We experimentally investigate the dynamical instability of a Bose Einstein condensate in an optical ring resonator for various cavity detuning and pump powers. The resulting phase diagram is asymmetric with respect to the cavity detuning and can be d
escribed by the coupling of two atomic modes with one optical mode. We compare the experimental data to a numerical simulation and to an analytic expression of the phase boundary. For positive and negative pump cavity detuning different coupling mechanisms are identified explaining the asymmetry of the phase diagram. We present a physical interpretation and discuss the connection to the Dicke quantum phase transition.
This paper reports on the excitation of surface plasmons on gold-coated nanofibre tips by side-illumination with a laser beam and the coupling of the surface plasmons to the optical fiber. The measurements show a strong dependence of the coupling eff
iciency on the incidence angle with a maximum coupling efficiency on the order of few percent. Moreover, the fibre tip was used as scanning probe device for measuring the beam waist of a focussed laser beam. This work is motivated by the goal to use such plasmonic nanofibre tips in quantum optics experiments with cold atoms.
Scattering of light at a distribution of scatterers is an intrinsically cooperative process, which means that the scattering rate and the angular distribution of the scattered light are essentially governed by bulk properties of the distribution, suc
h as its size, shape, and density, although local disorder and density fluctuations may have an important impact on the cooperativity. Via measurements of the radiation pressure exerted by a far-detuned laser beam on a very small and dense cloud of ultracold atoms, we are able to identify the respective roles of superradiant acceleration of the scattering rate and of Mie scattering in the cooperative process. They lead respectively to a suppression or an enhancement of the radiation pressure. We observe a maximum in the radiation pressure as a function of the induced phase shift, marking the borderline of the validity of the Rayleigh-Debye-Gans approximation from a regime, where Mie scattering is more complex. Our observations thus help to clarify the intricate relationship between Rayleigh scattering of light at a coarse-grained ensemble of individual scatterers and Mie scattering at the bulk density distribution.
We report on the observation of Bragg scattering of an ultracold Fermi gas of 6Li atoms at a dynamic optical potential. The momentum states produced in this way oscillate in the trap for time scales on the order of seconds, nearly unperturbed by coll
isions, which are absent for ultracold fermions due to the Pauli principle. In contrast, interactions in a mixture with 87Rb atoms lead to rapid damping. The coherence of these states is demonstrated by Ramsey-type matter wave interferometry. The signal is improved using an echo pulse sequence, allowing us to observe coherence times longer than 100 mus. Finally we use Bragg spectroscopy to measure the in-situ momentum distribution of the 6Li cloud. Signatures for the degeneracy of the Fermi gas can be observed directly from the momentum distribution of the atoms inside the trap.
We report on the first observation of Bragg scattering of an ultracold $^6$Li Fermi gas. We demonstrate a Ramsey-type matter-wave interferometer based on Bragg diffraction and find robust signatures of persistent matter wave coherences using an echo
pulse sequence. Because of the Pauli principle, the diffracted fermions oscillate nearly unperturbed in the trapping potential for long times beyond 2 s. This suggests extremely long coherence times. On these timescales, only the presence of a $^{87}$Rb cloud seems sufficient to induce noticeable perturbations.
