Servicio de Información Comunitario sobre Investigación y Desarrollo - CORDIS

Final Activity Report Summary - HOT (High performance optical and thermal properties of metallic nanostructures)

While the properties of metals are seemingly well understood there has been great excitement in scientific circles with the realisation that there are very interesting new phenomena associated with nanostructured metal. Particularly it is possible to confine visible light as electron oscillations on the metal surface. These so-called surface plasmon polaritions are confined to a few hundred nanometres of the interface between the metal and the adjacent material. As a result the properties are extremely sensitive to the surface region useful in biosensors to detect low levels antibodies for example. By periodic wavelength-scale structuring the metal in two and three dimensions additional properties can be obtained which could revolutionise our methods of providing general lighting with a new high efficient filament. The optical energy can be confined to dimensions much less that can be achieved with conventional optics permitting probing of objects at the nanoscale. To realise this potential it is necessary to develop practical structures which can make their way into real products. This means understanding of the underlying physics, the development of fabrication techniques and the investigation of the applications. These have been the goals of the project and have been achieved through the incoming experts working with the researchers at Tyndall.

Computer programs incorporating the underlying physics were developed to describe the evolution and distribution of optical energy on specific metallo-dielectric nanostructures. Concepts that are familiar in conventional optics such as interference and diffraction apply also to the plasmonic surface waves that hug the metal. The simulations demonstrate the critical importance of all aspects of the physical structure on the resultant optical properties. Simulations on plasmon guiding structures, enhancing the light emission from fluorescent particles and the role of structure of silver nanoparticles as a means of enhancing the photovoltaic properties of ultrathin cells were performed.

We investigated a number of processing strategies for nanostructuring surfaces. With the appropriate preparation a metal surface is nanostructured by use of heat treatment. Fabrication of large area 2-D quasiperiodic photonic crystals based on phase-mask lithography was investigated. An excimer laser was used to expose photoresist material through a 1-dimensional grating. By performing multiple exposures at different orientations between the grating and the sample it was possible to obtain 2-D quasiperiodic structures. This technique could be used to fabricate low cost and large area 2-D phase masks for generation of 3-D metal crystals. A bottom-up technique to create 3-D metal structures was successfully developed. In it we employed the natural sedimentation of submicron polystyrene spheres to form an opal structure.

We discovered a new method to infill the voids in the structure with pure silver. The infilling was more than sufficient to allow the removal of the spheres leaving a freestanding silver structure. Interestingly the reflection properties of the structure are equivalent to structuring air in 3 dimensions. These structures will enable new applications: The roughened gold surface in conjunction with the deposition of a dielectric layer leads to unique, polarisation dependent reflection properties. This has been explained as due to the coupling of the light associated with the nanostructured surface.

The structure is now being developed as a platform for high sensitivity detection of surface interactions. Nanostructured freestanding metal films have been investigated as a means of high sensitivity detection of changes in the local refractive index. An integrated metal platform has been developed for the excitation of surface plasmons on low cost lasers. This work will lead to a new generation of ultra-compact sensors and optical circuits.

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