Photoluminescence of metallic plasmonic nanostructures

The applications of plasmonic nanostructures provide a promising way to achieve enhanced light-matter interaction and energy harvesting in future. In this thesis, the single-photon luminescence (PL) of noble metallic plasmonic nanostructures has been systematically investigated, demonstrating the plasmon effect in the energy conversion from non-equilibrium electrons to photons. The main emphasis of this thesis is laid on the physical mechanism of photoluminescence in metal nanomaterials and the exact role of plasmon in this process. The photoluminescence spectra are collected by a micro-PL system from single isolated Au nanostructures fabricated by electron-beam lithography (EBL). It is well-known that EBL is able to produce plasmonic nanostructures with any arbitrary geometrical and dimensional nanostructures, promising the possibility of tunable plasmon resonance frequency within nanometer accuracy. Dark field scattering microscopy is used to obtain the plasmon resonance spectra of metal nanostructures. Strong shape-and size-dependent PL spectra are observed, showing very close correlation in peak position and spectral width with scattering spectra. These systematic results directly reveal that the PL peaks of individual Au nanostructures are indeed the consequence of radiative damping of plasmon resonance driven from optically excited electrons, instead of the direct recombination of sp electrons and d holes. The experimental results also show a universal blue shift in the PL peak positions with respect to the corresponding plasmon resonance peaks due to the inhomogeneous population distribution of non-equilibrium electrons available for the excitation of plasmon. More importantly, we have also observed the partial polarization of the PL emission spectra where the PL peaks show stronger intensity along the same polarization direction of the incident laser. The simulated absorption and scattering spectra provide strong theoretical support for our conclusions. The detailed plasmon-modulated PL process is described via a simple physical mechanism model, also giving a rough prediction of PL spectra of plasmonic nanostructures. The effect of plasmon coupling with single-photon luminescence is studied for the first time through two typical plasmonic nanostructures: isolated Au nanodisk dimers and single Ag nanowire-Al2O3-Au film. Strong luminescence spectra are determined by the radiative decay of coupled plasmon modes rather than intrinsic mode, for instance, longitudinal and transverse plasmon resonances in dimers, as well as the gap plasmonic nanocavity in nanowire-film system. It is found that tunable PL spectra could be achieved by adjusting the gap distance of nanodisk dimer or the diameter of Ag nanowire. The simulations of absorption and scattering spectra are conducted to confirm and understand our observations. These results provide more conclusive evidences about the origin of PL from plasmonic nanostructures and should also remove any doubts of the role of plasmons in PL. The plasmon-enhanced photoluminescence of Au nanoparticles array prepared by nanosphere lithography are visualized by using an optically trapped polystyrene microsphere. This method without the involvement of any plasmon allows us to not only obtain the near-field intrinsic PL images but also compare the PL spectra at different positions with higher spatial resolution (~100 nm). Strong PL hot spots are observed from the gap center of each nanotriangle dimer just parallel to the laser polarization direction. Obvious polarization dependence of near-field PL images is obtained due to the strong local electromagnetic field induced by plasmon resonance. This study provides the possibility to obtain near-field understanding about PL spectra and intensity distributions of plamsonic nanostructures. All experimental results and analyses presented in this thesis could benefit not only the fundamental understanding about the plasmon-modulated dynamics of energetic electrons but also the exploration of plasmon-based photonic or hot-electron devices.DOCTOR OF PHILOSOPHY (SPMS