Generation of single-charge optical vortices with an uniaxial crystal

We implement a novel experimental technique for generating mono- and polychromatic optical beams with on-axis single vortex by manipulating polarization singularities of light in biréfringent crystals. We demonstrate that, in contrast to the well-known optical quadrupoles generated by beams propagating along the optical axis of a uniaxial crystal, the beam bearing isolated single-charge on-axis vortex can be generated if the incident beam is tilted with respect to the optical axis at a certain angle.