Focusing performances of high numerical aperture Fresnel zone plates under different immersion media

The focusing performances of high-numerical-aperture (NA) Fresnel zone plates (FZP) under different immersion media are investigated by combining the vectorial angular spectrum (VAS) theory and the finite-difference time-domain method. It has been found that as the refractive index deviation increases, the focus approximately linearly shifts along the positive z-axis. Simultaneously, the transverse size of the focusing spot gradually decreases, and super-resolution focusing is realized. However, the peak intensity declines with an increase in the refractive index. In addition, when the deviation of the refractive index of the actual immersion medium from the design value is less than about 30%, VAS theory can correctly estimate the focusing light field distribution, except when the FZP has only one or two transparent annuli. On the contrary, VAS theory cannot achieve an accurate prediction when the deviation is larger. These findings will benefit the practical application of high-NA FZPs.