Plasmonic Silver Grating for Mid-Infrared Sensing

We numerically simulate and experimentally demonstrate the excitation of mid-infrared surface plasmon polaritons (SPPs) in one-dimensional silver grating structures. In particular, the feasibility to fabricate plasmonic grating structures for the mid-infrared region within an industrial fabrication environment by using standard microfabrication processes represents a decisive step towards plasmonic sensor technologies suitable for semiconductor mass-production. The experimental part covers angle-resolved, free-beam reflection measurements on silver-coated poly-silicon gratings, fabricated on eight-inch silicon wafers. Different grating depths in the range of 50 nm-375 nm are investigated, which lead to the optimal control of the SPP excitation depending on practical applications. In particular, the shallow gratings feature ultra-narrow SPP resonances in reflection of only 2.9 nm full width at half maximum (FWHM) at a wavelength of $4.12\ \mu\mathrm{m}$ . The data obtained indicate that the suggested structures constitute a promising platform for sensitive refractive index sensing. Simulations evidence a spectral shift of SPP resonances of about 4122 nm/RIU that translates to a Figure-Of-Merit of about 1421 RIU−1.