Optical vortices generated by dislocations in a cholesteric liquid crystal

We report the observation of optical vortices in a laser beam propagating t hrough the stripe pattern of a cholesteric liquid crystal. The liquid cryst al is confined in a cell with homogeneous boundary conditions and forms a d iffraction phase grating. Optical vortices are produced by edge dislocation s of the cholesteric grating. The vortices show up as spots of zero light i ntensity in the diffraction maxima. There is one spot in each +1 and -1 dif fraction maximum and two spots in diffraction maxima +2 and -2. (C) 2000 Op tical Society of America. OCIS codes: 160.3710, 050.1940, 180.1790, 070.469 0.