Vertical transport of subwavelength localized surface electromagnetic modes

Transport of subwavelength electromagnetic (EM) energy has been achieved through near-field coupling of highly confined surface EM modes supported by plasmonic nanoparticles, in a configuration usually on a two-dimensional (2D) substrate. Vertical transport of similar modes along the third dimension, on the other hand, can bring more flexibility in designs of functional photonic devices, but this phenomenon has not been observed in reality. In this paper, designer (or spoof) surface plasmon resonators ('plasmonic meta-atoms') are stacked in the direction vertical to their individual planes in demonstrating vertical transport of subwavelength localized surface EM modes. The dispersion relation of this vertical transport is determined from coupled-mode theory and is verified with a near-field transmission spectrum and field mapping with a microwave near-field scanning stage. This work extends the near-field coupled resonator optical waveguide (CROW) theory into the vertical direction, and may find applications in novel three-dimensional slow-light structures, filters, and photonic circuits.