NONLINEAR PLASMONICS WITH APPLICATIONS TO SENSING
Che, Franklin Mbende
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Recent progress in laser science has led to the development of ultrashort pulses, which have a broad spectral bandwidth and high peak intensities. When these pulses interact with nonlinear plasmonic materials, the combination of high peak powers and large bandwidth opens up new possibilities of novel nonlinear spectral effects and potential applications. In this dissertation, we study the nonlinear optical effects of generating second-order electromagnetic fields in reflection from a conducting thin film close to surface plasmon resonance using an ultrashort polychromatic light source. Suitable and realistic femtosecond polychromatic incident sources were formulated for these studies, and the nonlinear fields generated from these sources are explored. The potential of using such a system as a nonlinear refractive index sensor was also investigated leading to the following results. 1- It was demonstrated for the first time, to the best of our knowledge, the generation of plasmon enhanced sum- and difference-frequency waves from the surface of a metal thin film using a single polychromatic source. We used ultrashort femtosecond Gaussian and dual-wavelength Gaussian pulses to generate sum- and difference-frequency waves, respectively. Nonlinear waves in the visible and mid infra-red range were generated with a very strong enhancement of the nonlinear waves close to surface plasmon resonance of the incident light source. 2- Spectral shifts and switches in the nonlinear spectra reflected from a metal film, were observed close to surface plasmon resonance. We also demonstrated a strong correlation between these spectral effects and properties of the incident wave such as the pulse duration, with stronger effects observed for shorter pulses. Finally, we demonstrated a strong dependence of the reflected nonlinear spectral peak position on the material properties of the dielectric material surrounding the metal film. 3- A simple, robust and high sensitivity refractive index sensor was proposed and investigated, based on the second-order nonlinear processes of sum- and difference-frequency generation. The calculated sensor performance parameters showed very high sensitivity and figure of merit values with the ability to operate over a wide wavelength range. We demonstrated a spectral sensitivity at mid-infrared wavelengths, that was two orders of magnitude higher than current values available in the literature. 4- Successful estimation of the relative strengths of second-harmonic generation, reflected from a large variety of conducting thin films using an infrared femtosecond laser source. The films were sputtered on a glass substrate with thickness of 50nm and 100 nm. We also showed that the second-harmonic generation intensities from tungsten and antimony were comparable to that of silver and gold.