Mode matching methods for spectral and scattering problems

We present several applications of mode matching methods in spectral and scattering problems. First, we consider the eigenvalue problem for the Dirichlet Laplacian in a finite cylindrical domain that is split into two subdomains by a 'perforated' barrier. Using rather elementary methods, we prove that the first eigenfunction is localized in the larger subdomain, that is, its L2 norm in the smaller subdomain can be made arbitrarily small by setting the diameter of the 'holes' in the barrier small enough. This result extends the well-known localization of Laplacian eigenfunctions in dumbbell domains. We also discuss an extension to noncylindrical domains with radial symmetry. Second, we study a scattering problem in an infinite cylindrical domain with two identical perforated barriers. If the holes are small, there exists a low frequency at which an incident wave is almost fully transmitted through both barriers. This result is counterintuitive as a single barrier with the same holes would fully reflect incident waves with low frequencies.