  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.
