Calibration of focusing lens artifacts in a dual rotating-compensator Mueller matrix ellipsometer

A focusing lens consisting of two or more elements is widely used in ellipsometers for spatial resolution. In a typical ellipsometer layout, a lens is placed before the sample to focus light emerging to the sample, and another after the sample to collect the scattered light from the sample. Accurately calibrating the artifacts of the focusing lens is of great importance. In this paper, a method to improve calibration accuracy is proposed. A general analytical model is deduced to describe the artifacts of the focusing lens. This model can be applied to a system model of a dual rotating-compensator Mueller matrix ellipsometer. By adding a uniaxial crystal to calibration samples, with its optical axis neither parallel nor perpendicular to the incident plane, we can better separate artifacts from the lens before and after the sample. The system model also includes depolarization effects due to a finite numerical aperture (NA) that is related to the focusing lens. Similar effects due to the finite spectral bandwidth are also considered. Our simulation's results have validated the proposed method.