High-contrast gratings as generalized Fabry-Pérot interferometers

The extraordinary reflectivity and resonance features of high-contrast gratings are consequences of the interference of two grating modes. From this observation, an extension of the Fabry-P´erot interferometer to the bimodal case is proposed, in view of describing rigorously the physical mechanisms occurring in the grating. The closedform expressions of the interferometer reflection and transmission, obtained starting from a novel parametrization of the unitary symmetric 3 × 3 scattering matrix characterizing the bar-air interface, allow a complete exploration of the device parameter space, explaining and predicting phenomena such as ultra-broadband quasi-100% reflectivity.