Quasi-achromatic rhomb compensator: Mueller matrix analysis versus azimuthal angle

Abstract

A quasi-achromatic rhomb compensator has been developed to address shortcomings of the bi-plate compensator in rotating compensator spectroscopic ellipsometry (SE) and consequently to improve the accuracy of such measurements over broad spectral ranges. The compensator implemented here is based on a three-reflection in-line design comparable to that put forth by Krasilov in 1963, but altered for greater simplicity. Our design consists of 30°-60°-90° and 60°-60°-60° fused silica prisms in physical contact that generate total first and third internal reflections from their interfaces to the ambient. The 30°-60°-90° prism also generates the second internal reflection via an aluminum coating on the longer side opposite the hypotenuse. Prior to implementing the compensator as the rotating element of a SE instrument, its performance has been characterized versus azimuthal angle over the photon energy range from 1.2 eV to 5.4 eV. For this purpose, the compensator was aligned on a rotatable mount and its complete Mueller matrix was measured at selected azimuthal angles. We present an analysis of ψ for the aluminum coating, which generates a compensator dichroic angle ψC deviating from the ideal value of 45° by values ranging from 0.3° to 1.2° with increasing photon energy. This analysis also predicts a (fused silica)/Al interface contribution to the retardance of ∼ 176° to ∼ 160°. In addition, we present an analysis of the fused silica stress-induced deviations of the measured Mueller matrices from their ideal forms, the latter based on a dichroic compensator oriented at different azimuthal angles. The results of these analyses are expressions for the Mueller matrix elements which must be applied as functions of angle in order to completely characterize the polarization modifying properties of the device in rotating compensator instrument configurations.