Single-Particle Spectroscopic Studies on Two-Photon Photoluminescence of Coupled Au Nanorod Dimers

Noble metal nanoparticles exhibit interesting two-photon photoluminescence (2PPL) property which is important for their biomedical and photonic applications. Because of high-order sensitivity of nonlinear optical responses, 2PPL responses can serve as sensitive probe signals to study local electric-field amplification in plasmonic nanostructures. Here, we have conducted a systematic study on scattering and 2PPL properties of Au nanorod (NR) dimers with different geometries on the single-particle level to investigate the geometry effects on plasmon coupling and 2PPL responses. 2PPL responses of Au NR dimers displayed interesting polarization dependence and were strongly dependent on the alignment angle. 2PPL spectral profiles of all Au NR dimers at two different polarization angles (theta = 0 degrees and 90 degrees) resemble their corresponding scattering spectra. However, intensities of 2PPL and scattering signals of Au NR dimers displayed different behaviors as the alignment angle changed. In particular, 2PPL signal intensity of Au NR dimers under theta = 0 degrees first decreased as a decreases from 180 degrees to 20 degrees by 4 orders of magnitude but reversed the trend with its intensity increased sharply by 2 orders of magnitude when alpha= 20 changed to alpha = 0. The alpha = 0 dimer displayed distinctively different behaviors. Plasmon coupling between Au NRs with associated local electric amplification to significantly enhance two-photon excitation efficiency is the primary factor responsible for the observed orientation-dependent 2PPL. Hybridization of longitudinal surface plasmon resonance (SPR) modes of Au NRs is primarily responsible for the observed 2PPL behaviors for Au NR dimers with alpha = 0, whereas hybridization of transverse SPR modes plays an important role in 2PPL of the alpha = 0 dimer. These differences arise from transformation from point-to-point plasmon coupling in the a not equal 0 degrees dimer to face-to-face plasmon coupling in the alpha not equal 0 degrees dimer. The conclusions have been further supported by finite-difference time-domain simulation. This study will help to provide insight on understanding the excitation mechanism of nonlinear optical responses of Au nanoparticles, which will be useful in designing proper nanostructures for their applications in sensing and imaging, plasmon-based optoelectronics, and integrated photonic devices.