Spectroscopy and nonlinear microscopy of gold nanoparticle arrays on gold films

Regular arrays of rectangular gold nanoparticles placed on gold films are characterized by using linear reflection 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 730-820 nm) is detected. Experimental results are modeled using a finite-difference time-domain approach with the dielectric function of gold approximated by a Drude-Lorentz formula, showing a rather good agreement between the experimental and theoretical reflection and TPL enhancement spectra. The modeling is also used to optimize the array parameters for achieving strong and well-pronounced TPL enhancement maxima in the wavelength range accessible to the used experimental techniques, i.e., close to 800 nm. Accordingly designed samples are fabricated and characterized, corroborating the modeling predictions. We discuss the origin of TPL enhancement and its relation to local-field enhancements at the sample surface as well as its characterization with TPL microscopy. The implications of the obtained results are also discussed.