Spectroscopic study of gap-surface plasmons in a metallic convex groove array and their applications in nanofocusing and plasmonic sensing

Plasmonic tapered grooves have been proven to be good candidates for the excitation of gap surface plasmons (GSPs), surface plasmons trapped vertically inside a metallic tapered groove or slit. GSPs have attracted tremendous interest due to their unique properties of concentrating light in nanosized gaps with significant field enhancement, thus offering potential applications such as ultracompact nanocircuits, broadband light absorbers, and plasmonic sensors. In this paper, we focus on GSPs supported by periodic arrays of narrow convex grooves and study in detail their properties by using visible-near-infrared (VIS-NIR) spectroscopy. We identify strong second- and third-order GSP modes excited in ultrasharp convex grooves. The dependence of GSP resonances on the groove profile is analyzed with the help of detailed full-wave simulations, revealing the fact that an ultrasharp, but finite, gap exists at the groove bottom and plays a crucial role in determining both the GSP resonance positions and the nanofocusing capability with much improved field enhancement inside the grooves. Spectral shifts of the observed GSP resonances relative to the simulation results are found in a shorter wavelength range and are qualitatively explained as nonlocal effects originating from the nonclassical microscopic behavior of local currents and charges at imperfect interfaces. Utilizing such strong and distinguishable GSP resonance line shapes in an otherwise flat reflectivity spectral baseline, we experimentally demonstrate the capability of convex groove arrays to perform dual-band refractive index sensing in the VIS-NIR range.