Ultrafast Dynamics of Au Nanopyramid Interfaces Prepared by Nanosphere Lithography: Effect of Substrate Chemical Composition

This work describes ultrafast spectroscopy studies of Au triangular pyramid particle arrays deposited over glass (termed Au/glass), and 190 nm indium tin oxide (ITO) film (termed Au/ITO/glass) prepared by nanosphere lithography. The linear absorption spectra of Au/glass and Au/ITO/glass exhibit surface plasmon resonances at 800 and 870 nm, respectively, in good agreement with discrete dipole approximation simulations. Ultrafast pump-probe measurements at wavelengths below resonance, at resonance, and above the surface plasmon resonance for each of these two systems are presented. The pump-probe measurements on both systems can be well fit with a model accounting for electron-electron scattering, electron-phonon coupling, and acoustic oscillations on top of cooling of the gold lattice. Numerical simulations employing a two-temperature model are consistent with the single-color pump-probe exponential decays. The wavelength-dependent pump-probe results are interpreted in terms of the complex wavelength-dependent refractive index of gold. We show that this interpretation is consistent with diffractive-optic four-wave mixing spectroscopy measurements of absorptive and dispersive parts of the third-order nonlinear polarization at 800 nm.