Comparative investigation of nonparaxial mode propagation along the axis of uniaxial crystal

摘要

We compare nonparaxial propagation of Bessel and Laguerre-Gaussian modes along the axis of anisotropic media. It is analytically and numerically shown that the nonparaxial laser modes propagating along the crystal axis are periodically oscillating owing to polarization conversion. The oscillation period for Bessel beams is inversely proportional to the square of the spatial frequency of the laser mode and the difference between the dielectric constants of an anisotropic crystal. So, for higher spatial frequency of Bessel beams, we will get shorter period of oscillations. For a linearly polarized light, there is a periodic redistribution of the energy between the two transverse components, and for a beam with the circular polarization, the energy is transferred from the initial beam to a vortex beam and backward. Similar periodic behavior is observed for the high-order in radial index Laguerre-Gaussian beams. However, it is true only at short distances. As the distance increases, the frequency of periodicity slows down and the beam is astigmatically distorted. We show that high-spatial-frequency nonparaxial beams can provide spin-orbit conversion efficiency close to 100% on small distances (tens of microns) of propagation along the axis of uniaxial crystals. It provides an opportunity of miniaturization of mode optical converters.