Reduction of tip–sample interaction forces for scanning near-field optical microscopy in a liquid environment

Tip-to-sample shear force distance control of a scanning near-field optical microscope (SNOM) is more critical to implement in a liquid environment than in air. The higher viscosity of the medium increases the interaction forces between the tip and the sample and may even damage the sample in the case of soft materials. In this work we measure the decrease of the quality factor of a vibrating fiber when it is immersed in water. The main loss of quality is already observed for situations where the part of the fiber dipped in the water is only a few tens of microns long. We propose a new experimental set-up which maintains a small (<50 μm) and stable depth of the fiber in water, independent from the tip to sample distance. The quality factor of the fiber tip is then Q=52, only five times smaller than in air. Topographical and near-field optical images of a soft hydrogel are obtained without inducing any damage on the observed material. Such images are impossible to obtain with a SNOM tip totally immersed in water, due to the very soft surface of the hydrogel which contains 70% of water.