N. Vansteenkiste

Laser cooling below the one-photon recoil energy by velocity-selective coherent population trapping: theoretical analysis

We present a theoretical analysis of a new one-dimensional laser-cooling scheme that was recently demonstrated on a beam of metastable 4He atoms. Both internal and translational degrees of freedom are treated quantum mechanically. Unlike semiclassical approaches, such a treatment can be applied to situations in which the atomic coherence length is of the same order of or larger than the laser wavelength, which is the case for atoms cooled below the one-photon recoil energy. We introduce families of states that are closed with respect to absorption and stimulated emission, and we establish the generalized optical Bloch equations that are satisfied by the corresponding matrix elements. The existence of velocity-selective trapping states that are linear combinations of states with different internal and translational quantum numbers is demonstrated, and the mechanism of accumulation of atoms in these trapping states by fluorescence cycles is analyzed. From a numerical solution of the generalized optical Bloch equations, we study in detail how the final atomic-momentum distribution depends on the various physical parameters: interaction time, width of the initial distribution, laser detuning, laser power, and imbalance between the two counterpropagating waves. We show that the final temperature decreases when the interaction time increases, so that there is no fundamental limit to the lowest temperature that can be achieved by such a method. Finally, possible extensions of this method to two-dimensional cooling are presented.

Resonant enhancement of evanescent waves with a thin dielectric waveguide

Abstract A multilayer structure is realized where a laser beam is coupled to a dielectric waveguide by optical tunnelling through a solid gap. This provides an enhanced evanescent wave in the vacuum above the waveguide, that can be used as an atomic mirror. The enhancement factor has been indirectly measured with a generalized m -line type method.

Specular versus diffuse reflection of atoms from an evanescent-wave mirror

We tested the specularity of the ref lection of slow atoms from an evanescent-wave mirror at normal incidence. In two of the three prisms that we tested the atoms were ref lected diffusely. This nonspecular ref lection appears to be correlated with the rms roughness of the surface supporting the evanescent wave. Only the highest quality surface (rms roughness of the order of 0.1 nm) leads to specular ref lection. This discovery imposes stringent limits on the use of these mirrors in atomic-optics experiments.

Observation of intensity correlations in the fluorescence from laser cooled atoms

Abstract We report the observation of intensity correlations in the fluorescence from a three dimensional Rb optical molasses. This constitutes a new, non-destructive, in situ probe of laser cooled atoms. The power spectrum of the photocurrent displays a 100 kHz wide feature corresponding to the Doppler width of the atomic velocity distribution, and a narrow peak interpreted as being due to the localization of the atoms in wavelength sized potential wells. As the atoms rms velocity decreases, the height of the narrow peak increases while its width decreases to as little as 2 kHz. We discuss the potential for new applications of this method.