We study spatial optical solitons in a one-dimensional nonlinear photoniccrystal created by an array of thin-film nonlinear waveguides, the so-calledDirac-comb nonlinear lattice. We analyze modulational instability of theextended Bloch-wave modes and also investigate the existence and stability ofbright, dark, and ``twisted'' spatially localized modes in such periodicstructures. Additionally, we discuss both similarities and differences of ourgeneral results with the simplified models of nonlinear periodic mediadescribed by the discrete nonlinear Schrodinger equation, derived in thetight-binding approximation, and the coupled-mode theory, valid for shallowperiodic modulations of the optical refractive index.
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