Transversal optical vortex V.A. Pas'ko, M.S. Soskin, M.V. Vasnetsov * Institute of Physics, National Academy of Sciences of Ukraine, Prospect Nauki 46, Kiev 28, Ukraine Received 14 May 2001; accepted 1 August 2001 Abstract A structure of wavefront edge dislocations and associated ``transversal'' optical vortices in an interference ®eld of two two-dimensional Gaussian beams is analyzed. It was shown that the optical vortex rotation is directed toward the area of higher phase velocity in the interference ®eld the origin of the phase velocity variation is due to the Gouy eect). The conditions for the reversal of the sign of rotation were found as well as for annihilation of two edge dislocations. Topological reaction of ``unfolding'' of an edge dislocation, which happens when vortex collides with a phase saddle, is studied in details. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 42.25 Keywords: Optical vortex; Phase singularity; Wavefront dislocation 1. Introduction It was shown in last decades that light ¯ux can produce vortices, similar to vortices in a liquid [1]. The vortex appears around a wavefront disloca- tion, which is a continuous line in space, where the ®eld amplitude vanishes and the phase is unde- termined singular). According to the classi®cation introduced by Nye and Berry [2], a monochro- matic light wave can possess two main types of phase singularities: screw wavefront dislocation and edge dislocation, while mixed edge-screw dis- location is most common situation. Nowadays the term ``optical vortex'' OV) introduced in Ref. [3] became widely used, re¯ecting the general feature of phase singularities: phase circulation around the dislocation line. Therefore pure screw dislocation is a core of a ``longitudinal'' OV, and edge disloca- tion produces a ``transversal'' OV, with respect to the wave propagation direction. In our recent paper [4] we have analyzed how an edge dislocation of a wavefront can be created in an interference ®eld of two paraxial Gaussian beams. Destructive interference of co-axial beams results in appearance of a zero-amplitude circle, whose radius and position are determined by rela- tive phases and amplitudes of the beams. Around the circle, which is a circular edge dislocation, a sub-wavelength area of circular light ¯ow was detected within a loop of a separatrix, which di- vides the light current within and outside the dis- location. The separatrix possesses a self-crossing in a point of the light current stagnation phase saddle) which, depending on the ratio of the interfering beams amplitudes, can be located as 15 October 2001 Optics Communications 198 2001) 49±56 www.elsevier.com/locate/optcom * Corresponding author. Tel.: +380-44-265-14-22; fax: +380- 44-265-15-89. E-mail address: mvas@iop.kiev.ua M.V. Vasnetsov). 0030-4018/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0030-401801)01487-0