Absorbing mirrors are optical thin film systems consisting of an absorbing metal layer held by a transparent interlayer in defined distance to a (metal) mirror layer. In the reflected light, such three-layer systems show vivid color effects due to the absorption in the metal. This absorption is modified and enhanced by interference effects. The interference condition can be tuned by the thickness of the transparent interlayer. We show by experimentation and by stratified medium model calculations the importance of a nanoscale granular structure in the absorbing metal layer for obtaining a wide spectral range of the color effect.
By electron-beam lithography (EBL) 1D and 2D arrangements of metal nanoparticles can be fabricated with high control of particle shape, particle orientation and arrangement pattern. As the plasmon resonances in metal nanoparticles are primarily determined by the particle shape, their optical properties can be controlled within a wide range by design of the particle geometry parameters. Additionally, the control of local field effects, due to electrodynamic particle interaction, is possible by tailoring the interparticle distances and the specific arrangement pattern. Such EBL-produced metal nanoparticle thin films can be optimised for several optical properties, e.g. for defined dichroic behaviour.
In particular our metal nanoparticle films can be used very efficiently in the field of surface enhanced optical effects. By plasmon resonance control defined energetic interactions between fluorophors and the metal nanoparticle can be obtained, leading to a control of balance between radiating and non-radiating deexcitation pathways. Thus, the fluorophor-particle interaction modifies the fluorophor's absorption properties, its fluorescence intensities, fluorescence lifetime and photobleaching rates.
Spatially tuned resonant nano-clusters allow high local field enhancement when exited by electromagnetic radiation. A number of phenomena had been described and subsequently applied to novel nano- and bionano-devices. Decisive for these types of devices and sensors is the precise nanometric assembly, coupling the local field surrounding a cluster to allow resonance with other elements interacting with this field. In particular, the distance cluster-mirror or cluster-fluorophore gives rise to a variety of enhancement phenomena. High throughput transducers using metal cluster resonance technology are based on surface-enhancement of metal cluster light absorption (SEA). The optical property for the analytical application of metal cluster films is the so-called anomalous absorption. At a well defined nanometric distance of a cluster to a mirror the reflected electromagnetic field has the same phase at the position of the absorbing cluster as the incident fields. This feedback mechanism strongly enhances the effective cluster absorption coefficient. The system is characterised by a narrow reflection minimum.
Based on this SEA-phenomenon (licensed to and further developed and optimized by NovemberAG, Germany Erlangen) a number of commercial products have been constructed. Brandsealing(R) uses the patented SEA cluster technology to produce optical codings. Cluster SEA thin film systems show a characteristic color-flip effect and are extremely mechanically and thermally robust. This is the basis for its application as an unique security feature. The specific spectroscopic properties as e.g. narrow band multi-resonance of the cluster layers allow the authentication of the optical code which can be easily achieved with a mobile hand-held reader developed by november AG and Siemens AG. Thus, these features are machine-readable which makes them superior to comparable technologies.
Cluster labels are available in two formats: as a label for tamper-proof product packaging, and as a direct label, where label and logo are permanently applied directly and unremovable to the product surface. Together with Infineon Technologies and HUECK FOLIEN, the SEA technology is currently developed as a direct label for e.g. SmartCards.
We report on the optical excitation of plasmon modes in elongated gold nanoparticles. Beside dipolar plasmon modes higher order plasmon modes can be excited. As has been reported recently for elongated silver nanoparticles (Krenn et al., Appl.Phys.Lett 77, 3379 (2000)) this geometry gives rise to narrow extinction bands that are spectrally well separated.
Metal clusters excited by light exhibit high local field enhancement and nanoscale resonant behavior. Absorptive properties of these metal clusters bound to a surface are the basis of various new and highly promising setups to transduce biorecognitive interactions into an optical signal. Multilayered highly resonant systems had been proposed and recently demonstrated employing a metal mirror, a nanometric polymer distance layer, a biomolecule interaction layer and biorecognitively bound metal nano clusters. The optochips clearly exhibit strong reflection minima induced by the resonant behavior of the metal cluster layer. At least one narrow reflection minimum can be shifted to the red or infra red spectral range and therefore far away from spherical gold colloids (less than 520 nm) and human plasma absorption. The setup enabled us to replace conventional binding assays (like ELISA) overcoming the various technological limits as there are multiple incubation steps, harmful reagents and spatial resolution. A modified setup (the metal island coated swelling polymer over mirror system) employing an optical thin-layer system consisting of a metal mirror, an active analyte-induced swelling polymer, and a metal cluster (island) film as the topmost layer was used to transduce human plasma ion concentrations.
Surface enhanced absorption of metal nano-clusters enabled us to transduce bioaffinity interactions highly amplifying the optical effect of changes in sensor surface coverage. The sensors were built depositing multiple nanoscale layers: at first silver or gold were sputtered onto oxygen plasma activated polycarbonate substrates to obtain a semitransparent metal cluster layer. Alternatively the primary metal layer was built of gold colloids covalently coupled to the activated polycarbonate. Next a chemically inert distance layer was applied e.g. by polymer-spinning. Finally a second cluster layer of e.g. gold colloids was coupled via bioaffinity interactions to the surface of the inert distance layer. The optical properties of the senor were found to be dependent on the size, shape and number of the metal-clusters as well as the distance between both metal cluster layers. For biomedical sensing the number and the spatial arrangement of biorecognitive bound metal clusters was transduced into an optical signal with high sensitivity. Since the defined spatial approach of colloids to the sensor surface alone creates the signal we could visually follow molecular binding events in real time. The first setups constructed were based on lectin-hexose or antibody-antigen interaction. The analytes were quantified via a distinct change of the spectral reflectivity of the sensor chip visible to the eye or measured by a miniaturized photometric device.
In fluorescence labelled immunosensing the discrimination of labels remaining in the bulk solution from labels bound to the analyte at the sensor surface is a basic optical problem. It is shown that application of surface enhanced fluorescence at a layer of noble metal nano-particles can increase the surface-to-background signal ratio. We explain the enhancement mechanism by an electrodynamic model and discuss the interaction between metal particle and fluorophore for the excitation and emission process. We show the principal guidelines for optimization of that processes. We find that the obtained discrimination power increases with decreasing intrinsic quantum efficiency of the fluorophore, suggesting the application of new classes of labels, namely low-quantum efficiency fluorophores. This theoretical finding is shown by a practical model experiment.
An optical reflectivity change induced by a change of the micro environment around metal island is used to construct various sensors and biosensors. To obtain a sensitive micro sensor either the island density at the surface of the sensor device or the distance of an island layer film to a solid metal surface or to another island film can be varied. Polyvinylpyrrolidone crosslinked with sulfonated bisazidostilbenes shows chaotropic ion dependent nanometric shrinking and swelling which can be observed by using this polymer as interlayer in a metal island device. This volume change of the sensing polymer is transduced to an optical signal using a metal island film, followed by a thin layer of an optically transparent welling polymer and a further metal island film as the topmost layer, exposed to the analyte. This new set-up enables the spectroscopic monitoring of the reflectance change from the backside of the sensor chip not exposed to the analyte solution. For the construction of a biosensor the device was either covered by a photo-structured polyvinylpyrrolidone membrane incorporating the desired enzymes or combined with a micro enzyme reactor. The fully reversible response of the sensor is induced by carbonate and ammonium ions liberated from urea by immobilized urease.