Electromagnetic enhancement of one-dimensional plasmonic hotspots along silver nanowire dimer examined by ultrafast surface enhanced fluorescence

We investigated the spectral properties of electromagnetic (EM) enhancement of one-dimensional hotspots (1D HSs) generated between silver nanowire (NW) dimers. The EM enhancement spectra were directly derived by dividing the spectra of ultrafast surface-enhanced fluorescence (UFSEF) from single NW dimers with UFSEF obtained from large nanoparticle aggregates, which aggregate-by-aggregate variations in the UFSEF spectra were averaged out. Some NW dimers were found to exhibit EM enhancement spectra that deviated from the plasmon resonance Rayleigh scattering spectra, indicating that their EM enhancement was not generated by superradiant plasmons. These experimental results were examined by numerical calculation based on the EM mechanism by varying the morphology of the NW dimers. The calculations reproduced the spectral deviations as the NW diameter dependence of EM enhancement. Phase analysis of the enhanced EM near fields along the 1D HSs revealed that the dipole-quadrupole coupled plasmon, which is a subradiant mode, mainly generates EM enhancement for dimers with NW diameters larger than ~80 nm, which was consistent with scanning electron microscopic measurements

Unified evaluation of surface-enhanced resonance Raman scattering and fluorescence under strong coupling regime

We demonstrate importance of molecular multiple excitons and higher-order plasmons for both enhancement and quenching of resonance Raman and fluorescence of single dye molecule located at plasmonic hotspot under strong coupling regime. The multiple excitons induce complicated spectral changes in plasmon resonance and higher-order plasmons yield drastic quenching for both resonant Raman and fluorescence. A coupled oscillator model composed of plasmon and multiple excitons reproduces the complicated spectral changes. Purcell factors derived from higher-order plasmons reproduce the drastic quenching with considering ultra-fast surface enhanced fluorescence

Spectral correlation between surface-enhanced resonant Raman and far field scattering destructed by dipole quadrupole coupled plasmon resonance

The spectral relationships between surface enhanced resonant Raman scattering (SERRS) and plasmon resonance observed in far field scattering cross are investigated using single silver nanoparticle dimers with focusing on the lowest energy (superradiant) plasmon resonance. We find that these relationships can be classified into two types. The first is SERRS spectral envelopes with spectral shapes similar to those of plasmon resonance spectra. The second is SERRS envelopes exhibiting higher energy shifts from the plasmon resonance spectra. These results are examined as an effect of degree of morphological asymmetry in dimers based on an electromagnetic (EM) mechanism. The analysis of the first and second types reveals that dipole-dipole and dipole-quadrupole coupled plasmon resonance (subradiant Fano resonance) respectively determine the EM enhancement. This mechanism is commonly important for the development of plasmonic nanostructures for various surface enhanced spectroscopies

Contribution of Subradiant Plasmon Resonance to Electromagnetic Enhancement in Resonant Raman with Fluorescence Examined by Single Silver Nanoparticle Dimers

We investigate the spectral relationships between electromagnetic (EM) enhancement and surface-enhanced resonant Raman scattering (SERRS) with surface-enhanced fluorescence (SEF), which is observed as background emission of SERRS, in the context of light–matter interactions between subradiant plasmons and molecular excitons, using single silver nanoparticle dimers as a model system. We focus on the lowest-energy (superradiant) plasmon in far-field scattering to examine EM enhancement. We classify the spectral relationships into two types: those in which the spectral envelopes of SERRS with SEF have spectral shapes similar to those of plasmon resonance and those in which the spectral envelopes of SERRS with SEF exhibit higher energy shifts than the plasmon resonance. By examining these results, we aim to determine the degree of morphological asymmetry in the dimers based on an EM mechanism. Our analysis of the two types of spectral relationships reveals that dipole–dipole and dipole–quadrupole-coupled plasmon resonance (subradiant resonance) are responsible for EM enhancement

Surface Plasmon Resonance Near-Infrared Spectroscopy

Near-infrared (NIR) spectroscopy is ill-suited to microanalysis because of its low absorptivity. We have developed a highly sensitive detection method for NIR spectroscopy based on absorption-sensitive surface plasmon resonance (SPR). The newly named SPR−NIR spectroscopy, which may open the way for NIR spectroscopy in microanalysis and surface science, is realized by an attachment of the Kretschmann configuration equipped with a mechanism for fine angular adjustment of incident light. The angular sweep of incident light enables us to make a tuning of a SPR peak for an absorption band of sample medium. From the dependences of wavelength, incident angle, and thickness of a gold film on the intensity of the SPR peak, it has been found that the absorbance can be enhanced by ∼100 times compared with the absorbance obtained without the gold film under optimum conditions. This article reports the details of the experimental setup and the characteristics of absorption-sensitive SPR in the NIR region, together with some experimental results obtained by using it

Classification of La³⁺ and Gd³⁺ Rare-Earth Ions Using Surface-Enhanced Raman Scattering

In this study, surface-enhanced Raman scattering (SERS) spectra of different rare earth (RE) ion-citrate complexes were investigated for the first time for the qualitative classification of RE3+ ions. With the addition of RE3+ ions to citrate-capped silver nanoparticles in aqueous solutions, the Raman signals of RE-citrate complexes were enhanced, and characteristic peaks appeared near 1065 and 1315 cm–1. The I1065/I1315 ratios of La-citrate and Gd-citrate were approximately 1 and 0.55, respectively. Thus, different RE3+ ions were classified based on the ratio of characteristic SERS peaks near 1065 and 1315 cm–1. In addition, the effects of RE3+ ions in the RE-citrate complexes were analyzed based on density functional theory (DFT) calculations. Calculation results show that these characteristic peaks are attributed to the coordination of the C–O bond and COO– groups of citrates with the RE3+ ions, suggesting that these are spin-related bands of the RE-citrate complexes

Absorption cross-section spectroscopy of single strong coupling system between plasmon and molecular exciton resonance using single silver nanoparticle dimer generating surface enhanced resonant Raman scattering

This study investigated spectral changes in the absorption cross-sections of single strong coupling systems composed of single silver nanoparticle dimers and a few dye molecules during the quenching of surface-enhanced resonant Raman scattering (SERRS). The absorption cross-section was obtained by subtracting the scattering cross-section from an extinction cross-section. The spectral changes in these cross-sections were evaluated using a classical hybridization model composed of a plasmon and a molecular exciton including a molecular multi-level property. The changes in the scattering and extinction cross-sections exhibit blue-shifts in their peak energy and increased peak intensities, respectively, during SERRS quenching. These properties are effectively reproduced in the model by decreasing the coupling energy. In particular, the peaks in the scattering and extinction cross-sections appear as peaks or dips in the absorption cross-sections depending on the degree of scattering loss, which reflects the dimer sizes. These results are useful for optimizing photophysical and photochemical effects mediated by the electronic excited states of strong coupling systems

Propagation mechanism of surface-enhanced resonant Raman scattering light through one-dimensional plasmonic hotspot along silver nanowire dimer junction

We investigate the propagation of surface-enhanced resonant Raman scattering (SERRS) light by several micrometers through a one-dimensional hotspot (1D HS) located between a plasmonic nanowire dimer (NWD). The propagation exhibits the properties, e.g. an effective propagation induced by excitation and detection polarization orthogonal to the 1D HS long axis, the propagation profiles composed of bright short and dark long propagations, SERRS spectral shapes independent of localized plasmon (LP) resonance of NWDs, redshifts in the SERRS spectra at the edges of 1D HSs, and considerable NWD-by-NWD variations in the propagation lengths. These properties are well reproduced by numerical calculations based on electromagnetism. These calculations reveals the following propagation mechanism: excitation light resonantly coupled with LP at the edges of 1D HSs, and the light energy is transferred to two types of junction SP modes supporting the short and long propagations; these modes are attributed to the upper and lower branches of coupled two SP modes. This mechanism comprehensively clarifies the abovementioned properties

Surface-Enhanced Phosphorescence Measurement by an Optically Trapped Colloidal Ag Nanoaggregate on Anionic Thiacarbocyanine H‑Aggregate

A citrate-reduced Ag nanoaggregate was optically trapped on a fiber-shaped H-aggregate of an anionic thiacarbocyanine dye against Coulomb repulsion by focusing a near-infrared (NIR) laser beam. As the NIR laser power increased, namely, as the Ag nanoaggregate approaches the H-aggregate, phosphorescence from the H-aggregate with the Ag nanoaggregate excited moderately at 514 and 647 nm was strengthened, although that at 568 nm was weakened. By excitation at 568 nm, which was close to a surface plasmon resonance peak of the Ag nanoaggregate, surface-plasmon-enhanced optical trapping potential well might have deepened, and then the Ag nanoaggregate might have approached the H-aggregate too closely to enhance the phosphorescence because of energy transfer to the metal. As the excitation laser intensity increased, namely, as the surface-plasmon-enhanced optical trapping potential well was deepened, the phosphorescence enhancement factor trended upward and then downward by enhancement due to plasmon at a close distance from the Ag surface and the energy transfer at the closer distance, respectively

Truncated Power Law Analysis of Blinking SERS of Thiacyanine Molecules Adsorbed on Single Silver Nanoaggregates by Excitation at Various Wavelengths

From blinking surface-enhanced Raman scattering (SERS) of anionic thiacyanine adsorbed on single Ag nanoaggregates, the electromagnetic field and the molecular behavior in a nonemissive state were investigated by a truncated power law analysis. The power law that reproduces probability distribution of dark SERS events versus duration time was not truncated often by excitation at long wavelengths; otherwise it was truncated at the long tail. The truncation suggests a high energy barrier from nonemissive to emissive state and a short passage time of molecular random walk to overcome the energy barrier. The energy barrier in blinking SERS likely originates from a nanometer-ordered periodic optical trapping potential well, namely, electromagnetic field around a junction of the Ag nanoaggregate due to coupling of multipolar surface plasmon resonance, which is hardly induced by excitation at long wavelengths. This is consistent with the experimental excitation wavelength dependence of the truncation. At a low concentration of anionic thiacyanine, the power law was truncated at the short tail. The reason may be the short passage time of the molecule on the Ag surface adsorbing a small number of obstacles to reach the junction