Direct holographic measurement of torque and individual forces in ensembles of trapped particles

We present a generally applicable holographic force measurement method to simultaneously measure all components of the individual forces applied to a specific particle in a trapped ensemble, or to a specific site of an extended object. We show individual force measurements for up to ten traps. As this method does not require information about size, shape, or optical properties of the particles, it is well suited to study biological specimen. We demonstrate disentangling the individual forces on a red blood cell stretched by four optical traps. The detailed information (amplitude, phase, and polarization) about the scattered light field, which we gain by our method, allows us to also uncover the optical torque exerted on trapped particles. We experimentally confirm that our method delivers accurate results in various settings.