Forces in optical near-fields

In this article we use classical electrodynamics to derive the conservation law for linear momentum in an optical field. The net force exerted on an a rbitrary object is determined by Maxwell's stress tensor. It is shown that in the limiting case of an infinitely extended object, the formalism render s the known expressions for radiation pressure. Similarly, in the small obj ect limit, we obtain the familiar expressions for gradient and scattering f orces. The theory is applied to calculate the trapping forces near a laser- illuminated metal tip.