  We propose a new method based on variational principle for analysis of
photonic crystal (PC) slabs. Most of the methods used today treat PC slab as a
three-dimensional (3D) crystal and this makes them very time and/or memory
consuming. In this method we use Bloch theorem to expand the field on infinite
plane waves which their amplitudes depend on the component perpendicular to the
slab surface. By approximating these amplitudes with appropriate functions, we
can find modes of PC slabs almost as fast as we can find modes of a
two-dimensional (2D) crystal. Besides this advantage, we can also calculate
radiation modes with this method which is not feasible with 3D Plane Wave
Expansion (PWE) method.
