Hi, On a surface evanescent waves are created when total internal reflection occurs. However when this surface has periodic structures of appropriate periodicity (gratings, photonic crystals) the evanescent waves are "freed" from the surface and they propagate to the surrounding media. I am not able to intuitively see how a surface feature can change the intrinsic feature of a wave. By intrinsic feature, I mean the inplane wave vector of the wave. For evanescent waves, the inplane wave vector component will be greater than magnitude of wave vector. How does the presence of a surface periodic feature, decrease the inplane wavevector to a value lower than the magnitude of wave vector, so that it gets converted to a propagating wave ? Can somebody throw insight to this ? Thanks a lot!