We investigated the spectral and spatial characteristics of plasmons induced in chemically synthesized triangular gold nano- and microplates by aperture-type scanning near-field optical microscopy. Near-field transmission images taken at plasmon resonance wavelengths showed two-dimensional oscillating patterns inside the plates. These spatial features were well reproduced by the square moduli of calculated eigen functions confined in the two-dimensional triangular potential well. From the irreducible representations of the eigen functions, it was found that both the out-of-plane modes and in-plane modes were clearly visualized in the near-field images. We compared near-field transmission images of a triangular nanoplate to those of a truncated one with a similar dimension and revealed that the fine details of the geometrical shape of the apex on the plate strongly influence the experimentally observed eigen mode structures. We also performed near-field transmission measurements of micrometer-scale triangular plates and found that wavy patterns were observed along the edges of the plates. The wavy features can be interpreted as the superposition of eigen modes with similar eigen energy. These findings prove that near-field transmission imaging enables one to directly visualize plasmonic eigen modes confined in the particle and provide fruitful information not only for a deeper understanding of plasmons but also for the application of the design and active control of plasmonic optical fields.
A detailed characterization of plasmon modes is important not only for a deeper understanding of plasmons but also for their practical applications. In this study, we investigated the three-dimensional near-field characteristics of high-order plasmon modes excited in a gold hexagonal nanoplate. From the near-field spectroscopic images, we found that both in-plane and out-of-plane plasmon modes observed near 900 nm were spectrally and spatially overlapped. We performed three-dimensional near-field measurement to reveal the optical characteristics of the overlapped modes in detail. We found that the steric near-field distribution near the nanoplate strongly depended on the plasmon mode, and the out-of-plane mode confines electromagnetic fields more tightly than the in-plane mode. We also found that the in-plane mode was dominantly visualized as the probe tip–sample distance increased. These findings demonstrate that the three-dimensional near-field technique enables selective visualization of a single plasmon mode even if multiple modes are spatially and spectrally overlapped.
Precise understanding of the spatiotemporal characteristics of plasmons is essential for the development of applications of plasmonic nanoparticles. In this study, we investigated the spatiotemporal properties of high-order plasmon modes induced in a gold triangular nanoplate by static and dynamic near-field measurements. The near-field transmission measurements revealed that in-plane and out-of-plane polarized plasmon modes were simultaneously excited and these modes spectroscopically and spatially overlapped. The superposition of these modes was visualized in the near-field two-photon excitation image of the nanoplate. We performed time-resolved autocorrelation measurements on the nanoplate and found that the correlation width was broader than the excitation pulse due to the plasmon dephasing process. From the correlation width map of the nanoplate, we experimentally demonstrated that the out-of-plane plasmon mode exhibits a longer dephasing time than the in-plane plasmon mode. These findings indicate that the out-of-plane mode is desirable for improving the performance of plasmons in various applications.
A bright spot is observable in the center of Bull’s eye plasmonic pattern with a fluorescence microscope due to the plasmonic nanoantenna effect. In this effect, a propagating wave of surface plasmon resonance concentrates in the center. This study focused on the relationship between the center structure of Bull’s eye pattern and the nanoantenna effect in four fabricated Bull’s eye-type plasmonic chips with centers of different sizes (full- or half-pitch diameter) and shapes (convex or concave). The fluorescence intensity of the fluorescent nanoparticles adsorbed to these plasmonic chips was measured with an upright–inverted microscope to evaluate the plasmonic chip enhancement factor composed of the product of the excitation and emission enhancement and individual factors. When the emission enhancement factor was investigated under nonresonance excitation conditions, by the disappearance of a bright spot, excitation enhancement was found to contribute to the plasmonic nanoantenna effect. The concave Bull’s eye structure with a half-pitch diameter demonstrates the highest nanoantenna effect due to the formation of a larger constructive wave in the superposition of the diffraction wave of incident light under resonance conditions. In addition, the electromagnetic field intensity simulated by discrete dipole approximation agrees with the microscopy results. Overall, the results indicate that the plasmonic nanoantenna effect could be controlled depending on the resonance condition and center structure.
The geometrical shape of a metal nanostructure plays an essential role in determining the optical functionality of plasmonic cavity modes. Here, we investigate the geometrical modification effect on plasmonic cavity modes induced in two-dimensional gold nanoplates. We perform near-field transmission measurements on triangular and tip-truncated triangular nanoplates and reveal that the plasmonic cavity modes are qualitatively consistent with each other as long as the snipping size is not significant. To elucidate the tip-truncation effect on plasmonic cavity modes in detail, we carry out numerical simulations for nanoplates with various snipping sizes and find that tip truncation affects not only the optical selection rules but also the energy relation for the plasmonic cavity modes. These findings provide a foundation for the rational design of plasmonic cavities with desired optical functionality.
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