Engineering optical nonlinearities in metal nanoparticle arrays

Abstract

Metal nanostructures supporting localized surface plasmon (LSP) resonances are an emerging technology for sensing, optical switching, radiative engineering, and solar energy harvesting, among other applications. The unique property of LSP resonances that enable these technologies is their ability to localize and enhance the optical field near the surface of metal nanoparticles. However, many questions still remain regarding the effects of nanoparticle coupling on the linear and nonlinear optical properties of these structures. In this thesis, I investigate the role of long-range photonic and near-field plasmonic coupling on the linear and nonlinear optical properties of metal nanoparticles in periodic and deterministic aperiodic arrays within a combined experimental and theoretical framework. In particular, I have developed optical characterization techniques to study various properties of planar metal nano-cylinder arrays fabricated by electron beam lithography (EBL). These include the effect of Fano-type coupling between structural grating modes and LSP resonances on linear diffraction and second harmonic generation (SHG), the influence of near-field coupling on the efficiency of plasmon enhanced metal photoluminescence (PL), the dependence of two-photon PL (TPPL) on nanoparticle size, and the multi-polar nature of SHG from planar plasmonic arrays. Experimental results are fully supported by linear scattering theory of the near and far-field properties of particle arrays based on a range of analytical, semi-analytical, and fully numerical techniques. The breadth of computational methods used allows the investigation of a wide range of structures including large aperiodic arrays with hundreds of discrete particles and periodic arrays with realistic particle shapes, substrates, and excitation conditions. The technological potential of engineered plasmonic structures is demonstrated by enhanced vibrational sum frequency generation (VSFG) spectroscopy, a novel nonlinear sensing technique. These studies have revealed design principles for engineering the interplay of photonic and plasmonic coupling for future linear and nonlinear plasmonic devices for sensing, switching, and modulation. The optical characterization techniques developed in this thesis may additionally be used across a wide range of devices in photonics and nano-optics.