Phase-Sensitive Detection of Bragg Scattering at 1D Optical Lattices

Abstract

We report on the observation of Bragg scattering at 1D atomic lattices. Cold atoms are confined by optical dipole forces at the antinodes of a standing wave generated by the two counter-propagating modes of a laser-driven high-finesse ring cavity. By heterodyning the Bragg-scattered light with a reference beam, we obtain detailed information on phase shifts imparted by the Bragg scattering process. Being deep in the Lamb-Dicke regime, the scattered light is not broadened by the motion of individual atoms. In contrast, we have detected signatures of global translatory motion of the atomic grating.PACS numbers: 42.50. Vk, 05.45.Xt, 05.65+b, 05.70.Fh Elastic Rayleigh scattering is a phase-coherent process. In thermal atomic clouds however the photonic recoil transferred to the atoms by the scattering process introduces inhomogeneous line broadening washing out the phase-coherence. Furthermore, Rayleigh scattering only dominates at small intensities; stronger pumping causes power broadening, which merely increases the inelastic components of the resonance fluorescence. For these reasons it is difficult to directly measure the phase-shift induced by Rayleigh scattering.A phase-coherent study of Rayleigh scattering is facilitated by two measures: 1. Using cold and strongly confined atoms, and 2. arranging for long-range order in the atomic cloud. By cooling the atoms the Dopplerbroadening is reduced; by tightly confining them within a very small region of space, i.e. within the Lamb-Dicke limit, no recoil is imparted to individual atoms; and by creating density gratings the elastic part of the resonance fluorescence is concentrated into a very small solid angle. The resonant enhancement of the structure factor for elastic Rayleigh scattering by atomic long-range order is called Bragg scattering.Periodic structures are generally probed by Bragg scattering. A probe beam is shone onto the sample under a certain angle, the so-called Bragg angle, and the emergence of phase-coherent light at well-defined sharp solid angles is a signature of long-range order. This procedure can be applied to periodic arrangements of atoms in optical gratings. Bragg scattering of light at 3D atomic lattices has for the first time been observed by Birkl et al. and Weidemüller et al. [1,2].The elastic peak of the atomic response to incident laser light has been observed in several experiments. Westbrook et al. [3,4] used the heterodyne method to beat down the fluorescence of magneto-optically trapped atoms with a local oscillator to electronically accessible frequencies. It is in principle possible to probe the complete fluorescence spectrum, i.e. the Mollow triplet and the elastic Rayleigh peak by scanning the reference laser. This technique permitted Westbrook et al. to detect Dicke-narrowing of Rayleigh scattering in magnetooptical traps. However in this experiment the heterodyne signal was integrated over long times, so that the phasecoherence of the elastic scattering process is not directly shown.We report here the first detailed phas...

Self ordering threshold and superradiant backscattering to slow a fast gas beam in a ring cavity with counter propagating pump

“…Only recently, a first unambiguous observation of the (inverse-) CARL mechanism to accelerate a cold gas by coherent light scattering in a cavity was achieved in Tübingen using Cesium atoms [18,26]. These results are in very good agreement with predictions from microscopic dynamic models [19] as well as collective macroscopic descriptions of atoms in ring cavities [20].…”

Self-organization of atoms in a cavity field: Threshold, bistability, and scaling laws

“…There is no external finite-temperature heat bath to define the temperature which, instead, is set by the dynamical equilibrium of the dipole force fluctuations and the cavity cooling effect. This is a distinctive feature with respect to the recently demonstrated collective atomic recoil laser in a ring cavity (CARL) [22,23,24,25], where a magneto-optical trap is necessary to stabilize the organized phase and the otherwise transient gain [26], and also to inject noise for obtaining the phase transition-like behavior [27,28,29].…”

Self-organization of a laser-driven cold gas in a ring cavity

“…If the polarized gas roughly forms a lattice with period λ (x 0 giving the position of the lattice modulo λ/2), mode h − will only be weakly coupled to the pump laser, since it has nodes at the lattice points. On the contrary, mode h + has antinodes at the atomic positions and thus is driven via stimulated Bragg scattering off the gas [7,10]. The possibility of optical trapping of high-field seekers with negligible spontaneous emission loss requires large red detuning, whereby the resonance shift U 0 is negative.…”

“…This is due to the fast photon round-trips in the resonator on the time scale given by the other interaction strengths [4,5], i.e., the longitudinal mode spacing is much larger than the atomic and cavity linewidth as well as the relevant detunings. To be specific, we assume a one-dimensional ring resonator with two counterpropagating modes, which corresponds to the experimental setups in [6][7][8]. As a difference to the correlated atomic recoil laser (CARL), we consider a transverse pump scheme, i.e., the particles are laser driven from a direction orthogonal to the resonator axis.…”