Inverse optimization for designing wide angle diffractive optical element

The most frequently-used design method for diffractive optical elements (DOE) is scalar diffraction theory, but it is unsuitable for the design of sub-wavelength DOE with large diffraction angles. In this paper, we propose a hybrid iterative design method, which effectively utilizes the large-scale global optimization characteristics of scalar diffraction theory and the accuracy of the rigorous coupled wave analysis (RCWA) theory. A 5*7 beam splitter was designed to verify the proposed model. The design resolution is around 200nm with diffraction angle at 55.61°x61.82°. Several initial solutions were obtained by the non-paraxial scalar diffraction theory. Those solutions were later used as the inputs for continuous optimization through the Genetic Algorithm (GA). The RCWA model was used to analyze the diffraction efficiency and uniformity of the beam splitter DOE. All these structures were fabricated by lithography and duplicated by nano-imprinting process. The optical uniformity of 5*7 beam splitter pattern is improved to 46.30% from its initial values which is bigger than 70%. The testing data from RCWA optimized pattern matches with the design value from scatter plot analysis. This provides an effective method for the design of sub-wavelength DOE with large diffraction angle.