Higher-order extremely asymmetrical scattering

Extremely asymmetrical scattering (EAS) is a type of Bragg scattering in periodic gratings, that occurs when the diffracted order satisfying the Bragg condition (scattered wave) propagates parallel to the grating boundaries. Previous studies were concerned only with first-order EAS that was shown to be a highly unusual type of scattering, characterised by a strong resonant increase of the scattered wave amplitude (i.e. the amplitude of the first diffracted order). In this paper, a rigorous numerical study of higher-order EAS is presented for the case of bulk TE electromagnetic waves in planar holographic gratings of various amplitudes and widths. In particular, it is demonstrated that typical scattered wave amplitudes in higher-order EAS are significantly smaller than those in first-order EAS, and display strongly different dependencies on grating width, grating amplitude, distance from the front grating boundary, etc. Similar to first-order EAS, second-order EAS is shown to be strongly sensitive to small variations of mean structural parameters at the grating boundaries. EAS in non-sinusoidal gratings is investigated in detail with special focus on the transition between first-order EAS and second-order EAS in such gratings. Tolerance of second-order EAS to the presence of the second grating harmonic is analysed. The effect of phase of the second grating harmonic on transitional EAS is investigated. Physical explanation of the predicted effects is presented.