Reflective free-form optical elements for focusing of high-power THz radiation
Appearance of the sources of coherent and high power THz radiation  opened new horizons for investigations in this frequency range . High attention is focused on silicon diffractive optical elements (DOE), which are used for the beam manipulation [3-6]. The lithographic etching of a silicon substrate has been used in [3, 4] to fabricate binary relief of diffractive optical elements. Lithographic etching has disadvantages in the case of multilevel elements, because an expensive and complicated procedure of alignment of photomask is required. Binary (two-level) elements, in turn, have limited energy efficiency. The laser ablation technology has been used in [5, 6] for the fabrication of multilevel diffractive lens with high energy efficiency. However, diffractive optical elements are designed for working with monochromatic radiation of a fixed wavelength  only. The present talk is devoted to the fabrication of terahertz reflective free-form elements for transformation of high-power beams. Aluminium elements (spherical and cylindrical mirrors, reflective axicons) were fabricated by technology of micromilling. These optical elements were tested in the beam of the Novosibirsk Free Electron Laser at the wavelength of 129.5 μm. The measured diffractive efficiency of the spherical mirror (>94%) is in good agreement with both numerical calculations and theoretical predictions. Specific features and perspectives of the fabrication of optical elements by micromilling are discussed.
 B.A Knyazev, G.N. Kulipanov, N.A. Vinokurov, Novosibirsk terahertz free electron laser: instrumentation development and experimental achievements, Measurement Science and Technology, 21 (5), 054017, pp. 1-13 (2010)  G.N. Kulipanov, E.G. Bagryanskaya, E.N. Chesnokov, Yu.Yu. Choporova,V.V. Gerasimov, Ya.V. Getmanov, S.L. Kiselev, B.A. Knyazev, V.V. Kubarev,S.E. Peltek, V.M. Popik, T.V. Salikova, M.A. Scheglov, S.S. Seredniakov, O.A. Shevchenko, A.N. Skrinsky, S.L. Veber, N.A. Vinokurov, Novosibirsk Free Electron Laser—Facility Description and Recent Experiments, IEEE Transactions on Terahertz Science and Technology 5.5 pp. 798-809 (2015)  A.N. Agafonov, B.O. Volodkin, D.G. Kachalov, B.A. Knyazev, G.I. Kropotov, K.N. Tukmakov, V.S. Pavelyev, D.I. Tsypishka, Yu.Yu. Choporova, A.K. Kaveev, Focusing of Novosibirsk Free Electron Laser (NovoFEL) radiation into paraxial segment, Journal of Modern Optics, 63, 11,pp. 1051-1054 (2016)  A.N. Agafonov, B.O. Volodkin, A.K. Kaveev, B.A. Knyazev, G.I. Kropotov, V.S. Pavel’ev, V.A. Soifer, K.N. Tukmakov, E.V.Tsygankova, Yu.Yu. Choporova, Silicon diffractive optical elements for high-power monochromatic terahertz radiation, Optoelectronics, Instrumentation and Data Processing, 49, pp. 189-195 (2013)  B.O. Volodkin, B.A. Knyazev, V.V. Kononenko, T.V. Kononenko, V.I. Konov, V.S. Pavelyev, V.A. Soifer, K.N. Tukmakov, Yu.Yu.Choporova, Fabrication of a multilevel THz Fresnel lens by femtosecond laser ablation, Quantum Electronics, 45, 10, pp. 933 –936 (2015)  L. Minkevičius, S. Indrišiūnas, R. Šniaukas, B. Voisiat, V. Janonis, V. Tamošiūnas, I. Kašalynas, G. Račiukaitis, G. Valušis, Terahertz multilevel phase Fresnel lenses fabricated by laser patterning of silicon, Optics Letters, 42, 10, pp. 1875-1878 (2017).  V. A. Soifer, Computer Design of Diffractive Optics (Cambridge International Science & Woodhead, 2012).