Thanks David !!
But the photonic crystal slab situation is little bit different. It is not a 2D or 3D photonic crystal. Instead it is referred to the quasi 3D photonic crystal. Do you understand what I am trying to say? The band diagram of that slab differs significantly as compared to 2D or 3D photonic crystal. This is very important as for real systems you cant use 2D phc. It will definitely have some finite thickness in the plane perpendicular to the plane of periodicity and this thickness is the main problem to consider.
So what I need is “How to define a cell of photonic crystal slab and corresponding band diagram? Give me a tutorial.”