Accessing Surface Plasmons with Ni Microarrays for Enhanced IR Absorption by Monolayers

Abstract

Surface plasmons have been accessed with mid-IR light on Ni microarrays of subwavelength apertures using zeroth-order FTIR transmission spectra. A number of transmission resonances are observed throughout the mid-IR region, which is particularly interesting because nickel’s dielectric properties are unfavorable in the visible. On the strongest resonance, these microarrays transmit about 3 times the intensity of light that is directly incident upon the holes (i.e., they exhibit Ebbesen’s extraordinary transmission effect). A study is presented of the dispersion and line shape of resonances for a mesh of well-defined geometry (6.5-Im-wide square holes, square lattice with 12.7-Im hole-to-hole spacing, and 5-Im thickness). The dispersion diagram reveals large band gaps (as a fraction of the resonance energy) that are complicated by the lifting of nominally degenerate resonances. The line shape of the most isolated resonance was fit to a damped harmonic oscillator model revealing the complex dielectric parameters including the damping constant. Lifetimes obtained from the damping constants range from 80 to 510 fs and exhibit an exponential dependence on the resonance wavelength. A great enhancement of resonant transmission relative to the fractional open area has also been observed upon stacking two or more meshes on top of each other. The metallic microarray transmission geometry converts the photon energy to surface plasmon polaritons traveling along the metal surface and therefore through coatings, membranes, or monolayers on the mesh. This dramatically changes the path length in absorption experiments from about twice the thickness of the coating (as it is in reflection absorption experiments) to the thickness of the microchannel and more, which could amount in practice to a 1000-fold enhancement in the fraction of light absorbed. These observations suggest opportunities for using these resonances in sensitive detection schemes with vibrational spectroscopies to detect molecular surface species. We show one exciting outcome that involves some unusually large absorbances by 1-dodecanethiolate monolayers on these Ni meshes.