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Chinese Physics B

Atomic motion in the magneto–optical trap consisting of partially spatially coherent laser

Baowu Zhang, Zhaoying Wang, De-Long Kong, Qiang Lin

Published Aug 1, 2015• 0 citations
Publisher

Abstract

Rb atom motion in a magneto–optical trap (MOT) consisting of a partially spatially coherent laser (PSCL) is investigated theoretically. The spatial coherence of the laser is controlled by the electro–optic crystal. The instantaneous spatial distribution of the dissipative force induced by the PSCL on an Rb atom is varying with time stochastically. The simulated results indicate that compared with a fully coherent laser, the spatial coherent laser has effects on the atomic trajectories; however, the capture velocity and the escape velocity are kept the same. The main reason is that the spatial coherence of the laser fluctuates temporally and spatially, but the average photon scattering rate varies little, which makes the total number of atoms and the atomic density distribution unchanged.

DOI
10.1088/1674-1056/24/8/083701

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Rb atom motion in a magneto–optical trap (MOT) consisting of a partially spatially coherent laser (PSCL) is investigated theoretically. The spatial coherence of the laser is controlled by the electro–optic crystal. The instantaneous spatial distribution of the dissipative force induced by the PSCL on an Rb atom is varying with time stochastically. The simulated results indicate that compared with a fully coherent laser, the spatial coherent laser has effects on the atomic trajectories; however, the capture velocity and the escape velocity are kept the same. The main reason is that the spatial coherence of the laser fluctuates temporally and spatially, but the average photon scattering rate varies little, which makes the total number of atoms and the atomic density distribution unchanged.
Atom cooling and trapping by partially spatially coherent lasers is investigated both experimentally and theoretically. The degree of spatial coherence of the laser beams is controlled by the electro-optic crystal. The characteristics of the trapped atomic cloud are studied. The experimental results indicate that the atom number and atomic Gaussian density distribution stay unchanged due to the fact that the average photon scattering rate is independent of the spatial coherence of the laser beams. However, the measured temperature of the atomic cloud increases as the degree of spatial coherence of the laser beams decreases. The reason is that the scattering force of the partially spatially coherent lasers acting on the atoms fluctuates temporally and spatially, which broadens the velocity distribution of the atoms.
Since the first proposal and the first experiment on magneto-optical trapping of atoms, magneto-optical traps (MOT) became the most popular traps for cold atoms. Despite the numerous successful applications of MOT it is still unclear what ultimate parameters could be expected for atoms trapped in MOT. We develop the theory of MOT for cold atoms under the multilevel dipole interaction schemes. Theory is based on consideration of one-dimensional motion of atoms which have three ground-state sublevels (F/sub g/=1) and five upper-state sublevels (F/sub e/=2) and are in inhomogeneous magnetic field and in the field of two counter-propagating circular polarized laser waves. It is shown that in multilevel interaction schemes two-photon processes are responsible for formation of a narrow velocity and spatial structure in the radiation force. It is found that two-photon coherent process enhances considerably the friction force and the oscillation frequency at negative detuning. Single-photon and two-photon processes are shown to produce the two-component structure of the MOT potential and accordingly the two-component coordinate distribution.
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.
We present an experimental and theoretical study of controlling the atomic spatial distributions in a magneto-optical trap (MOT). With a diaphragm we can vary the waist and power of one of the cooling laser beams and we can change parameters of large-diameter, parallelogram-shaped atomic orbits. We show that the radiative force generated by the repumping MOT laser has to be taken into consideration. Computer simulations of atomic trajectories explain the observed spatial structures, and we employ these simulations to present potential applications of controlling the diaphragm diameter as a function of time. A potential use of controlled vortex forces seems to have a great significance in recently presented important new methods to investigate cold atom collisions in the MOT, which were recently published.
Pulsed photoionization of laser-cooled atoms in a magneto-optical trap (MOT) has the potential to create cold electron beams of few meV bandwidths and few ps pulse lengths. Such a source would be highly attractive for the study of fast low-energy processes like coherent phonon excitation. To study the suitability of MOT-based sources for the production of simultaneously cold and fast electrons, we study the photoionization dynamics of trapped Cs atoms. A momentum-microscope-like setup with a delay-line detector allows for the simultaneous measurement of spatial and temporal electron distributions. The measured patterns are complex, due to the Lorentz force inducing spiral trajectories. Ray-tracing simulations reproduce the main features. We find that the production of electron bunches with bandwidths of a few meV is straightforward; however, pulses in the ps-range are more demanding and require beam blanking or partial blocking.
Here, we present our studies on intra-trap dynamics of an optical dipole trap (ODT) loaded from a Magneto-optical trap (MOT) of 87^{87}87Rb atoms. A spatio-temporal Monte Carlo (MC) simulation approach has been employed in conjunction with the semi-classical theory based calculations for estimation of various intra-trap two-body loss parameters in the ODT. The temporal MC method based on estimated parameters from spatial MC, gives a close estimation of the experimentally observed atom-number evolution in ODT. We also find that, the decay rate of atoms in an ODT is dominated by momentum-transfer elastic collisions in the absence of MOT beams. However, in presence of MOT beams, the radiative escape process is the dominant two-body loss mechanism in the trap, which surpasses the fine-structure changing and hyperfine changing collisional losses by nearly an order of magnitude.