A Sparse Finite-Element Method for Modeling Evanescent Modes in the Stopband of Periodic Structures

A novel approach is presented to solve an algebraic problem related to complex finite element analysis of periodic structures. This approach turns the quadratic eigen problem into a linear eigen problem of the same dimension and also exploits the sparsity of matrices which reduces the computation time enormously. An algorithm is also proposed that generates the dispersion diagram automatically. The method is applied to fairly simple singly, doubly, and triply periodic structures and the results agree well with those obtained by the dense-matrix approach. A more complicated device, a mushroom structure for use in the power distribution networks of electronic circuits, is also analyzed. The computational complexity of the new method is estimated to be n^1.6, where n is the dimension of the matrix problem.