Theoretical calculations of optical force exerted on a dielectric sphere in the evanescent field generated with a totally-reflected focused gaussian beam

Abstract

We investigate theoretically the optical force exerted on a dielectric sphere in the evanescent field, by which the small particles can be pushed or pulled on the scale of the optical wavelength. In our approach we derive an expression of the evanescent field formed at the plane interface with a totally-reflected gaussian laser beam, assuming that the sphere does not affect the boundary conditions. We then solve the scattering problem for the evanescent field incident on a sphere to find the analytical formulas of the cartesian components of optical force. Numerical results show that the magnitude of the optical force for a sphere of radius a=0.5 λ is ≈ 10 −6 (dyne) in the evanescent field of a 1 W gaussian laser beam and the z-component of optical force F z changes the sign according to the refractive index n of the sphere.