Parveen,
Yeah..Definitely it is periodic in two dimension, but it has finite thickness in the plane perpendicular to the plane of the periodicity.
In 2D photonic crystal structure we generally consider that the structure is infinitely extended in the third direction. But if we restrict it into finite thickness then it becomes photonic crystal slab.
Therefore my query is whether it is possible to compute its band structure in optiFDTD as it differs significantly with its 2D counterpart. If it is yes, then provide a tutorial.