Observation and explanation of polarization-controlled focusing of terahertz surface plasmon polaritons

Polarization-controlled focusing of surface plasmon polaritons (SPPs) is experimentally and theoretically investigated in the terahertz (THz) frequency range. The spiral phase distribution and the phase singularities, which, respectively, lead to the transverse shift of the focal spot for the circularly polarized THz radiation and the split of the focal spot for the linearly polarized THz radiation, are experimentally observed. Furthermore, an alternative theoretical model based on the Huygens-Fresnel principle and Fourier transformation of THz SPPs is proposed to analyze the transverse shift of the focal spot, which provides a simple way to understand the polarization-related SPP phenomena. This research is instructive for the design of polarization-based plasmonic devices.