Sign-indefinite transmission conditions and near-zero eigenvalues and resonances

Abstract

We consider the linear operator for metamaterial modeling $$L_{c,d }=(χ_{\RE^{3}\backslash\overline Ω}+c)\nabla\!\cdot (χ_{\RE^{3}\backslash\overline Ω}+d)\nabla\,,$$ where the material parameters $c>0$ and $d$ are constant on each connected component of the bounded domain $\Omega$, $d$ is negative on at least one component of $\Omega$ and transmission boundary conditions hold at the interfaces. It turns out that $L_{c,d}$ has, as in the sign-definite case, an absolutely continuous spectrum equal to $[0,+\infty )$; however, contrary to the latter, it has a non-empty point spectrum; in particular, we find conditions on the parameters so that zero is a threshold eigenvalue. In the case $\Omega$ is connected, we study how such a threshold eigenvalue behaves under $\epsilon$-small analytic perturbations of the parameters. We show that, depending on the position of the number $\frac12\,\frac{1+d(\ve)}{1-d(\ve)}$ relative to the nearest eigenvalue of the Neumann-Poincaré operator of the interface, the zero eigenvalue splits into two branches (analytic in ${\epsilon }^{1/2}$), consisting either of two branches of resonances, or of one branch of negative eigenvalues and one of resonances.

29 pages, latex