In general, the substrate film is included in a practical Fresnel zone plate (FZP). The dependence of focusing characteristics on the incident direction of light illuminating the binary phase sub-wavelength FZP on substrate film are studied by using the finite-different time-domain method. The simulation results show that, in the range of effective etch depth, the intensity and size of FZP's focusing spot in the far-field region are insensitive to the incident direction. However, the focal length for the light illuminating from the etched structure side of the FZP is larger than that from the substrate film side of the FZP. For these two different incident directions, focal length decreases as the increase of etch depth. And for some special value of etch depth, for example, when the value equals to 700 nm, the depth of focus is quite great in the situation of light illuminating from the etched structure side and the reduction of focusing intensity and resolution of spot is within an acceptable range. The simulation results in this paper are useful for the FZP's applications in microscopy and photolithography.

sub-wavelength Fresnel zone plate, focusing, finite-different time-domain method, direction of incidence