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

Space-Selective Fabrication of Light-Emitting Carbon Dots in Polymer Films Using Electron-Beam-Induced Chemical Reactions

Nanocarbon-based materials have excellent properties, including high electrical conductivity as well as charity-dependent optical absorption and luminescence; therefore, the materials are promising in applications for nanoelectric devices, nanophotonics, and so on. Carbon dots (CDs) are one of the carbon materials recently fabricated. Optical properties of CDs have been reported to be similar to those of polycyclic aromatic hydrocarbons (PAHs). For this reason, the CDs are considered to be composed of PAH. Synthesis of CDs has previously been accomplished through hydrothermal synthesis and microwave irradiation. These methods require a long synthesis time, and the processes involve multiple steps. In this study, we developed a fabrication method of CDs in simple and spatially selective ways, by using radical reactions in an organic polymer film with focused electron-beam irradiation. We investigated various organic polymers as reaction materials and found that polystyrene has a higher efficiency for CD formation than other organic polymers investigated. Absorption, photoluminescence, and Raman scattering properties of the electron-beam-irradiated sample were in good agreement with those reported for the CDs. The technique developed in this study is promising for fabricating light-emitting CDs and photonic crystals in a simple and flexible manner

Enhanced and Polarized Photoluminescence from Carbon Dot–Metal Nanoparticle Composites

Carbon dots are attracting much attention because of their low toxicity, biocompatibility, and photostability. In this study, we fabricate carbon dot–metal nanoparticle composites using electron-beam-induced chemical reactions to improve the photoluminescence characteristics. We investigate the spectral characteristics of the composites by linear and nonlinear optical spectroscopy. The composites show plasmon resonance depending on the material used. The linear and nonlinear photoluminescence from the composites are enhanced by plasmon excitations, and the enhancement reaches nearly several times and 20 times, respectively. We also fabricate nanowire structures containing the composites. The structures show very intense photoluminescence and unique polarization characteristics depending on the wire width. We reveal that the polarization characteristics originate from the plasmonic coupling along the short axis of the wire. The technique developed in this study is promising for the polarization control of light-emitting elements and the creation of new functional nanomaterials

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

Development of Absorption and Scattering Microspectroscopy Using a Phase-Stepping Method

Nanoparticles with subwavelength dimensions exhibit unique optical properties due to strong light-matter interactions and have received much attention in not only fundamental science but also applied science. The optical properties of the nanoparticles include both light scattering and absorption, and for practical applications, detailed knowledge of the scattering and absorption properties at the single nanoparticle level is indispensable. For this purpose, the development of a novel spectroscopic method for achieving simultaneous measurement of the absorption and scattering properties is desirable. The phase-stepping modulation technique enables the demodulation of a spectroscopic component for overlapped components and is promising for this purpose. In this study, we developed a novel spectroscopic technique by combining bright-field optical microscopy with a phase-step modulation method to simultaneously measure the absorption and scattering properties of nanoparticles

Plasmon Mode Imaging of Single Gold Nanorods

We have investigated two-photon-induced photoluminescence images and spectra of single gold nanorods by using an apertured scanning near-field optical microscope. The observed PL spectrum of single gold nanorod can be explained by the radiative recombination of the electron−hole pair near the X and L symmetry points. PL images reveal characteristic features reflecting an eigenfunction of a specific plasmon mode as well as electric field distributions around the nanorod

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

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

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

Observation of Plasmon Wave Packet Motions via Femtosecond Time-Resolved Near-Field Imaging Techniques

The generation and dynamics of plasmon wave packets in single gold nanorods were observed at a spatiotemporal scale of 100 nm and 10 fs via time-resolved near-field optical microscopy. Following simultaneous excitation of two plasmon modes of a nanorod with an ultrashort near-field pulse, a decay and revival feature of the time-resolved signal was obtained, which reflected the reciprocating motion of the wave packet. The time-resolved near-field images were also indicative of the wave packet motion. At some period of time after the excitation, the spatial features of the two modes appeared alternately, showing motion of plasmonic wave crests along the rod. The wave packet propagation was clearly demonstrated from this observation with the aid of a simulation model. The present experimental scheme opens the door to coherent control of plasmon-induced optical fields in a nanometer spatial scale and femtosecond temporal scale