Optical forces on arbitrary shaped particles in optical tweezers

Lin Ling, Fei Zhou, Lu Huang, and Zhi-Yuan Li

Discrete dipole approximation (DDA) method is an efficient method for computing electromagnetic (EM) field of nanometer/micrometer-sized dielectric particles with arbitrary geometric shape and topology. In this work we employ the DDA method to calculate the optical force of dielectric shaped particles embedded in optical tweezers made from focused Gaussian laser beams. The EM force is calculated based on the self-consistent solution of EM field distribution and discrete dipole moment distribution within the particles. The DDA method agrees well with the Mie theory for spherical dielectric particles and this supports the effectiveness of the DDA method in handling optical forces in optical tweezers. The optical force for shaped particles such as cubes, rectangles, cylinders, and core-shell composite particles shows many interesting features. The force strongly depends on the orientation of the particle with respect to the laser beam propagation and polarization direction and the aspect ratio of the anisotropic particle. For a core-shell composite particle the zero-force balance point shifts from the particle center to its two sides. When an additional particle comes close a trapped particle, the perturbation effect strongly depends on the relative location of the center of the focused laser beam with respect to the two particles. Furthermore, the geometry of shaped particles not only affects the magnitude of the optical force but also influences the optical trap stiffness.

DOI