Direct measurement of individual optical forces and torque with optical tweezers

Manipulating particles with optical tweezers in combination with the ability to measure force and torque provides a powerful way to investigate objects on the microscale. In this thesis we present an optical-tweezers-based generally applicable holographic measurement method that overcomes many limitations and issues of existing techniques. Specifically, in the presence of several trapping spots it enables one to simultaneously measure all components of the optical force and the optical (spin and orbital) torque applied at an individual trapping site. Our approach uses the scattered light in the farfield to deduce the force and torque from the change of linear and angular momentum of light induced by the trapped particle. The holographic measurement relies on a detailed analysis of a single interference pattern formed in the back focal plane of a high-NA condenser lens to recover amplitude and phase of the far-field by means of an iterative algorithm, using the known positions of the optical traps as constraints. (author)