Photonic Force Microscopy: Interferometric tracking behind phase relief structures

Optical trapping systems have been widely used for manipulation of small particles, for measurements of ultra-soft forces and in the last years also for surface probing. Similar to an AFM-cantilever, an optically trapped probe is scanned across a surface. This provides a surface relief map or –and this offers various new applications – a map of local interaction or local diffusion. A very powerful method to measure the displacement of the probe and therefore the interaction is a technique called Back Focal Plane Interferometry. Here the interference between unscattered light and light scattered at the trapped probe is used to measure its 3D position in the focal region of the laser beam with nm-resolution and at scan-rates of several hundred kilohertz. However, behind phase relief structures the interferometric tracking of the probe is disturbed by the additional phase shifts induced by the surface. In this talk we present a method to correct for these phase disturbances. We prove its effectiveness by an experiment, where one nano-sphere is rolled along a defined trace over another nano-sphere.