Helical-wavefront laser beams produced with a spiral phaseplate

We demonstrate experimentally that a spiral phaseplate can convert a TEM00 laser beam into a helical-wavefront beam with a phase singularity at its axis. The diffractive-optical effect of the spiral phaseplate is implemented by index matching a macroscopic structure in an optical immersion. We discuss the optical properties of a helical wavefront beam produced this way by means of a mode analysis and by Fraunhofer diffraction calculations.