V.N. Karazin Kharkiv National University, 4 Svobody Square, 61022 Kharkiv, Ukraine
^*Corresponding author e-mail: mykola.dubinin@karazin.ua

Abstract. Analytical expressions to describe nonparaxial diffraction of vortex laser beams during their propagation in free space are obtained. The beams excited by radially polarized TM_0m (m = 1, 2, 3) modes of a dielectric waveguide resonator of a terahertz laser, which interact with a spiral phase plate with an arbitrary topological charge (n) are considered. The simulation is performed using the Rayleigh–Sommerfeld integral theory. It is shown that a phase plate with topological charges n = 1, 2 forms a field in the Fresnel zone, in which an azimuthal component appears. In this case, preservation of the radial and longitudinal components of the field is observed and the contribution of the longitudinal component to the total beam power remains insignificant. For a phase plate charge n = 1, the maximum of the field intensity is located on the axis, whereas the field distribution for n = 0 and n = 2 becomes ring-shaped. The maximum values of the field intensity are observed at a charge n = 1, while they decrease at n = 2. Phase distributions of the field transverse components of vortex laser beams are visualized. At a topological charge n = 1, they become a one-lobe helical structure, and a two-lobe structure with more pronounced helical twists of the azimuthal component for all the studied modes is observed at n = 2.

Keywords: vortex beam, terahertz laser, dielectric resonator, modes, phase plate, propagation.

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