Efficient atomization of cesium metal in solid helium by low energy (10 μ\muJ) femtosecond pulses

Metal atoms in solid and liquid helium-4 have attracted some interest either as a way to keep the atoms in a weakly perturbing matrix, or using them as a probe for the helium host medium. Laser sputtering with nanosecond pulsed lasers is the most often used method for atom production, resulting however in a substantial perturbation of the matrix. We show that a much weaker perturbation can be obtained by using femtosecond laser pulses with energy as low as 10 $\mu$J. As an unexpected benefit, the atomic density produced is much higher

Condensation of helium in aerogels and athermal dynamics of the Random Field Ising Model

High resolution measurements reveal that condensation isotherms of $^4$He in a silica aerogel become discontinuous below a critical temperature. We show that this behaviour does not correspond to an equilibrium phase transition modified by the disorder induced by the aerogel structure, but to the disorder-driven critical point predicted for the athermal out-of-equilibrium dynamics of the Random Field Ising Model. Our results evidence the key role of non-equilibrium effects in the phase transitions of disordered systems.Comment: 5 p + suppl. materia

Critical behavior of the liquid gas transition of 4 He confined in a silica aerogel

6 pagesInternational audienceWe have studied 4 He confined in a 95% porosity silica aerogel in the vicinity of the bulk liquid gas critical point. Both thermodynamic measurements and light scattering experiments were performed to probe the effect of a quenched disorder on the liquid gas transition, in relation with the Random Field Ising Model (RFIM). We find that the hysteresis between condensation and evaporation present at lower temperatures disappears at a temperature T ch between 25 and 30 mK below the critical point. Slow relaxations are observed for temperatures slightly below T ch , indicating that some energy barriers, but not all, can be overcome. Above T ch , no density step is observed along the (reversible) isotherms, showing that the critical behavior of the equilibrium phase transition in presence of disorder, if it exists, is shifted to smaller temperatures, where it cannot be observed due to the impossibility to reach equilibrium. Above T ch , light scattering exhibits a weak maximum close to the pressure where the isotherm slope is maximal. This behavior can be accounted for by a simple model incorporating the compression of 4 He close to the silica strands

Implantation d'atomes de césium dans un cristal d'hélium

This manuscript deals with the production and spectroscopic studies of cesium atoms embbeded in a hcp helium single crystal near 1.1 K in view of possibly using them for measuring Cs nucleus anapole moment. We have built up an optical cryostat able to keep single oriented crystals for several months. Crystal doping with cesium atoms was made by laser sputtering from metal grains, first with a second harmonic YAG laser and then with an amplified femtosecond Ti:Sa laser.With this new process, cesium atom density is significantly higher while putting less energy inside the crystal. At 1.1 K, optical detection of these atoms reveals that they are trapped in atomic form for a longer time than at 1.6 K. The D2 (6S1/2 --> 6P3/2) absorption line shape suggests an energy level splitting between sub-levels MJ = ±3/2 and ±1/2 around 200 cm−1, in good agreement with theoretical predictions of the energy splitting for a static deformation (6%)of the atomic bubble. From the intensities of D2 components, we deduce that the high frequency one could be the | MJ |= 1/2 sub-level and as a consequence that the atomic bubble deformation in the ground state should be oblate (like a cushion). We searched for a dependency of the line shape with light polarization. The effects are weaker than expected. This could result either from incident light depolarization by the cryostat windows or from an isotropic distribution of the local anisotropic axis felt by the atoms.Ce mémoire de thèse décrit l'implantation et l'étude spectroscopique d'atomes de césium dans un monocristal d'hélium hcp à une température de 1.1 K dont une des applications possible pourrait être une mesure du moment anapolaire du noyau de Cs. Nous avons mis en place un cryostat optique permettant de travailler pendant plusieurs mois avec un monocristal d'hélium orienté. L'implantation des atomes de césium a été réalisée par atomisation laser à partir de grains métalliques, d'abord avec un laser YAG doublé, puis avec un laser Ti:Sa femtoseconde amplifié. Ce nouveau procédé a permis d'obtenir une densité atomique supérieure tout en déposant moins d'énergie dans le cristal. La détection optique des atomes a mis en évidence un allongement notable de la durée de piègeage de la population atomique à 1.1 K par rapport à 1.6 K. La forme de raie de la transition D2 (6S1/2 --> 6P3/2) en absorption est compatible avec une levée de dégénérescence entre les sous-niveaux MJ = ±3/2 et ±1/2 de 200 cm−1, en bon accord avec les prévisions théoriques donnant l'écart en énergie entre ces sous-niveaux dans une bulle atomique présentant une anisotropie statique de 6%. L'analyse de l'intensité relative des composantes de la raie D2 permet d'inférer que la composante haute énergie correspondrait aux sous-niveaux | MJ |= 1/2 et que la déformation de la bulle autour de l'état fondamental serait négative (en forme de coussin). Nous avons cherché à vérifier la dépendance de la forme de raie avec la polarisation de la lumière. La faiblesse des effets observés laisse supposer soit que la lumière incidente est dépolarisée, soit l'axe local de l'anisotropie du cristal autour des atomes est désorienté de manière isotrope

Implantation d'atomes de césium dans un cristal d'hélium

PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

On the Efficiency of Infrared Filters in Optical Cryostats

8 pagesInternational audienceIn an optical cryostat, the sample temperature is limited by the 300 K ther- mal radiation transmitted through the observation windows. As discussed in this pa- per, the corresponding heat load should be brought much below the μW level by using suitable cold IR filters. However, there are situations where one wants to di- rectly check the absence of IR heating by running the same experiment with the cold observation windows either open or totally blocked. We describe a particular example which is the study of helium condensation in a silica aerogel. On this aim, we devel- oped a simple and compact magnetic system able to move metallic shutters inside our optical cryostat. This allows us to set an upper bound of 1 μW for the effect of 300 K thermal radiation on our sample. We discuss the relevance of this result to the operation of optical dilution refrigerators