Two-temperature momentum distribution in a thulium magneto-optical trap

Abstract

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.

Deep Laser Cooling of Thulium Atoms to Sub-µK Temperatures in Magneto-Optical Trap

“…The transfer efficiency of atoms from the 1st to 2nd stage MOT is near 100%, and the final temperature of atoms is about 20 µK. More details can be found in [26,27].…”

Subnanometer optical linewidth of thulium atoms in rare-gas crystals

“…Even though the collisional transfer of angular momentum in Tm-He collisions was very small, the spin relaxation rates for Tm-Tm collisions was found to be large [26], ruling out the possibility of evaporative cooling in a magnetic trap. However, thulium has been successfully captured in a magneto-optical trap (MOT) using a strong transition at 410 nm and a secondary cooling transition at 530 nm [27][28][29]. The lifetime of the 2 F 5/2 state was found to be about 75 ms in solid helium [5] and 112 ms in an optical lattice [30].…”