Near-field optical microscopy at the liquid/air interface

We have used an uncoated, tapered single-mode optical fiber to measure evanescent decay lengths above the liquid/air interface for the liquids glycerol, mineral oil, and water. The ability to detect evanescent fields from above demonstrates that near-field microscopy of the liquid/air interface is possible. The evanescent decays are compared with predicted decay lengths and with the decay above a quartz prism. Gravity waves are seen in all three liquids, with wave amplitudes inversely proportional to the viscosity of the liquid. These waves interfere with but do not preclude near-field approach to the interface. We also describe the simultaneous measurements of evanescent decay and shear force strength as the probe approaches a quartz prism.