Arbitrary Polarization Syntheses Based on Spin-Momentum Locking in Spoof Surface Plasmon Polaritons

Spin-momentum locking is universal and an inherent property of evanescent electromagnetic (EM) waves, which transfers the spinning or handedness of electromagnetic waves onto their propagation direction. This motivates an approach to investigate the polarization-controlled directional transmission or emission. Here, the spin-momentum locking is demonstrated in spoof surface plasmon polariton (SSPP) waveguides, and momentum-controlled radiation is further realized in SSPP-patch antennas. The study shows that the circular polarization of the SSPP-patch antennas originates from the spin-momentum locking in the SSPP waveguides and hence is determined by the propagation direction of the SSPP modes. Two examples are presented to achieve +/- 45 degrees dual-polarizations and vertical/horizontal dual-polarizations by combining the SSPP-patch antenna with hybrid couplers. Finally, a straightforward method is proposed for synthesizing the polarization emitted by the SSPP-patch antenna, allowing access to arbitrary polarization states on the Poincare sphere. The SSPP-patch antenna is polarization responsive, shedding light on chiral sensors, Stokes polarimetry, and polarization measurement. Hence, the spin-momentum locking and the related momentum-controlled radiations provide additional freedom to regulate the EM waves by engineering the helicity in the SSPP waveguide.