Resonant nanocluster technology—from optical coding and high quality security features to biochips

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Abstract
Metal clusters deposited on a substrate and positioned at a nanometric distance from a wave-reflecting layer act as nanoresonators able to receive, store and transmit energy within the visible and infrared range of the spectrum. Among the unique effects of these metal nanocluster assemblies are high local field enhancement and nanoscale resonant behaviour driving optical absorption in the visible and infrared range of the spectrum. In these types of devices and sensors the precise nanometric assembly coupling the local field surrounding a cluster is critical for allowing resonance with other elements interacting with this field. In particular, the cluster–mirror distance or the cluster–fluorophore distance gives rise to a variety of enhancement phenomena (e.g. resonant-enhanced fluorescence, REF). Depending on the desired application this 'resonance' distance is tuned from 5 up to 500 nm. High-throughput transducers using metal cluster resonance technology are based on surface enhancement of light absorption by metal clusters (surface-enhanced absorption, SEA). These devices can be used for detection of biorecognition binding as well as structural changes in nucleic acids, proteins or any polymer. The optical property made use of in the analytical application of metal cluster films is so-called anomalous absorption. An absorbing film of clusters is positioned 10–400 nm from an electromagnetic wave-reflecting layer. At a well-defined mirror–cluster distance the reflected electromagnetic field has the same phase at the position of the absorbing cluster as the incident field. This feedback mechanism strongly enhances the effective cluster absorption coefficient. These systems are characterized by a narrow reflection minimum whose spectral position shifts sensitively with interlayer thickness, because a given cluster–mirror distance and wavelength defines the optimum phase.