Optical properties of photonic crystal slabs with an asymmetrical unit cell

Using the unitarity and reciprocity properties of the scattering matrix, we analyze the symmetry and resonant optical properties of the photonic crystal slabs (PCS) with non-trivial unit cell. We show that the reflectivity does not change upon the 180^-rotation of the sample around the normal axis, even in PCS with asymmetric unit cell, whereas the transmissivity of the asymmetric PCS becomes asymmetrical if the diffraction or absorption are present. The PCS reflectivity peaks to unity near the quasiguided mode resonance for normal light incidence in the absence of diffraction, depolarization, and absorptive losses. For the oblique incidence the full reflectivity is reached only in symmetric PCS.