Optimising forces and torques for optical micromanipulation

The motion of a colloidal particle in an optical field depends on a complex interplay between the structure of the field, and the geometry and composition of the particle. There are two complementary approaches to generating a particular force field. The first, involving shaping the optical field with e.g. a spatial light modulator, has been extensively developed. A second method, highlighted recently [J. Gluckstad, Nature Photonics, 5, 7-8 (2011)] involves sculpting of the particles themselves, and has received less attention. However, as modern two-photon polymerisation methods advance, this avenue becomes increasingly attractive for micromanipulation. In this paper we will show how computational methods may be used to optimise particle geometries to produce desirable patterns of forces and torques. In particular, we will examine the design of a constant force optical spring for use as a passive force clamp, and the effect of particle size on the trapping of prolate spheroids.