Analysis of optical force and optical potential well for trapping particles near a light-illuminated slit using a finite-difference time-domain method

We analyze numerically the optical force and the optical potential well for trapping a circular-cylinder type object in a near zone of a light-illuminated metal slit. Our analysis is based on the finite-difference formulas of the optical force and the optical potential energy to he developed, a finite-difference solution of Maxwell's curl equations, and an absorbing boundary condition to define the problem in a finite region. We show that trapping forces induced by the field gradient produced in the vicinity of a slit can provide a two-dimensional potential well for trapping nanometric particles against a gravitational force. The results may be used to predict the properties of gradient-force traps in a more complicated system.