Double-resonant extremely asymmetrical scattering of electromagnetic waves in periodic arrays separated by a gap

Very recent theoretical analysis of extremely asymmetric scattering (EAS) in nonuniform periodic arrays with step-like variations in the grating phase has led to a discovery of a radically new type of Bragg scattering-double-resonant extremely asymmetrical scattering (DEAS). DEAS in narrow nonuniform arrays is characterized by a unique combination of two simultaneous sharp resonances, one of which occurs at a certain resonant frequency determined by the Bragg condition, and the other takes place at a resonant value of the phase shift in the grating. The second resonance with respect to phase shift takes place on the background of an already resonantly large scattered wave amplitude. As a result, typical scattered wave amplitudes in DEAS are much larger (tens or hundreds of times) than those in EAS. It has also been shown that the main physical reason for DEAS is related with the diffractional divergence of the scattered waves inside the nonuniform array. The aim of this work is to demonstrate that the strong DEAS also takes place in two parallel strip-like oblique periodic arrays separated by a gap. The analysis is carried out for bulk TE electromagnetic waves, as well as for TE and TM modes guided by a slab.