We propose a general theoretical approach to the modelling of complex dispersion characteristics of leaky optical modes operating in photonic crystal slab composed of two coupled high-index contrast gratings. Our analytical model, based on a non-Hermitian Hamiltonian, allows for a unified description of the wide family of optical modes which may be generated within uni-dimensional photonic crystal. Our theory stands for a variety of illustrative examples relating to the manipulation of bound states in the continuum and exceptional points, and can be used as a powerful enabler for the discovery of novel photonic species. Finally, as proof-of-concept, we demonstrate experimentally the formation of a Dirac point at the merging of three bound states in the continuum that is the most achieved photonic specie discussed in this work.
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