Fabrication and characterization of diffractive phase plates for forming high-power terahertz vortex beams using free electron laser radiation

Abstract

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.