Force measurement in the manipulation of complex samples with holographic optical tweezers

Holographic optical tweezers are a powerful tool for the dynamic manipulation of micron-sized objects with light. The measurement of the forces exerted by the traps by direct detection of beam momentum is not restricted to spherical samples, viscous environments or the assumption of a harmonic trapping potential. Here, we demonstrate that forces on complex beams, such as a multiple trap system or a cogwheel beam, or samples such as cylinders, can be measured with this method. It also allows quantitative studies inside the cell, where molecular motors exert characteristic forces within a heterogeneous and viscoelastic medium such as the cytoplasm.