Coherent terahertz beams with radial polarization of the 1st, 2nd, and 3rd orders have been generated with the use of silicon subwavelength diffractive optical elements (DOEs). Silicon elements were fabricated by a technology similar to the technology used before for the fabrication of DOEs forming laser terahertz beams with pre-given mode content. The beam of the terahertz Novosibirsk Free Electron Laser was used as the illuminating beam. The experimental results are in good agreement with the results of the computer simulation.

A technique of high-frequency laser ablation using a 2D beam scanner has been designed and applied to creation of a silicon diffractive optical element (DOE) with a continuous profile to focus a terahertz Gaussian beam into a square region. The microrelief of the resulting silicon DOE has been analyzed with a white-light interferometer and a scanning electron microscope. The distribution of radiation behind the DOE illuminated by a high-power beam of the Novosibirsk free-electron laser at a wavelength of 130 µm, recorded with a pyroelectric camera, has been demonstrated. The measured diffraction efficiency of the DOE is equal to 97 ± 2 % .

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In this paper, we experimentally demonstrated excitation of terahertz vortex surface plasmon polaritons by end-fire coupling of radially-polarized annular beams with orbital angular momentum (the wavelength was 141 µm, and the topological charges were ±3 and ±9) to a 70 mm long, simply connected axis-symmetric transmission line and their propagation to the end of the line and diffraction into a free wave possessing the same topological charge as the input beam. The diameter of the line exceeded greatly the radiation wavelength, and, in contrast to experiments with nanowires, no azimuthal electromagnetic modes existed. We observed that 18 plasmons, locally excited on the input face perimeter by a wave with topological charge of ±9, traveled rotating over the tapering cylindrical line and transformed into 18 lobes of the decoupled free wave. The evidence of the possibility of the transformation of a vortex beam into vortex plasmons and back into vortex beam with the conservation of the topological charge opens a way for the development of plasmonic communication lines with coding and decoding of information at the free-wave stage. Application of this technique to complex beams consisting of a combination of different vortex modes can be a technological base for the development of multiplex plasmonic communication lines in the terahertz range. Since wave characteristics are easier to record than plasmon characteristics, sorting of free waves by the topological charges can be used for demultiplexing combined plasmons in multiplex systems.