Optical trapping of dielectric particles in arbitrary fields

We present a new method to calculate trapping forces of dielectric particle s with diameters D less than or equal to lambda in arbitrary electromagneti c, time-invariant fields. The two components of the optical force, the grad ient force and the scattering force, are determined separately. Both the ar bitrary incident field and the scatterer are:represented by plane-wave spec tra. The scattering force is determined by means of the momentum transfer i n either single- or double-scattering processes. Therefore the second-order Born series is evaluated and solved in the frequency domain by Ewald const ructions. Numerical results of our two-force-component approach and an esta blished calculation method are compared and show satisfying agreement. Our procedure is applied to investigate axial trapping by focused waves experie ncing effects of aperture illumination and refractive-index mismatch. (C) 2 001 Optical Society of America.