Doctor of Philosophy
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.
Summary for Lay Audience
Nanomaterials are structures with dimensions comparable to the size of light waves. These materials are known to interact efficiently with a field of light. Through a variety of fabrication techniques, these materials can be made to interact with light of a specific color up to a multitude of colors, depending on the features and sizes of the designed material, generating what is known as plasmons. These nanomaterials can then be employed to improve existing analytical techniques to allow the characterization of minute details at their surfaces.
While there are many light-based techniques to characterize materials, in this thesis we are particularly interested by a two-photon process known as second-harmonic generation. This technique has a selection rule that makes it forbidden in materials that have a center of inversion. Herein, we study how plasmons affect the second-harmonic generation output of materials with specific geometries. The study of a two-dimensional material, which are a category of materials that are approximately 1 nm thick, or 1/70,000th of a human hair, is also studied. A dye is then added to this two-dimensional material to study its effects. By studying and improving these properties, light-based applications, such as fiber optic communications and polarization sensing techniques, can be optimized.
Furthermore, when a field of light excites these plasmons, they can interact with molecules at the surface of the nanomaterials to induce chemical reactions. These materials are designed in such a way to generate patterns at the surface using different shapes and features when fabricating the materials. This would allow the production of large surfaces with potential applications in biological trapping and diagnostic tools that require smaller sample sizes and higher densities of light-emitting surface areas are possible.
Therien, Denis AB, "Plasmonically-Active Nanomaterials for Enhanced Second-Harmonic Generation and Chemical Reactions" (2022). Electronic Thesis and Dissertation Repository. 8536.
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