Spectroscopy and nonlinear microscopy of Au nanoparticle arrays: Experiment and theory

Regular arrays of rectangular gold nanoparticles on glass substrates are characterized by using linear extinction spectroscopy (in the wavelength range of 450-950 nm) and nonlinear scanning optical microscopy, in which two-photon photoluminescence (TPL) excited with a strongly focused laser beam (in the wavelength range of 720-800 nm) is detected. The dimensions of the nanoparticles (similar to 150x150x50 nm(3)) are chosen to realize the localized-surface-plasmon (LSP) resonance at the wavelength of similar to 750 nm, which is clearly seen on the obtained extinction spectra as well as with the recorded TPL images. Extinction spectra are modeled using a finite-difference time-domain approach with the dielectric function of gold approximated by a Drude-Lorentz formula, showing rather good agreement between the experimental and theoretical spectra simulated for the nominal geometrical parameters of gold nanoparticles. The developed modeling tool is further used to evaluate the field intensity enhancement at the particles, which is then compared to the intensity enhancement estimated from the TPL images. We find good agreement between the intensity enhancement levels and indications that the LSP resonance wavelengths seen in the extinction spectra might differ from those deduced from the intensity enhancement spectra. The implications of the results obtained are discussed.