Radiation forces on an absorbing micrometer-sized sphere in an evanescent field

The vertical radiation force on an absorbing micrometer-sized dielectric sphere situated in an evanescent field is calculated, using electromagnetic wave theory. The present work is a continuation of an earlier paper [E. Almaas and I. Brevik, J. Opt. Soc. Am. B {\bf 12}, 2429 (1995)], in which both the horizontal and the vertical radiation forces were calculated with the constraint that the sphere was nonabsorbing. Whereas the horizontal force can be well accounted for within this constraint, there is no possibility to describe the {\it repulsiveness} of the vertical force, so distinctly demonstrated in the Kawata-Sugiura experiment [Opt. Lett. {\bf 17}, 772 (1992)], unless a departure from the theory of pure nondispersive dielectrics is made in some way. Introduction of absorption, i.e. a complex refractive index, is one natural way of generalizing the previous theory. We work out general expressions for the vertical force for this case and illustrate the calculations by numerical computations. It turns out that, when applied to the Kawata-Sugiura case, the repulsive radiation force caused by absorption is {\it not} strong enough to account for the actual lifting of the polystyrene latex or glass spheres. The physical reason for the experimental outcome is, in this case, most probably the presence of surfactants making the surface of the spheres partially conducting