Comparison of electromagnetic and scalar methods for evaluation of diffractive lenses

We compare scalar and electromagnetic methods for computing fields in the f ocal regions of diffractive lenses (DLs). The DL is treated locally as a li near grating with slowly varying period and groove orientation. Both the st andard complex-amplitude transmission method and rigorous electromagnetic d iffraction theory are employed to obtain the field immediately behind the D L. The field in the focal region is then evaluated using the first Rayleigh -Sommerfeld diffraction formula. It is shown that local optimization of the diffraction efficiency by modification of the grating profile improves the intensity at the focal point significantly for DLs faster than f: 2. Impro vements of 18% and 41% are obtained for four-level f: 1 and f: 0.5 lenses o f focal length 1 mm at lambda = 633 nm.