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Quantum Electronics and Laser Science Conference

Anisotropic velocity distributions in 3D optical lattices

Johan Jersblad, Harald Ellmann, R. Velasco Fuentes, Anders Kastberg

Published Nov 13, 2002• 0 citations
Publisher

Abstract

Summary form only given. We have measured velocity distributions in two dimensions, as a function of intensity, in a 3D near resonant optical lattice (NROL) in Cs. The NROL is created by two orthogonally polarized pairs of laser beams that propagate in the yz and xz plane respectively. The angle between the beams of each pair is 90/spl deg/ and each beam forms an angle of /spl theta/=45/spl deg/ with the (vertical) quantization (z-) axis. This results in a tetragonal structure in which the lattice constants are a/sub z/=/spl lambda//(2/spl radic/2) and a/sub x,y/=/spl lambda//2, where /spl lambda/=852 nm. The difference in lattice constants means that lattice sites have anisotropy trapping potentials, where the oscillation frequencies differ with a factor of 2 along x and z, in a harmonic approximation.

DOI
10.1109/qels.2001.962228

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Quantum Electronics and Laser Science Conference•Jan 1, 2000•0 citations
Summary form only given. We have measured velocity distributions in two dimensions, as a function of intensity, in a 3D near resonant optical lattice (NROL) in Cs. The NROL is created by two orthogonally polarized pairs of laser beams that propagate in the yz and xz plane respectively. The angle between the beams of each pair is 90/spl deg/ and each beam forms an angle of /spl theta/=45/spl deg/ with the (vertical) quantization (z-) axis. This results in a tetragonal structure in which the lattice constants are a/sub z/=/spl lambda//(2/spl radic/2) and a/sub x,y/=/spl lambda//2, where /spl lambda/=852 nm. The difference in lattice constants means that lattice sites have anisotropy trapping potentials, where the oscillation frequencies differ with a factor of 2 along x and z, in a harmonic approximation.
In a one-dimensional polarization-gradient cooling scheme, with two counter-propagating laser beams having mutually orthogonal linear polarization, a linear array of potential wells for cold atoms are formed. These optical potential wells are created by the spatially varying light shift of the atomic ground state. Transitions between quantized vibrational states in such wells have been observed by stimulated1 and spontaneous2 Raman spectroscopy. Recently, this was extended to three dimensions using both four beams3 and six beams.4 In this work, we intersect four travelling-wave laser beams, all polarized in the same plane, as indicated in Fig. 1. This produces a threedimensional body-centered cubic lattice of potential wells for cesium atoms. The laser wavelength is tuned 25 linewidths below the 62S1/2(F = 4) → 62P3/2(F = 5) transition at 852 nm; and the resultant lattice constants (distances between wells of the same character) are λ / 2 perpendicular to the plane of polarization, and λ × 2 in the plane of polarization.
A complete laser cooling setup was built, with focus on threedimensional near-resonant optical lattices for cesium. These consist of regularly ordered micropotentials, created by the interference of four laser beams. One key feature of optical lattices is an inherent ”Sisyphus cooling” process. It efficiently extracts kinetic energy from the atoms, leading to equilibrium temperatures of a few µK. The corresponding kinetic energy is lower than the depth of the potential wells, so that atoms can be trapped.We performed detailed studies of the cooling processes in optical lattices by using the time-of-flight and absorption-imaging techniques. We investigated the dependence of the equilibrium temperature on the optical lattice parameters, such as detuning, optical potential and lattice geometry. The presence of neighbouring transitions in the cesium hyperfine level structure was used to break symmetries in order to identify, which role “red” and “blue” transitions play in the cooling. We also examined the limits for the cooling process in optical lattices, and the possible difference in steady-state velocity distributions for different directions. Moreover, in collaboration with ´Ecole Normale Sup´erieure in Paris, numerical simulations were performed in order to get more insight in the cooling dynamics of optical lattices.Optical lattices can keep atoms almost perfectly isolated from the environment and have therefore been suggested as a platform for a host of possible experiments aimed at coherent quantum manipulations, such as spin-squeezing and the implementation of quantum logic-gates. We developed a novel way to trap two different cesium ground states in two distinct, interpenetrating optical lattices, and to change the distance between sites of one lattice relative to sites of the other lattice. This is a first step towards the implementation of quantum simulation schemes in optical lattices.
We study the propagation of an atomic beam flow in an optical potential generated by a two-dimensional (2D) standing wave produced by two interfering laser beams with a frequency that is close to the resonant frequency of the propagated atoms. We extended the equations of motion of an atom in a light field to three-dimensional (3D) coordinate system and calculated the 3D trajectories of chromium atoms that are thermally evaporated and deflected by the gradient force of the standing wave laser field. The polarization and polarity of detuning of the frequency of the interfering beams are varied to study the formation of chromium atoms on a substrate. Our results show that a combination of parallel polarization and positive detuning forms a 2D grid structure. The rest of the combination of polarization and polarity of detuning yields a 2D array of point structures. An optimum size of the point structure with a diameter of approximately 50 nm can be formed with an orthogonal polarization and positive detuning of the laser frequency.
Summary form only given. We have performed a detailed study of the steady-state temperature and other parameters in a three-dimensional optical lattice for a wide range of detunings. Specifically we have investigated the situation with the detuned blue light of a J-J transition, and configurations where several allowed transitions may influence the results. We load a chirp cooled Cs-beam into a magneto-optical trap, cool the atoms in an optical molasses and load them into a four-beam three-dimensional optical lattice. The lattice beams are detuned below the transition between the states 6s/sup 2/S/sub 1/2/, F=4 and 6p/sup 2/P/sub 3/2/, F' =5, at 852 nm. The detuning is varied from -5/spl Gamma/ (where /spl Gamma//2/spl pi/=5.22 MHz is the natural linewidth of the transition) all the way to the F=4/spl rarr/F'=4 transition, located -48.3/spl Gamma/ away. For each detuning we let the atoms equilibrate in the lattice, and measure the steady-state temperature by a time of flight technique.