Huge Enhancement in Two-Photon Photoluminescence of Au Nanoparticle Clusters Revealed by Single-Particle Spectroscopy

Aggregated metal nanoparticles have been known to display significantly enhanced two-photon photoluminescence (TPPL) compared to nonaggregated nanoparticles, which could be utilized to develop platforms for two-photon sensing and imaging applications. Here we have conducted single-particle spectroscopic studies on gold (Au) nanoparticle clusters of different sizes to understand the enhancement mechanisms and explore the limit of maximum achievable enhancement. Our studies show that the TPPL intensity of Au nanoparticle clusters significantly increases from monomer to trimer. The averaged intensity of the Au nanosphere dimers and linear trimers is similar to 7.8 x 10(3) and similar to 7.0 x 10(4) times that of Au nanosphere monomers, respectively. A highest enhancement of 1.2 x 10(5) folds was obtained for the linear trimer. The TPPL spectra of these single Au nanosphere clusters closely resemble their corresponding scattering spectra, suggesting strong correlation between their TPPL with plasmon resonance. The scattering spectra of dimers and linear trimers displayed cos(2) dependence on the detection polarization, while their TPPL displayed cos(4) dependence on the excitation polarization, which are very similar to Au nanorods. These results suggest that two-photon excitation of dimer and linear trimer is strongly coupled to their longitudinal plasmon resonance modes. These studies help to provide insight on fundamental understanding of the enhancement mechanisms as well as development of biomedical and photonic applications.