Computational Electrodynamics and Second hyper-Raman Scattering

Surface-enhanced second hyper-Raman spectroscopy is a type of nonlinear light scattering in which three photons are simultaneously annihilated to produce one photon whose color is slightly shifted from its third harmonic. This process, while incredibly weak, can be magnified by many orders of magnitude with the aid of plasmonic substrates. In this dissertation, three different studies are presented probing both the nature of plasmonics and nonlinear Raman scattering. In the first, nanoparticle aggregates were dosed with two isotopologues of Rhodamine 6G to determine single-molecule activity. Plasmon maps of the aggregates were then generated from electron energy-loss spectroscopy (EELS) and 3D computational electrodynamics simulations. The results from this study suggest that electron beam excitation of the single molecule hot spot is only possible when the electron beam from the scanning transmission electron microscope (STEM) is located outside the junction region. In the second study, the Fano interferences between localized surface plasmon resonances (LSPRs) of single silver nanocubes were probed using dark-field optical microscopy, EELS, and computational electrodynamics. From this study, it was shown that hybrid plasmon modes induced from an electron beam are the same as those produced by resonance-Rayleigh scattering from individual nanocubes. In the third study, silver colloids were dosed with Rhodamine 6G and tested for second hyper-Raman scattering (2HRS) by laser illumination. Simulations of this scattering mechanism were extended from previous vibronic coupling models based on the time-dependent wavepacket approach. This work showed the incredible enhancement factors required for 2HRS and that the scattering mechanism for 2HRS followed the same A-term mechanism as normal surface enhanced Raman scattering (SERS).