Plasmonics

Project Description

Interaction of light with nanostructured objects consisting of noble metals such as Au or Ag leads to an oszillation of electrons in the conduction band. At the resonance frequency most of the incident light is absorbed by this resonator, which dimensions are typically smaller than the wavelength of light. The position of the resonant frequency depends on the dielectric environment and can therefore be used for sensing devices.

Using colloidal lithography we have fabrcated a range of plasmonic superstructures, which have been investigated with respect to their fundamental optical properties as well as their application for sensing and light management in solar cells.

One of the simplest access to colloidal arrays represents the evaporation of a noble metal through a mask of a hexagonally ordered colloidal monolayer.

This leads directly to a two-dimensional array of pyramidal plasmonic nanostructures, where the size of the nanotriangle plays a decisive role in the position of the plasmonic resonance. Combining this method with our capability to fabricate heterostructured colloidal monolayers, we were able to deposit plasmonic resonator of different sizes and therefore distinct plasmonic resonances at predefined position with a lateral resolution of several ten µm.

Instead of using close-packed colloidal monolayers, separating the polymer spheres into a non-close packed structure opens the space for a whole range of more intricate plasmonic structures such as nanoholes, elliptical shapes, and even crescent shaped objects. Such nanocrescents can be regarded as nano-antennae for light and feature polarization dependent plasmonic resonances. Furthermore, their relatively sharp tips are locations of strong electromagnetic fields, which can lead to plasmon coupling.

Arranging plasmonic nanoparticles on a regular lattice with large interparticle distance can also lead to an enhanced absorption due to long-range dipolar coupling within this regular lattice. The combination of large area colloidal monolayers with evaporation of noble metals yields macroscopic arrays of plasmonic resonators, which can be characterized by conventional far-field spectroscopy. The position of the plasmonic resonance depends sensitively on the local dielectric environment. It can therefore be used to sense the deposition of a polyelectrolye for instance. Another application lies in the field of light management in photovoltaic cells, where an improvement in efficiency could be observed.

Financial Support

• SFB 840 TP B7