Classical electrodynamics of surface enhancing nanoparticles

The optical properties of silver and gold nanoparticles are studied in regards to their use in surface enhanced spectroscopies and in sensing applications. Two properties are of primary interest: (1) peaks in absorption and scattering spectra in the visible and near-infrared wavelength range, and (2) enhancement of electric fields near the surface. These two properties are studied as a function of particle size, shape, and external environment.; Four computational methods of classical electrodynamics are applied: (1) Mie theory, which is the exact solution to Maxwell's equations for a spherical scattering body, (2) the Modified Long Wavelength Approximation (MLWA), which is an analytic method applicable to spheroidal particles that are considerably smaller than the wavelength of light, (3) the Discrete Dipole Approximation (DDA), which is a rigorous finite element method capable of arbitrary particle shape, and (4) the Multiple Multipole Program (MMP), which is a surface matching technique for arbitrary shapes. Mie theory and MLWA are useful for surveying the general trends of experimental systems, however they fail to quantitatively reproduce peak wavelengths for particles that are not spheres or spheroids. These analytic results are useful for comparisons with the two numerical methods. Careful application of DDA can match experimental results quantitatively, as shown for triangular lithographic particles and colloidal trigonal prisms. However fields at the surface are not numerically stable in DDA. The MMP is applied to spheres, ellipsoids, and cubes, and this method shows promise for the calculation of smoothly varying fields near particle surfaces.