Analysis of Enhanced Two-Photon Absorption in Tapered Optical Fibers
  We analyze the rate of two-photon absorption in tapered optical fibers with diameters less than the wavelength of the incident light. The rate of two-photon absorption is shown to be enhanced due to the small mode volume of the tapered fiber and the relatively large overlap of the evanescent field with an atomic vapor that surrounds the tapered region. The two-photon absorption rate is optimized as a function of the diameter of the tapered region. 
Hao You, S.M. Hendrickson, and J.D. Franson