Characteristics of unconventional Rb magneto-optical traps

Abstract

We study several new magneto-optical trapping configurations in ⁸⁷Rb. These unconventional MOTs all use type-II transitions, where the angular momentum of the ground state is greater than or equal to that of the excited state. Some use red-detuned light, and others blue-detuned light. The properties of these MOTs are strongly influenced by the balance between opposing Doppler and Sisyphus forces, and vary widely from one configuration to another. In the blue-detuned MOT, Sisyphus cooling dominates over Doppler heating for all relevant speeds and magnetic fields. We measure the capture velocity of this MOT as a function of intensity and detuning, finding a maximum of 3.8 ± 0.1 m/s. Atomic densities are particularly high in the blue-detuned MOT, and its lifetime is limited by collisions between the trapped atoms. We present measurements of the loss rate due to these ultracold collisions as a function of laser intensity and detuning. In the red-detuned MOTs, Sisyphus heating dominates at low speeds and Doppler cooling at higher speeds. Consequently, temperatures in the red-detuned MOTs are up to a thousand times higher than in the blue-detuned MOTs. One MOT forms large ring structures, with no density at the centre, showing how atoms driven towards a non-zero equilibrium speed remain trapped by orbiting around the centre. Another MOT demonstrates that magnetic mixing of the excited-state hyperfine levels can be an important mechanism in type-II MOTs.