Numerical evaluation of binary optical elements with subwavelength structures

The diffraction characteristics of a diffractive device with binary subwavelength gratings are evaluated by the use of FDTD method. The field patterns are computed for several grating parameters. The transmitted fields are analyzed in terms of the effective refractive indices, which are determined by the duty ratio and grating period and depend on the polarization of the incident light. Numerical results show that the device works as an optical lens.