Sub-pico-Newton shear-force feedback system in air and liquid for scanning probe microscopy

Scanning near-field optical microscopy requires a performant sensor to measure the tip-to-sample distance. In this article, we report on a shear force detection scheme for scanning near-field optical microscopy applications. It is based on an all fiber low-coherence interferometer. This setup makes possible the measurements of the tip oscillation amplitude of less than 50 pm both in air and aqueous environment with a precision of 160 fm/Hz1/2, thus demonstrating the ability to perform topographic measurements both in air and in liquids with a resolution better than 1 nm in tip-sample distance. Stable feedback in air and fluids is obtained with tip-sample interaction forces below 1 pN.