Localised modes due to defects in high contrast periodic media via two-scale homogenization

Abstract: The spectral problem for an infinite periodic medium perturbed by a compact defect is considered. For a high contrast small $\ve$-size periodicity and a finite size defect we consider the critical $\ve^2$-scaling for the contrast. We employ (high contrast) two-scale homogenization for deriving asymptotically explicit limit equations for the localised modes (exponentially decaying eigenfunctions) and associated eigenvalues. Those are expressed in terms of the eigenvalues and eigenfunctions of a perturbed version of a two-scale limit operator introduced by V.V. Zhikov with an emergent explicit nonlinear dependence on the spectral parameter for the spectral problem at the macroscale. Using the method of asymptotic expansions supplemented by a high contrast boundary layer analysis we establish the existence of the actual eigenvalues near the eigenvalues of the limit operator, with %tight "$ \ve$ square root" error bounds. An example for circular or spherical defects in a periodic medium with isotropic homogenized properties is given and displays explicit limit eigenvalues and eigenfunctions. Further results on improved error bounds for the eigenfunctions are discussed, by combining our results with those of M. Cherdantsev ({\it Mathematika. 2009;55:29--57}) based on the technique of strong two-scale resolvent convergence and associated two-scale compactness properties.