Nearly free-photon approximation for two-dimensional photonic crystal slabs

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Abstract
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.