Plasmonically-Active Nanomaterials for Enhanced Second-Harmonic Generation and Chemical Reactions

Upon excitation by an electromagnetic field, metallic nanomaterials will produce highly localized areas of electromagnetic enhancement, a phenomenon known as localized surface plasmon resonance (LSPR), which can be applied to a variety of techniques including second-harmonic generation (SHG) and surface chemistry. These tunable LSPRs can be modelled prior to fabrication by finite-difference time-domain (FDTD) calculations and observed experimentally by SHG microscopy (SHGM). In this thesis, two types of nanomaterials were characterized using SHGM: plasmon-active dendritic fractals (dendrimers) and transition-metal dichalcogenides (TMDs). Dendrimers with specific geometries and LSPRs were used to demonstrate how nanomaterial symmetry affects SHG as well as how local non-centrosymmetric instances can induce SHG responses in otherwise forbidden circumstances. This introduces an in-depth analysis on the effects of plasmon active dendrimers with specific geometries, and how to enhance their nonlinear optical properties. TMDs were analyzed using polarization-dependent SHG and were subsequently functionalized with an organic dye to visualize its effect on the SHG signal. Commercial applications of nonlinear optical processes from plasmonic metamaterials or crystalline materials open a wealth of applications in fiber-optic communications, sensing in biology, photonics filters as well as efficient light conversion and tuning. Laser-induced periodic surface structures (LIPSS) are generated upon irradiation at the surface of a material with repeating features. Reported in this thesis are LIPSS created on glass with a single wavelength, followed by substrates prepared by nanosphere lithography, and the effects of the structure shape and size by the resulting LSPRs was analyzed. Crosshair structures were also computationally modelled and fabricated to exhibit up to three distinct LSPRs in the visible spectral region and irradiated at three distinct wavelengths and two polarizations. The resulting grafting patterns were compared to the FDTD-modelled hotspots, which demonstrated three different grafting patterns at both incident polarizations. These could lead to applications such as surface specific and polarization dependent diagnostic tools, variable gratings on a single substrate, multi-coloured nanoemitters, and more

Single-Beam Inscription of Plasmon-Induced Surface Gratings

The formation of gratings on gold nanoprisms arrays by plasmon-mediated reduction of a diazonium salt is investigated. Nanosphere lithography (NSL) is used to produce large surfaces of gold nanoprisms that are effective at reducing diazonium salts by producing hot electrons through excitation of localized surface plasmon resonances (LSPRs). Using single beam irradiation, we report here on the formation of periodic structures formed from the diazonium salts and that follow the NSL structures. On plasmonically active nanoprism substrates, the electric field enhancement promotes chemical reduction and hence modifies the grafting direction and grating properties of the ripples. The nanoprisms act as a plasmon guide which widens the pitch of the self-organized gratings and can even alter it from straight lines into a crisscross pattern