Evanescent straight tapered-fiber coupling of ultra-high Q optomechanical micro-resonators in a low-vibration helium-4 exchange-gas cryostat

We developed an apparatus to couple a 50-micrometer diameter whispering-gallery silica microtoroidal resonator in a helium-4 cryostat using a straight optical tapered-fiber at 1550nm wavelength. On a top-loading probe specifically adapted for increased mechanical stability, we use a specifically-developed "cryotaper" to optically probe the cavity, allowing thus to record the calibrated mechanical spectrum of the optomechanical system at low temperatures. We then demonstrate excellent thermalization of a 63-MHz mechanical mode of a toroidal resonator down to the cryostat's base temperature of 1.65K, thereby proving the viability of the cryogenic refrigeration via heat conduction through static low-pressure exchange gas. In the context of optomechanics, we therefore provide a versatile and powerful tool with state-of-the-art performances in optical coupling efficiency, mechanical stability and cryogenic cooling.Comment: 8 pages, 6 figure

Even harmonic generation in isotropic media of dissociating homonuclear molecules

Isotropic gases irradiated by long pulses of intense IR light can generate very high harmonics of the incident field. It is generally accepted that, due to the symmetry of the generating medium, be it an atomic or an isotropic molecular gas, only odd harmonics of the driving field can be produced. Here we show how the interplay of electronic and nuclear dynamics can lead to a marked breakdown of this standard picture: a substantial part of the harmonic spectrum can consist of even rather than odd harmonics. We demonstrate the effect using ab-initio solutions of the time-dependent Schr\"odinger equation for $H$$_2$$^+$ and its isotopes in full dimensionality. By means of a simple analytical model, we identify its physical origin, which is the appearance of a permanent dipole moment in dissociating homonuclear molecules, caused by light-induced localization of the electric charge during dissociation. The effect arises for sufficiently long laser pulses and the region of the spectrum where even harmonics are produced is controlled by pulse duration. Our results (i) show how the interplay of femtosecond nuclear and attosecond electronic dynamics, which affects the charge flow inside the dissociating molecule, is reflected in the nonlinear response, and (ii) force one to augment standard selection rules found in nonlinear optics textbooks by considering light-induced modifications of the medium during the generation process.Comment: 7 pages, 6 figure

Autoionizing decay of H 2 doubly excited states by using xuv-pump-infrared-probe schemes with trains of attosecond pulses

We present a theoretical study of H2 ionization by a pump-probe scheme consisting on an attosecond pulse train (APT) and a near-infrared (IR) pulse.We focus on the autoionization dynamics of the first series of resonant states of the molecule, the Q1 doubly excited states. The APT central frequency is tuned to populate the 1∑ + u resonant states. The trace of autoionization is clearly visible in the two-dimensional (2D) proton-electron coincidence spectra and in the proton kinetic energy spectra. The dynamics of the autoionization process is clearly visible in the movie obtained by plotting the 2D spectrum as a function of the time delay between the APT and IR pulses. An analysis of the final symmetries ∑g and ∑u allows us to track the origin of the different structuresThis work was partially supported by the MICINN Projects No. FIS2010- 15127, No. ACI2008-0777, and No. CSD 2007-00010, the ERA-Chemistry Project No. PIM2010EEC-00751, the European grants MC-ITN CORINF and MC-RG ATTOTREND, the European COST Action CM0702, and the Advanced Grant of the European Research Council XCHEM 29085

Boundary regularity via Uhlenbeck-Rivi\`ere decomposition

We prove that weak solutions of systems with skew-symmetric structure, which possess a continuous boundary trace, have to be continuous up to the boundary. This applies, e.g., to the H-surface system with bounded H and thus extends an earlier result by P. Strzelecki and proves the natural counterpart of a conjecture by E. Heinz. Methodically, we use estimates below natural exponents of integrability and a recent decomposition result by T.Rivi\`ere.Comment: 20 page

Molecular resolvent-operator method: Electronic and nuclear dynamics in strong-field ionization

We present an extension of the resolvent-operator method (ROM), originally designed for atomic systems, to extract differential photoelectron spectra (in photoelectron- and nuclear-kinetic energy) for diatomic molecules interacting with strong, ultrashort laser fields in the single active electron approximation. The method is applied to the study of H2+ photodissociation and photoionization by femtosecond laser pulses in the XUV-IR frequency range. In particular, the method is tested (i) in the perturbative regime, for few-photon absorption and bound-bound electronic transitions, and (ii) in the strong-field regime, in which multiphoton absorption and tunneling are present. In the latter case, we show how the differential ROM allows one to track the transition between both regimes. We also analyze isotopic effects by comparing the dynamics of H2+ and D2+ ionization for different pulses. © 2014 American Physical Society.This work was accomplished with an allocation of computer time from Mare Nostrum BSC and CCC-UAM and was partially supported by the MICINN Projects No. FIS2010- 15127 and No. CSD 2007-00010, ERA-Chemistry Project No. PIM2010EEC-00751, the European grants No. MC-ITN CORINF and No. MC-RG ATTOTREND, the European COST Action No. CM0702, and European Research Council Advanced Grant No. XCHEM 290853. R.E.F.S. acknowledges a Ph.D. contract from ITN CORINF and Grant No. SFRH/BD/84053/2012 from the Portuguese government. P.R. acknowledges a Juan de la Cierva contract grant from the Spanish MICIN

Les laits tropicaux. Etude de la composition chimique et des variations de composition des laits de vaches au Mali

Les auteurs poursuivent l'étude du lait des vaches de races N'Dama et Zébu du Centre de Recherches Zootechniques de Sotuba-Bamako. Après avoir envisagé, dans un article précédent, les variations des taux de matières protéiques totales, de la caséine et du rapport caséine/protéines totales, ils considérent celles de la densité, de la matière grasse et des différents extraits secs: - extrait sec, extrait sec dégraissé, extrait sec calculé et extrait sec dégraissé rectifi

Emploi pratique des analyses fourragères pour l'appréciation des pâturages tropicaux

Les taux de matières sèches, de matières azotées brutes, de cellulose et de cendres constituent les éléments de calcul de la valeur bromatologique des fourrages. A partir de résultats expérimentaux, les tables hollandaises donnent directement la valeur bromatologique d'un kg de matières sèches de fourrage en fonction du taux de cellulose et de cendres pour la valeur énergétique et en fonction du taux de matières azotées brutes pour les matières azotées digestibles. Par analogie à l'unité gros bétail (U.G.B.) des pays tempérés, un animal de référence de 250 kg pour les pays tropicaux peut être adopté comme Unité-Bétail tropical (U.B.T.). La consommation journalière théorique étant évaluée à 2,5 kg de matière sèche pour 100 kg de poids vif, les besoins de l'U.B.T. sont rapportés au kg de matière sèche ingérée sous le nom d'Equivalent-Rati on exprimé en unité fourragère et matière azotée digestible. Le dépouillement de plus de 500 analyses bromatologiques de plantes fourragères tropicales a mis en évidence: l'importance des pâturages aériens comme complément de la ration en matières azotées digestibles au cours de la saison sèche tant sur steppes que sur savanes; l'importance périodique des espèces d'appoint pour assurer la nourriture du bétail sur steppes; - le nombre limité d'espèces assurant une ration d'entretien pendant une longue période sur steppes; - l'importance du temps de croissance pour les espèces vivaces de savanes, la valeur bromatologique des repousses dépendant plus de ce facteur que de la date de récolt

Les laits tropicaux : étude de la composition chimique et des variations de composition des laits de vaches au Mali

Disposant, au Centre de Recherches Zootechniques de Sotuba-Bamako, d'un important cheptel bovin de races N'Dama et Zébu, les auteurs ont entrepris l'étude systématique du lait de ces animaux et en particulier des variations de sa composition en fonction de l'espèce, de la saison et de la date du vêlage. Cette étude comprendra deux parties: 1) Composition et variations des laits moyens; 2) Variations de la Composition des laits individuels. Le milieu et les conditions d'entretien des animaux sont décrits. Les méthodes d'analyse employées sont précisées. Les principaux constituants du lait sont considérés ainsi que les différents rapports ou constantes classiques calculés à partir de ces éléments. Une analyse statistique est faite des résultats qui sont comparés aux données fournies par d'autres auteurs en pays tropicau