Spectroscopic Mueller polarimeter based on liquid crystal devices

Abstract

A new spectroscopic Mueller matrix polarimeter based on liquid crystal devices is presented. Fast spectroscopic measurements (approx. 1 s) of a complete Mueller matrix with 1 nm resolution in the range from the visible to the near infrared can be obtained by the use of a CCD array coupled to a dispersion grid. The optical set-up is based on a polarization states generator (PSG)-sample-polarization states analyzer (PSA) configuration that can be adapted to work in transmission as well as in reflection mode. PSG and PSA are both identical and consist of a linear polarizer, two voltage driven liquid crystal devices and a waveplate. The liquid crystals were chosen to be ferroelectric, working essentially as retardation waveplates. The design of the polarimeter was based on an objective criterion that allowed to optimize the accuracy of the measurements. As a result, we measured a complete Mueller matrix with a relative error lower than 0.5%. The eigenvalue method previously reported provided a straightforward and robust way to calibrate the polarimeter without ambiguities using a small set of three simple reference samples (a linear polarizer, a waveplate and a reflecting surface). In this article, we present a detailed description of the optical setup, the calibration of the polarimeter and some examples of measurements. Whenever the polarimeter is used as an ellipsometer, it is possible to profit the redundant information given by the 16 Mueller matrix coefficients to enhance the accuracy and precision of the measured ellipsometric angles Ψ and Δ. For illustration, we compare the measurements obtained with the spectroscopic polarimeter to those of a standard spectroscopic ellipsometer.