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This paper introduces an efficient domain decomposition algorithm for the solution of time-harmonic electromagnetic fields arising in three dimensional, finite-size photonic band gap and electromagnetic band gap structures. The method is based on the finite element approximation and a nonoverlapping domain decomposition method. A set of "cement" unknowns on the inter-domain interfaces has been explicitly introduced to enforce the appropriate field continuities. The introduction of these extra unknowns allows for nonconforming/nonmatching triangulations across domain, eliminating the need for periodic mesh. To ensure and improve the convergence of the outer iteration loop, Robin transmission condition is used to communicate information across interfaces. The resulting system of equations is solved with a fast algorithm that loosely resembles the well known finite element tearing and interconnecting algorithm. In this algorithm, the method solves, in the preprocessing step, for the Robin primal subdomain problems multiple times, by exciting one dual unknown at a time. This step generates an iteration matrix that is then used to update the dual unknowns in the outer-loop iteration. The present method becomes extremely efficient for problems with geometric repetitions, such as, photonic and electromagnetic band gap structures.