**2. Two dimensional metallic PhCs for maximum full band gap**

One of the main issues when dealing with two dimensional PhCs is the maximum full band gap that the crystal can provide. Specifically, in 2D the crystal manifests two non degenerative polarizations known as TE and TM. The former refers to solutions with the electric field *in* the plane of symmetry of the crystal, the latter with the magnetic field in the same plane. In order to produce a *full* band gap, both TE and TM must provide an optical gap in the *same* spectral region. This is not an easy task and different solutions have been proposed (Joannopoulos et al., 1995), however mostly referring to dielectric PhCs. Here we shall examine how metallic PhCs can improve the chance of a common band gap between TE and TM with respect to a dielectric structure.
