Grating couplers fabricated by e-beam lithography for long-range surface plasmon waveguides embedded in a fluoropolymer

Long-range surface plasmon polariton waveguides consisting of Au stripes integrated with input and output grating couplers embedded in thick Cytop claddings are proposed and demonstrated experimentally. Under the right conditions, grating couplers enable broadside (top) coupling with good efficiency while producing a low level of background light. The scheme does not require high-quality input and output edge facets, and it simplifies optical alignments. We demonstrate coupling using a cleaved bow-tie fiber and a lensed fiber, and we determine the grating coupling efficiencies in both cases over a broad operating wavelength range. The lensed fiber produces a better overlap with the long-range surface plasmon mode of interest and thus results in a better coupling efficiency with essentially no background light as observed on an infrared camera. The measurements are compared with theoretical results obtained using a realistic model of the structures, including out-of-plane curvature in the grating profile resulting from our fabrication process. The coupling scheme along with the surface plasmon waveguides hold strong potential for biosensing applications.