Atom-Molecule Collisions in an Optically Trapped Gas

Journal of Physics: Conference Series, 2007

We discuss the results of measurements of the temperature and density distribution of cold Rubidium atoms trapped and cooled in an optical dipole trap formed by focussed CO2 laser beams at a wavelength of 10.6 µmfrom a cold, collimated and intense atomic beam of flux 2 × 10 10 atoms/s produced using an elongated 2D + MOT. A large number of rubidium atoms (≥ 10 10) were trapped in the MOT and the number density of atoms were further increased by making a temporal dark MOT to prevent density-limiting processes like photon rescattering by atoms at the trap centre. Subsequently, between 10 7 to 10 8 cold atoms at a temperature below 30 µK were transferred into a Quasi-Electrostatic trap (QUEST) formed by focussed CO2 laser beams at the MOT centre. Both single beam and crossed dual beam dipole traps were studied with a total output power of 50 W from the CO2 laser with focal spot sizes less than 100 microns. Various measurements were done on the cold atoms trapped in the dipole trap. The total atom number in the dipole trap and the spatial atom number density distribution in the trap was measured by absorption imaging technique. The temperature was determined from time-of-flight (TOF) data as well as from the absorption images after ballistic expansion of the atom cloud released from the dipole trap. The results from measurements are used to maximize the initial phase-space density prior to forced evaporative cooling to produce a Bose-Einstein Condensate.

We describe the results of the pump-probe spectroscopy performed with 85Rb atoms trapped in a magneto-optical trap (:NIOT). We show how various processes associated with light-atom interactions shape the observed spectra and how their features can be utilized for obtaining information about properties of cold-atom sample. In particular, we emphasize the important role of the atomic-recoil phenomenon and use it for efficient and reliable velocimetry of the working IvIOT. of a ground atomic state (Raman-Zeeman Resonances, RZR) [2,3], vibrational energy levels of atoms localized in an optical lattice (Raman-Vibrational Resonances, RVR) [4], or kinetic momentum states of unbound atoms (Recoil-Induced Resonances, RIR) [5-7].

Physical Review A, 1992

We develop an optical-Bloch-equation approach for modeling ultracold-atom collisions in optical traps. The method incorporates a molecular picture of the atomic collision, laser-field dressing of the molecular states participating in the dynamics, and decay of the population and polarization due to spontaneous emission of excited states. The last is important to incorporate because the duration of the cold collisions is longer than the excited-state lifetimes. The relative motion of the atoms during the course of the collision is treated semiclassically with corrections for the time-dependent relative motion of the atoms in the various channels. An application of the method to Cs trap loss due to fine-structurechanging collisions is presented. Good agreement with experiment is obtained.

Laser Physics, 2005

Theoretical study and computer simulation results for the stochastic dynamics of atoms localized in an optical dipole trap are presented. This dynamics is governed by the optical trap potential, cooling due to the Doppler effect, and heating due to the emission and absorption of virtual photons, i.e., due to the resonant dipole-dipole interactions (RDDI). It is shown that the RDDI becomes essential for closely spaced atoms, but the effect can be significantly improved by irradiating the atoms in the trap with an additional resonance probe laser beam. By varying both the optical dipole trap parameters and intensity of the probe laser field, the role of RDDI in the atomic dynamics in the trap is clarified in detail.

2007

Pump-probe spectroscopy of cold, trapped atoms is discussed with particular attention to mechanisms specific for cold atoms and potential diagnostics applications. The discussion is illustrated with experimental results obtained with 85 Rb atoms trapped in a magneto-optical trap. Most important applications are non-destructive, real-time velocimetry (thermometry) and analysis of optical lattice dynamics.

Physical Review Letters, 1998

Journal de Physique IV (Proceedings), 2004

We present a brief overview of optical trapping experiments of individual neutral atoms. Then we describe in more details an experiment using a very small optical dipole trap, that is designed to store and manipulate individual atoms. Due to the very small dipole trap volume, a "collisional blockade" mechanism locks the average number of trapped atoms on the value 0.5 over a large range of loading rates. We study this regime experimentally, and we describe methods to measure the oscillation frequencies and the temperature of a single atom in the trap with a high accuracy.

Physical Review A, 1999

We derive a simple formula for the heating rate that arises from quantum-diffractive background gas collisions in atom traps. This result appears to explain the residual heating rates reported for recent experiments with a Cs vapor-loaded, far-detuned optical trap at Ӎ10 Ϫ9 Torr ͓Phys. Rev. Lett. 81, 5768 ͑1998͔͒. Diffractive collisions may determine the minimum heating rates achievable in shallow all-optical or magnetic atom traps operating at low temperature and high density. ͓S1050-2947͑99͒50307-7͔ PACS number͑s͒: 32.80.Pj

Physical Review A, 2013

The semiclassical theory of atomic dynamics in a three-dimensional pulsed optical dipole trap formed by superimposed trains of short laser pulses (down to a few fs duration), which is based on a stochastic formulation for the dynamics of an open quantum system, is considered in detail. It covers all key features of the atomic dynamics in the trap, including the dipole-dipole interaction (DDI) between trapped atoms due to the exchange of virtual photons between the atoms. Analytical solutions are obtained for the relaxation and laser Liouvillians, which describe the dissipation and laser excitation in the system, respectively. The probabilities of single-atom and two-atom escapes from the trap are analyzed. As an example, the theory is applied to computer simulation of Rb atoms preliminarily cooled in a magneto-optical trap that are trapped in a femtosecond optical dipole trap (pulse duration 100 fs). Our simulations prove that such a trap effectively confines atoms at the pump laser power in the range from a few mW to several kW. It is also shown that a near-resonant DDI, through which atoms that are closely spaced in the micropotential wells interact with each other, can be significantly increased by illuminating the atoms with a near-resonant probe laser beam. By varying both the parameters of the trap and the intensity of the probe laser field, the role of the DDI in the atomic dynamics in the trap and its influence on the single-atom and two-atom escape rates are clarified in detail.

Physical Review Letters, 2003

We report on the production of a pure sample of up to 3 × 10 5 optically trapped molecules from a Fermi gas of 6 Li atoms. The dimers are formed by three-body recombination near a Feshbach resonance. For purification a Stern-Gerlach selection technique is used that efficiently removes all trapped atoms from the atom-molecule mixture. The behavior of the purified molecular sample shows a striking dependence on the applied magnetic field. For very weakly bound molecules near the Feshbach resonance, the gas exhibits a remarkable stability with respect to collisional decay.

Physical Review A, 2011

Studies of quantum gases have been extended to divalent-atom gases including ytterbium (Yb) and alkaline-earth atoms, such as strontium (Sr) used in our experiment.

Physical Review Letters, 1993

A new magneto-optical trap is demonstrated which confines atoms predominantly in a "dark" hyperfine level, that does not interact with the trapping light. This leads to much higher atomic densities as repulsive forces between atoms due to rescattered radiation are reduced and trap loss due to excitedstate collisions is diminished. In such a trap, more than 10' sodium atoms have been confined to densities approaching 10' atoms cm

Physical Review Letters, 1996

We have determined the rate of loss of atoms from a Bose-Einstein condensed gas due to binary processes in the presence of a far-detuned laser field. In this limit, the binary loss rate spectrum is markedly different from, and can greatly exceed, the basic atomic loss rate. We suggest that measurements of the loss rate spectrum can be used to determine the nature of atom interactions in a condensate. [S0031-9007(96)00919-2]

ICONO 2001: Quantum and Atomic Optics, High-Precision Measurements in Optics, and Optical Information Processing, Transmission, and Storage, 2002

Theoretical study and computer simulation results for stochastic dynamics of two atoms trapped in an optical dipole trap under action of a probe resonant radiation are presented. The radiation force correlations resulting from our model lead, in addition to cold collisions, to a tendency for atoms escape in pairs from the trap.

Advances In Atomic, Molecular, and Optical Physics, 2002