Avoided crossings and bound states in the continuum in low-contrast dielectric gratings

We consider bound states in the continuum (BICs) in low-contrast dielectric gratings. It is demonstrated that the BICs originate from the reduced guided modes on the effective dielectric slab with the permittivity equal to the average permittivity of the dielectric grating. In the case of isolated resonances, the positions of BICs can be found from two-wave dispersion relationships for guided leaky modes. In the case of the degeneracy between the two families of leaky modes, the system exhibits an avoided crossing of resonances. In the spectral vicinity of the avoided crossing the transmittance as well as the emergence of BICs is described in the framework of the generic formalism by Volya and Zelevinsky [Phys. Rev. C 67, 054322 (2003)] with a single fitting parameter.