Gilbert Grynberg

Observation of instabilities due to mirrorless four-wave mixing oscillation in sodium

Abstract We report the observation of an instability when two counter-propagating pump beams interact with sodium atoms. The instability appears as a coherent emission around the pump beams axis. This oscillation has only been observed on the self-focusing side of the resonance in agreement with theory. The characteristics of the oscillation are very different from those of the usual conical emission.

Mirrorless four-wave mixing oscillation in atomic vapors

Abstract The conditions of observation of a mirrorless four-wave mixing oscillation in atomic vapors are described. If a non-linear medium interacts with two pump beams propagating in opposite directions, a mirrorless oscillation can occur for a phase conjugate reflectivity of the order of 2.4. In the case of a set two-level atoms, such an oscillation is only possible on the self-focusing side of the resonance.

Phase control of directed diffusion in a symmetric optical lattice.

We demonstrate the phenomenon of directed diffusion in a symmetric periodic potential. This has been realized with cold atoms in a one-dimensional dissipative optical lattice. The stochastic process of optical pumping leads to a diffusive dynamics of the atoms through the periodic structure, while a zero-mean force which breaks the temporal symmetry of the system is applied by phase modulating one of the lattice beams. The atoms are set into directed motion as a result of the breaking of the temporal symmetry of the system.

Photometric observations of a polychromatic laser guide star

We report the photometric observation of a polychromatic laser guide star (PLGS) using the AVLIS laser at the Lawrence Livermore National Laboratory (LLNL). The process aims at providing a measurement of the tilt of the incoming wave front at a telescope induced by atmospheric turbulence. It relies on the two-photon coherent excitation of the 4D5/2 energy level of sodium atoms in the mesosphere. We used two laser beams at 589 and 569 nm, with a maximum total average output power of approximately 350 W. For the purpose of photometric calibration, a natural star was observed simultaneously through the same instrument as the PLGS at the focus of the LLNL 50-cm telescope. Photometric measurements of the 330-nm return flux confirm our previous theoretical studies that the PLGS process should allow us at a later stage to correct for the tilt at wavelengths as short as approximately 1 microm at good astronomical sites. They show also that, at saturation of two-photon coherent absorption in the mesosphere, the backscattered flux increases by a factor of approximately 2 when the pulse repetition rate decreases by a factor of 3 at constant average power. This unexpected behavior is briefly discussed.

Quantum Picture of Optical Patterns: Complementarity and Wave-Particle Aspects

We consider a model of degenerate optical parametric oscillator which includes the possibility of an arbitrary level of breaking of the translational symmetry. On the basis of the example of the stripe pattern, we examine the impact of quantum noise on nonlinear optical patterns. We define and focus on the phenomenon of quantum image and show that, in the present example, it is related to the wave and particle aspects of the e.m. radiation.

Brillouin Propagation Modes in Optical Lattices: Interpretation in Terms of Nonconventional Stochastic Resonance

We report the first direct observation of Brillouin-like propagation modes in a dissipative periodic optical lattice. This has been done by observing a resonant behavior of the spatial diffusion coefficient in the direction corresponding to the propagation mode with the phase velocity of the moving intensity modulation used to excite these propagation modes. Furthermore, we show theoretically that the amplitude of the Brillouin mode is a nonmonotonic function of the strength of the noise corresponding to the optical pumping, and discuss this behavior in terms of nonconventional stochastic resonance.

Stochastic resonance in periodic potentials: Realization in a dissipative optical lattice

We have observed the phenomenon of stochastic resonance on the Brillouin propagation modes of a dissipative optical lattice. Such a mode has been excited by applying a moving potential modulation with phase velocity equal to the velocity of the mode. Its amplitude has been characterized by the center-of-mass (CM) velocity of the atomic cloud. At Brillouin resonance, we studied the CM velocity as a function of the optical pumping rate at a given depth of the potential wells. We have observed a resonant dependence of the CM velocity on the optical pumping rate, corresponding to the noise strength. This corresponds to the experimental observation of stochastic resonance in a periodic potential in the low-damping regime.

Spatial diffusion in a periodic optical lattice: revisiting the Sisyphus effect

Abstract:We numerically study the spatial diffusion of an atomic cloud experiencing Sisyphus cooling in a three-dimensional linlin optical lattice in a broad range of lattice parameters. In particular, we investigate the dependence on the size of the lattice sites which changes with the angle between the laser beams. We show that the steady-state temperature is largely independent of the lattice angle, but that the spatial diffusion changes significantly. It is shown that the numerical results fulfill the Einstein relations of Brownian motion in the jumping regime as well as in the oscillating regime. We finally derive an effective Brownian motion model from first principles which gives good agreement with the simulations.

Temperature and spatial diffusion of atoms cooled in a 3D lin⊥lin bright optical lattice

Abstract:We present a detailed experimental study of a three-dimensional lin⊥lin bright optical lattice. Measurements of the atomic temperature and spatial diffusion coefficients are reported for different angles between the lattice beams, i.e. for different lattice constants. The experimental findings are interpreted with the help of numerical simulations. In particular we show, both experimentally and theoretically, that the temperature is independent of the lattice constant.

Drift Instability for a Laser Beam Transmitted through a Rubidium Cell with Feedback Mirror

We present the characteristics of the spatio-temporal instability that occurs when rubidium atoms interact with an incident laser beam and with the beam reflected from a tilted feedback mirror. We show that there is a drift of the roll pattern that appears above threshold. Measurements of the drift frequency are in excellent agreement with recent predictions. An experiment done with two incident waves shows a more complex spatio-temporal behaviour.