Near-field Optical Measurements at the Liquid/Air Interface

Near-field optical microscopy offers the benefit of dramatically improved optical resolution [1] in exchange for working in close proximity (10-50nm) to a thin sample (<20nm). Monolayers at the liquid/air interface fulfill the thin sample criterion and are spectroscopically disctict from the supporting liquid. However, the mobility of the interface suggests that when a tapered probe approaches closely, attractive forces may cause the interface to jump up and submerge the probe at probe-sample spacings much greater than the required working distance for near-field optics. We detect the evanescent field decay above the liquid/air interfaces for glycerol, mineral oil, and water. The ability to measure these evanescent decays demonstrates that near-field approach to the liquid/air interface is possible. The measured evanescent decay lengths agree well with the predicted values. The presence of gravity waves in deep (1-2 cm) liquid samples sets a lower limit on the distance of closest approach but does not prevent near-field measurements.