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Journal of the Physical Society of Japan

Optimal Design of Dipole Potentials for Efficient Loading of Sr Atoms

Hidetoshi Katori, T. Ido, Makoto Kuwata‐Gonokami

Published Aug 15, 1999• 156 citations
Publisher

Abstract

We propose and demonstrate a novel far-off-resonance optical dipole trap (FORT) that is compatible with Doppler cooling. Strontium atoms are magneto-optically cooled and trapped by the spin-forbidden transition 1 S 0 - 3 P 1 at 689 nm, while the tight-focused infrared laser couples these two states to the respective dipole-allowed singlet and triplet states to generate the same amount of light shift. These designed light shifts for the cooling transition, enabling simultaneous Doppler cooling and dipole trapping, lead to the efficient loading of cooled atoms into the FORT with a few recoil energies. This scheme provides an important step towards the realization of quantum degenerate strontium atoms by purely optical means.

DOI
10.1143/jpsj.68.2479

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Cooling and trapping of ultracold strontium isotopic mixtures

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We present the simultaneous cooling and trapping of an isotopic mixture in a magneto-optical trap and describe the transfer of the mixture into a conservative, far-off-resonant dipole trap. The mixture is prepared with a technique that applies to intermediate and heavy alkaline-earth-metal-like atoms. In this work, $^{88}\mathrm{Sr}$ and $^{86}\mathrm{Sr}$ are simultaneously loaded first into the magneto-optical trap operated on the $^{1}S_{0}\text{\ensuremath{-}}^{3}P_{1}$ spin-forbidden line at 689 nm, and then transferred into the dipole trap. We observe fast interspecies thermalization in the dipole trap which allows one to set a lower bound on the $^{88}\mathrm{Sr}\text{\ensuremath{-}}^{86}\mathrm{Sr}$ elastic cross section.

arXiv:physics/0608233•Physical Review A•Jun 13, 2005•53 citations
We propose and demonstrate a novel far-off-resonance optical dipole trap (FORT) that is compatible with Doppler cooling. Strontium atoms are magneto-optically cooled and trapped by the spin-forbidden transition 1 S 0 - 3 P 1 at 689 nm, while the tight-focused infrared laser couples these two states to the respective dipole-allowed singlet and triplet states to generate the same amount of light shift. These designed light shifts for the cooling transition, enabling simultaneous Doppler cooling and dipole trapping, lead to the efficient loading of cooled atoms into the FORT with a few recoil energies. This scheme provides an important step towards the realization of quantum degenerate strontium atoms by purely optical means.
Employing a far-off resonance optical-dipole trap (FORT), we attained a phase-space density exceeding 0.1, or an order to quantum degeneracy. Strontium atoms were magneto-optically cooled and trapped using the spin-forbidden ${}^{1}{S}_{0}{\ensuremath{-}}^{3}{P}_{1}$ transition and then compressed into a FORT that was designed to allow simultaneous Doppler cooling. We discussed that the phase-space density was finally limited by the light-assisted collisions occurring in the optical cooling.
We report on a narrow-line laser cooling and trapping of strontium atoms near quantum degeneracy. Employing a magneto-optical trap (MOT) on the spin-forbidden transition 1S0−3P1 at 689 nm, we have laser-cooled an atomic sample down to the photon recoil temperature of 400 nK with a phase space density of 10−2. The atoms were then compressed into a new type of far-off resonance optical dipole trap that was designed to allow simultaneous Doppler cooling, resulting in a phase space density of 10% to that required for quantum degeneracy. It is shown that this value is finally limited by light-assisted collisions occurring in the optical cooling. To reduce these inelastic losses, we applied evaporative cooling and demonstrated a creation of two-dimensional atomic gases.
This paper reports results of cooling and trapping of88Sr for future use in an optical lattice clock. Strontium atoms are cooled to ultra-cold temperatures using a two-stage cooling process. In the first stage, atoms are captured from a Zeeman-slowed atomic beam and cooled to 2 mK in a magneto optical trap operated on the1So-1P1transition at 461 nm. The second cooling stage utilizes the spin forbidden1So-3P1transition at 689 nm. To reach ultra-cold temperatures, 50 ms of broadband cooling is followed by 28 ms long single frequency cooling. The transfer efficiency from the first to the second cooling stage was measured to be 23%, leaving 7 x 106 atoms at a temperature of 1 muK. The atomic cloud is overlapped with a horizontally oriented 1-D far-off resonance optical dipole trap (FORT) at 813 nm with an effective potential depth of 25 muK. Due to equal light shifts of the1Soand the3P1state simultaneous Doppler cooling during the atom loading into the FORT is possible which leads to a transfer efficiency into the dipole trap of up to 60%.
Summary form only given. All-optical production of quantum degenerate atomic gases has been one of the long-standing targets in laser cooling. We report a novel approach towards this direction based on a narrow-line laser cooling for strontium atoms. By employing two transitions with markedly different dipole moments, i.e., the allowed /sup 1/S/sub 0/-/sup 1/P/sub 1/ (/spl lambda/=461 nm, /spl gamma/=2/spl pi//spl times/32 MHz) and the spin-forbidden /sup 1/S/sub 0/-/sup 3/P/sub 1/ (/spl lambda/=689 nm, /spl gamma/=2/spl pi//spl times/7.6 kHz) transition, thermal strontium atoms were Doppler-cooled down to 400 nK or the photon recoil temperature, in a magneto-optical trap (MOT). Presents the corresponding energy levels for cooling and trapping. The use of a narrow spin-forbidden transition successfully reduced the radiation trapping effects at a high atom density over 10/sup 12//cm/sup 3/, thus enabling us to attain the hitherto high phase space density of 0.01 in a MOT.
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 present the simultaneous cooling and trapping of an isotopic mixture in a magneto-optical trap and describe the transfer of the mixture into a conservative, far-off-resonant dipole trap. The mixture is prepared with a technique that applies to intermediate and heavy alkaline-earth-metal-like atoms. In this work, 88Sr^{88}\mathrm{Sr}88Sr and 86Sr^{86}\mathrm{Sr}86Sr are simultaneously loaded first into the magneto-optical trap operated on the $^{1}S_{0}\text{\ensuremath{-}}^{3}P_{1}$ spin-forbidden line at 689 nm, and then transferred into the dipole trap. We observe fast interspecies thermalization in the dipole trap which allows one to set a lower bound on the $^{88}\mathrm{Sr}\text{\ensuremath{-}}^{86}\mathrm{Sr}$ elastic cross section.