When the diameter of an optical fiber is reduced below the wavelength L of the light it guides, a portion of the light energy resides in an evanescent field outside the fiber. Further reducing the diameter to about L/4 results in an evanescent field that extends as far as 5L away from the fiber and practically carries all the mode energy. This extended mode is weakly bound but nevertheless can be guided over distances much longer than the corresponding Rayleigh range in free space.

We immerse such a nanofiber (200-nm diameter) in a warm atomic vapor (Rubidium) and study their interaction. We show that the extreme evanescent field provides for a unique combination of high optical depth and low transit-time broadening. We demonstrate lifetime-limited spectroscopy, coherent and narrow-band two photon excitations (EIT and EIA), and extreme optical nonlinearity. This configuration can be highly compatible with Rydberg-level excitations, thus paving the way towards quantum nonlinear optics at room temperature.