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 arbitrary object is determined by Maxwell's stress tensor. It is shown that in the limiting case of an infinitely extended object, the formalism renders the known expressions for radiation pressure. Similarly, in the small object limit, we obtain the familiar expressions for gradient and scattering forces. The theory is applied to calculate the trapping forces near a laser-illuminated metal tip.