Prospects for rapid deceleration of small molecules by optical bichromatic forces

M. A. Chieda and E. E. Eyler

Phys. Rev. A 84, 063401 – Published 2 December 2011

Abstract

We examine the prospects for utilizing the optical bichromatic force (BCF) to greatly enhance laser deceleration and cooling for near-cycling transitions in small molecules. We discuss the expected behavior of the BCF in near-cycling transitions with internal degeneracies, then consider the specific example of decelerating a beam of calcium monofluoride molecules. We have selected CaF as a prototype molecule both because it has an easily accessible near-cycling transition and because it is well suited to studies of ultracold molecular physics and chemistry. We also report experimental verification of one of the key requirements, the production of large bichromatic forces in a multilevel system with significant energy level splittings, by performing tests in an atomic beam of metastable helium.

Received 17 August 2011

DOI:https://doi.org/10.1103/PhysRevA.84.063401

Authors & Affiliations

M. A. Chieda and E. E. Eyler

Physics Department, University of Connecticut, Storrs, Connecticut 06269, USA

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Vol. 84, Iss. 6 — December 2011

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Images

Figure 1
Simplified sketch of the optical bichromatic force on a diatomic molecule. Two-color beams impinge from each direction, offset by frequencies $\pm δ$ from resonance at $ω_{0}$ . For large molecular velocities $v$ , Doppler offsets of $\pm k v$ must also be added. The frequency pairs at $\pm δ$ give rise to a series of rf beat notes, each with a pulse area of approximately $π$ . Alternating cycles of excitation from the right and stimulated emission from the left produce a large decelerating force directed to the left. The direction is controlled by the relative phase of the beat notes from the right and the left.Reuse & Permissions
Figure 2
Rotationally closed transition from the $N^{''}$ = 1 level of the $X^{2} Σ^{+}$ , $v^{''}$ = 0 ground state of CaF to $J^{'}$ = 1/2 in the $A^{2} Π_{1 / 2}$ , $v^{'}$ = 0 state. The corresponding transition in SrF was used for laser cooling in Ref. [3]. The ground-state fine structure and hyperfine structure give rise to a total of 12 $m_{F^{'}}$ and $m_{F^{''}}$ levels, not shown. In traditional notation [8] this is a combination of the near-degenerate $Q_{12}$ (0.5) and $P_{11}$ (1.5) branches.Reuse & Permissions
Figure 3
Arrows show the four quasicycling transitions between the $m_{F^{''}}$ and $m_{F^{'}}$ sublevels of the $Q_{11}$ (0.5) branch in CaF, when illuminated with $π$ -polarized light. All four line strengths are the same. Energies are not drawn to scale. Radiative decay to $N^{''} = 2$ is allowed, so a rotational repumping laser tuned 62 GHz to the red is required.Reuse & Permissions
Figure 4
(a) Relevant energy levels in metastable helium, showing the two BCF frequency components and an off-resonant coupling to the $2^{3} P_{1}$ state, discussed in the text. The distant $J^{'} = 0$ fine-structure level plays no significant role and is not shown. (b) Zeeman sublevels, with vertical solid arrows indicating allowed transitions for $π$ -polarized light, labeled with their relative transition strengths [31]. The vertical dashed lines indicate transitions giving rise to off-resonant excitation and ac Stark shifts, and the angled dashed line indicates the transition excited by $σ^{+}$ polarization in the conventional BCF configuation for He*.Reuse & Permissions
Figure 5
Experimental configuration for observing the BCF in atomic He with $σ^{+}$ - and $π$ -polarized light. The $λ / 4$ retarders inside the vacuum chamber are removed to produce $π$ polarization. Polarizing beamsplitter cubes are denoted by PBC, and a beamsplitter by BS.Reuse & Permissions
Figure 6
Comparison of bichromatic slowing of metastable helium using $σ^{+}$ polarization (top) and $π$ polarization at twice the laser irradiance (bottom). Effects of the BCF are observed between about 700 and 1050 m/s, but are confined to about 20 $%$ of the atoms within the BCF velocity range, because of limited transverse overlap with the tightly focused lasers. The additional shifts in the velocity profile at 1050–1300 m/s are not bichromatic in nature—they arise from the strongly saturated force from one of the four component beams acting alone, producing small shifts for a large number of atoms.Reuse & Permissions

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