Single Particle Studies on Two-Photon Photoluminescence of Gold Nanorod–Nanosphere Heterodimers

Abstract

The localized surface plasmon resonance (SPR) property of metal nanoparticles has numerous potential applications. Coupling between different SPR modes of adjacent metal nanoparticles can result in significantly enhanced optical responses such as surface-enhanced Raman scattering (SERS) and two-photon photoluminescence (2PPL). In this work, gold (Au) nanorod (NR)–nanosphere (NS) heterodimers with side-linked and end-linked spatial arrangements have been systematically studied on the single-particle level to investigate the plasmon coupling effects on their 2PPL properties. Compared to a single Au NR, both end-linked and side-linked Au NR-NS heterodimers displayed red-shifted scattering spectra with similar cos2θ excitation polarization dependence, which can be understood in terms of plasmon hybridization theory. However, the 2PPL intensities of end-linked and side-linked Au NR-NS heterodimers displayed strikingly different excitation polarization dependence. The 2PPL signals of the end-linked Au NR-NS heterodimer displayed cos4θ excitation polarization dependence, similar to that of Au NR monomer. The 2PPL intensity of the end-linked heterodimer is the largest with polarization along the long axis of Au NR, which is ∼300 times that of the Au NR monomer component and even ∼5 times that of a longer Au NR with a similar overall size to the heterodimer. In a striking contrast, 2PPL intensity of the side-linked heterodimer is largest for excitation along the transverse direction of Au NR, which is ∼25 times that under excitation along the longitudinal direction. Despite these differences, an interesting feature is that the largest 2PPL signals occur along the coupling directions for both end-linked and side-linked heterodimers. Plasmon-coupling-enhanced 2PPL is generally believed to arise from enhanced two-photon excitation efficiency through two effects: improved resonance effects due to the plasmon-coupling-induced red-shifted SPR mode and plasmon-coupling-induced giant local electric field amplification. Stronger 2PPL signals have been observed for coupled nanostructures with excitation along the coupling direction but less favorable overlap between the SPR scattering spectra and excitation wavelength, which unambiguously supports that plasmon-coupling-induced local electric field amplification is the dominant effect responsible for the big difference in 2PPL intensities of different nanostructures. This conclusion has been further confirmed by the excellent agreement between the numerically calculated integrated |E/E0|4 and the experimentally obtained 2PPL signal intensities for various nanostructures. These studies offer some fundamental understanding of plasmon coupling effects on optical responses and excitation mechanisms of 2PPL of metal nanostructures, which provide insight on designing nanostructures with tailored optical properties for various potential photonic and optoelectronic applications.