Several works have already shown that the excitation of plasmonic structures through waveguides enables a strong light confinement and low propagation losses . This kind of excitation is currently exploited in areas such as biosensing , nanocircuits and spectroscopy.
Efficient excitation of surface plasmon modes (SPP) with guided modes supported by high-index-contrast waveguides, such as silicon-on-insulator waveguides, had already been shown [1,5], however, the use of weak-confined guided modes of an ion exchanged waveguide on glass as a source of excitation of SPP represents a scientific and technological breakthrough. This is because the integration of plasmonic structures into low-index-contrast waveguide increases the bandwidth of operation and compatibility with conventional optical fibers.
In this work, we describe how an adiabatic tapered coupler formed by an intermediate high-index-contrast layer placed between a plasmonic structure and an ion-exchanged waveguide decreases the mismatch between effective indices, size, and shape of the guided modes. This hybrid structure concentrates the electromagnetic energy from the micrometer to the nanometer scale with low coupling losses to radiative modes. The electromagnetic mode confined to the high-index-contrast waveguide then works as an efficient source of SPP supported by metallic nanostructures placed on its surface.
We theoretically studied the modal properties and field distribution along the adiabatic coupler structure. In addition, we fabricated a high-index-contrast waveguide by electron beam lithography and thermal evaporation on top of an ion-exchanged waveguide on glass. This structure was characterized with the use of near field scanning optical microscopy (NSOM). Numerical simulations were compared with the experimental results.
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