Lateral force measurement using a probe fiber as a microlens

Abstract

We present a lateral force sensing method for small lateral forces that uses an optical fiber probe as a microlens and measures displacement of the laser spot on a position sensitive detector. We demonstrate that arranging the focus point near the object focal point of the fiber improves the signal intensity and our method combined with synchronous detection can provide a detection limit of the order of 1 pm at a bandwidth of 1 Hz. This indicates that the method enables us to measure lateral force of the order of 1 pN. The method is not restricted by measurement frequency as is the case with tuning fork-based lateral force sensing, and will be a useful method in applications that aim to clarify the intermolecular interaction between the probe and sample by measuring the frequency response, such as nanotribology, nanorheology, interfacial science, and nanobiology.