146 research outputs found
Veränderungen des schulischen Handlungsfeldes: Neue und alte Aufgaben für LehrerInnen als Herausforderungen der Lehrerausbildung
No full textWischer B. Veränderungen des schulischen Handlungsfeldes: Neue und alte Aufgaben für LehrerInnen als Herausforderungen der Lehrerausbildung. In: Krüger A-K, Nakamura Y, Rotermund M, eds. Schulentwicklung und Schulpraktische Studien - Wie können Schulen und Lehrerbildung voneinander profitieren?. Schriftenreihe der Bundesarbeitsgemeinschaft Schulpraktische Studien. Vol 5. Leipzig: Leipziger Univ.-Verl.; 2010: 12-33
Ultrafast third-order optical nonlinearities of vertically-aligned ZnO nanorods
No full textVertically-aligned ZnO nanorods with different rod diameters of 50 nm, 110 nm and 240 nm were synthesized by a low-temperature wet-chemical method. Morphologies and structures of ZnO nanorods were examined by scanning electron microscopy and X-ray diffraction. Third-order optical nonlinearities such as nonlinear refraction and absorption were determined by closed- and open-aperture z-scan measurements at 815 nm using femtosecond laser pulses. Enhanced optical nonlinearities were observed with increasing rod diameters and aligned degrees of ZnO nanorods along the c-axis. Nonlinear response time, measured by using a time-resolved optical Kerr-gate technique, was less than 95 fs. (c) 2007 Elsevier B.V. All rights reserve
Steady state mode-locking of a 1.34 μm Nd:YVO4 laser using a single-walled carbon nanotube saturable absorber
No full textSteady state mode-locked operation of a neodymium laser operating on the F-4(3/2) -> I-4(13/2) transition around 1.3 mu m is achieved for the first time using transmitting single-walled carbon nanotube (SWCNT) saturable absorber. The Nd:YVO4 laser cavity was optimized for large fundamental mode volume generating an average power of 0.8 W at a repetition rate of 127 MHz in a stable train of 16.5 ps long pulse
Superior characteristics of organic chalcone single crystals as efficient nonlinear optical material
No full textHigh-quality biaxial single crystals of chalcone derivatives were grown by solution growth technique. Their molecular structures were designed to possess large second-order nonlinearities by choosing proper donor/acceptor groups while retaining a high transparency in the visible and infrared spectral regions. The second-order nonlinear optical coefficients, determined by applying the Maker fringe technique, were found to be much larger than those of LiB(3)O(5), KTiOPO(4), KH(2)PO(4), and urea. The advantages, such as easy synthesis and crystal growth, low cutoff wavelength (< 450 nm), large optical nonlinearity, and high damage threshold (>7.2 GW/cm(2)), make these organic crystals promising for efficient frequency doubling. (C) 2008 American Institute of Physic
Growth and characterization of an efficient nonlinear optical D-pi-A-pi-D type chalcone single crystal
No full textA chalcone molecule, p-methoxy dibenzylideneacetone (PDBA), with molecular formula C19H18O3 having D-pi-A-pi-D type, lambda (Lambda)-shape structure has been synthesized by Claisen-Schmidt condensation reaction. High-quality single crystals of PDBA were grown by slow evaporation solution growth technique using acetone, methanol and dimethyl formamide (DMF) as solvents. PDBA easily crystallizes in orthorhombic system with noncentrosymmetric space group, Aba2, and the cell parameters are a = 7.2756(9) angstrom, b = 33.583(6) angstrom, c = 6.132(5) angstrom and v = 1498.3 angstrom(3). The powder second harmonic generation (SHG) efficiency of PDBA was determined to be 15.5 times that of urea. This large SHG efficiency of PDBA is due to (i) the A-shape of the molecule resulting in a head-to-tail molecular columns along the crystallographic c-direction where dipole moment of the individual molecule add up to establish a net macroscopic polarization and (ii) the C-H...pi hydrogen bond interactions in PDBA crystal structure (which can extend molecular charge transfer into supramolecular realm in the exited state) which may partly contribute to the observed SHG. PDBA crystal is highly transparent in the visible to near infrared region and has lower optical cutoff of 440 nm. The thermal stability of PDBA is comparable with that of urea. High optical transparency down to blue region, better Vickers hardness value, higher powder SHG efficiency and laser damage resistance than that of urea, and the existence of phase-matching property in this chalcone make it a potential candidate for SHG applications. (C) 2008 Elsevier B.V. All rights reserve
Designing novel chalcone single crystals with ultrafast nonlinear optical responses and large multi-photon absorption coefficients
No full textA chalcone single crystal, 1-(4-chlorophenyl)-3-(4-methoxyphenyl)prop-2-en-1-one that is transparent over the visible to infrared region is introduced as a new potential material to third-order nonlinear optical applications. The crystal exhibits ultrafast optical response(<= 90 fs) and large optical nonlinearity in the wavelength range 800-1200 nm. A very large effective two-photon absorption coefficient beta(eff) exceeding 120 cm/GW can be obtained with this chalcone crystal, at a low intensity threshold of 41 MW/cm(2). The mechanism of nonlinear absorption at different levels of intensity has been discussed. The crystal shows no damage against the laser pulse intensity as high as 8 GW/cm(2). We discuss the molecular and crystal designing of chalcones with large and ultrafast optical nonlinearity combined with low optical cut-off (<450nm). (C) 2010 Elsevier B.V. All rights reserve
Synthesis, crystal growth and characterization of a phase matchable nonlinear optical single crystal: p-chloro dibenzylideneacetone
No full textGood quality single crystals of p-chloro dibenzylideneacetone (CDBA) of size 13 mm x 8 mm x 2 mm were grown by slow evaporation solution growth technique. The grown crystals were confirmed by elemental analysis, Fourier transform infrared (FTIR) analysis and single crystal X-ray diffraction techniques. From the thermo gravimetric/differential thermal (TG/DT) analysis, the CDBA was found to be thermally stable up to 250 degrees C. The mechanical stability of the crystal is comparable with that of the other reported chalcones. The lower optical cut-off wavelength for this crystal was observed at 440 nm. The laser damage threshold of the crystal was 0.6 GW/cm(2) at 532 mm. The second harmonic generation conversion efficiency of the powder sample of CDBA was found to be 4.5 times greater than that of urea. We also demonstrate the existence of the phase matching property in this crystal using Kurtz powder technique. (C) 2008 Elsevier B.V. All rights reserve
GaS0.4Se0.6: Relevant properties and potential for 1064 nm pumped mid-IR OPOs and OPGs operating above 5 μm
No full textWe present measurements of the transparency, refractive index dispersion, nonlinear coefficient, damage threshold, and two-photon absorption of mixed GaS (x) Se1 - x crystals and show that GaS0.4Se0.6 is a promising nonlinear material for down conversion of pulsed 1064 nm radiation to the mid-IR above 5 mu m without significant two-photon absorptio
Optimal sigmoidal tuning curves for intensity encoding sensory neurons with quasi-Poisson variability
No full textRate-coding neurons are often characterized by their tuning
curve, that is, the average firing rate, T(x), as a function
of stimulus intensity, x. However the substantial natural
variability in firing rate that often occurs for a fixed stimulus
provides a limitation on the fidelity of firing rate
encoding of stimuli. Consequently, stimulus-dependent
variance in firing rate, V(x), is crucial in studies of tuning
curve optimality. Information theory can be used to quantify
such limits and to address the question of finding the
tuning curve that maximizes information rate
Passively Q-switched Er,Yb:GdAl<sub>3</sub>(BO<sub>3</sub>)<sub>4</sub> laser with single-walled carbon nanotube based saturable absorber
No full textWe demonstrate a passively Q-switched Er,Yb:GdAl3(BO3)(4) diode-pumped laser emitting near 1.5 mu m. By using a single-walled carbon nanotube (SWCNT) as a saturable absorber, Q-switched laser pulses with energy of 0.8 mu J and duration of 130 ns at a maximum repetition rate of 500 kHz were obtained at 1550 nm.
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