Abstract
Magneto-optical trapping forces for molecules are far weaker than for alkali atoms because the photon scattering rate is reduced when there are multiple ground states, and because of optical pumping into dark states. The force is further reduced when the upper state has a much smaller Zeeman splitting than the lower state. We use a rate model to estimate the strength of the trapping and damping forces in a magneto-optical trap (MOT) of CaF molecules, using either the A$^{2}\Pi_{1/2}$ - X$^{2}\Sigma^{+}$ transition or the B$^{2}\Sigma^{+}$ - X$^{2}\Sigma^{+}$ transition. We identify a new mechanism of magneto-optical trapping that arises when, in each beam of the MOT, two laser components with opposite polarizations and different detunings address the same transition. This mechanism produces a strong trapping force even when the upper state has little or no Zeeman splitting. It is the main mechanism responsible for the trapping force when the A$^{2}\Pi_{1/2}$ - X$^{2}\Sigma^{+}$ transition is used.
Highlights
There is currently great interest in cooling molecules to very low temperatures, motivated by a diverse range of applications [1]
For the real molecule the acceleration is greatly reduced. This is partly because the molecule has multiple levels in the ground state that all need to be driven with separate laser frequencies, and this reduces the maximum achievable scattering rate relative to that of a two-level system [9]
In this paper we explore this question and we identify a mechanism of magneto-optical trapping that can give strong trapping forces irrespective of the upper-state g factor, and that sidesteps the problem of optical pumping into dark states
Summary
There is currently great interest in cooling molecules to very low temperatures, motivated by a diverse range of applications [1]. Laser cooling, which has been used to cool atoms to ultracold temperatures for decades, is difficult to apply to molecules because it is necessary to address multiple vibrational branches, each requiring a separate laser Despite this difficulty, laser cooling has been demonstrated for the diatomic radicals SrF [2,3], YO [4], and CaF [5], and most recently a magneto-optical trap (MOT) of SrF molecules was demonstrated [6,7]. This is partly because the molecule has multiple levels in the ground state that all need to be driven with separate laser frequencies, and this reduces the maximum achievable scattering rate relative to that of a two-level system [9] This is compounded by optical pumping into states that are dark to the polarization of the laser beam that pushes displaced molecules back to the center.
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