Gap plasmon resonances in metal film-coupled nanoparticles for enhanced photoluminescence and nonlinear optical emission

Abstract

Placing a noble metal nanoparticle near a metallic substrate can significantly modify its optical properties, for which the metal film-coupled nanoparticle system has received extensive research interests in the past decade. In this structure configuration, the capacitive electromagnetic coupling between the particle plasmon resonance and its induced charges distributed in the metal film results in a gap plasmon mode featured by substantially enhanced electromagnetic field intensity at the gap junction, which offers a promising platform for various plasmon-enhanced optical phenomena. Particularly, the ease in fabrication and readily control over the particle-film gap distance down to sub-nanometer scale enable the realization of an ultrasmall plasmonic nanocavity, which has triggered a host of recent breakthroughs in fundamental nanophotonic research and energy harvesting applications such as photocatalysis. This thesis reports our studies on the linear and nonlinear plasmonic properties of the metal film-coupled nanoparticle system. Firstly, our studies focus on experimental characterization and theoretical understanding on the fundamental properties of the gap plasmon modes in gold film-coupled nanoparticle monomers and dimers, with particular attention on the plasmon hybridization upon coupling the nanoparticles to the metal film. Secondly, our studies explore several fascinating gap plasmon resonance-enhanced linear and nonlinear optical phenomena, including photoluminescence (PL) and nonlinear optical emission such as second-harmonic generation (SHG) and two-photo absorption induced luminescence (TPL). In the first part of this thesis, the plasmonic scattering properties of the gap plasmon modes of the gold film-coupled nanoparticle monomers and dimers are revealed experimentally. By using an improved dark-field spectroscopy and imaging methodology - polarization resolved spectral decomposition and color decoding, one can "visualize" and distinguish unambiguously the spectral and far-field radiation properties of the complex gap plasmon modes in the two systems. Together with the full-wave numerical simulation results, it is found that while the monomer-film system supports two hybridized dipole-like plasmon modes having different oscillating orientations and resonance strengths, the scattering spectrum of the dimer film-system features two additional peaks, one strong yet narrow resonant mode resembling a plasmonic dipolar mode, and one hybridized higher-order resonance mode, both polarized along the dimer axis. In particular, the stronger radiation efficiency and much narrower spectral linewidth of the dimer plasmon resonance, compared to its counterpart on silica substrate, are further addressed with an analytical multipole expansion model. The calculation results confirm that these new features are originated from an intense plasmon hybridization between a bonding dipolar mode along the dimer axis and a quadrupolar mode resulted from an anti-paralleled dipole bonding in direction perpendicular to the dimer axis, which reduces the far-field radiation loss and thus leads to the linewidth shrinking effect. These findings not only shed new lights on the plasmon hybridization between individual plasmon resonance modes but also open up the prospect for engineering resonance linewidth of coupled plasmonic modes in general nanoclusters by metal substrate mediated mode hybridization. Based on the depended understanding of the gap plasmon properties as unraveled in the first part, we extend my work to explore their spectroscopy enhancement applications in the second half of this thesis. In light of the strong radiation efficiency and enhanced near-field intensity associated with the excitation of the dimer plasmon resonance, I have performed photoluminescence spectroscopy measurements on the gold film-supported nanoparticle dimer system, and an emission intensity enhancement up to ~200-fold is demonstrated as compared to that of its counterpart on the glass substrate, showing excellent agreement with the calculation results (253-fold). The experimentally observed similar spectral characteristics of the plasmonic scattering and the photoluminescence emission indicate that the radiative decay of the hybridized dimer plasmons is the origin of the detected photoluminescence, which is further verified by the calculations with a proposed phenomenological model. Moreover, the calculation identifies that the particle-film gap junctions are dominantly responsible for the enhanced emission, implying that the metal film-coupled nanoparticle dimer system can be used as a versatile plasmonic platform simultaneously possessing an improved quality factor and a nanoscale mode volume for realizing light -matter interaction in the strong coupling regime that is of fundamental importance in cavity quantum electrodynamics (QED). Finally, we turn our attention to the nonlinear optics aspect of the metal film-coupled gold nanospheres. In this part, we firstly demonstrate a large spectral tunability of the coupled structure in the visible and near infrared region by simply controlling the diameter of the gold nanosphere, and then we use a multiphoton spectroscopy and imaging system to explore its nonlinear optical properties. Upon resonant excitation by a pump femtosecond (fs) laser spectrally overlapping with the gap plasmon resonance, it is found that the nonlinear optical emission spectrum of the structure is featured with a pronounced narrow SHG emission peak and a broad TPL emission band, both of which shows significant enhancement in emission intensity compared to that of a gold nanosphere of the same diameter on silica substrate. More interestingly, the two-lobes shaped emission pattern of the observed SHG and TPL excited by the tightly focused linearly polarized fs laser beam, unambiguously evidences that the invoking of the dipolar gap plasmon resonance at the nanosphere-film vertical junction is dominantly responsible for the large nonlinear emission enhancement. Excellent agreement between the nonlinear optical emission patterns with the calculated focal field distributions based on the vector diffraction theory suggests that the metal film-couple nanoparticles can be used as a sensitive plasmonic virtual probe for mapping the longitudinal electromagnetic fields in the focal plane.