We present analytical and numerical results on the formation and properties of the leaky stop band in one-dimensional photonic lattices. At the second stop band, one band edge mode suffers radiation loss generating guided-mode resonance whereas the other band edge mode becomes a non-leaky bound-state in the continuum. We show that the frequency location of the leaky band edge, and correspondingly the bound-state edge, is determined by superposition of Bragg processes generated by the first two Fourier harmonics of the spatial dielectric constant modulation. At the closed-band state, we discover an analytic condition for the exceptional point where frequency is fully degenerate.
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