摘要： Light polarization is a key factor of modern photonics. Tailoring surface plasmons (SPs) in anisotropically-shaped metallic nanostructures introduces the prospect of polarization control at small scale [1]. However, the resulting components remain much larger than the wavelength of light. Here, we present a travelling-wave helical plasmonic antenna (TW-HPA) that overcomes this limit [2]. Due to its non-resonant nature, it differs from existing helical plasmonic structures [3-5], thus extending the concept of travelling-wave helical antenna [6] to optics. Our TW-HPA consists of a narrow gold-coated wire wound up in a screw-like shape forming a tiny helix (Fig. 1a). The gold-coated wire sustains a cutoff-free axially symmetric travelling SPs [7], locally excited with the dipolar mode of a rectangular aperture nanoantenna right at the helix's pedestal. In the course of propagation, the plasmon wire mode acquires orbital angular momentum (OAM). Due to the sharp curvatures, the OAM of the SP mode match the spin angular momentum (SAM) of free-space propagating photons [8]. On the basis of this OAM-to-SAM transfer, individual TW-HPA can produce circularly polarized directional light on the subwavelength scale through a swirling-plasmon effect. Such TW-HPAs can then be closely packed to build micron scale arrangements of tiny circularly polarized light sources of desired handedness and tunable intensities, which could open new perspectives in a large panel of photonic applications requiring local addressing, such as detectors, displays, optomagnetic recording as well as quantum information. By optically coupling four TW-HPAs of opposite handedness (Fig.1b), we obtained a phase plate occupying a volume smaller than a cubic wavelength whose polarization properties have never previously been demonstrated. Switching between left and right circular polarizations (LCP and RCP) occurs when the incident linear polarization is rotated by an angle of 52°, instead of 90° as for standard quarter wave plates. Based on the spin-orbit interaction of light, our method is versatile, robust and leads to ultracompact plasmonic polarizers and unconventional phase plates. Taken as individual or coupled structures, TW-PHAs may pave the way towards highly integrated polarization-encoded optics, particularly for the generation and control of spin-encoded photon qubits in quantum information and optical spintronics.