A silicon subwavelengh terahertz axicon has been designed, fabricated, and investigated by methods of numerical and optical experiments. The research has been performed on a free-electron laser workstation NOVOFEL (Budker Institute of Nuclear Physics of SB RAS, Novosibirsk). Diffractive and polarization features of realized element have been investigated.
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.
The diffractive optical element (DOE) for transforming of linearly polarized THz radiation beam into a cylindrically polarized beam is investigated. Optimal diffractive microrelief height is determined by numerical simulation.
A new approach to the implementation of sectorial plates for beam polarization conversion is proposed. Using a newly developed etching technology, a four-sector polarization converter integrated into a calcite crystal is implemented. A four-sector polarization converter, which provides a pairwise orthogonal polarization state of the sectors, is fabricated and experimentally characterized. A comparison is made of inter-sector joints of the integrated converter with those of a converter composed of individual wave-plate fragments. Analysis of the spectral properties of such a converter is carried out, wavelengths at which the necessary polarization conversion takes place are determined. The quasi-periodic repetition of phase matching conditions is experimentally shown to slow down with increasing wavelength.
An increase in the radiation power of terahertz (THz) sources requires the development of new optics working with it. The laser-assisted replication technique is proposed to fabricate the diamond cylindrical diffractive lens with a continuous profile for the THz range. The procedure involves the inverted structuring of a silicon substrate by laser ablation for its further replication to the diamond surface utilizing the chemical vapor deposition process. Testing of the fabricated lens performed with a free-electron laser at the wavelength of 141 µm has demonstrated high diffraction efficiency (
95
±
5
%
) and a good agreement between the measured and expected intensity distribution in the focal plane.
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