BIC in waveguide arrays within a symmetry classification scheme

We study a photonic analog of a modified Fano-Anderson model -- a waveguide array with two additional waveguides and by using the coupled mode theory we calculate its spectral and scattering properties. We classify eigenomodes according to vertical symmetry of the structure given by self-coupling coefficients of the additional waveguides and establish the conditions for BIC existence. We use the Weierstrass factorization theorem and interpret the scattering spectra of the systems with broken symmetry in terms of the eigenmodes. The Fano resonance related with excitation of quasi-BIC is explained as arising from the interference between this mode and another leaky mode.