Anisotropic edge enhancement with spiral zone plate under femtosecond laser illumination

Fractional and de-centered phase spiral zone plates (SZPs) are proposed for anisotropic edge enhancement using a femtosecond laser. The transmission functions of the two types of phase SZPs are deduced and the diffraction distributions are theoretically analyzed and simulated as well. By setting the fractional topological charge p and the orientation angle ϑ of a fractional SZP (FSZP), the intensity and the direction of the anisotropic edge enhancement can be controlled. A de-centered SZP (DSZP) can be obtained by shifting the coordinates of the traditional phase SZP while the topological charge equals to 1. The intensity and direction of the anisotropic edge enhancement can be controlled by setting the displacement distance r and the azimuthal angle φ of a DSZP. The anisotropic edge enhancement of the two phase SZPs was experimentally demonstrated with a phase pattern and living biological cells under femtosecond laser illumination.