299 research outputs found

    Highly tunable low-threshold optical parametric oscillation in radially poled whispering gallery resonators

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    Whispering gallery resonators (WGR's), based on total internal reflection, possess high quality factors in a broad spectral range. Thus, nonlinear optical processes in such cavities are ideally suited for the generation of broadband or tunable electromagnetic radiation. Experimentally and theoretically, we investigate the tunability of optical parametric oscillation in a radially structured WGR made of lithium niobate. With a 1.04 /mum pump wave, the signal and idler waves are tuned from 1.78 to 2.5 \mum - including the point of degeneracy - by varying the temperature between 20 and 62 {\deg}C. A weak off-centering of the radial domain structure extends considerably the tuning capabilities. The oscillation threshold lies in the mW-power range.Comment: 4 pages, 5 figure

    Adsorption of 2,2 '-dithiodipyridine as a tool for the assembly of silver nanoparticles

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    Silver nanostructured thin films stabilized by 2,2’-dithiodipyridine (2dtpy) were prepared. The Ag nanoparticles were obtained by treating the complex [Ag(2dtpy)]NO3 with NaBH4 in a methanol–toluene mixture. The films were transferred to borosilicate glass slips by a dip-coating method and were found to consist of Ag nanoparticles possibly linked via 2dtpy molecules. Surface-enhanced Raman scattering (SERS) studies have offered the possibility of investigating the adsorption modes of 2dtpy at the Ag nanoparticle surfaces in the fil

    Semences et séquences génétiques en open source ? Expériences récentes et stratégies futures

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    Le concept d'open source appliquĂ© aux semences vĂ©gĂ©tales a un avenir prometteur. Ce concept inverse la logique du systĂšme de propriĂ©tĂ© intellectuelle pour une ressource renouvelable que l'on rend disponible en la sortant du domaine exclusif de la propriĂ©tĂ© intellectuelle. Aujourd'hui, les instruments juridiques font encore dĂ©faut pour Ă©tablir un cadre lĂ©gal complet. Toutefois, le concept d'open source, que l'on pourrait traduire ici par licence libre et ouverte, engendre au fil du temps une masse critique d'utilisateurs et de soutiens qui entraĂźne une lĂ©gitimitĂ© sociale grandissante. À terme, cette lĂ©gitimitĂ© pourrait renforcer le pouvoir juridique. L'extension future Ă  d'autres matĂ©riels dans diffĂ©rents contextes doit ĂȘtre rĂ©flĂ©chie en s'appuyant sur les expĂ©riences actuelles de semences sous licences libres et ouvertes conduites aux États-Unis, en Europe et en Afrique

    Synthesis of CdS and CdSe nanocrystallites using a novel single-molecule precursors approach

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    The synthesis of CdS and CdSe nanocrystallites using the thermolysis of several dithioor diselenocarbamato complexes of cadmium in trioctylphosphine oxide (TOPO) is reported. The nanodispersed materials obtained show quantum size effects in their optical spectra and exhibit near band-edge luminescence. The influence of experimental parameters on the properties of the nanocrystallites is discussed. HRTEM images of these materials show well-defined, crystalline nanosized particles. Standard size fractionation procedures can be performed in order to narrow the size dispersion of the samples. The TOPO-capped CdS and CdSe nanocrystallites and simple organic bridging ligands, such as 2,2Âą-bipyrimidine, are used as the starting materials for the preparation of novel nanocomposites. The optical properties shown by these new nanocomposites are compared with those of the starting nanodispersed materials

    First measurement of the Gerasimov-Drell-Hearn integral for Hydrogen from 200 to 800 MeV

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    A direct measurement of the helicity dependence of the total photoabsorption cross section on the proton was carried out at MAMI (Mainz) in the energy range 200 < E_gamma < 800 MeV. The experiment used a 4π\pi detection system, a circularly polarized tagged photon beam and a frozen spin target. The contributions to the Gerasimov-Drell-Hearn sum rule and to the forward spin polarizability Îł0\gamma_0 determined from the data are 226 \pm 5 (stat)\pm 12(sys) \mu b and -187 \pm 8 (stat)\pm 10(sys)10^{-6} fm^4, respectively, for 200 < E_\gamma < 800 MeV.Comment: 6 pages, 3 figures, 3 table

    Quantum dots coordinated with conjugated organic ligands: new nanomaterials with novel photophysics

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    CdSe quantum dots functionalized with oligo-(phenylene vinylene) (OPV) ligands (CdSe-OPV nanostructures) represent a new class of composite nanomaterials with significantly modified photophysics relative to bulk blends or isolated components. Single-molecule spectroscopy on these species have revealed novel photophysics such as enhanced energy transfer, spectral stability, and strongly modified excited state lifetimes and blinking statistics. Here, we review the role of ligands in quantum dot applications and summarize some of our recent efforts probing energy and charge transfer in hybrid CdSe-OPV composite nanostructures

    The VIMOS Public Extragalactic Redshift Survey (VIPERS): Ωm0\Omega_{\rm m_0} from the galaxy clustering ratio measured at z∌1z \sim 1

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    We use a sample of about 22,000 galaxies at 0.65<z<1.20.65<z<1.2 from the VIPERS PDR-1 catalogue, to constrain the cosmological model through a measurement of the galaxy {\it clustering ratio} ηg,R\eta_{g,R}. This statistic has favourable properties, being defined as the ratio of two quantities characterizing the smoothed density field in spheres of given radius RR: the value of its correlation function on a multiple of this scale, Ο(nR)\xi(nR), and its variance σ2(R)\sigma^2(R). For sufficiently large values of RR, this is a universal number, capturing 2-point clustering information independently of the linear bias and linear redshift-space distortions of the specific galaxy tracers. In this paper we discuss in detail how to extend the application of ηg,R\eta_{g,R} to quasi-linear scales and how to control and remove observational selection effects which are typical of redshift surveys as VIPERS. We verify the accuracy and efficiency of these procedures using mock catalogues that match the survey selection process. These results evidence the robustness of ηg,R\eta_{g,R} to non-linearities and observational effects, which is related to its very definition as a ratio of quantities that are similarly affected. We measure Ωm,0=0.270−0.025+0.029\Omega_{m,0}=0.270_{-0.025}^{+0.029}. In addition to the great precision achieved on our estimation of Ωm\Omega_m using VIPERS PDR-1, this result is remarkable because it appears to be in good agreement with a recent estimate z≃0.3z\simeq 0.3, obtained applying the same technique to the SDSS-LRG catalogue. It, therefore, suports the robustness of the present analysis. Moreover, the combination of these two measurements at z∌0.3z\sim 0.3 and z∌0.9z\sim 0.9 provides us with a very precise estimate Ωm,0=0.274±0.017\Omega_{m,0}=0.274\pm0.017 which highlights the great consistency between our estimation and other cosmological probes such as BAOs, CMB and Supernovae.Comment: 18 pages, 17 figures, accepted for publication in A&A, references adde
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