Using rigorous coupled wave analysis (RCWA) and finite element method (FEM) simulations together, many interesting ellipsometric measurements can be investigated. This work specifically focuses on simulating copper grating structures that are plasmonically active. Looking at near-field images and Mueller matrix spectra, understanding of physical phenomena is possible. A general strategy for combatting convergence difficulties in RCWA simulations is proposed and applied. The example used is a copper cross-grating structure with known slow convergence. Baseline simulations on simple samples are provided for comparison and determination of FEM accuracy.