Controllable Singular Skeleton Formation By Means Of The Kummer Optical-Vortex Diffraction At A Rectilinear Phase Step

We study positions and morphology of optical vortices (OVs) within the field obtained after transmitting a circular single-charged OV-beam through the transparent screen with a rectilinear pi-phase step. Experimental results are obtained with the help of a programmable spatial light modulator which is used for the Kummer beam formation and for introduction of the pi-step phase difference at a desirable position within the incident beam cross section. The theory based on the Kirchhoff-Fresnel approximation shows a good agreement with the experimental data; peculiar features of the Kummer beam diffraction are elucidated in the course of confrontation against the results involving the Laguerre-Gaussian beam model with the same transverse size and spherical wavefront component. The diffracted field contains a system of interacting OVs (singular skeleton) which demonstrate a regular evolution (migration) within the diffracted beam cross section while the pi-phase step translates across the incident beam; depending on the step position, new OV pairs may emerge and/or annihilate in the topological reactions. The morphology parameters of the separate diffracted-field OVs (orientations and form-factors of the near-core equal-intensity ellipses) also depend on the stage of the OV evolution and indicate conditions favorable for the efficient trapping and guiding of microparticles. The results may be useful for the diagnostics of OVs, experimental measurements of phase objects and in micromanipulation techniques.