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.
Metal films with patterns of subwavelength holes (grids or meshes) have interesting optical properties including the extraordinary transmission effect. These optically thick metal films transmit more radiation than that incident on the holes owing to the excitation of surface plasmons (SPs). Meshes present a new and simple way to excite SPs at perpendicular incidence (i.e., without the need to vary the angle of the incident beam). This represents a new opportunity to integrate SPs with experiments and devices-a new instrument in the toolbox of SP techniques that may broaden the range of SP applications. This review discusses the discovery, basic optical physics, the role of SPs, and applications of the extraordinary transmission of subwavelength hole arrays.
The extraordinary infrared transmission effect of metal films with arrays of tiny holes (microarrays or mesh)
is mediated by surface plasmons. This work describes the optical physics of propagating infrared surface
plasmons on mesh and reviews initial applications of the effect. In particular, surface-plasmon-enhanced
infrared absorption spectroscopy is featured. Contents include: a brief history of surface plasmons, mesh-based surface plasmons, propagating surface plasmon basics, surface plasmon resonance response to
nanocoatings, enhanced IR absorption spectroscopy, and the interaction of surface plasmon resonances with
molecular vibrations.
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.
The surface-plasmon-mediated, extraordinary transmission of metallic arrays of subwavelength apertures has been used as the light source for absorption studies of self-assembled monolayers on metal. Enhanced infrared absorption spectra of a sequence of alkanethiol self-assembled monolayers on copper were recorded for carbon chain lengths varying from 8 to 18 atoms. Transition positions and intensities are presented over a large range of the infrared region. The connection between the vibrational modes of the CH(2) wagging progression and the infinite methylene chain is explored using a traditional coupled oscillator approach and a new cluster perspective.
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