Closed-form bases for the description of monochromatic, strongly focused, electromagnetic fields

A procedure is proposed for generating rigorous closed-form orthonormal bases for the expansion of strongly focused (high-numerical-aperture), monochromatic, electromagnetic fields. The performance of three such bases is tested in terms of a parameter that determines their directional spread, for several truncation orders. Simple example fields corresponding to beams with differing polarizations focused by a thin lens are expanded in terms of these bases.