FDFD with eigenvalue formulation

Hi everyone,

does anyone have experience or even a code for an FDFD eigenvalue formulation (using yee scheme of course)? I basically want to find cut off frequencies (eigenvalues) for an empty, dielectric waveguide using this formulation. I'm not sure how to construct my main Matrix for the eigenvalue equation, since I have a primary and a dual mesh.

Does the matrix row for the first point of the dual mesh come right after the last point of the primary mesh?

thanks a lot