Generation of Bessel vortex beams in the subterahertz range using reflecting diffractive optical elements

In this work, we propose a simple method for generating Bessel vortex beams in the subterahertz (subTHz) range with the orbital angular momentum with l = 1 based on reflecting metal diffractive optical elements with a continuous helical microrelief. The elements are fabricated by micromilling in a polished duralumin substrate and by tin casting, and tested using a backward wave oscillator (wavelength lambda = 855 mu m). When using the micromilled element, Bessel vortex beams are shown to be generated and retain a Bessel intensity profile at a distance of 20 - 50 mm from the reflecting element, which is in good agreement with the results of numerical simulation. An experimental estimate of the energy efficiency of this element is 63%. When using elements made by tin casting, the vortex beams are generated with a distorted profile due to the presence of residual deformations of tin, which has plasticity. Due to their high conductivity, metallic reflecting elements can be used with high power density sub-THz radiation sources such as free electron lasers and gyrotrons.