Fabrication and characterization of diffractive phase plates for forming high-power terahertz vortex beams using free electron laser radiation
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Beams with orbital angular momentum are widely used in many spectral ranges, including the visible light, radio-frequency band and even soft X-rays. In this regard, the terahertz range is still underinvestigated; very few studies were devoted to the generation of terahertz vortex beams, and only the Lagguerre–Gaussian beams were generated to date. In this paper we describe the design and fabrication of silicon binary phase plates with a spiral pattern, which transform an incident plane wave with a wavelength of 141 μm into a vortex one. Using the Novosibirsk free electron laser as a source of CW radiation, non-diffractive Bessel vortex beams with topological charges of l = ±1 and l = ±2 and the average power of 30 W were first produced in the terahertz spectral range. The spatial characteristics of the beams were examined using a microbolometer array. Path-length/radius ratios of 180 and 90, respectively, were attained experimentally for these beams. The self-healing ability of the beams obtained was demonstrated.
KeywordsDiffractive phase plates Terahertz radiation Beams with orbital angular momentum Free electron laser
Operation of the user station “Terarad”, belonging to the Novosibirsk State University, was supported by the Ministry of Education and Science of the Russian Federation (MES RF). The diffractive optical elements (DOEs) were designed with the support of the MES RF (Project 1879) and fabricated with the support of RFBR Grant No. 13-02-97007. The equipment for characterization of DOEs was developed with the support of Russian Science Foundation (Project 14-50-00080). The study of beams with the orbital angular momentum was supported by RFBR Grant No. 15-02-06444. The experiments were carried out with the application of equipment belonging to the Siberian Center of Synchrotron and Terahertz Radiation.
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