1,721,035 research outputs found
Generation of 15-nJ pulses from a highly efficient, low-cost multipass-cavity Cr[superscript 3+]:LiCAF laser
No full textWe describe the generation of enhanced pulse energies using a multipass-cavity (MPC) Cr[superscript3] +:LiCAF laser, pumped by inexpensive, single spatial mode laser diodes. A semiconductor saturable absorber was used for stable mode-locked operation. The MPC reduced the pulse repetition rate to the ~10 MHz level, scaling pulse energies and intensities. With only 540 mW of absorbed pump power, 98 fs pulses with energies of 9.9 nJ and peak powers of ~101 kW, corresponding to 95 mW of average power at a repetition rate of 9.58 MHz, were generated. By increasing the intracavity negative dispersion, 310 fs pulses with energies of 15.2 nJ and peak powers of ~49 kW, corresponding to 160 mW of average power at 10.51 MHz repletion rate, were also generated. These results demonstrate that low-cost MPC Cr[superscript 3+]-doped colquiriite lasers can generate pulse energies and intensities comparable to more expensive Ti:sapphire lasers
Low-cost, highly efficient, and tunable ultrafast laser technology based on directly diode-pumped Cr:Colquiriites
Full text linkThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 335-352).This doctoral project aims to develop robust, ultra low-cost (150) as the pump source, we have obtained continuous-wave (cw) output powers >250-mW with slope efficiencies >50%. Record cw tuning ranges were demonstrated for Cr:LiSAF (775-1042 nm), Cr:LiSGaF (777-977 nm), and Cr:LiCAF (754-871 nm). For femtosecond pulse generation, semiconductor saturable absorber mirrors (SESAMs/SBRs) were developed, which were used to initiate and sustain mode-locking. Typical performance was ~25-100 fs pulses, with an optical spectrum in the 770-920 nm range, with ~1-2 nJ of pulse energies from ~100-MHz repetition rate cavities. Record electrical-to-optical conversion efficiencies of ~10% were demonstrated in the cw mode-locked regime. A mode-locked tuning range of 767-817 nm, with ~130-fs long pulses was obtained by using Cr:LiCAF as gain medium. With the Cr:LiSAF gain medium, using regular SESAMs/SBRs centered around 800 nm, 850 nm, and 910 nm, mode-locked tuning ranges of 803-831 nm, 828-873 nm, and 890-923 nm were demonstrated, respectively. By using a broadband oxidized SESAM/SBR, a record tuning range of 800-905 nm was demonstrated with ~150-fs long pulses. Using an extended cavity Cr:LiCAF laser, pulse energies >15-nJ with peak powers exceeding 100-kW were obtained. We performed the first cavity-dumping experiments with a Cr:Colquiriite laser and demonstrated pulse energies >100-nJ, and peak powers approaching MW level, at repetition rates up to 50-kHz. Cr:LiCAF gain media were also pumped by single-emitter multimode diodes, where we obtained >2-W output power in cw operation, and ~100-fs pulses with 390-mW of average power at a repetition rate of 140 MHz in cw mode-locked operation. As an example application area for this low cost technology, we performed multiphoton microcopy experiments with a single-mode diode-pumped Cr:LiCAF laser. We also performed attosecond-resolution timing jitter characterization experiments of the femtosecond Cr:LiSAF laser, and measured a record-low upper limit for the integrated timing jitter of the Cr:LiSAF laser (137-attoseconds in 10 kHz-10 MHz range).by Umit Demirbas.Ph.D
Chromium-doped zinc selenide gain media: From synthesis to pulsed mid-infrared laser operation
No full textThis paper provides an overview of the experimental work performed in our research group on the synthesis, spectroscopic investigation, and laser characterization of chromium-doped zinc selenide (Cr[superscript 2+]:ZnSe). By using diffusion doping, 40 polycrystalline Cr[superscript 2+]:ZnSe samples with ion concentration in the range of 0.8 × 10[superscript 18] to 66 × 10[superscript 18]ions/cm[superscript 3] were prepared. From the absorption data, temperature-dependent diffusion coefficient of chromium and losses at the lasing wavelength were measured. In luminescence measurements, the concentration dependence of the fluorescence lifetime and fluorescence quantum efficiency was determined. During continuous-wave operation, the optimum concentration for lasing was determined to be 8.5 × 10[superscript 18] ions/cm[superscript 3] at an incident pump power of 2.1 W for 1800-nm pumping. During gain switched operation, intra-cavity pumping with a 1570-nm optical parametric oscillator resulted in continuous tuning between 1880 and 3100 nm. By employing dispersion compensation with a MgF[superscript 2] prism pair, Kerr-lens mode-locked operation was also demonstrated at 2420 nm, resulting in the generation of 95-fs pulses with an average output power of 40 mW and spectral bandwidth of 69 nm. The time-bandwidth product of the pulses was further measured to be 0.335 close to the expected value of 0.315 for sech[superscript 2] pulses.Network of Excellence on Micro-Optics (Organization)Sixth Framework Programme (European Commission) (2004-2008)Türkiye Bilimsel ve Teknolojik Araştırma Kurumu (TBAG-2030)Türkiye Bilimsel ve Teknolojik Araştırma Kurumu (108T028
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
Modeling and optimization of tapered-diode pumped Cr:LiCAF regenerative amplifiers
No full textWe present a numerical study, which investigates the potential of diode pumped Cr:LiCAF regenerative amplifiers in detail. Special attention has been given to relevant material properties of the gain media, like the Auger energy transfer upconversion (ETU) process, to utilize the full potential of the material and to develop guidelines in choosing optimal material properties like chromium doping concentration and length. Moreover, importance of pulsed pumping, rather than continuous-wave (cw) pumping, in obtaining higher small signal gain values is discussed in detail. Effects of pump power, cavity losses, excited-state absorption, seed pulse energy, optical damage and ETU on obtainable pulse energies will also be presented. The modeling results have shown that, Cr:LiCAF regenerative amplifiers pumped by two 675 nm state-of-the-art 1-W tapered diodes have the potential to produce 50-fs long pulses around 800 nm with 70 mu J pulse energy and 1.4 GW peak power at repetition rates up to 5 kHz. Moreover, a 20-W diode pumped Cr:LiCAF amplifier has the potential to produce pulse energies of 1.1 mJ and peak powers of 20 GW. Expected optical-to-optical conversion efficiencies of the systems are about 30%. These results demonstrate that, with ongoing progress in laser diode technology, low-cost and efficient Cr:Colquiriite amplifiers has the potential to replace the expensive Ti:Sapphire technology in the future. (C) 2013 Elsevier B.V. All rights reserved
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
Electrochemistry, electropolymerization and electrochromism of novel phthalocyanines bearing morpholine groups
No full textIn this work, 4-(4-((4-morpholinophenylimino)methyl)phenoxy)phthalonitrile (3) and it's peripherally tetra substituted metal-free (4), zinc(II) (5), copper(II) (6) and nickel(II) (7) phthalocyanines were synthesized for the first time. The structural characterization of these novel compounds were performed with FT-IR, H-1 NMR, C-13 NMR, UV-Vis and mass spectroscopy. Electrochemical and spectroelec-trochemical characterizations of phthalocyanines (4-7) showed that while all phthalocyanines went to characteristics phthalocyanine ring and/or metal based reduction processes during the negative potential scans, all complexes were coated on the working electrode during the oxidation processes due to the cationic electropolymerizations of the 4-(4-((4-morpholinophenylimino)methyl)phenoxy) substituents. Changing the metal centers of phthalocyanine ring influenced the redox processes due to the different redox activities and effective nuclear charges of the metal centers. In addition to the electropolymerization ability of 4-(4-((4-morpholinophenylimino)methyl)phenoxy) substituent slightly affected the reversibility and peak positions of the redox processes. Indium tin oxide coated glass electrode (ITO) was coated with electropolymerized films of phthalocyanines to construct ITO/Poly-Phthalocyanine and these electrodes were tested as a potential electrochromic material. ITO/Poly-Phthalocyanine provided color change between green and blue with 1.2 s switching time, 18% optical contrast and 100% optical stability with 50 cronoamperometric (CA) cycles. These studies indicated possible usage of the electropolymerized films of phthalocyanines as possible building blocks for advanced electrochromic devices needing green-blue color change. (C) 2020 Elsevier B.V. All rights reserved
The photophysical and photochemical properties of new unmetallated and metallated phthalocyanines bearing four 5-chloroquinolin-8-yloxy substituents on peripheral sites
No full textThe synthesis and characterization of novel peripherally tetrakis-(5-chloroquinolin-8-yloxy) substituted metal-free (4), zinc(II) (5), lead(II) (6), cobalt(II) (7), copper(II) (8) and nickel(II) (9) phthalocyanines are described for the first time in this study. The spectroscopic, photophysical (fluorescence quantum yields and lifetimes) and photochemical properties (singlet oxygen production and photodegradation under light irradiation) of metal-free (4), zinc(II) (5) and lead(II) (6) phthalocyanines are investigated in N,N-dimetilformamid (DMF). The newly synthesized cobalt(II) (7), copper(II) (8) and nickel(II) (9) phthalocyanine compounds were not evaluated for this purpose due to open shell nature of these central metals in the phthalocyanine cavity. The influence of various the nature of the central metal ion (zinc, lead or without metal) on these properties has also been investigated and compared. (C) 2013 Elsevier B.V. All rights reserved
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