Fractional optical vortices in a uniaxial crystal

TL;DR

This paper analyzes fractional optical vortices in uniaxial crystals, revealing their structure, stability, and energy efficiency, with implications for beam generation and manipulation in optical systems.

Contribution

It introduces a detailed analysis of fractional optical vortices in uniaxial crystals, highlighting their formation, stability conditions, and the influence of the K parameter on their properties.

Findings

Fractional vortices can exist in small crystals when K is real.

Imaginary K parameter prevents fractional vortices in free space but allows vortex generation in far field.

Energy efficiency is high for beams with imaginary K due to their conical wave structure.

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 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 the holistic structure within a small crystal lengths much smaller the Rayleigh while the beams with the imaginary K-parameter cannot maintain fractional optical vortices in free state. However, such beam types can generate the singly charged optical vortices in 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 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 define a very high value of the energy efficiency.