Investigation of a 85Rb dark magneto-optical trap using an optical nanofibre

We report here the first measurements on a dark magneto-optical trap (DMOT) of ⁸⁵Rb atoms using an optical nanofibre (ONF) with a waist of ∼1 μm. The DMOT is created using a doughnut-shaped repump beam along with a depump beam for efficient transfer of cold atoms from the bright hyperfine ground state (F = 3) into the dark hyperfine ground state (F = 2). The fluorescence from the cold ⁸⁵Rb atoms of the DMOT is detected by coupling it into the fibre-guided modes of the ONF. The measured fractional population of cold atoms in the bright hyperfine ground state (p) is as low as ∼0.04. The dependence of the loading rate of the DMOT on cooling laser intensity is investigated and also compared with the loading rate of a bright MOT. This work lays the foundation for the use of an ONF for probing a small number of atoms in an optically-dense cold atomic cloud.