ATOMISTIC ELECTRODYNAMICS-QUANTUM MECHANICAL METHODS TO MODEL SURFACE-ENHANCED NONLINEAR OPTICAL SPECTROSCOPIES

Spectroscopy, the study of interactions between radiation and matter, often becomes more complicated for intense radiation due to the cause of nonlinearity. At the molecular level, the nonlinear optical (NLO) properties can be described using higher-order polarizability tensors, such as the first and second hyperpolarizabilities. Providing an insight into these properties is essential for designing NLO devices, and thus theoretical tools that can accurately and efficiently model them are greatly needed. The sum-over-states model within a time-dependent density functional theory (TDDFT) framework is probably the most commonly used tool, and has shown success in calculating resonant NLO properties. However, this approach often assumes that a few states dominate the response, and consequently becomes less reliable when far from resonances or for systems that are characterized by a high density of states, such as metal clusters. To overcome this limitation, we have adopted response theory that takes all states into account by construction. With a phenomenological damping factor embedded into it, termed damped response theory, a balanced description of all off-, near-, and on-resonance optical properties can be enabled for both molecules and metal clusters. With the damped nonlinear response theory, we have performed simulations for the resonance hyper-Rayleigh scattering (HRayS) of both molecules and small Ag clusters, the frequency-scanned hyper-Raman scattering (HRS) of the octupolar molecule crystal violet (CV), and the two-photon absorption (TPA) of the thiolate-protected Au25 cluster. These achievements allow for evaluating two-photon resonance enhanced HRayS of Ag clusters, indicate the HRS of CV is dominated by the Franck-Condon effects at the lowest two excitation energies, and reveal the one- and two-photon double resonance effect is not the main cause of the huge TPA cross sections of Au25(SH)18− found experimentally. However, widespread of the NLO techniques is still impeded by their inherent low cross sections. One routine solution is to intensify the spectroscopic signals via chemical mechanism (CM), e.g., the molecular resonance effects; alternatively, electromagnetic mechanism (EMM) that arises from surface localized plasmons can also significantly enhance the signal intensities. For surface-enhanced Raman scattering (SERS), it has been found that the EMM plays a more important role in the signal enhancement as compared to the CM. To this end, we focus on the EMM and have developed two atomistic electrodynamics- quantum mechanical models to include the surface effects on HRS, termed surface-enhanced HRS (SEHRS). The first is the discrete interaction model/quantum mechanical model, which combines an atomistic electrodynamics model of the nanoparticle with a TDDFT description of the molecule. The second is a dressed-tensors method that accounts for the interactions between the molecule and the inhomogeneous local fields. With these methods, we have shown that the field gradient effects mainly determine the…