Lens Design for High Numerical Aperture Achromatic Diffraction Imaging

An efficient design method based on the vector diffraction theory is presented for high numerical aperture achromatic imaging lenses (HADLs). Simultaneously achieving high numerical aperture and achromatic performance puts extremely demanding requirements on the design of HADLs. Therefor an efficient numerical technique is devised by combining the finite-difference time-domain (FDTD) method with improved binary search algorithm and thus sets up a routing to facilitate the smooth connecting between vector diffraction computing with optimizing iteration. Furthermore by taking advantage of lens symmetry when optimizing the phase transmission function, tremendous computational resources saves. By taking advantage of FDTD rigorous electromagnetic computing and choosing carefully parameters, search strategy and objective function, optimal solution is obtained and the search time is very short. To validate the proposed method, a HADLs with focal length f=3.5um and numerical apertures (NA) of 0.753 is designed and focusing efficiency of over 30% across the visible light wavelength is obtained by numerical evaluation. A brief comparing with scalar diffraction theory is also carried out.