intense and broadband THz radiation. Although many attentions have been paid to THz emission from two-color plasma filaments, one-color plasma emission has been scarcely investigated. In particular, the polarization state of one-color THz emission is still controversial due to the limitations of the existing THz detection techniques, which are incapable of simultaneously detecting elliptically and radially polarized THz radiation. In this manuscript, we develop a novel detection method and unambiguously demonstrate for the first time that one-color laser-induced plasma filament simultaneously emits elliptically and radially polarized THz radiation. These polarization states suggest that the generation mechanism results from electric quadrupole, showing a new route for producing more complex polarization states and THz vortex beams
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
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