Electromagnetic waves carrying orbital angular momentum (OAM), namely, vortex beams, have a plethora of applications ranging from rotating microparticles to high‐capacity data transmissions, and it is a continuing trend in manipulating OAM with higher degrees of freedom. Here, an approach to control terahertz (THz) near‐field plasmonic vortex based on geometric and dynamic phase is proposed and experimentally demonstrated. By locally tailoring the orientation angle (geometric phase) and radial position (dynamic phase) of aperture arrays embedded in an ultrathin gold film, the excited surface waves can be flexibly engineered to form both spin‐independent and spin‐dependent THz plasmonic vortex field distributions, resulting in multi‐degree of freedom for controlling OAM of THz surface plasmon polaritons (SPPs). Arbitrary OAM values of THz plasmonic vortex and coherent superposition between two OAM states are investigated based on near‐field scanning terahertz microscopy (NSTM) system. The proposed approach provides unprecedented freedom to modulate THz near‐field plasmonic vortex, which will have potential applications in THz communications and quantum information processing.
Metasurfaces have shown unprecedented capabilities in manipulating the phase, intensity, and polarization of electromagnetic waves. The coupling efficiency of surface plasmon polaritons is overcome by polarization sensitivity metasurfaces, but they face challenges in the application of high-intensity-based SPPs confined to the surface of a metal. Based on spiral arrays combined with a concentric groove, we experimentally demonstrate the application-oriented and polarization-controlled terahertz superfocusing by emitting high-efficiency radially convergent SPPs into free space to form a focal spot beyond the diffraction limit. The full wave at half maximum of the focal spot is 0.38λ, and it shows tunable intensity (the overall intensity of the focused spot can be tuned) by controlling the polarization state of the incident waves. This work paves a way towards imaging, data storage, and lithography.
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