Guided Modes in Periodic Slabs: Existence and Nonexistence

For homogeneous lossless 3D periodic slabs of fixed arbitrary geometry, we characterize guided modes by means of the eigenvalues associated to a variational formulation. We treat robust modes, which exist for frequencies and wavevectors that admit no propagating Bragg harmonics and therefore persist under perturbations, as well as nonrobust modes, which can disappear under perturbations due to radiation loss. We prove the nonexistence of guided modes, both robust and nonrobust, in "inverse" structures, for which the celerity inside the slab is less than the celerity of the surrounding medium. The result is contingent upon a restriction on the width of the slab but is otherwise independent of its geometry. c S.P. Shipman and D. Volkov