Experimental studies on a dark magneto-optical trap for 85Rb

Laura Russell, Rajesh Kumar, Vibhuti Bhushan Tiwari, Síle Nic Chormaic

Published Jun 25, 2013• 0 citations

Abstract

This work is concerned with density enhancements in a laser-cooled sample of 85Rb using a dark magneto-optical trap (MOT). The density of atoms in a standard MOT is limited to approximately ~1011 atoms/cm3 due to radiation pressure and collisions. The dark MOT (DMOT) circumvents problems associated with radiation pressure in the atom cloud. To create a DMOT, a donut-shaped repump beam is projected on to the cloud of 85Rb. The dark region in the center of the donut is attenuated by ~10^(-4) compared to the bright outer ring. Atoms which encounter this dark region in the repump beam are scattered to the lower hyperfine ground level, F = 2, otherwise known as the dark hyperfine level. Additionally, a depump beam is used to force more atoms into F = 2. Results using free-space spectroscopy are presented to demonstrate the effect of the donut-shaped repump beam and the depump beam on the population distribution of the atoms in the hyperfine ground states. A tapered optical fiber is used to measure the quality of the DMOT via a parameter, p, and a strong population transfer from the bright, F = 3, to the dark, F = 2, hyperfine ground level is demonstrated. The tapered fiber, with a waist of ~ 1 micron, is made from commercially-available optical fiber and is used to detect fluorescence from the cold 85Rb coupled into the fiber guided modes. This work lays the foundation for optical nanofiber absorption spectroscopy with a small number of atoms in an optically-dense system.

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