The Design and Optimization of Focal Shift in Focusing Grating Mirrors

This paper utilizes the rigorous coupled wave analysis (RCWA) method and finite-difference time-domain (FDTD) algorithm to investigate the focal shift of focusing grating mirrors. By designing and optimizing various dimensions, preset focal length, winding number, and other parameters, the focusing effect of the mirror is observed. The results indicate that the phase focus shift is solely dependent on Fresnel number, while lens size, preset focal length, and incident wavelength are influential factors of Fresnel number. Based on this finding, a new guiding parameter FNg is proposed to better guide and optimize the focal shift phenomenon of grating mirrors and further enhance research on this phenomenon.