GCforce: Decomposition of optical force into gradient and scattering parts

A MATLAB function GCforce is presented for the calculation of gradient and scattering parts of optical force (OF). The decomposition of OF into the gradient and scattering parts, or, equivalently, the conservative and nonconservative components, is of great importance to the physical understanding of optical micromanipulation. In this paper, we propose a formulation to decompose the OF acting on a spherical particle immersed in an arbitrary monochromatic optical field, based on the generalized Lorenz-Mie theory and the Cartesian multipole expansion approach. The expressions for the gradient and scattering forces are given explicitly in terms of the partial wave expansion coefficients of the optical field shining on the particle and the Mie coefficients of the particle. A MATLAB function GCforce.m is also presented for the calculation. The explicit and rigorous decomposition of the OF into conservative and nonconservative forces shed light on the understanding of light-matter interaction as well as contribute significantly to the designing of optical fields to achieve various optical micromanipulation.