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Journal of Physics Conference Series

Characterisation of focused-beam trap: FORT loading optimisation

Nuttanan Tanasanchai, K Srakowl, Apimook Watcharangkool

Published Dec 1, 2018• 1 citation
Publisher

Abstract

We investigate the properties of the focused-beam magneto-optical trap (FBT) of Rb 85 atoms, which provides more optical accessibility in contrast to typical magneto-optical trap (MOT). We have characterised the relations between temperature, number and density of trapped atoms versus trap parameters. In this paper, we discuss some distinctive features of the FBT comparing to those of standard MOTs, and determine a suitable condition in which loading atoms into a far-off-resonance optical dipole trap (FORT) is possible. Although the trap can capture fewer atoms comparing to the standard MOTs, due to its asymmetric geometry, it may offer an alternative trap configuration for preparing single atom.

DOI
10.1088/1742-6596/1144/1/012076

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Physical Review A•Jun 13, 2000•185 citations
We investigate the properties of the focused-beam magneto-optical trap (FBT) of Rb 85 atoms, which provides more optical accessibility in contrast to typical magneto-optical trap (MOT). We have characterised the relations between temperature, number and density of trapped atoms versus trap parameters. In this paper, we discuss some distinctive features of the FBT comparing to those of standard MOTs, and determine a suitable condition in which loading atoms into a far-off-resonance optical dipole trap (FORT) is possible. Although the trap can capture fewer atoms comparing to the standard MOTs, due to its asymmetric geometry, it may offer an alternative trap configuration for preparing single atom.
We present an asymmetric focused-beam trap (FBT) of cold Rubidium-85 atoms.Unlike the cold atom preparation using the magneto-optical trap, the FBT has its path of cooling beam occupying only one axis.Since the experiments on these cold atoms usually require additional laser beams or other atom interacting sources, the FBT allows more optical and physical accessibility for these experiments.In our experiment, the cooling beams only occupied one axis, and the crossed repumping beams were in the transverse plane to the cooling beams.By varying the intensity of the repump laser, we have shown that the atomic cloud density can be monotonically manipulated from zero to the maximum value limited by the cooling power.With this monotonic control, we then computationally introduced a scheme for the precision loading of 1-6 atoms into the far-off-resonance optical dipole trap (FORT).Using the FBT for precision FORT loading could enable extensions toward the research areas, such as Rydberg atom, quantum simulation, and quantum computing.
This dissertation reports on the design, construction, and investigation of a pulsed optical dipole force trap which uses laser light to confine ultracold rubidium (Rb) atoms. Because the laser frequency is detuned far from the atomic resonance frequency, the optical dipole force trap is also called a "far-off-resonance trap" (FORT). The use of pulsed laser light to create an optical trap may find application in expanding the number of atomic species which can be confined. The experiments reported here are principally aimed, however, at understanding the physics of pulsed FORT dynamics in anticipation of using the free electron laser (FEL) at Jefferson Lab (Jlab) to construct a FORT. The Jlab FEL will provide a tunable, high power laser light source enabling the realization of spatially large and/or energetically deep traps which are not presently accessible with table-top laser sources. Here, a mode-locked Nd:YAG laser is used as a pulsed laser source. Since the conservative optical dipole force trap does not cool Rb atoms, ultracold atoms must be loaded into the FORT. The optical dipole force trap is formed at the focus of a Gaussian, 1.06 μm Nd:YAG laser beam which is located at the center of a high vacuum chamber and superimposed onto the center of a pre-cooled Rb atom cloud that has been previously accumulated in a magneto-optic trap (MOT). The performance of the pulsed FORT is compared to a continuous wave mode (cw) FORT which is built using the same laser beam; the operation of the two kinds of FORTs can be switched easily without disturbing the experimental alignment. The dependencies of FORT loading efficiency to FORT and MOT parameters such as FORT laser power, loading time, storage/holding time, detuning of the primary MOT laser frequency, and repump laser intensity are investigated. There are about 1.5×107 ultracold rubidium atoms in the MOT. At 7 Watts FORT laser power, about 8% of the atoms are loaded successfully into the cw FORT and about 5% into the pulsed FORT under similar, but not identical, conditions. In most respects both the cw and pulsed FORTs show comparable behavior. The behavior of both FORTs depends strongly on FORT laser power. As expected, at higher power, more atoms can be loaded into the FORT. Both FORTs also depend on the loading time and holding time.
We report on an investigation of loading characteristics of deep microscopic dipole traps. The dipole trap is loaded from a low density magneto optical trap (MOT) containing ≈5×106\approx 5\times 10^{6}≈5×106 atoms. We determine the loading parameters that maximize the trapped atom number for a trap of waist 5 μm with trap depths ranging from 3.5 mK to 10 mK. We determine the optimal trap loading conditions and the loading rates, loss coefficients and temperature of the trapped atoms under these conditions. We show that it is possible to load a few hundred to thousand atoms in dipole traps of depth 3.5 mK to 8.5 mK under the optimal loading conditions.
We have examined loading of 85Rb^{85}\mathrm{Rb}85Rb atoms into a shallow far-off-resonance trap (FORT) from an optical molasses stage following a magneto-optical trap (MOT) stage and compared it to loading from a MOT stage only. Substantially more atoms could be loaded into the FORT using an optical molasses. To determine why this was the case, we measured the rate of atoms loaded into the FORT and the losses from the FORT during both loading processes over a range of detunings and hyperfine pump powers. We found that the losses induced during MOT loading were essentially the same as the losses induced during molasses loading and decreased with increased red detuning; however, we found that the differences in load rate caused an optical molasses to optimize at a greater detuning, improving the number of atoms we could trap by a factor of 2 over that of optimal loading from a MOT.
We present the properties of a magneto-optical trap (MOT) of CaF molecules. We study the process of loading the MOT from a decelerated buffer-gas-cooled beam, and how best to slow this molecular beam in order to capture the most molecules. We determine how the number of molecules, the photon scattering rate, the oscillation frequency, damping constant, temperature, cloud size and lifetime depend on the key parameters of the MOT, especially the intensity and detuning of the main cooling laser. We compare our results to analytical and numerical models, to the properties of standard atomic MOTs, and to MOTs of SrF molecules. We load up to molecules, and measure a maximum scattering rate of s −1 per molecule, a maximum oscillation frequency of 100 Hz, a maximum damping constant of 500 s −1 , and a minimum MOT rms radius of 1.5 mm. A minimum temperature of 730 μ K is obtained by ramping down the laser intensity to low values. The lifetime, typically about 100 ms, is consistent with a leak out of the cooling cycle with a branching ratio of about . The MOT has a capture velocity of about 11 m s −1 .
We present a detailed experimental study of the physics involved in transferring atoms from a magneto-optical trap (MOT) to an optical dipole trap. The loading is a dynamical process determined by a loading rate and a density dependent loss rate. The loading rate depends on cooling and the flux of atoms into the trapping volume, and the loss rate is due to excited state collisions induced by the MOT light fields. From this study we found ways to optimize the loading of the optical dipole trap. Key ingredients for maximum loading are found to be a reduction of the hyperfine repump intensity, increased detuning of the MOT light, and a displacement of the optical dipole trap center with respect to the MOT. A factor of 2 increase in the number of loaded atoms is demonstrated by using a hyperfine repump beam with a shadow in it. In this way we load $8\ifmmode\times\else\texttimes\fi{}{10}^{6}{}^{85}\mathrm{Rb}$ atoms into a 1 mK deep optical dipole trap with a waist of $58 \ensuremath{\mu}\mathrm{m},$ which is 40% of the atoms initially trapped in the MOT.