Sub-Doppler Laser Cooling of Thulium Atoms in a Magneto-optical Trap

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We have experimentally studied sub-Doppler laser cooling in a magneto-optical trap for thulium atoms working at the wavelength of 410.6nm. Without any dedicated molasses period of sub-Doppler cooling, the cloud of 3× 10⁶ atoms at the temperature of 25(5)μK was observed. The measured temperature is significantly lower than the Doppler limit of 240μK for the cooling transition at 410.6nm. High efficiency of the sub-Doppler cooling process is due to a near-degeneracy of the Landé-g factors of the lower 4f¹³6s² (J=7/2) and the upper 4f¹²5d₃/₂6s² (J=9/2) cooling levels.