Voici les éléments 1 - 10 sur 794
  • Publication
    Accès libre
    Bound-to-continuum and two-phonon resonance, quantum-cascade lasers for high duty cycle, high-temperature operation
    (2002)
    Faist, Jérôme
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    Beck, Mattias
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    Aellen, Thierry
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    Rochat, Michel
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    Blaser, Stéphane
    Recent advances in quantum-cascade (QC) laser active-region design are reviewed. Based on a rate equation model of the active region, we show why new gain regions. based on a two-phonon resonance or a bound-to-continuum transition exhibit significantly better performance than the traditional design based on a three-quantum-well active region. Threshold current densities as low as 3 kA/cm2 at T = 300 K, operation with a peak power of 90 mW at 425 K, single-mode high-power operation up to temperatures above 330 K at λ ≈ 16 μm and continuous wave operation up to T = 311 K are demonstrated. QC lasers able to operate at high duty cycles (50%) on a Peltier cooler were used in a demonstration of a 300-MHz free-space optical link between two buildings separated by 350 m.
  • Publication
    Métadonnées seulement
    4-Chlorobenzoyl-meso-octamethylcalix[2]pyrrolidino[2]pyrrole: an acyl chloride derivative of a partially reduced calix[4] pyrrole
    In the title compd., C35H47ClN4O, the two pyrrolidine rings have envelope conformations. The conformation of the macrocycle is stabilized by N-H···N H bonds and a C-H···N interaction. The benzoyl ring is inclined to the adjacent pyrrole ring by 11.66(11)°, with a centroid-centroid distance of 3.7488(13) Å. In the crystal, mols. are linked by N-H···O H bonds into helical chains propagating in [010] and C-H···O and C-H···? interactions are also obsd. [on SciFinder(R)]
  • Publication
    Accès libre
    Protonation and Zn(II) complexation with versatile valine and glycylglycine N-pyrimidines derivatives: crystal structures of layered {[Zn(HL1)2] • 2H2O}n and [Zn(HL2)2 (H2O)4]
    (2004)
    López-Garzón, R.
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    Godino-Salido M. L.
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    Arranz-Mascarós, P.
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    Fontecha-Cámara, M.A.
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    Gutiérrez-Valero, M. D.
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    Cuesta, R.
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    Moreno, J.M.
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    Protonation and Zn(II) complexation of N-substituted amino acids, valine (H2L1) and glycylglycine (H2L2), with 4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxopyrimidin-2-yl as substituent, were studied by potentiometric and UV–Vis measurements. Bianions L1 and L2 suffer three protonation steps in aqueous medium corresponding to the amide and carboxylate groups of the amino acidic moiety, and the nitrogen atom of the nitroso group of the pyrimidine fragment. Both ligands form mononuclear Zn(II) complexes in aqueous solutions. The binding donor groups are the nitroso and/or the oxo groups of the pyrimidinic moiety or the carboxylate group, depending on whether the ligands are neutral or anionic, respectively. Weak metal-to-ligand interactions were observed independently of the functionality used by the corresponding ligand on bonding to Zn(II). The reaction of ZnCl2 with the monodeprotonated ligands (1:1) yields a polynuclear 2D {[Zn(HL1)2] • 2H2O}n and a mononuclear [Zn(HL2)2(H2O)4] complexes, showing the influence of the susbtituent on the amino acids fragment as well as the versatility of this class of compounds when acting as ligands.
  • Publication
    Accès libre
    Cross-influence between the two servo-loops of a fully-stabilized Er:fiber optical frequency comb
    We present a study of the impact of the cross-coupling between the two servo loops used to stabilize the repetition rate frep and the carrier-envelope offset (CEO) frequency fCEO in a commercial Er:fiber frequency comb, based on the combination of experimental measurements and a model of the coupled loops. The developed theoretical model enables us to quantify the influence of the servo-loop coupling on an optical comb line, by simulating the hypothetic case where no coupling would be present. Numerical values for the model were obtained from an extensive characterization of the comb, in terms of frequency noise and dynamic response to a modulation applied to each actuator, for both frep and fCEO. To validate the model, the frequency noise of an optical comb line at 1.56 μm was experimentally measured from the heterodyne beat between the comb and a cavity-stabilized ultranarrow-linewidth laser and showed good agreement with the calculated noise spectrum. The coupling between the two stabilization loops results in a more than 10-fold reduction of the comb mode frequency noise power spectral density in a wide Fourier frequency range.
  • Publication
    Accès libre
    Synthesis and structural analysis of palladium biscarbene complexes derived from bisimidazolium ligand precursors
    (2006)
    Heckenroth, Marion
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    Neels, Antonia
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    Albrecht, Martin
    The palladation of potentially chelating bisimidazolium ligand precursors with palladium acetate gives bridging bimetallic, chelating monometallic, and homoleptic tetracarbene complexes. The coordination mode of the biscarbene ligand has been identified by spectroscopic analysis and crystallographic characterization of representative complexes, including the first example of a biscarbene A-frame structure. Substantial concentrations of free acetate favor the formation of tetracarbene over biscarbene palladium complexes, while in the absence of a base, the concentration of reactants influences the selectivity for bridging bimetallic versus chelating monometallic species. Preliminary kinetic and mechanistic studies indicate that chelating biscarbene palladium acetate complexes are intermediates in the formation of the homoleptic tetracarbene complexes. Probably due to the high trans effect of the biscarbene ligand, such complexes are more efficient palladating agents for bisimidazolium salts than palladium acetate.
  • Publication
    Métadonnées seulement
  • Publication
    Accès libre
    All-optical microwave generation using frequency combs
    Optical frequency combs from modelocked femtosecond lasers provide a direct, phase-coherent link between the radio frequency and optical domains. This thesis describes the development and realization of an ultra-stable microwave oscillator based on optical-to-microwave frequency transfer using two different optical frequency comb technologies.
    An optical reference oscillator is realized based on Pound-Drever-Hall stabilization of a laser to a high finesse Fabry-Perot cavity. A multilayer vacuum enclosure system was designed, built, assembled and characterized, with the aim of achieving a high filtering of any external thermal perturbation that might affect the cavity, leading to a thermal time constant of more than six days.
    In this thesis the noise properties of a commercial Er:fiber optical frequency comb and of an Er:Yb:glass laser oscillator frequency comb, referred to as ERGO and developed in ETH Zurich, have been investigated. The benefit of the ERGO diode-pumped solid-state laser (DPSSL) in terms of low noise operation is demonstrated by a 20-fold improvement in the residual fractional frequency stability of the stabilized CEO beat as compared to the Er:fiber comb. The measured CEO integrated phase noise of only 0.72 rad rms is one of the lowest values reported for a self-referenced comb the 1.5 µm spectral region. Furthermore, this thesis gives new insights into the noise in a fiber frequency comb not only from the experimental point of view but also by providing a theoretical model addressing the cross-influence between the CEO and repetition rate stabilization loops.
    Finally, generation of microwave signals via optical-to-microwave frequency division using both fiber and DPSSL frequency comb technologies is discussed. The evaluation of the generated microwave signals was made with a transportable ultra-stable cryogenically-cooled sapphire oscillator. The microwave signals generated with both combs showed a similar fractional frequency stability of 5×10 15 at 1 s and several possible present limitations have been investigated. However, the use of the ERGO comb proved to be beneficial compared to the Er:fiber comb in terms of close-to-carrier phase noise of the microwave signal, resulting in a 20 dB improvement in a large Fourier frequency range (100 Hz – 100 kHz) offset from a 10 GHz carrier.
  • Publication
    Métadonnées seulement
    High harmonic generation in a gas-filled hollow-core photonic crystal fiber
    (2009)
    Heckl, Oliver Hubert
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    Baer, Cyrill Roman Emmanuel
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    Kr?nkel, C
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    Marchese, Sergio V
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    Schapper, F
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    Holler, M
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    Robinson, JS
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    Tisch, JWG
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    Couny, F
  • Publication
    Métadonnées seulement
    Continuous-wave operation of quantum cascade laser emitting near 5.6
    (2003)
    Yarekha, Dmitri A
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    Beck, Mattias
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    Blaser, Stéphane
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    Aellen, Thierry
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    Gini, Emilio
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    ;
    Faist, Jérôme
    Buried heterostructure quantum cascade lasers emitting at 5.64 mum are presented. Continuous-wave (CW) operation has been achieved at -30degreesC for junction down mounted devices with both facets coated. A 750 mum-long laser exhibited 3 mW of CW power with a threshold current density of 5.4 kA/cm(2).