Controlling the three-dimensional shape of a focal spot with a three-annular-zones-polarizing filter

Abstract

A linear polarization through a high numerical aperture yields an elliptical spot on the focal plane due to depolarization, which is explained by the vector diffraction theory. Depolarization results from orthogonal polarizations are generated after diffraction by a high numerical aperture lens. In this paper, it has been shown that the elliptical focal shape can be suppressed by adding perpendicular polarization to the incident beam. By a specially designed three-annular-zones-polarizing filter which modifies the linear polarization properly, the ellipticity and volume size of the focal spot are reduced from 0.395 to 0.10 and 0.4053 to 0.3813, respectively. Furthermore, the filter elongates the focal spot longitudinally by 50.9% and divides the focus into two spots longitudinally separated by 0.754.