Resonant excitation analysis on asymmetrical lateral leakage of light in finite zero-contrast grating mirror

Resonant excitation analysis on asymmetrical lateral leakage of light in the finite zero-contrast grating (ZCG) mirror is demonstrated, and the size effect of the excited source is investigated. The dispersion equation of slab waveguide for the TM mode is proposed to evaluate the resonant condition of the ZCG mirror, and it is shown that the mirror effect of the infinite ZCG is resulted from the overlapping of order-mode resonance pairs. However, two leaky channels are excited in spectra of the finite ZCG mirror for the normal-incident Gaussian source, where bound states in the continuums (BICs) turn into quasi-BICs as the angular component is nonzero. The leaky channels are confined in the grating layer and the sublayer, and they are all transferred from the symmetrical mode to the asymmetrical mode along the lateral direction from the center of the ZCG. The asymmetrical loss of the lateral leakage of light relative to incidence direction, in principle, can break reflection reciprocity of the symmetrical system of the finite ZCG mirror. By changing the excited source from the grating side to the sublayer side, the lateral leakage of light of the finite ZCG mirror can be efficiently reduced.