This thesis explores the optical response of systems made from metallic nanoparticles, combining numerical simulations and experimental studies. Metallic nanoparticles, which are much smaller than the wavelength of light, interact with optical fields through plasmonic resonances. These resonances depend strongly on the size, shape and environment of the particles, and the wavelengths that they scatter can be tuned across the whole visible spectrum using particles made from gold, silver and aluminum. The sensitivity to the environment has lead to applications for instance in chemical sensing, where particles floating in a solution aggregate when the target chemical is introduced, leading to a change in color observable without any specialized instrumentation.
The change in color of the aggregates comes from the near-field interactions between the particles. Instead of uncontrolled aggregation, the particles can also be self-assembled around other nanoscale objects acting as scaffolds for the construction. The self-assembled structures have been studied in the Publications III and IV of this thesis, where gold nanoparticles were assembled around twisting, stick-like nanostructures. The resulting structures were found to show large plasmonic circular dichroism, while the individual gold nanoparticles used to make the structures do not have this property. The structures were simulated numerically in the work conducted for this thesis, and found to match the experiments well.
The color of the nanoparticles can be changed also by making periodic lattices from them. In reflection or transmission through the structure, a narrow wavelength range can be boosted due to constructive interference, while others are diminished by destructive interference. This wavelength range can be chosen by tilting the incident angle or changing the particle spacing. The cover image shows a number of aluminum nanoparticle arrays with different periodicities, reflecting light across the whole visible spectrum. This type of structures have been studied in Publications I and II of the thesis as a platform for realizing minituarized laser sources.
The optical properties of the particle lattices are discussed in detail, starting from simplified models and moving to numerical simulations and experiments done. Both one dimensional and two dimensional lattices are shown to act as lasers when covered with organic dye molecules. Curiously, both structures show lasing in an optical dark mode -- a mode where light typically cannot be coupled to from the far field.
|Translated title of the contribution||Nanopartikkeleista koostuvat rakennelmat|
|Publication status||Published - 2018|
|MoE publication type||G5 Doctoral dissertation (article)|
- plasmonics, periodic structures, nanoparticle arrays, chiral structures