The nearly free-photon approximation for two-dimensional photonic crystal slabs is investigated. Diffraction loss and mixing of different polarizations in the photonic crystal slabs are quantitatively estimated. The selection rules among irreducible representations of k groups are shown to be essential in these phenomena. Numerical calculations in terms of the finite-difference time-domain method for a photonic crystal slab with a weak periodic modulation show good agreement with the results of the nearly free-photon approximation.

• Received 25 January 2001

DOI:https://doi.org/10.1103/PhysRevB.64.045108