High-efficiency surface plasmon meta-couplers: concept and microwave-regime realizations.

Surface plasmon polaritons (SPPs) and their low-frequency counterparts (i.e., spoof SPPs on artificial surfaces) have recently found numerous applications in photonics, but traditional devices to excite them (such as gratings and prism couplers) all suffer from problems of inherent low efficiency because the generated SPPs can decouple, returning to free space, and reflections at the device surface can never be avoided. Here, we propose a new SPP excitation scheme based on a transparent gradient metasurface and numerically demonstrate that it exhibits inherently high efficiency (~94%) because the designed meta-coupler suppresses both decoupling and surface reflections. As a practical realization of this concept, we fabricated a meta-coupler for operation in the microwave regime and performed near-field and far-field experiments to demonstrate that the achieved excitation efficiency for spoof SPPs reaches ~73%, which is several times higher than that achieved by other available devices in this frequency domain. Our findings can motivate the design and fabrication of high-performance plasmonic devices to harvest light-matter interactions, particularly those related to spoof SPPs in the low-frequency domain.