Fresnel zone plate outperforms the metalens at high NA.

Abstract

Rigorous simulations challenge recent claims that metalenses outperform conventional diffractive lenses, such as fresnel zone plates (FZPs), in focusing efficiency at high numerical apertures (NAs). Across various lens diameters, FZPs exhibit a pronounced asymmetry in the shadow effect, leading to significantly higher focusing efficiency when optimally oriented. Extending this analysis, we show that conventional blazed gratings also surpass meta-gratings in efficiency. Since any linear optical element can be decomposed into local gratings, these findings broadly underscore the superiority of blazed structures over binary metastructures. Experimental characterization of an FZP with a diameter = 3 mm, a focal length = 0.2 mm, operating at λ = 634 nm, confirms the dependence of efficiency on illumination direction. We attribute this sensitivity to the axial symmetry of the FZP geometry-a feature not present in the metalens. Our results emphasize the need for rigorous, direct comparisons between meta-optics and traditional diffractive optics to ensure accurate performance assessments.