Alignment and controlled rotation of microparticles in Gaussian beam laser traps

It has been recognised that the ability to controllably rotate and orient optically trapped microparticles is a major advance in the manipulation possible with a laser trap, and a variety of schemes to achieve this have been proposed and tested. However, these generally require particular types of particles or spe-cial/multiple beams, greatly restricting their applicability. We present experimental results showing that it is possible to achieve controlled rotation and alignment of at least some non-absorbing isotropic particles in a single-Gaussian-beam trap. Our results suggest that this method might be of particular value in the manipulation of naturally occuring particles, including biological specimens. 1 Laser trappingoptical force and torque Optical forces have been widely used to trap and manipulate microscopic particles for many years (Ashkin 1970). More recently, there has been strong interest in the rotation of microscopic objects, trapped or otherwise. The ability to optically generate torque introduces the possibility of true three