Scalar and electromagnetic diffraction point-spread functions for high-NA microlenses

Abstract

Scalar diffraction theories are often used to characterize optical imaging systems in terms of their scalar diffraction point-spread functions (PSFs). This works well at large f-numbers (low numerical apertures (NA)), since polarization effects can then be ignored. But as the f-number decreases, polarization effects become more important and a fully vectorial diffraction theory is required to determine the electromagnetic diffraction PSF of the system. In this paper we study a variety of low f-number refractive microlenses and characterize each in terms of its scalar as well as its electromagnetic diffraction PSF using the combined method of raytracing and diffraction (CMRD). We find that a polynomial aspherical surface gives less spherical aberration than a spherical or ellipsoidal surface. For the polynomial surface both the scalar and the electromagnetic PSFs are found to be asymmetric about the focal plane and to give focal shifts due to high-order spherical aberrations. The differences between scalar and electromagnetic diffraction PSFs are found to be small on the axis, due to symmetry, but for an f-number of 0.39, differences of up to are found off-axis.