Preprint of: T. A. Nieminen, H. Rubinsztein-Dunlop, and N. R. Heckenberg “Laser trapping of non-spherical particles” pp. 304–307 in G. Videen, Q. Fu, and P. Ch´ ylek (eds) Light Scattering by Nonspherical Particles: Halifax Contributions Army Research Laboratory, Adelphi, Maryland (2000) presented at 5th Conference on Electromagnetic and Light Scattering by Nonspherical Particles: Theory, Measurements, and Applications, Halifax, Canada (2000) Laser trapping of non-spherical particles T. A. Nieminen, H. Rubinsztein-Dunlop, and N. R. Heckenberg, Centre for Laser Science, Physics Department, The University of Queensland, Brisbane QLD 4072, Australia. tel: +61-7-3365-3405, fax: +61-7-3365-1242, e-mail: timo@physics.uq.edu.au Abstract Optical trapping, where microscopic particles are trapped and manipulated by light [1] is a powerful technique. The single-beam gradient trap (also known as optical tweezers) is widely used for a large number of biological and other applica- tions [2, 3]. The forces and torques acting on a trapped particle result from the transfer of mo- mentum and angular momentum from the trapping beam to the particle. Despite the apparent simplicity of a laser trap, with a single particle in a single beam, exact calculation of the optical forces and torques acting on particles is difficult, and a number of approximations are normally made. Approximate calculations are per- formed either by using geometric optics, which is appropriate for large particles, or using small particle approximations. Neither approach is adequate for particles of a size comparable to the wavelength. This is a serious deficiency, since wave- length sized particles are of great practical interest because they can be readily and strongly trapped and can be used to probe interesting microscopic and macroscopic phenomena. The lack of suitable theory is even more acute when the trapping of non-spherical particles is considered. Accurate quantitative calculation of forces and torques acting on non-spherical particles is of particular interest due to the suitability of such particles as microscopic probes. These calculations are also important because of the frequent occurrence of non-spherical biological and other structures, and the possibility of rotating or controlling the orientation of such objects. The application of electromagnetic scattering theory to the laser-trapping of wave- length sized non-spherical particles is presented. 1 Introduction Optical trapping, the trapping and manipulation of microscopic particles by a focussed laser beam, is a widely used and powerful tool. The most common optical trap, the single-beam gradient trap (optical tweezers) consists of a laser beam focussed by a lens, typically a high-numerical aperture 1 brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by University of Queensland eSpace