RESUMEN
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.
RESUMEN
The first demonstration of grating-coupled long range surface plasmon polaritons in cladded free-standing membrane waveguides is presented. Two different waveguide structures are explored: the first is a gold (Au) stripe embedded in a thin Cytop free-standing membrane, the other being the same structure but with a thin palladium (Pd) over-layer. The waveguides are excited with integrated grating couplers designed for a working wavelength of 1550 nm. The waveguides are characterized by applying a cutback technique with the Au waveguide loss measured as 3.4 dB/mm and the Pd/Au waveguide loss as 57 dB/mm. The wavelength dependency of the weakly reflecting optical cavity is also observed with a free spectral range of ~3.6 nm and a finesse of 2.1.
RESUMEN
A low power, reusable optical hydrogen sensor using long-range surface plasmon polariton (LRSPP) cladded membrane waveguides is demonstrated. The sensor incorporates a 5 µm wide, 20 nm thick gold stripe embedded in a 160 nm thick free-standing Cytop membrane with a 5 nm thick Pd over-layer. Input and output coupling is achieved with directly integrated broadside grating couplers. The sensor is tested with hydrogen concentrations up to 3% and demonstrates a strong response with an estimated detection limit of 290 ppm, and a response time of 7 s to a 0.6% H2 step - this level of performance is among the best reported for optical H2 sensors. Cycling of the hydrogen exposure shows a significant hysteresis response, however no film deformation or delamination was observed over many cycles. The high stress that is induced in clamped films during hydrogenation is relaxed in due to the film being deposited on the flexible and elastic Cytop membrane. This could result in improved lifetimes for this sensor and increased uptake ability.