1,622 research outputs found

    Self-calibrating d-scan: measuring ultrashort laser pulses on-target using an arbitrary pulse compressor

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    In most applications of ultrashort pulse lasers, temporal compressors are used to achieve a desired pulse duration in a target or sample, and precise temporal characterization is important. The dispersion-scan (d-scan) pulse characterization technique usually involves using glass wedges to impart variable, well-defined amounts of dispersion to the pulses, while measuring the spectrum of a nonlinear signal produced by those pulses. This works very well for broadband few-cycle pulses, but longer, narrower bandwidth pulses are much more difficult to measure this way. Here we demonstrate the concept of self-calibrating d-scan, which extends the applicability of the d-scan technique to pulses of arbitrary duration, enabling their complete measurement without prior knowledge of the introduced dispersion. In particular, we show that the pulse compressors already employed in chirped pulse amplification (CPA) systems can be used to simultaneously compress and measure the temporal profile of the output pulses on-target in a simple way, without the need of additional diagnostics or calibrations, while at the same time calibrating the often-unknown differential dispersion of the compressor itself. We demonstrate the technique through simulations and experiments under known conditions. Finally, we apply it to the measurement and compression of 27.5 fs pulses from a CPA laser.Comment: 11 pages, 5 figures, Scientific Reports, in pres

    Letter from J. Helder to John Muir, 1906 Jun 18.

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    PersonalPort Washington, OhioJune 18, 1906My dear Mr. Muir:-I expected to attend the N.E.A. meeting at San Francisco this summer, and hence, have the great pleasure of paying you a visit before leaving California. But the sad misfortune which befell your city has also caused us to postpone our meeting. I have not seen you for a long time, and I am also very anxious to talk over with you a matter concerning my present welfare. Under the circumstances may I present the matter by letter? - The present wide-spread disbelief in individual immortality has disturbed me very much. I cannot understand why so many great minds have come to the same conclusion as was reached by Prof. Dr. Os[illegible]wald in his recent Harvard lecture on Individuality and Immortality — that there is no personal immortality after death. How can the human soul abandon all aspiration for persistence of personal identity after death? How can reason feel satisfied with the belief that immortality consists in the effect each individual has on the advancement of the race?(over)03743 & think from such a conclusion, and yet I cannot shake off the distress which it brings to my mind. My soul longs to view this great problem in the light of your own illumination. I have long admired your rare mental poise, and profound spiritual insight to the serenity of your soul remained undisturbed. Pray, express your views and convictions as fully as your time may permit. Also please state how you account for the present wide-spread disbelief in individual immortality especially in the academic circles of Germany. You do not speak from the chair of Theology - but you speak in the name of the Eternal. Hope I may be able to serve you in return.In conclusion permit me to express a long-cherished desire. We are very anxious to have your autographed photograph on our walls. It will be an inspiration to students, and we will feel highly honored. Pray, do not disappoint us! - May I hope to hear from you at your earliest convenience? Hoping that you continue to enjoy good health, I beg to remain, with cordial greetings,Ever your faithful friend,Prof. J. Helder

    Effects of Neutral Hydrogen on Cosmic Ray Precursors in Supernova Remnant Shock Waves

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    Many fast supernova remnant shocks show spectra dominated by Balmer lines. The Hα\alpha profiles have a narrow component explained by direct excitations and a thermally Doppler broadened component due to atoms that undergo charge exchange in the post-shock region. However, the standard model does not take into account the cosmic-ray shock precursor, which compresses and accelerates plasma ahead of the shock. In strong precursors with sufficiently high densities, the processes of charge exchange, excitation and ionization will affect the widths of both narrow and broad line components. Moreover, the difference in velocity between the neutrals and the precursor plasma gives rise to frictional heating due to charge exchange and ionization in the precursor. In extreme cases, all neutrals can be ionized by the precursor. In this paper we compute the ion and electron heating for a wide range of shock parameters, along with the velocity distribution of the neutrals that reach the shock. Our calculations predict very large narrow component widths for some shocks with efficient acceleration, along with changes in the broad- to-narrow intensity ratio used as a diagnostic for the electron-ion temperature ratio. Balmer lines may therefore provide a unique diagnostic of precursor properties. We show that heating by neutrals in the precursor can account for the observed Hα\alpha narrow component widths, and that the acceleration efficiency is modest in most Balmer line shocks observed thus far.Comment: 9 pages, 3 figure

    Proper Motions of H-alpha filaments in the Supernova Remnant RCW 86

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    We present a proper motion study of the eastern shock-region of the supernova remnant RCW 86 (MSH 14-63, G315.4-2.3), based on optical observations carried out with VLT/FORS2 in 2007 and 2010. For both the northeastern and southeastern regions, we measure an average proper motion of H-alpha filaments of 0.10 +/- 0.02 arcsec/yr, corresponding to 1200 +/- 200 km/s at 2.5kpc. There is substantial variation in the derived proper motions, indicating shock velocities ranging from just below 700 km/s to above 2200 km/s. The optical proper motion is lower than the previously measured X-ray proper motion of northeastern region. The new measurements are consistent with the previously measured proton temperature of 2.3 +/- 0.3 keV, assuming no cosmic-ray acceleration. However, within the uncertainties, moderately efficient (< 27 per cent) shock acceleration is still possible. The combination of optical proper motion and proton temperature rule out the possibility that RCW 86 has a distance less than 1.5kpc. The similarity of the proper motions in the northeast and southeast is peculiar, given the different densities and X-ray emission properties of the regions. The northeastern region has lower densities and the X-ray emission is synchrotron dominated, suggesting that the shock velocities should be higher than in the southeastern, thermal X-ray dominated, region. A possible solution is that the H-alpha emitting filaments are biased toward denser regions, with lower shock velocities. Alternatively, in the northeast the shock velocity may have decreased rapidly during the past 200yr, and the X-ray synchrotron emission is an afterglow from a period when the shock velocity was higher.Comment: Accepted for publication in MNRA

    Taking Synchrony Seriously: A Perceptual-Level Model of Infant Synchrony Detection

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    Synchrony detection between different sensory and/or motor channels appears critically important for young infant learning and cognitive development. For example, empirical studies demonstrate that audio-visual synchrony aids in language acquisition. In this paper we compare these infant studies with a model of synchrony detection based on the Hershey and Movellan (2000) algorithm augmented with methods for quantitative synchrony estimation. Four infant-model comparisons are presented, using audio-visual stimuli of increasing complexity. While infants and the model showed learning or discrimination with each type of stimuli used, the model was most successful with stimuli comprised of one audio and one visual source, and also with two audio sources and a dynamic-face visual motion source. More difficult for the model were stimuli conditions with two motion sources, and more abstract visual dynamics—an oscilloscope instead of a face. Future research should model the developmental pathway of synchrony detection. Normal audio-visual synchrony detection in infants may be experience-dependent (e.g., Bergeson, et al., 2004)

    Історія України в джерелознавчих дослідженнях З. Анусіка

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    Рецензія на монографію: Zbigniew Anusik, Studia i szkice staropolskie, Łódź 2011, Wydawnictwo Uniwersytetu Łódzkiego, s. 853 (Збігнев Анусік. Старопольські дослідження і нариси (Лодзь, 2011)

    A system to manage the allocation of MSc Dissertations at University of Minho

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    This paper describes the basis for the specification of an IT infrastructure, supported by a website, designed to manage the entire paperwork process associated with the management of Masters Dissertations. With this infrastructure it is possible to disclose the subjects offered, to allocate each subject to a candidate - "dissertations in progress", browse dissertations that are awaiting review and consult the archive of dissertations with exams already carried out. PostgreSQL technology was used to support the databases. This technology was chosen to be robust, with easy integration with PHP and its code is open source. The website is housed in a Linux server based infrastructure ready to run continuously.(undefined

    Real-time human activity monitoring exploring multiple vision sensors

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    In this paper, we describe the monitoring of human activity in an indoor environment through the use of multiple vision sensors. The system described in this paper is made up of three cameras. Two of these cameras are active and are part of a binocular system. They operate either as a set of three static cameras or as a set of one fixed camera and an active binocular vision system. The human activity is monitored by extracting several parameters that are useful for their classification. The system enables the creation of a record based on the type of activity. These logs can be selectively accessed and provide images of the humans in specific areas.http://www.sciencedirect.com/science/article/B6V16-4379G7B-B/1/216c3ddb70ad57a4491b08864d1b296