Techniques to Estimate Zone of Formation of Terahertz Bessel Vortex Beams

In this work, quantitative techniques to identify the zone of the Bessel beam formation using diffractive optical elements are proposed and tested. The methods are based on the analysis of the formation of the first ring of the Bessel mode in the so-called “non-diffractive” zone and the energy efficiency of diffractive optical elements to form the Bessel mode at different distances from the element. These methods were tested on numerical simulations of Bessel beams with orbital angular momentum and topological charge |l|= 3 and 9 at a wavelength λ = 141 µm. The Bessel beams were formed by reflective binary phase spiral axicons with diffractive grating period p = 2 and 3 mm. The experimental verification was performed with the terahertz radiation of the Novosibirsk free electron laser, and the test samples were reflective axicons, the relief of which was formed on silicon by reactive ion etching with subsequent metallization. The results of the simulation and experiments are in good agreement with each other, as well as with analytical estimates, which indicates the reliability of the proposed methods. It is expected that the methods considered in this work will help to simplify and automatize the search for the zone of the Bessel beam formation in different spectral ranges.