The collective response of closely spaced metal particles in non-periodic arrangements has the potential to provide a beneficial angular and frequency dependence in sensing applications. In this paper, we investigate the optical response of a Sierpinski fractal arrangement of gold nanoparticles and show that it supports a collective resonance similar to the surface lattice resonances that exist in periodic arrangements of plasmonic resonators. Using back focal plane microscopy, we observe the leakage of radiation out of a surface lattice resonance that is efficiently excited when the wavenumber of the incident light matches a strong Fourier component of the fractal structure. The efficient coupling between localized surface plasmons leads to a collective resonance and a Fano-like feature in the scattering spectrum. Our experimental observations are supported by numerical simulations based on the coupled-dipole approximation and finite-difference time-domain methods. This work presents a first step towards the application of fractal arrangements for plasmonic applications

We describe experimental and theoretical investigations of the properties of a fractal optical antenna-the Sierpinski carpet optical antenna. Fractal optical antennas are inspired by fractal antennas designed in radio frequency (RF) region. Shrinking the size of fractal optical antennas from fractal antennas in RF regions by a factor of lE-5 arises challenges of fabrication, characterization and modelling their response to incident light. The comparison between optical antennas with the Sierpinski carpet geometry and other geometries is also an interesting topic. Generally speaking, this thesis covers the fabrication, the theoretical investigations of the properties of eigenmodes and the characterization of Sierpinski carpet optical antennas by optical techniques. The broadband response and hot-spot distribution of a Sierpinski carpet optical antenna have been investigated by the white-light dark field and two-photon photoluminescence microscopy respectively, and results are compared with two other morphologies: the pseudo-random and periodic nanostructures. The subradiant surface lattice resonance of a Sierpinski carpet optical antenna has been investigated by the back focal plane microscopy. Due to the versatile application of optical antennas, the generalization and application of the theoretical and experimental techniques to other plasmonic arrays, or even the application of the Sierpinski carpet optical antenna itself to other areas of light­ matter interaction, can be expected.

We present fabrication, characterization, and simulation results on an optical antenna inspired by the Sierpinski carpet fractal geometry for operation in the visible and near-infrared wavelength regions. Measurements and simulations of the far-field scattering efficiency indicate a broadband optical response. Two-photon photoluminescence images provide maps of the near-field intensity distribution, from which we extract an enhancement factor of ∼70. To explore the effect of morphology on the optical response of a large assembly of particles, we also present results on an arbitrarily chosen pseudo-random configuration as well as a periodic array.

We investigate the benefits of estimating the group delay spectrum (as opposed to the phase spectrum) on the performance of an adaptive laser pulse shaping experiment. We use both numerical and experimental approaches, and consider different figures of merit such as efficiency, noise robustness, convergence speed, repeatability, fitness sensitivity, and parameter insensitivity. While primarily focused on the process of broadband second harmonic generation, we also study a resonant third order nonlinear process. Our results show an overall superiority of delay-based formulation in the cases under study (compared to a conventional phase-based formulation) for adaptive laser pulse shaping.