We produce a quantum degenerate mixture composed by two Bose-Einstein condensates of different atomic species, 41 K and 87 Rb. We study the dynamics of the superfluid system in an elongated magnetic trap, where off-axis collisions between the two interacting condensates induce scissors-like oscillation.The long-standing interest in mixtures of superfluids, originally focused on helium systems [1], has recently been renewed by the achievement of Bose Einstein condensation (BEC) in dilute atomic gases [2]. Already using a single atomic species, multiple condensates were realized by exploiting the magnetic structure of the ground electronic state of alkali atoms. Mixtures of two hyperfine spin states of 87 Rb in magnetic traps allowed to study the effect of the mutual interaction in the dynamic of miscible BECs [3]. Superposition of spinor condensates of 23 Na in an optical trap led to a first observation of both weakly miscible and immiscible superfluids [4] and of the occurrence of metastable states [5]. These experimental achievements stimulated an extensive theoretical research on the properties of a mixture of two BECs, and the role of the interparticle interaction in determining its static and dynamical properties has been recognized [6][7][8][9][10].As early suggested [6], an even wider scenario for the study of superfluid systems would be opened by BEC in mixtures of different atoms. Considering the species condensed so far, the use of different isotopes of the same species would be restricted to the case of rubidium [11], while a wider choice would be offered the use of different atomic species. Recently, two-species mixtures were successful for the realization of Fermi-Bose degenerate gases [12].In this Letter, we report the realization of a mixture of Bose-Einstein condensates of different atomic species, using potassium and rubidium. Simultaneous condensation of 41 K and 87 Rb is achieved by means of two-species sympathetic cooling [13] in a magnetic trap. The stability against collapse of the degenerate mixture, already * also at

A degenerate gas of identical fermions is brought to collapse by the interaction with a Bose-Einstein condensate. We used an atomic mixture of fermionic potassium-40 and bosonic rubidium-87, in which the strong interspecies attraction leads to an instability above a critical number of particles. The observed phenomenon suggests a direction for manipulating fermion-fermion interactions on the route to superfluidity.

Elaborating reliable and versatile strategies for efficient light coupling between free space and thin films is of crucial importance for new technologies in energy efficiency. Nanostructured materials have opened unprecedented opportunities for light management, notably in thin-film solar cells. Efficient coherent light trapping has been accomplished through the careful design of plasmonic nanoparticles and gratings, resonant dielectric particles and photonic crystals. Alternative approaches have used randomly textured surfaces as strong light diffusers to benefit from their broadband and wide-angle properties. Here, we propose a new strategy for photon management in thin films that combines both advantages of an efficient trapping due to coherent optical effects and broadband/wide-angle properties due to disorder. Our approach consists of the excitation of electromagnetic modes formed by multiple light scattering and wave interference in two-dimensional random media. We show, by numerical calculations, that the spectral and angular responses of thin films containing disordered photonic patterns are intimately related to the in-plane light transport process and can be tuned through structural correlations. Our findings, which are applicable to all waves, are particularly suited for improving the absorption efficiency of thin-film solar cells and can provide a new approach for high-extraction-efficiency light-emitting diodes.

We report on the achievement of simultaneous quantum degeneracy in a mixed gas of fermionic 40 K and bosonic 87 Rb. Potassium is cooled to 0.3 times the Fermi temperature by means of an efficient thermalization with evaporatively cooled rubidium. Direct measurement of the collisional crosssection confirms a large interspecies attraction. This interaction is shown to affect the expansion of the Bose-Einstein condensate released form the magnetic trap, where it is immersed in the Fermi sea.PACS numbers: 05.30. Fk, 05.30.Jp, 32.80.Pj The recently demonstrated quantum degeneracy of Fermi-Bose (FB) mixtures of dilute atomic gases [1,2,3] promises to further enrich the field of the physics of degenerate matter at ultralow temperatures [4,5]. When a Bose-Einstein condensate (BEC) interacts with a Fermi gas, novel phenomena are expected to occur. The most appealing one is certainly BCS-like fermionic superfluidity, since a BEC could affect interactions between fermions [6,7]. Furthermore, different FB interaction regimes could allow studies of phase-separation [8,9] or of the stability properties of the binary mixtures [10].The mixtures so far reported have in common the use of fermionic 6 Li, combined with a BEC of 7 Li [1, 2], or of 23 Na [3]. For the 6 Li-7 Li mixtures the FB interaction is repulsive, with possible consequences for the separation of the components, and eventually for the thermal contact. For the 6 Li-23 Na case, the interaction has not been measured, however the theoretical predictions are again in favor of a repulsive character [11].A different, promising scenario would be offered by mixtures combining species with attractive interaction, because the absence of a phase separation would allow efficient cooling well below the Fermi temperature and would favor the interaction between the two components in the degenerate regime. . This cooling scheme represents also an alternative to the single-species evaporation approach [14] that was early demonstrated to produce a Fermi gas of K atoms. We observe signatures of the large interaction of the two components also in the degenerate regime.The degenerate mixture is produced using the apparatus described in Ref. [13]. In brief, about 10 5 40 K atoms and 5×10 8 87 Rb atoms at a temperature around 100 µK are loaded in an elongated magnetostatic trap using a double magneto-optical trap apparatus. As opposed to the case of 41 K [13], combined magneto-optical trapping of 40 K and 87 Rb is efficient, as was also shown in Ref. [15]. Prior to magnetic trapping, both species are prepared in their doubly polarized spin state, |F = 9/2, m F = 9/2 for K and |2, 2 for Rb. These states experience the same trapping potential, with axial and radial harmonic frequencies ω a = 2π × 24 s −1 and ω r = 2π × 317 s −1 for K, while those for Rb are a factor (M Rb /M K ) 1/2 ≈ 1.47 smaller. Evaporative cooling is then performed selectively on the Rb sample. Due to the different gyromagnetic factors of the two species a radio-frequency evaporation scheme could be implemented [16]...

We compare the experimental stability diagram of a Fermi-Bose mixture of K-40 and Rb-87 atoms with attractive interaction to the predictions of a mean-field theoretical model. We discuss how this comparison can be used to give a better estimate of the interspecies scattering length, which is currently known from collisional measurements with larger uncertainty.Comment: 5 pages, 4 figure

Mind the Gap Near-field microscopy has benefited from subwavelength near-field plasmonic probes that make use of the field-concentrating properties of gaps. These probes achieve maximum enhancement only in the tip-substrate gap mode, which can yield large near-field signals, but only for a metallic substrate and for very small tip-substrate gap distances. Bao et al. (p. 1317 ) designed a probe that unites broadband field enhancement and confinement with bidirectional coupling between far-field and near-field electromagnetic energy. Their tips primarily rely on the internal gap modes of the tip itself, thereby enabling it to image nonmetallic samples.

We investigate the spatial patterns of the ground state of two interacting Bose-Einstein condensates. We consider the general case of two different atomic species (with different mass and in different hyperfine states) trapped in a magnetic potential whose eigenaxes can be tilted with respect to the vertical direction, giving rise to a non trivial gravitational sag. Despite the complicated geometry, we show that within the Thomas-Fermi approximations and upon appropriate coordinate transformations, the equations for the density distributions can be put in a very simple form. Starting from this expressions we give explicit rules to classify the different spatial topologies which can be produced, and we discuss how the behavior of the system is influenced by the interatomic scattering length. We also compare explicit examples with the full numeric Gross-Pitaevskii calculation.

The ability to mold the flow of light at the wavelength scale has been largely investigated in photonic-crystal-based devices, a class of materials in which the propagation of light is driven by interferences between multiply Bragg scattered waves and whose energy dispersion is described by a photonic band diagram [1]. Light propagation in such structures is defined by Bloch modes, which can be engineered by varying the structural parameters of the material [2][3][4]. In disordered media, both the direction and phase of the propagating waves are randomized in a complex manner, making any attempt to control light propagation particularly challenging. Disordered media are currently investigated in several contexts, ranging from the study of collective multiple scattering phenomena [5,6] to cavity quantum electrodynamics and random lasing [7,8], to the possibility to provide efficient solutions in renewable energy [9], imaging [10], and spectroscopy-based applications [11]. Transport in such systems can be described in terms of photonic modes, or quasi-modes, which exhibit characteristic spatial profiles and spectra [12,13]. In diffusive systems, these modes are spatially and spectrally overlapping while in the regime of Anderson localization, they become spatially and spectrally-isolated [14]. Unlike Bloch modes in periodic systems, the precise formation of photonic modes in a single realization of the disorder is unpredictable.Control over light transport can be obtained by shaping the incident wave to excite only a specific part of the modes available in a given system [15][16][17][18]. For fully exploiting the potential of disordered systems, however, a mode control is needed. It was shown 3 theoretically that isolated modes could be selectively tuned and possibly coupled to each other by a local fine modification of the dielectric structure [19,20].In this Article, we demonstrate experimentally the ability to fully control the spectral properties of an individual photonic mode in a two-dimensional disordered photonic structure [21], in a wavelength range that is relevant for photonic research driven applications. A statistical analysis of individual spatially-isolated random photonic modes is performed by multi-dimensional near-field imaging, leading to a detailed determination of intensity fluctuations, decay lengths and mode volumes. We then demonstrate that individual modes can be fine-tuned either by near-field tip perturbation or by local sub-micrometer-scale oxidation of the semiconductor slab [22]. The resonant frequency of a selected mode is gradually shifted until it is in perfect spectral superposition with the frequency of other two modes, located a few micrometers apart and spatially overlapping with the tuned mode. On spectral resonance, we observe frequency crossing and anti-crossing behaviours, respectively, the latter indicating mode interaction. This provides the experimental proof-of- (e) and (f), respectively). The main difference between the two spectra normalized to the average intensity i...