We use a rigorous method for diffraction by a finite set of parallel cylinders to study the influence of defects in a photonic crystal. The method allows us to give an accurate description of all the characteristics of the electromagnetic field (near-field map, scattered field, and energy flow). The localized resonant modes can also be computed. We show some of their symmetry properties and the influence of coupling between two neighboring defects. Finally, an example is given, which shows that a slight local change in the crystal period can 1 used for the realization of devices that radiate energy in a very narrow angular range. # 1997 Optical Society of America [S0740-3232(97)01911-X]
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