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.
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.
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