Directional coupling in long-range dielectric-loaded plasmonic waveguides

Directional couplers (DCs) based on long-range dielectric-loaded surface plasmon-polariton waveguides (LR-DLSPPWs) operating at telecom wavelengths are studied both numerically and experimentally. The investigated LR-DLSPPWs are formed by ~1.2-µm-high and 1-µm-wide polymer ridges fabricated atop of 15-nm-thick and 500-nm-wide gold stripes supported by a 288-nm-thick Ormoclear polymer deposited on a low-index (n(s) ≈1.34) layer of Cytop. DC structures consisting of sine-shaped S-bends (having an offset of ~10 µm over a distance of ~20 µm) and ~100-µm-long parallel LR-DLSPPWs with a center-to-center separation of 2 µm are characterized using scanning near-field microscopy. The experimentally obtained values of the propagation length (~400 µm), S-bend loss (~4 dB) and coupling length (~100 µm) are found in good agreement with the numerical simulations, indicating a significant potential of LR-DLSPPWs for the realization of various plasmonic components.