Observation of Two-Photon Absorption at Low Power Levels Using Tapered Optical Fibers in Rubidium Vapor

Nonlinear optical effects can be enhanced in tapered optical fibers with diameters less than the wavelength of the propagating light. Here we report on the observation of two-photon absorption using tapered fibers in rubidium vapor at power levels of less than 150 nW. Transit-time broadening produces two-photon absorption spectra with sharp peaks that are very different from conventional line shapes.

Figure 1

Two-photon absorption in a three-level atom. We typically held the detuning $\delta$ of the intermediate state constant while scanning the detuning $\Delta$ of the final state.

Figure 2

Optical setup used to observe Doppler-free, two-photon absorption in a tapered fiber. Counterpropagating beams passed through feedthroughs into a vacuum chamber containing a tapered fiber and rubidium atomic vapor and then back out again. $50/50$ couplers allowed the beams to pass through a free-space reference cell for comparison purposes.

Figure 3

A typical single-photon absorption scan used to verify the evanescent interaction between the guided mode in a tapered optical fiber and the surrounding Rb atoms. The transmission through a tapered optical fiber is plotted as a function of the detuning from the first excited state. The spacing of the dips is in agreement with the hyperfine splitting of the $5S_{1/2}$ ground state.

Figure 4

Doppler-free, two-photon absorption in a free-space rubidium reference cell with an off-resonant intermediate state. The fluorescence intensity from the decay of the upper atomic state is plotted as a function of the detuning (arbitrary offset). The broader peaks show the Doppler broadening for the copropagating beams while the sharp peaks correspond to the Doppler-free spectrum for the counterpropagating beams.

Figure 5

Resonant two-photon absorption in a tapered optical fiber. The percent transmission of the 776 signal through the tapered fiber is plotted as a function of its detuning from the upper atomic state. (a) 780 nm power level of 146 nW. (b) 780 nm power level of 726 nW.

Figure 6

Off-resonant, two-photon absorption observed in a tapered optical fiber. (a) Counterpropagating two-photon absorption with high input power showing a near-resonant dip and an off-resonant dip (labeled by the arrow). (b) Copropagating two-photon absorption showing increased linewidth and the inability to resolve the two lines.