Florian Flossmann

Light beams with fractional orbital angular momentum and their vortex structure

Light emerging from a spiral phase plate with a non-integer phase step has a complicated vortex structure and is unstable on propagation. We generate light carrying fractional orbital angular momentum (OAM) not with a phase step but by a synthesis of Laguerre-Gaussian modes. By limiting the number of different Gouy phases in the superposition we produce a light beam which is well characterised in terms of its propagation. We believe that their structural stability makes these beams ideal for quantum information processes utilising fractional OAM states.

Stokes parameters in the unfolding of an optical vortex through a birefringent crystal.

Following our earlier work [F. Flossmann et al., Phys. Rev. Lett. 95 253901 (2005)], we describe the fine polarization structure of a beam containing optical vortices propagating through a birefringent crystal, both experimentally and theoretically.We emphasize here the zero surfaces of the Stokes parameters in three-dimensional space, two transverse dimensions and the third corresponding to optical path length in the crystal. We find that the complicated network of polarization singularities reported earlier -lines of circular polarization (C lines) and surfaces of linear polarization (L surfaces) - can be understood naturally in terms of the zeros of the Stokes parameters.

Optical vortices in a Laguerre–Gaussian beam

We have investigated experimentally the evolution of the intensity distribution of an initial off-axis vortex with topological charge 1 in a Laguerre–Gaussian background beam. It was found that the non-generic dislocation surface of the Laguerre–Gaussian beam destroyed and new vortices are created during propagation in agreement with calculations. The innermost vortex moves to the centre of the beam, in contrast to the other vortices, which propagate to the periphery. The motion of the innermost vortex is explained by a model employing phase and intensity gradients where the changes of the background beam during propagation are taken into account.