We demonstrate sub-terahertz vortex beam generation using a spiral metal reflector that can be used for both polarizations. A vortex beam is a ring-shaped beam that possesses sub-wavelength null in the center formed by angular phase variation. While the sub-terahertz vortex beams have gained increasing attention for a wide range of applications in sensing and communications, techniques for generating them are still accompanied by challenges. For example, the use of a phase plate, which is common in the optical regime, suffers from intrinsic losses of dielectric materials in the sub-terahertz regime. Moreover, holographic diffraction gratings, which could replace transmissive components, are inefficient and sensitive to the polarization. To reconcile these challenges, here we design a reflector type metal component with a spiral surface shape. We firstly derive a direct equation to design its shape. We then experimentally validate the design by mapping the radiation pattern of a vortex beam for the WR10 frequency band (75 to 110 GHz) in both of the orthogonal polarizations. The result confirms an inexpensive and versatile approach to generate a vortex beam in the sub-terahertz regime.
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