THEORETICAL CALCULATIONS OF OPTICAL FORCE EXERTED ON A DIELECTRIC SPHERE IN THE EVANESCENT FIELD GENERATED WITH A TOTALLY-REFLECTED FOCUSEDGAUSSIAN-BEAM

We investigate theoretically the optical force exerted on a dielectric sphere in the evanescent field, by which the small particles can be p ushed or pulled on the scale of the optical wavelength. In our approac h 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 fi nd the analytical formulas of the cartesian components of optical forc e. Numerical results show that the magnitude of the optical force for a sphere of radius a = 0.5 lambda is approximately 10(-6) (dyne) in th e evanescent field of a 1 W gaussian laser beam and the z-component of optical force F(z) changes the sip according to the refractive index n of the sphere.