Spatial Dynamics of a Radially Polarized Terahertz Laser Beam with a Phase Singularity

Analytical expressions are obtained that describe the nonparaxial diffraction in free space of the TM01 mode with radial polarization of the field of the dielectric waveguide resonator of a terahertz laser during its interaction with a spiral phase plate with different topological charge (n). The physical features of the obtained vortex beams during their propagation and tight focusing are studied by numerical simulation. The integral diffraction Rayleigh-Sommerfeld transforms are used to simulate the propagation and focusing of the obtained beams. In free space the use of the spiral phase plate at the waveguide output with a topological charge of n = 1 leads to a change in the transverse beam profile from annular to a beam that has a field maximum on the axis, and then for n = 2 again to annular. During focusing the transverse distribution of the total field intensity in the absence of a spiral phase plate has a ring structure. In this case there is a slight intensity on the axis due to the contribution of the longitudinal component of the field. The transverse profile of the beam changes in the same way as during its propagation when using a phase plate. In this case the phase front changes from spherical to spiral with the presence of two (n = 1) and four (n = 2) branching vortices.