Photoluminescence Properties of Gold Nanorod and <i>J</i>‑Aggregate Hybrid Systems Studied by Scanning Near-Field Optical Microscopy

Plasmons excited in metal nanostructures couple strongly with excitons in organic aggregates in the vicinity of the structure. The photoluminescence properties of plasmon–exciton hybrids have been studied, and peak splitting of the photoluminescence has been reported. However, the origin of the splitting is under discussion and remains to be solved. In this study, we investigate the photoluminescence properties of single-gold nanorod and J-aggregate hybrids using dark-field scattering and near-field optical microscopy. We reveal from the dark-field scattering and near-field transmission measurements that the hybrids are under a strong coupling regime. Near-field photoluminescence microscopy demonstrates that photoluminescence enhancement at the hybrid reaches more than 15-fold, and the enhancement is correlated with the reduced damping in the coupled states

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

Selective Excitation of Dark Plasmon Modes Using Cylindrical Vector Beams Studied by Microscopic Imaging of Nonlinear Photoluminescence

Noble metal nanostructures exhibit multiple plasmon modes with different spatial characteristics and resonance energies. Plasmons confine electromagnetic fields in the vicinity of nanostructures, and the confined field has been utilized for various applications, such as sensing and chemical reactions. The plasmon mode with no net polarization is optically dark, and thus, it is not accessible by plane wave excitation. The dark plasmon mode exhibits a long dephasing time due to suppression of the radiative decay process. As this feature is advantageous for applications, development of a novel excitation scheme for dark modes is indispensable. Optical selection rules of a plasmon mode are determined by the spatial symmetry of the plasmon mode and excitation field. Cylindrical vector beams possess unique spatial polarization characteristics that are different from the linearly and circularly polarized light and are capable of the excitation of the dark plasmon modes. This study examines the nonlinear photoluminescence properties of gold nanoplates excited by radially and azimuthally polarized beams and demonstrates that the spatial characteristics of the excitation images are strongly dependent on the excitation field. Electromagnetic simulations support the findings that selective excitation of the dark plasmon mode is feasible by the cylindrical vector beams

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

Nanoantenna Effect at the Center of the Bull’s Eye Pattern by Controlling the Refractive Indices and Layer Thicknesses of Dielectric Media on a Silver Surface

The light that is illuminated on a silver-film-coating substrate with a periodic structure, i.e., a plasmonic chip, can couple to plasmon polaritons and enhance the electric field on the surface of the chip. Fluorescent molecules fixed to the plasmonic periodic pattern are excited by an enhanced electric field, enhancing their fluorescence. Particularly, a bright fluorescence point appears at the center of a concentric circle pattern called a Bull’s eye pattern. This nanoantenna effect has been studied in various types of concentric circles and has been comprehended by a constructive wave superposed with diffraction light on the grooves of a plasmonic pattern. Here, the antenna effect of fluorescent nanoparticles immobilized on the chip surface was studied based on the controlling factors of the surface plasmon resonance wavelengths, such as the pitch of a pattern, the refractive index, and the layer thickness of the dielectric media on the silver film, and it was improved by their factors. The pitches of the plasmonic patterns were set at 400 and 480 nm, and the nanoantenna rate (Ap) of the 480 nm pitch was higher than that of the 400 nm pitch when a 20 nm thick SiO2 layer was used. By changing the refractive index of the dielectric media on the silver film from 1.45 (silica layer) to 2.10 (zinc oxide layer), Ap increased at a 400 nm pitch. These results were well explained by a constructive wave that was formed by the superposition of the diffraction waves on the grooves at the center of the pattern. The most enhanced antenna effect was found to be obtained by controlling the pitch of a plasmonic chip as the resonance wavelength is adjusted to the excitation wavelength. Conversely, the distance from the silver surface was controlled using silica layer thicknesses of 20 and 80 nm, and Ap increased remarkably at 480 nm pitch for the 80 nm thick silica layer. This result was supported by the electric field intensities at the center and edge calculated by discrete-dipole approximation, revealing that the distance factor can contribute to the electric field intensities of propagated waves. The nanoantenna effect could be enhanced by the pitch and dielectric media prepared on the silver film