A coupled-Bloch-mode model for explaining various optical properties of strongly modulated subwavelength lamellar gratings is proposed. The model evidences the key role played by propagative Bloch modes, which bounce inside the grating and couple at the grating interfaces. Many optical properties of these gratings, such as their broadband reflectance, their polarization effect, and the presence of sharp anomalies, are understood as resulting from vertical resonances of the coupled-resonator modes. The approach provides a new insight on the physical mechanisms or optical behaviors observed, which is different from the classical study of the poles and zeros of the scattering operator.
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