Experimental study of internal conical refraction in a biaxial crystal with Laguerre-Gauss light beams

The effect of internal conical refraction (CR) in a biaxial crystal was studied using Laguerre-Gauss light beams LG(0)(l) with l = 1 and 2, while the lowest-order LG(0)(0) beam was used as a reference. The transition from ordinary double refraction to CR was examined. It has been shown that double refraction of an LG(0)(l) beam forms two focal spots containing l dark stripes. These stripes evolve into l + 1 dark rings over an annular focal image when CR is established, and it results in a fine-structure of l + 2 bright focal rings with different intensities. In a sharp contrast to the lowest-order reference, the multiring focal structure has a distinct asymmetry with respect to the focal image plane. It has been shown that bright off-axis 'hot spot' can be formed on the far-field profiles of outgoing light beams when the biaxial crystal is slightly tilted, and a small angle between the propagation axis of the beam and the optic axis of the crystal arises. These off-axis light structures emerge as either a charge-one optical vortex or a zero-charge spot with annihilated vorticity. Polarization selection reveals J(1) or J(0) Bessel-like profiles of the corresponding 'hot spots', and a complex pattern of forked fringes in the dark region near the beam core.