Rigorous analytical modeling of high-aperture focusing through a spherical interface

High-aperture focusing through a spherical interface has been employed in optical data storage, photolithography, and especially microscopy. This paper first forms an approximate model, based on geometrical optics and Fourier optics, for evaluating focal fields of the focusing systems. This approximate model helps to clarify some doubts existing in literature. We then propose a rigorous model that is applicable to more general systems. Our model is based on multipole theory, which expands the electromagnetic fields into spherical harmonics.