Nanoscopic Imaging of Photonic Modes Excited in Square-Shaped Perylene Microcrystals

Organic microcrystals have attracted much attention because of their light confinement and transport capability on the micrometer scale. The capability is closely related to the photonic modes resonantly excited in the organic microcrystal, and therefore, visualization of photonic modes is important not only for a deeper understanding of organic microcrystals but also for their practical application. Here, we directly visualized the photonic modes excited in square-shaped perylene microcrystals by using a scanning near-field optical microscope. From the near-field optical images and the electromagnetic simulations, we demonstrate that the Fabry–Pérot modes are predominantly excited compared with the whispering gallery modes in two-dimensional organic microcrystals. The findings provide a deeper understanding of the photonic modes and should be beneficial for the design of microscale waveguides and photonic integrated circuits

Imaging of Plasmonic Eigen Modes in Gold Triangular Mesoplates by Near-Field Optical Microscopy

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

Plasmon-Enhanced Fluorescence Near Single Gold Nanoplates Studied by Scanning Near-Field Two-Photon Excitation Microscopy

Plasmonic optical fields have been applied for surface-enhanced spectroscopy, chemical sensing, and bioimaging. Spatial distributions of optical fields are critical for optimizing their functionalities. In plasmon-enhanced fluorescence, both incoming and outgoing fields excited by the plasmon should contribute to the enhancement of the fluorescence. Spatial characteristics of plasmons are critical not only for the fundamental understanding of the plasmon but also for their practical applications. Here, we investigate the spatial characteristics of the excitation and relaxation processes near the gold nanoplate using time-resolved near-field two-photon microscopy. We reveal from near-field optical microscopy that the incident field is locally enhanced by the plasmon resonance effect and the lightning rod effect. Near-field time-resolved fluorescence imaging demonstrates that the fluorescence decay is accelerated entirely over the surface of the plate regardless of the spatial distribution of the incident field. These results provide deep insight into plasmonic optical fields and are of great importance for designing plasmon-based substrates for surface-enhanced spectroscopy and photochemical reactions

Static and Dynamic Near-Field Measurements of High-Order Plasmon Modes Induced in a Gold Triangular Nanoplate

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

Visualization of Plasmon–Exciton Interactions by Scanning Near-Field Optical Microscopy

The electronic properties of a substance are perturbed by interactions of elementary excitations. The optical properties of the interacting states have been extensively studied and revealed to be correlated with the eigenfunctions of the isolated systems. On the other hand, the spatial characteristics of the states have been little studied because of the diffraction limit of light. In this study, we examine plasmon and exciton interactions in silver nanoplate and organic J-aggregate hybrid structures using scanning near-field optical microscopy. We reveal that the light transmission is enhanced when the plasmon and exciton resonantly interact with each other. We visualize the spatial distribution of the interacting states and find that the interaction of the high-order plasmons with the exciton enables manipulation of the electronic states in a spatially resolved manner. This study demonstrates that the optical field can be spatially controlled via coupling of the elementary excitations

Plasmon Dephasing in Single Gold Nanorods Observed By Ultrafast Time-Resolved Near-Field Optical Microscopy

We applied time-resolved near-field optical microscopic measurements with ultrashort light pulses of ∼16 fs duration to observe plasmon dephasing processes in single gold nanorods. The correlation widths of the time-resolved signals obtained at each position on the nanorods were broadened compared with the autocorrelation width of the pulse because of the plasmon lifetime. The correlation width maps of the rods showed spatially oscillating patterns that look similar to the plasmon mode structures observed in the static near-field optical images. The spatial variation of the correlation widths was explained as arising from the position-dependent contribution of the resonant plasmon excitation in the time-resolved signals relative to that of the nonresonant excitation. This finding indicates that the dephasing times of the resonant plasmon modes were constant regardless of the excitation position. This result is understood to be a consequence of the spatial coherence of the plasmon mode that causes the local excitation to be immediately delocalized across the rod after irradiation. A comparison between the time-resolved signals of the inner parts and the outer parts of the nanorods suggests that the nonresonant contribution to the time-resolved signals may be driven by the lower-order plasmon modes having resonances in a much longer wavelength region

Plasmon Dephasing and Near-Field Enhancement of Periodical Arrays of Au Nanogap Dimers

The manipulation of near-field enhancements in plasmonic nanostructures is essentially significant in boosting the performance of plasmon-enhanced near-fields in various applications. Far-field coupling induced in the periodically arrayed plasmonic nanostructures offers a promising platform for manipulating not only the near-field enhancements but also the dephasing dynamics in plasmonic nanostructures. In this study, we fabricated periodic arrays of Au nanoblock dimers with various pitch sizes and systematically investigated the far-field coupling effect on the plasmon dephasing time. Ultrafast time-resolved measurements revealed that the pitch size of the arrays crucially influences the plasmon dephasing of the Au nanoblock dimers. The observed pitch-size dependency of the dephasing time was qualitatively reproduced by electromagnetic simulations. We also simulated near-field distributions on the arrays and found that the far-field coupling enables us to manipulate the near-field enhancement without impairing the mode volume of the plasmonic nanostructures. Our study provides a deeper understanding of the plasmon dephasing of the periodically arrayed nanostructures and gives fruitful information for optimizing plasmonic near-field enhancements in various applications

Exploring Hybrid States and Their Ultrafast Dynamics in Exciton–Plasmon Strong Coupling Systems

To enhance the interaction between light and matter, it is crucial to confine light into minute spaces while simultaneously slowing it down. Plasmon resonance has been a principle used to amplify the interaction between light and matter by acting as a nanoscale optical resonator. However, their light confinement capability is limited, indicating a short phase relaxation time. Here, we explored the possibility of extending this phase relaxation time through strong coupling to long-lived excitons. Initially, estimation from the width of the far-field spectrum suggested that the spectral width of the exciton–plasmon strong coupling system narrowed compared to the plasmon bandwidth, hinting at an extension of the phase relaxation time. In the excitation spectrum measurements, we not only demonstrated the extended phase relaxation time similar to the analysis results from the far-field spectrum but also successfully highlighted the clear formation of hybrid states based on strong coupling. Ultrafast time-resolved measurements and electromagnetic simulations employing the finite-difference time-domain method further revealed the extended lifetime of the exciton–plasmon hybrid structure compared to the precoupled plasmon, foreseeing applications in nonlinear photochemical reaction fields based on enhanced electromagnetic field derived from the extension of phase relaxation time