Electromagnetic Resonances and Local Fields in the Linear and Nonlinear Optical Response of Metal Nanostructures

The unique and tailorable properties of metal nanostructures show great prospects for future nanophotonics applications. However, the complicated interaction between the nanostructures and the electromagnetic field has many aspects still not known. Therefore, the basic research on the optical properties of metal nanostructures is essential for building a solid base for the future development towards real applications. In this Work, T-shaped nanodimers and L-shaped nanoparticles, both made of gold, have been investigated. The goal has been to better understand the linear and nonlinear optical properties of the structures. The main focus has been in the plasmon resonances and the local electric fields, which depend on various parameters of the samples, and which are intimately connected. In T-shaped nanodimers the local fields are very sensitive to the smallest structural details. Furthermore, the changes in the local-field distribution lead to variations in the second-order nonlinear response of the samples. In the linear response of L-shaped nanoparticles, we experimentally observed more higher-order resonances than anyone before. We also explained the origin of a short wavelength resonance, which had not been well understood earlier. In each case, the experimental observations were confirmed by numerical simulations. Over the past years, the sample quality has improved significantly enabling fabrication of metal nanostructures with designable optical properties. Our first demonstrations of the new possibilities are related to resonance-domain structures. By utilizing the long-range diffractive coupling between the particles, we introduce a new concept for tailoring both the linear and nonlinear optical properties of arrays of metal nanostructures. The details in the mutual ordering of the particles can remarkably affect the diffractive coupling leading to unexpectedly large differences in the optical response of the samples