Fractional optical vortices in a uniaxial crystal

Abstract

We have analyzed the solutions to the vector paraxial wave equation in the unbounded uniaxial crystal in the form of the transverse electric (TE) and transverse magnetic (TM) mode beams transporting the fractional optical vortices in the circularly polarized components. We revealed that the TE and TM beams have an asymmetric structure in distributing the local elliptic polarization over the beam cross-section and form two sets of singular beams depending on the real or imaginary value of the free K parameter. We found that the fractional optical vortex born in the left-handed circularly polarized component of the beam with the real K parameter can exist in the form of a holistic structure within small crystal lengths much smaller than the Rayleigh length while the beams with the imaginary K parameter cannot maintain fractional optical vortices in the free state. However, such beam types can generate singly charged optical vortices in a far field. We also revealed that the energy efficiency and spin–orbit coupling are defined by the angular spectrum of the beam. The beam with the real K parameter is characterized by a broad spectrum of plane waves propagating at small angles to the crystal optical axis. The beams with the imaginary K parameter are shaped by two conical fans of plane waves. It is this circumstance that defines a very high value of the energy efficiency.