Controllable singular skeleton formation by means of the Kummer optical-vortex diffraction at a rectilinear phase step

Abstract

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 π-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 π-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 π-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.

Design of broadband terahertz vector and vortex beams: II. Holographic assessment

“…As the beam propagates further ( = 250 and 300 mm in Fig. 6), it is possible to observe the THz field transformation into the second form, where the smoothing of the granular field structure begins: (i) speckles become larger and rotate according to the azimuth, in the direction of the energy circulation vector; (ii) simultaneously, an octagonal low-intensity zone is formed around the central beam point; (iii) the phase distribution reveals the births of a large number of singular point dipoles (pairs of helical phase dislocations with opposite topological charges) [49,50]. Finally, the speckled beam structure disappears, and the amplitude-phase distribution of the field becomes smoother.…”