Direct measurement of individual optical forces in ensembles of trapped particles

Abstract

Optical tweezers are a powerful tool to hold and manipulate particles on the microscale. The ability to measure tiny forces enables detailed investigations, e.g., of the mechanical properties of biological systems. Here we present a generally applicable method to simultaneously measure all components of the force applied to a specific particle in a trapped ensemble, or to a specific site of an extended object. This holographic force measurement relies on a detailed analysis of a single interference pattern formed in the far field to recover amplitude and phase of the field. It requires no information about size, shape, or optical properties of the particles and can be scaled to many traps—we show individual force measurements for up to 10 particles. In addition, we demonstrate force measurements when stretching a red blood cell, held directly by four traps. This method opens up a wealth of new opportunities made possible by localized quantitative force measurements in complex biological settings.