Linear and Nonlinear Optical Spectroscopy on Two-Dimensional Arrays of Silver Nanoparticles

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The work presented in this thesis describes investigations into linear and nonlinear optical properties of two-dimensional silver nanoparticle arrays. The linear optical measurements - extinction measurements were performed on two-dimensional square arrays of L-shaped silver nanoparticles fabricated by electron beam lithography in order to study the effect of dipole coupling to the plasmon resonance of the arrays. A single L-shaped nanoparticle localized surface plasmon resonance (LSPR) had two polarized components which independently coupled to create plasmon resonances in the arrays. The resonance location and bandwidth of the arrays were dependent on grid spacing and number of particles in the array. The array plasmon resonance had a minimum bandwidth of 700 - 800 cm<sup>-1</sup> at a grid spacing ~ 75 nm smaller than the grid having the largest red shift of the plasmon resonance. The birefringence property of the arrays was also investigated. The maximum phase difference of 30° for two orthogonal optical axes was observed in between two main dipole resonance wavelengths. The observation suggested the possible application of the two-dimensional nanoparticle arrays as wavelength-tunable, extremely thin birefringence materials. The nonlinear optical measurements - frequency-scanned excitation profiles of coherent second harmonic generation (SHG) were measured for silver nanoparticle arrays prepared by nanosphere lithography. The second harmonic (SH) emission from the arrays was compared with a smooth silver film to identify an enhancement of SH emission efficiency. The polarization and orientation dependence of the enhancement suggested that it is related to a dipolar LSPR mode polarized normal to the plane of the substrate. In-plane dipoles that dominate linear extinction spectra did not contribute to the SHG process because of the centrosymmetric orientation of arrays which allowed observation of the weak out-of-plane component. Laser power stability of Al<sub>2</sub>O<sub>3</sub> coated silver nanoparticles was tested. It was demonstrated that the atomic layer deposited Al<sub>2</sub>O<sub>3</sub> layers provide enhanced stability of the silver nanoparticles against femtosecond laser exposure, therefore the Al<sub>2</sub>O<sub>3</sub> coated nanoparticles can serve as a stable platform for surface enhanced laser spectroscopy, including nonlinear spectroscopy.

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  • 08/27/2018
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