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We demonstrate nanowatt-level saturated absorption using a sub-wavelength diameter tapered optical fiber (TOF) suspended in a gas of metastable xenon atoms. This ultralow-power nonlinearity is enabled by a small optical mode area propagating over a relatively long distance through the Xe gas. The use of inert noble gasses in these kinds of TOF experiments may offer practical advantages over the use of reactive alkali vapors such as rubidium.PACS numbers: 42.81.Qb, 42.62.Fi,42.50.Gy Sub-wavelength diameter tapered optical fibers (TOF's) and small hollow-core photonic bandgap fibers (PBGF's) enable low-loss propagation of evanescent and air-guided modes with very small mode areas over very long distances [1,2]. The interaction of these highly confined fields with atomic vapors can allow the realization of optical nonlinearities at remarkably low power levels [3]. For example, TOF's surrounded by rubidium vapor, and PBGF's filled with rubidium vapor, have recently been used to demonstrate saturated absorption, two-photon absorption, and a variety of other nonlinear effects at nanowatt and even "few-photon" power levels [4][5][6][7][8][9][10].Unfortunately, the tendency of Rb to accumulate on silica surfaces [11] severely limits the performance of these devices. In the case of TOF's, Rb accumulation causes a drastic loss of transmission [12], while in PBGF's it can limit the penetration depth into the hollow-core to several cm's [10]. The observation of these difficulties suggests the use of inert noble gases, rather than reactive Rb vapor, to improve these systems. Here we specifically investigate the use of xenon in TOF experiments. We observe saturated absorption at nanowatt power levels, which indicates the suitability of this system for further ultralow-power nonlinear optics applications.An overview of one particular set of Xe energy levels for these applications is shown in Figure 1. A weak electric discharge is used to excite Xe to the 6s[3/2] 2 metastable state, which has a long intrinsic lifetime of ∼43 s [13]. This metastable state serves as an effective "ground state" for an optical ladder transition at 823 nm and 853 nm that can then be used for the various two-photon nonlinearities. The transition rates of these lines are 3 × 10 7 s −1 and 2 × 10 6 s −1 , respectively, which are comparable to those of the well-known 5S 1/2 → 5P 3/2 → 5D 5/2 ladder transition at 780 nm and 776 nm in Rb [14]. In principle, metastable state atomic densities of 10 13 cm −3 can be achieved in Xe [15], which would allow the high optical depths (OD's) desirable for many ultralow-power nonlinear optics applications.The purpose of this initial work was to perform saturation spectroscopy of the 823 nm transition using a TOF surrounded by a relatively low-density gas of metastable Xe atoms. The ability to saturate this transition at ultralow power levels is an indicator of the overall strength of the atom-field interaction in this system. Figure 2 shows a measured transmission spectrum of the 823 nm line obtained by passing a...

has been co-authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

No abstract

We experimentally investigate ultralow-power saturation of the rubidium D 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.