Symmetry-Broken Metamaterial Absorbers as Reflectionless Directional Couplers for Surface Plasmon Polaritons in the Visible Range

Gradient-index meta-surfaces have been shown to have the ability to manipulate wavefronts at will in a reflectionless manner in the GHz range, including the extreme example of converting freely propagating waves into surface waves with high efficiency. Upon approaching the optical regime, the gradient-index concept encounters difficulties due to fabrication limitations. Here, it is theoretically and experimentally demonstrated that asymmetric, periodic, two-element metal-insulator-metal structures can serve as reflectionless directional converters between freely propagating visible photons and surface plasmon polaritons (SPPs). Coupling between propagating modes caused by the broken symmetry and localized modes generated by individual elements is shown to be the mechanism of this high-efficiency process, yielding an observed Rabi splitting of approximate to 135 meV. Direct experimental evidence is obtained for reflectionless (<8% measured reflectance) directional SPP coupling in the visible range, with directionality approaching 100%. These results are meaningful for varied applications in integrated nanoplasmonics, plasmonic logic, and plasmonic light harvesting.