Light transmission through a slab of two-dimensional photonic crystal is known to present forbidden gaps. By a particular choice of he surface cut of the slab, it is possible to introduce modes guided in the vicinity of the crystal surface, which can have propagation constants lying inside the forbidden gap. These modes can be excited using additional diffraction gratings positioned onto the surfaces. This resonant excitation introduces defects in the gap that can lead to a narrow-band transmission with a 100% maximum. By working near normal incidence, it is possible to use the flattening of the mode dispersion curve near the Bragg cell boundaries and to reduce the requirements for beam parallelism, while preserving the strong spectral selectivity.
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