Hybrid-level Fresnel zone plate for diffraction efficiency enhancement

Abstract

The Fresnel Zone Plate (FZP) is widely used in a variety of applications. However, the most concerning drawback of the FZP is its low diffraction efficiency limited by fabrication capability. The diffraction efficiency of an FZP with a surface-relief profile can reach 100% at a specified diffracted order, however, this is difficult to engineer in a precise manner. In this paper, a hybrid-level FZP (HLFZP) is proposed to maximize the diffraction efficiency under a certain lithography condition. To theoretically verify this enhancement, the diffracted field of the HLFZP is derived by establishing a diffraction model. According to different machining capabilities, the design criteria for the level number distribution on the HLFZP are presented. Simulated results show significant improvement in imaging quality while comparing the diffraction efficiency of the specifically designed 8-4-2 HLFZP to that of the traditional 2-level FZP. Fabrication processes for the 8-4-2 HLFZP are developed, with experimental results indicating that the diffraction efficiency could be improved by 54% using the proposed HLFZP, compared to the traditional 2-level FZP. The root mean square (RMS) of the wavefront error for the 8-4-2 HLFZP is 1/20λ (λ = 632.8 nm), and imaging results are good as the predicted results. It is concluded that the proposed hybrid-level method is also promising for improving the diffraction efficiency of other diffractive optical elements.