Resonance behavior of diffraction on encapsulated guided-mode grating of subwavelength thickness

We study the resonances in transmission of a subwavelength dielectric lossless structure, periodic in one direction and infinite in the orthogonal (i.e. the effectively 2D problem). We are interested in the case of an encapsulated grid, deeply embedded into an optically-homogeneous surrounding medium. In the case, even a thin periodic structure, characterized by a positive relative excess of the refractive index, demonstrates full reflectance in a narrow bandwidth due to its waveguide capability. Here we systematically study all existing types of the diffraction spectra of the grids. We show that the diffraction type of the grid is determined by the geometric filling factor and the degree of asymmetry of the grid's cross section profile. These two parameters are some linear functions of the complex amplitude of the second spatial Fourier harmonic of the material distribution in the grid. The amplitude determines the coupling between two resonant guiding modes. The coupling is relevant for relatively small angles of incidence, for which it substantially modifies the diffraction spectra in the vicinity of the resonances. Also, we revealed a particular type of diffraction grids, which show suppressed resonances at a relatively large angle of incidence but have full reflectance in one resonance if the angle of incidence is sufficiently small.