Three-Level Modeling of Third-Nonlinear Optical Resonance in Light-Emitting Au Nanoclusters within Silica Host

In this work, we study the third-order nonlinear optical response of Au nanoclusters with size smaller than 2 nm embedded in a silica host, using a standard Optical Kerr Gate configuration with ultraviolet picosecond pulses at 355 nm.  Depending on the annealing temperature, the Au clusters exhibit either orange (photoluminescence peak 570 nm) or blue emitting (one of photoluminescence peak at 430 nm) upon excitation at 355 nm.  At higher annealing temperatures, larger plasmonic nanoparticles are nucleated; although the sample still emits light, its nonlinear optical response is approximately four times lower than that of samples containing only emitting nanocluster.  All the emitting samples also exhibit a transition from a two-photon absorption process to saturate absorption behavior at higher irradiances.  These findings are analyzed by considering that the emitting metallic nanoclusters behaves as a molecular-like system with discretized energy levels. A three-level model interacting with a monochromatic laser beam is proposed to account for the experimental observations.