Littrow resonances in broad corrugated ribbons: Quasi-optics in the no man´s land between resonators and waveguides

We shall present the properties of "Littrow modes" in broad periodic waveguides that we recently demonstrated in SoI samples. In the band picture they correspond to multimode slow light. In the resonator picture, they mainly behave as Fabry-Perot resonators. However, there are several subtleties in their quasi-optics behaviour that we will discuss, from forbidden coupling to nonlinear conversion effects. We will stress also that they represent a general wave phenomenon.