Saturation of atomic transitions using subwavelength diameter tapered optical fibers in rubidium vapor

We experimentally investigate ultralow-power saturation of the rubidium D sub(2) transitions using a tapered optical fiber (TOF) suspended in a warm Rb vapor. A direct comparison of power-dependent absorption measurements for the TOF system with those obtained in a standard free-space vapor cell system highlights the differences in saturation behavior for the two systems. The effects of hyperfine pumping in the TOF system are found to be minimized due to the short atomic transit times through the highly confined evanescent optical mode guided by the TOF. The TOF system data are well-fit by a relatively simple empirical absorption model that indicates nanoWatt-level saturation powers.