Achromatic phase retardation by subwavelength gratings in total internal reflection

Achromatic phase retarders based on subwavelength dielectric diffraction gratings are studied numerically in total-internal-reflection configuration in both the visible and near-infrared regions. Rigorous diffraction theory is used to show that the examined gratings, with reasonable fabrication parameters, can act as quarter-wave plates over a wide range of wavelengths. The best results are obtained if the grating vector is perpendicular to the plane of incidence and the period is slightly below the cutoff period for zero-order operation. Properties of S matrices are used to demonstrate that the main contribution to the broadband effect is not the adjustment of the dispersion of effective refractive indices (like in the previously introduced achromatic diffractive elements), but phase-shift compensation effects caused by the back-boundary reflection.