Trapping and tracking a local probe with a photonic force microscope

Abstract

An improved type of scanning probe microscope system able to measure soft interactions between an optically trapped probe and local environment is presented. Such a system that traps and tracks thermally fluctuating probes to measure local interactions is called a photonic force microscope (PFM). The instrument can be used to study two-dimensional and three-dimensional surface forces, molecular binding forces, entropic and viscoelastic forces of single molecules, and small variations in particle flow, local diffusion, and viscosities. We introduce and characterize a PFM, and demonstrate its outstanding stability and very low noise. The probe’s position can be measured within a precision of 0.2–0.5 nm in three dimensions at a 1 MHz sampling rate. The trapping system facilitates stable trapping of latex spheres with diameter ${\mathrm{D=\lambda }}_0\text{/2}$ at laser powers as low as 0.6 mW in the focal plane. The ratio between the trapping stiffness and laser power was able to be optimized for various trapping conditions. The measured trap stiffnesses coincide well with the calculated stiffnesses obtained from electromagnetic theory. The design and the features of the novel PFM setup are discussed. The optical and thermodynamical principles as well as signal analysis are explained. Applications for three-dimensional, hard-clipping interaction potentials are shown. The technique discussed in this article and the results presented should be of great interest also to people working in the fields of classical optical tweezing, particle tracking, interferometry, surface inspection, nanotechnology, and scanning probe microscopy.