Generation of non-diffractive terahertz Bessel beams using 3D printed spiral zone plates

Due to their unique non-diffracting properties, Bessel beams have been studied across microwave to optical frequency regimes. Although several schemes for generating Bessel beams at microwave and terahertz (THz) frequencies have been demonstrated, these works have mainly focused on symmetric Bessel beams with little attention being given to asymmetric forms. In addition, existing methods are often technically involved, bulky, and costly. To address the gap in the generation of asymmetric beams and the identified challenges with contemporary generation schemes, we show that it is possible to produce both asymmetric and symmetric Bessel beams in a relatively simple and cost-effective manner using a planar Archimedean spiral zone plate. We fabricate this device using 3D printing and use it as a Bessel beam launcher operating at 0.2 THz. The zone plate generates an asymmetric Bessel beam close to the aperture through superposition of Bessel modes. As the axial distance is increased, the beam evolves into a symmetric Bessel beam.