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-μm-wide square holes, square lattice with 12.7-μm hole-to-hole spacing, and 5-μm 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.

Ordered arrays of subwavelength holes in thin metallic films have been produced that exhibit extraordinary transmission resonances throughout the infrared including the range of wavelengths that excite fundamental molecular vibrations. This phenomenon is attributed to the excitation of surface plasmons which propagate along the surface and tunnel through the holes without being scattered out of an incident beam. Commercial nickel mesh has been coated with copper to reduce the hole size enhancing surface plasmon lifetimes (narrowing resonances) and enabling the study of catalytic processes on the Cu surface. The copper oxide catalyzed reaction of methanol has been studied by recording enhanced zero-order FTIR transmission spectra of the copper mesh activated with a drop of water and then exposed to a drop of methanol. Transformation of CH 3 OH(ads) to CH 3 O(ads) and then to the product CH 2 O(ads) is observed at room temperature and in air with a very simple procedure. The spectrum of adsorbed formaldehyde is analyzed with regard to its orientation on the copper oxide surface.

Self-assembled monolayers, phospholipid bilayers, and membrane bound proteins are assembled on a subwavelength metallic array. These assemblies are assayed with direct infrared absorption spectroscopy which is greatly enhanced due to the extraordinary infrared transmission of the arrays. Stacking the arrays, one upon another, accentuates the surface plasmon properties and provides the basis of a nanospaced capacitive sensor.

Articles you may be interested inPropagation lengths of surface plasmon polaritons on metal films with arrays of subwavelength holes by infrared imaging spectroscopy Influence of unsymmetrical periodicity on extraordinary transmission through periodic arrays of subwavelength holes Appl. Phys. Lett. 90, 161112 (2007); 10.1063/1.2724914 Surface-plasmon resonance with infrared excitation: Studies of phospholipid membrane growth J. Appl. Phys. 98, 093506 (2005); 10.1063/1.2123370 Enhanced infrared absorption spectra of self-assembled alkanethiol monolayers using the extraordinary infrared transmission of metallic arrays of subwavelength apertures