In this paper we address the question how to design photonic crystals that have photonic band gaps around a finite number of given frequencies. In such materials electromagnetic waves with these frequencies can not propagate; this makes them interesting for a large number of applications. We focus on crystals made of periodically ordered thin rods with high contrast dielectric properties. We show that the material parameters can be chosen in such a way that transverse magnetic modes with given frequencies can not propagate in the crystal. At the same time, for any frequency belonging to a predefined range there exists a transverse electric mode that can propagate in the medium. These results are related to the spectral properties of a weighted Laplacian and of an elliptic operator of divergence type both acting in L 2 (R 2 ). The proofs rely on perturbation theory of linear operators, Floquet-Bloch analysis, and properties of Schrödinger operators with point interactions.Here, the three-dimensional vector fields E and H are the electric and the magnetic field, respectively, ω > 0 is the frequency of the wave, ε is the relative dielectric permittivity and c > 0 stands for the Combining the statements of Theorem 3.1 and of Proposition 2.4 with the perturbation result [W, Satz 9.24 b)], we obtain the following claim on the spectrum of H r,λn . Proposition 3.3. Let 0 < λ 1 < λ 2 < · · · < λ N , let a > 0 be fixed and define η := 2π |Ω| + 1. Let the operator H r,λn be as in (1.7). Then there exist a lattice Λ and constants c 1 , . . . , c N (that appear in (1.5)) such that λ n − η − a, λ n + η + a ⊂ ρ (H r,λn ) for all sufficiently small r > 0 and all n ∈ {1, . . . , N }.