Direct Emission of Focused Terahertz Vortex Beams Using Indium‐Tin‐Oxide‐Based Fresnel Zone Plates

Terahertz (THz) vortex beams with helical wavefronts promise many important applications ranging from high‐speed wireless communications to super‐resolution imaging. Conventional methods to generate THz beams mostly rely on manipulating and converting the incident THz beams. Herein is proposed and experimentally demonstrated how the patterned indium tin oxide (ITO) film, i.e., ITO Fresnel zone plate (ITO‐FZP), enables direct emission of focused THz vortex beams with desired orbital angular momentums. Such THz vortex‐beam emitters are based on the straightforward nonlinear THz emission through the patterned ITO film under femtosecond laser excitation. The proposed approach greatly integrates THz emission and vortex‐beam generation processes, which is low cost, robust, and easy to fabricate.