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 by use of electromagnetic wave theory. The present study is a continuation of an earlier paper [J. Opt. Soc. Am. B 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 of describing the repulsiveness of the vertical force that was so distinctly demonstrated in the Kawata-Sugiura experiment [Opt. Lett. 17, 772 (1992)] unless a departure from the theory of pure nondispersive dielectrics is made in some way. Introduction of absorption, i.e., of a complex refractive index, is one natural way to generalize 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 it is applied to the Kawata-Sugiura case, the repulsive radiation force caused by absorption is 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, which make the surfaces of the spheres partially conducting. (C) 2003 Optical Society of America.