Plasmonic Luneburg and Eaton lenses for terahertz spoof surface plasmon polaritons

Combining the principle of gradient index optics and spoof surface plasmon polaritons (SSPPs), plasmonic Luneburg and Eaton lenses in the terahertz frequency range are demonstrated and dynamical modulation of SSPPs field is also realized. The lenses are composed of subwavelength metallic pillars. By analyzing the dispersion characteristics of the pillars, the relationship between the effective mode index and the width of the pillars is obtained. The desired index distribution of the lenses can be achieved by arranging metallic pillars with different widths. For the Luneburg lens, the SSPPs field can be effectively focused to a point around perimeter of the lens with a bandwidth of 100 GHz. By changing the incident direction of THz beam, the SSPPs fields are imprinted with oblique phases and thus can be dynamically coupled to different waveguides. Conversely, the plasmonic Luneburg lens can transform a SSPPs point source into a SSPPs plane wave. Moreover, because of the spin-dependent spiral phase introduced by the circularly polarized THz beam, the propagation direction can be adjusted by changing the polarization state of the beam. Similarly, plasmonic Eaton lenses are also designed to realize the 30 and 60 degrees bending of the SSPPs plane wave. This study may advance the development of miniaturized THz devices and on-chip THz communications.