Optical forces and torques: Theory and experiment

My work involved using a single beam optical trap commonly called optical tweezers to hold and manipulate mirco/nano-scale particles. We attempted to trap several asymmetric particles made of various interesting materials. We successfully trapped C60 polymer nanorods with diameters of 800 to 1000 nm and lengths of 1-3 mum in water in a single beam optical trap. While in the trap, the nanorods were optically torqued by rotating the plane of polarization of the trapping light. The optical torque caused the rods to rotate at an angular speed determined by the rate of rotation of the trapping laser polarization. The torque on the rod that is needed to overcome the drag torque due to the medium can be found by measuring the rod rotation rate as a function of the polarization rotation rate, and then finding a theoretical fit to this curve. For comparison, we developed a method of computing optical forces and torques from the electric field inside the particle. Our method relies on a different expression, that we derived from the Maxwell equations, than that used by others. It consists of a single expression, involving a volume integral, that simultaneously includes the scattering and gradient forces that are frequently calculated in other methods. To evaluate our volume integral expression, we primarily used the discrete dipole approximation and finite difference time domain numerical techniques to find the electric field inside the particle. We tested our technique against published results, that use other theoretical methods, to good agreement. However, when applied to our experimental conditions, there was a discrepancy between experimental values and computed values for the forces and torques.