1,720,973 research outputs found
Time-domain measurement of optical activity by an ultrastable common-path interferometer
Full text linkWe introduce a novel configuration for the broadband measurement of the optical activity of molecules, combining time-domain detection with heterodyne amplification. A birefringent common-path polarization-division interferometer creates two phase-locked replicas of the input light with orthogonal polarization. The more intense replica interacts with the sample, producing a chiral free-induction decay field, which interferes with the other replica, acting as a time-delayed phase-coherent local oscillator. By recording the delay-dependent interferogram, we obtain by a Fourier transform both the circular dichroism and circular birefringence spectra. Our compact, low-cost setup accepts ultrashort light pulses, making it suitable for measurement of transient optical activity
Broadband pump-probe spectroscopy at 20-MHz modulation frequency
Full text linkWe introduce an innovative high-sensitivity broadband pump-probe spectroscopy system, based on Fourier-transform detection, operating at 20-MHz modulation frequency. A common-mode interferometer employing birefringent wedges creates two phase-locked delayed replicas of the broadband probe pulse, interfering at a single photodetector. A single-channel lock-in amplifier demodulates the interferogram, whose Fourier transform provides the differential transmission spectrum. Our approach combines broad spectral coverage with high sensitivity, due to high-frequency modulation and detection. We demonstrate its performances by measuring two-dimensional differential transmission maps of a carbon nanotubes sample, simultaneously acquiring the signal over the entire 950-1350 nm range with 2.7·10-6 rms noise over 1.5 s integration time
Scanning Fourier transform spectrometer in the visible range based on birefringent wedges
Full text linkWe introduce a spectrometer capable of measuring sample absorption spectra in the visible regime, based on a time-domain scanning Fourier transform (FT) approach. While infrared FT spectrometers typically employ a Michelson interferometer to create the two delayed light replicas, the proposed apparatus exploits a compact common-mode passive interferometer that relies on the use of birefringent wedges. This ensures excellent path-length stability (∼λ/300) and accuracy, with no need for active feedback or beam tracking. We demonstrate the robustness of the technique measuring the transmission spectrum of a colored bandpass filter over one octave of bandwidth and compare the results with those obtained with a commercial spectrophotometer
Broadband stimulated Raman scattering spectroscopy by a photonic time stretcher
Full text linkStimulated Raman scattering spectroscopy is a powerful technique for label-free molecular identification, but its broadband implementation is technically challenging. We introduce and experimentally demonstrate a novel approach based on photonic time stretch. The broadband femtosecond Stokes pulse, after interacting with the sample, is stretched by a telecom fiber to ≈15ns, mapping its spectrum in time. The signal is sampled through a fast analog-to-digital converter, providing single-shot spectra at 80-kHz rate. We demonstrate ≈10-5 sensitivity over ≈500cm-1 in the C-H region. Our results pave the way to high-speed broadband vibrational imaging for materials science and biophotonics
A compact and rugged hyperspectral camera for remote sensing based on FT spectroscopy
No full textSpectral imaging is a method to acquire the spectrum of the light for each point in the image of a scene. By combining classical imaging with Fourier-Transform spectrometry it is possible to acquire hyperspectral images with higher spectral accuracy and lower times compared to standard dispersive optical systems. The technique is based on interferometry, and is hence technically challenging as it requires to generate field replicas with delay controlled within a small fraction (1/100 or better) of the optical cycle. Standard FT spectrometers are heavy, cumbersome and too sensitive to mechanical and thermal perturbations for use in portable devices or for deployment in space applications. Here we propose and experimentally validate a compact FT-based hyperspectral camera, in which the FT module is an innovative ultra-stable birefringent common-path interferometer (the Translating-Wedge-Based Identical Pulses eNcoding System, TWINS). TWINS has intrinsic interferometric stability, it is lightweight and is ultracompact, making our FT-based hyperspectral camera an ideal device for portable on-field and spaceborne applications. Our prototype camera is able to measure absolute reflectance and fluorescence with very high spectral accuracy in the visible and nearinfrared spectral range and can be extended to the spectroscopically rich thermal infrared range (3-14 μm) using suitable birefringent materials and detectors. We present some examples of application in the visible and TIR ranges
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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