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Journal of Physics B Atomic Molecular and Optical Physics

Optical dipole trap using a Fabry–Perot interferometer as a power buildup cavity

Seung‐Koo Lee, Hyun Sook Lee, Jang Myun Kim, D Cho

Published Apr 8, 2005• 9 citations
Publisher

Abstract

We construct an optical dipole trap using a Fabry–Perot interferometer as a power buildup cavity. Large power enhancement allows us to produce 9 mK deep potential wells with 48 µm spot size and 100 nm detuning from the rubidium D 1 resonance. The optical trap is loaded from a dark-spot magneto-optical trap which, in turn, is loaded from a low-velocity intense source of 85 Rb atoms. Under typical experimental conditions, there are 1.4 × 10 6 atoms in 2000 antinodes of the optical trap. Average atom density is 1.1 × 10 12 cm -3 . The number of trapped atoms is limited by the atom density, or trapping volume. The standing-wave configuration with tight longitudinal confinement has much smaller trapping volume compared with the equivalent travelling wave. A method for converting the trap to a travelling wave-like configuration using phase modulation is proposed.

DOI
10.1088/0953-4075/38/8/023

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•Jan 1, 2006•0 citations
We construct an optical dipole trap using a Fabry–Perot interferometer as a power buildup cavity. Large power enhancement allows us to produce 9 mK deep potential wells with 48 µm spot size and 100 nm detuning from the rubidium D 1 resonance. The optical trap is loaded from a dark-spot magneto-optical trap which, in turn, is loaded from a low-velocity intense source of 85 Rb atoms. Under typical experimental conditions, there are 1.4 × 10 6 atoms in 2000 antinodes of the optical trap. Average atom density is 1.1 × 10 12 cm -3 . The number of trapped atoms is limited by the atom density, or trapping volume. The standing-wave configuration with tight longitudinal confinement has much smaller trapping volume compared with the equivalent travelling wave. A method for converting the trap to a travelling wave-like configuration using phase modulation is proposed.
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.
Trapping particles is a key starting point for a range of physics experiments. The ability to confine, manipulate and interrogate a trapped species enables an invaluable degree of control and precision. Each trap offers different advantages and possibilities, and emerging experiments often demand different methods of trapping. This thesis describes a new three-dimensional microwave trap for atoms and molecules. Like an optical dipole trap, it can trap ground state polar molecules, but its depth and volume are larger by factors of 10^3 and 10^9 respectively. The experiment consists of a lithium oven, Zeeman slower, magneto-optical trap (MOT), a moving magnetic trap, and a high-quality Fabry-Perot cavity which serves as the microwave trap. Starting from a MOT of 1×10^8 lithium-7 atoms, the atomic cloud is cooled from a temperature of around 1mK to 41(1)μK with the recently developed Raman grey molasses method. Next, the population is optically pumped to a magnetically trappable state, and up to 85(2)% are loaded into a quadrupole magnetic trap. These atoms are then transferred to a moving magnetic trap and translated 600mm to the centre of the microwave cavity. After optimizing the transport to minimize atom heating and loss, we can produce samples of 2×10^7 atoms in the microwave chamber, at a phase space density of 3.58(15)×10^7. Finally, loading of the microwave trap is demonstrated and its basic properties studied. When the input power to the cavity is 610W, the electric field amplitude at the centre reaches roughly 20kV/cm. At this input power, centre-of-mass oscillation frequencies in the microwave trap were measured as 28.55(5)Hz along the axis of the cavity, and 8.81(8)Hz along the orthogonal direction. The observed lifetime was 1.76(12)s, limited by the vacuum quality. The atomic cloud expands and cools to 22(3)μK when loaded into the microwave trap, with a phase space density of 4(1)×10^7. This is consistent with adiabatic loading from the magnetic trap.
We propose a new scheme for constructing a single-beam dark optical trap that minimizes light-induced perturbations of the trapped atoms. The proposed scheme optimizes the trap depth for given trapping laser power and detuning by creating a light envelope with (a) an almost minimal surface area for a given volume and (b) the minimal wall thickness that is allowed by diffraction. The stiffness of the trap’s walls, combined with the large detuning allowed by the efficient distribution of light intensity, yields a low spontaneous photon scattering rate for the trapped atoms. Our trap also optimizes the loading efficiency by maximizing the geometrical overlap between a magneto-optical trap and the dipole trap. We demonstrate this new scheme by generating the proposed light distribution of a single-beam dark trap with a trap depth that is ∼33 times larger than that of existing blue-detuned traps and ∼13 times larger than that of a red-detuned trap with the same diameter, detuning, and laser power. Trapped atoms are predicted to have a decoherence rate that is >200 times smaller than in existing single-beam dark traps and ∼1800 times smaller than in a red-detuned trap with the same diameter, depth, and laser power.
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.
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.