An efficient on-chip coupling mechanism is essential for nanoplasmonic circuits and elements. We show theoretically that end-fire coupling is a promising candidate to deliver light into regions with subwavelength dimension on flat metal surfaces. A design and optimization principle is presented for a flat metal surface and further demonstrated in a plasmonic Mach-Zehnder interferometer platform. The physical mechanism is discussed based on reciprocity. By considering the radiation pattern and position of the incidence, the coupling efficiency at the metal/air interface can be enhanced up to 77.6%–95.4%, which is promising to develop energy-efficient applications for on-chip plasmonic waveguide networks and sensors.

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