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Physical Review A

Double-structure potential due to multiphoton processes in a magneto-optical trap

Jin Woo Jun, Soo Chang, Ho Seong Lee, Vladimir G. Minogin, Wonho Jhe

Published Dec 1, 1999• 7 citations
Publisher

Abstract

We have derived ab initioanalytical results for the shape of the double-structure potential well for a (3+5)-level atoms in the magneto-optical trap (MOT). We show that for a case of a (3+5)-level atom and a one-dimensional ${\ensuremath{\sigma}}^{+}\ensuremath{-}{\ensuremath{\sigma}}^{\ensuremath{-}}$ laser field, the sharp bottom part of the MOT potential well results from the two-photon processes between the atomic ground-state magnetic sublevels. These nonlinear processes are, in particular, responsible for atomic temperatures below the Doppler limit which is due to the one-photon processes in the MOT. We also find that in more complicated atomic schemes, even-order multiphoton processes contribute to the sharp and narrow MOT potential well.

DOI
10.1103/physreva.60.4738

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We have derived ab initioanalytical results for the shape of the double-structure potential well for a (3+5)-level atoms in the magneto-optical trap (MOT). We show that for a case of a (3+5)-level atom and a one-dimensional \ensuremathσ+\ensuremath−\ensuremathσ\ensuremath−{\ensuremath{\sigma}}^{+}\ensuremath{-}{\ensuremath{\sigma}}^{\ensuremath{-}}\ensuremathσ+\ensuremath−\ensuremathσ\ensuremath− laser field, the sharp bottom part of the MOT potential well results from the two-photon processes between the atomic ground-state magnetic sublevels. These nonlinear processes are, in particular, responsible for atomic temperatures below the Doppler limit which is due to the one-photon processes in the MOT. We also find that in more complicated atomic schemes, even-order multiphoton processes contribute to the sharp and narrow MOT potential well.
A single-particle kinetic theory of the magneto-optical trap (MOT) for cold atoms is developed for a multilevel dipole interaction scheme. The theory analyzes the one-dimensional motion of (3+5)(3+5)(3+5)-level atoms in an inhomogeneous magnetic field and in the field of two counterpropagating circularly polarized laser waves. It is shown that in multilevel interaction schemes two-photon and generally even-order multiphoton processes are responsible for the formation of the narrow velocity and spatial structures in the radiation pressure force. The structures are located at zero velocity and near the origin of the trap. It is found that in the case of a (3+5)(3+5)(3+5)-level atom, single- and two-photon processes produce the two-component structure of the MOT potential well. Near the bottom of a conventional spatially broad potential well caused by one-photon processes, there is a much narrower well caused by two-photon processes. The double-structure MOT potential well is shown to be responsible for the two-component spatial distribution of the trapped atomic cloud. It is proposed that tight confinement of atoms near the narrow bottom of the MOT potential well can be used for the creation of a pointlike source of cold atoms for various applications in atom optics.
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 double structure potential predicted in the atomic cloud of a magneto-optical trap by one- (Doppler part) and two-photon process (sub-Doppler part) is experimentally observed and studied through the parametric excitation with the modulation of cooling laser intensities. Modulating the intensity of the cooling laser at around twice its resonant frequency of Doppler part, the atoms in Doppler part are divided and they oscillate reciprocally with finite amplitude due to the nonlinearity of the trap, and the others in sub-Doppler part are not influenced. By the modulation, we can investigate the sub-Doppler part spatially without Doppler part and also measure the temperature of sub-Doppler part independently.
Second-stage laser cooling of thulium atoms at the 530.7 nm transition with a natural linewidth of 350 kHz offers an interesting possibility to study different regimes of a magneto-optical trap (MOT). The intermediate value of the spectral linewidth of the cooling transition allows the observation of three distinct regimes depending on the intensity and detuning of the cooling beams, namely, the ``bowl-shaped'' regime when light pressure force competes with gravity, the ``double structure'' regime with interplay between the Doppler and polarization-gradient (sub-Doppler) cooling, and the ``symmetric'' regime when Doppler cooling dominates over sub-Doppler cooling and gravity. The polarization-gradient cooling manifests itself by a two-temperature momentum distribution of atoms resulting in a double-structure of the spatial MOT profile consisting of a cold central fraction surrounded by a hot halo. We studied the double structure regime at different saturation parameters and compared observations with calculations based on semiclassical and quantum approaches. The quantum treatment adequately reproduces experimental results if the MOT magnetic field is properly taken into account.