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Review of Scientific Instruments

A deep optical cavity trap for atoms and molecules with rapid frequency and intensity modulation

P. D. Edmunds, P. F. Barker

Published Aug 1, 2013• 8 citations
Publisher

Abstract

We describe a deep far-off resonance trap for metastable argon atoms utilizing a medium finesse cavity and a high input power (30 W) to produce trap depths of up to 11 mK. The depth can be rapidly modulated allowing efficient loading of the trap, characterization of trapped atom temperature, and reduction of intensity noise. We measure the change in radius of curvature of the mirrors due to heating by the high circulating intensity and show that trapping is not adversely effected by this for all input powers.

DOI
10.1063/1.4817311

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We describe a deep far-off resonance trap for metastable argon atoms utilizing a medium finesse cavity and a high input power (30 W) to produce trap depths of up to 11 mK. The depth can be rapidly modulated allowing efficient loading of the trap, characterization of trapped atom temperature, and reduction of intensity noise. We measure the change in radius of curvature of the mirrors due to heating by the high circulating intensity and show that trapping is not adversely effected by this for all input powers.
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This thesis describes the dipole trapping of both metastable and ground state argon atoms. Metastable argon atoms are first Doppler-cooled down to ∼80 μK in a magneto- optical trap (MOT) on the 4s[3/2]2 to 4p[5/2]3 transitions. These were loaded into dipole traps formed both within the focus of a high-power CO2 laser beam and within an optical build-up cavity. The optical cavity’s well depth could be rapidly modulated: allowing efficient loading of the trap, characterisation of trapped atom temperature, and reduction of intensity noise. Collisional properties of the trapped metastable atoms were studied within the cavity and the Penning and associative losses from the trap calculated. Ground state noble gas atoms were also trapped for the first time. This was achieved by optically quenching metastable atoms to the ground state and then trapping the atoms in the cavity field. Although the ground state atoms could not be directly probed, we detected them by observing the additional collisional loss from co-trapped metastable argon atoms. This trap loss was used to determine an ultra-cold elastic cross section between the ground and metastable states. Using a type of parametric loss spectroscopy we also determined the polarisability of metastable argon at the trapping wavelength of 1064 nm.
We trap cold, ground state argon atoms in a deep optical dipole trap produced by a buildup cavity. The atoms, which are a general source for the sympathetic cooling of molecules, are loaded in the trap by quenching them from a cloud of laser-cooled metastable argon atoms. Although the ground state atoms cannot be directly probed, we detect them by observing the collisional loss of cotrapped metastable argon atoms and determine an elastic cross section. Using a type of parametric loss spectroscopy we also determine the polarizability of the metastable 4s[3/2]24s[3/2{]}_{2}4s[3/2]2​ state to be $(7.3\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}39}\text{ }\text{ }\mathrm{C}\text{ }{\mathrm{m}}^{2}/\mathrm{V}$. Finally, Penning and associative losses of metastable atoms in the absence of light assisted collisions, are determined to be $(3.3\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\text{ }\text{ }{\mathrm{cm}}^{3}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$.
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 demonstrate a direct, fluorescence-free measurement of the oscillation frequency of cold atoms in an optical dipole trap based on a high-finesse optical cavity strongly coupled to atoms. The parametric heating spectra of the trapped atoms are obtained by recording the transmitted photons from the cavity with the trap depth is modulated by different frequency. Moreover, in our method the oscillation can be observed directly in the time scale. Being compared to the conventional fluorescence-dependent method, our approach avoids uncertainties associated with the illuminating light and auxiliary imaging optics. This method has the potential application of determining the motion of atoms with stored quantum bits or degenerate gases without destroying them.