General formulations for computing the optical gradient and scattering forces on a spherical chiral particle immersed in generic monochromatic optical fields

We present the Cartesian multipole expansion theory for computing the optical force acting on a spherical chiral particle immersed in generic monochromatic optical fields. The theory enables us to develop the general formulations for individually calculating the optical gradient and scattering forces (also known as the conservative and nonconservative forces) on a spherical chiral particle of arbitrary size. A set of analytical expressions are then derived for the gradient and scattering forces acting on a chiral particle in arbitrary optical field modeled by a series of homogenous plane waves. As examples of applications, we reveal that, in optical lattice composed of three interferential plane waves, the profiles of the in-plane optical gradient and scattering force acting on a spherical chiral particle show higher degree of symmetry and exhibit invariance with respect to the particle size, material composition, and chirality. The remarkable characteristics are totally masked in the undecomposed optical total force. The rigorous analytical decomposition of optical force sheds some more light on the physical understanding of light-matter interaction, it may also contribute significantly to the design of optical beams for achieving more diversified optical micromanipulation on chiral particles.