Azimuthal prism effect with partially blocked vortex-producing lenses

We report a new effect in which the diffraction from a partially blocked angular phase pattern occupying half of the input plane produces a partial vortex output pattern that is rotated by 90 degrees compared with the input. The energy is sent into a different quadrant of the output plane from the input plane. The rotation direction depends on whether the angular phase pattern is clockwise or counterclockwise. When we combine clockwise and counterclockwise angular phase patterns on separate horizontal halves of the input plane, we create an interference effect in one half of the output plane, while the other half remains dark.