Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications.

Surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS) have opened a variety of exciting research fields. However, although a vast number of applications have been proposed since the two techniques were first reported, none has been applied to real practical use. This calls for an update in the recent fundamental and application studies of SERS and TERS. Thus, the goals and scope of this review are to report new directions and perspectives of SERS and TERS, mainly from the viewpoint of combining their mechanism and application studies. Regarding the recent progress in SERS and TERS, this review discusses four main topics: (1) nanometer to subnanometer plasmonic hotspots for SERS; (2) Ångström resolved TERS; (3) chemical mechanisms, i.e., charge-transfer mechanism of SERS and semiconductor-enhanced Raman scattering; and (4) the creation of a strong bridge between the mechanism studies and applications.

Electromagnetic enhancement spectra of one-dimensional plasmonic hotspots along silver nanowire dimer derived via surface-enhanced fluorescence.

We developed a spectroscopic method for directly obtaining the spectra of electromagnetic (EM) enhancement of plasmonic hotspots (HSs). The method was applied to one-dimensional (1D) HSs generated between silver nanowire (NW) dimers. The EM enhancement spectra were derived by dividing the spectra of surface-enhanced fluorescence (SEF) from single NW dimers with SEF obtained from large nanoparticle aggregates, where aggregate-by-aggregate variations in the SEF 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 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.

Demonstration of electromagnetic enhancement correlated to optical absorption of single plasmonic system coupled with molecular excitons using ultrafast surface-enhanced fluorescence.

The relationship between the electromagnetic (EM) enhancement of the optical responses of molecules and plasmon resonance has been investigated using Rayleigh scattering or the extinction spectra of plasmonic systems coupled with molecular excitons. However, quantum optics predicts that the EM enhancement of such optical responses, e.g., fluorescence, Raman, and their nonlinear counterparts, is related directly to optical absorption and indirectly to Rayleigh scattering and extinction. To demonstrate this prediction, a micro-spectroscopic method for obtaining Rayleigh scattering, extinction, absorption, and EM enhancement is developed using single-coupled plasmonic systems composed of silver nanoparticle dimers and dye molecules. The EM enhancement is derived from ultrafast surface-enhanced fluorescence. An evaluation of the spectral relationships demonstrates that the EM enhancement can be reproduced better by absorption than by Rayleigh scattering or extinction. This reproduction is phenomenologically confirmed by numerical calculations based on classical electromagnetism, indicating the importance of absorption spectroscopy in coupled plasmonic systems for evaluating EM enhancement.

Anti-crossing property of strong coupling system of silver nanoparticle dimers coated with thin dye molecular films analyzed by electromagnetism.

Evidence of strong coupling between plasmons and molecular excitons for plasmonic nanoparticle (NP) dimers exhibiting ultra-sensitive surface enhanced resonant Raman scattering is the observation of anti-crossing in the coupled resonance. However, experimentally tuning the plasmon resonance of such dimers for the observation is difficult. In this work, we calculate the anti-crossing property of dimers coated with thin dye films according to the classical electromagnetism. This property is quantitatively evaluated according to the coupled oscillator model composed of a plasmon and a molecular exciton representing the molecular multi-level system. A comparison of the film thickness dependences of dimer spectral changes with those of silver ellipsoidal NPs indicates that the dipole plasmons localized in the dimer gap are coupled with molecular excitons of the film much stronger than the dipole plasmons of ellipsoidal NPs. Furthermore, the anti-crossing behavior of coupled resonances is investigated while tuning plasmon resonance by changing the morphology and refractive index of the surrounding medium. The spectral changes observed for ellipsoidal NPs clearly exhibit anti-crossing property; however, the anti-crossing behavior of dimers is more complex due to the strong coupling of dipoles and higher-order plasmons with multiple molecular excitons. We find that the anti-crossing for dimers is clearly confirmed by the refractive index dependence of coupled resonance.

Reproduction of surface-enhanced resonant Raman scattering and fluorescence spectra of a strong coupling system composed of a single silver nanoparticle dimer and a few dye molecules.

The spectral changes in surface-enhanced resonant Raman scattering (SERRS) and surface enhanced fluorescence (SEF) of single silver nanoparticle dimers adsorbed by near-single dye molecules are reproduced under strong coupling regimes. For the reproduction, the enhancement and quenching factors in SERRS and SEF are derived from the Purcell factors including both radiative and nonradiative plasmon modes. The Purcell factors are estimated using the coupling energies obtained by analyzing the spectral changes in plasmon resonance during SERRS and SEF decay processes on the basis of a classical hybridization model. The model is composed of a plasmon and a molecular exciton with phonon replicas accurately representing the molecular multi-level system. The reproduced SERRS spectral changes are consistent with the experimental ones. Furthermore, the calculated SEF spectral changes can reproduce the experimental ones by phenomenologically assuming transitions from ultra-fast SEF to conventional SEF with decreasing coupling energies.

Plasmon-enhanced spectroscopy of absorption and spontaneous emissions explained using cavity quantum optics.

The purpose of this tutorial review is to provide a comprehensive explanation of plasmon-enhanced spectroscopies, such as plasmon-enhanced Raman scattering, fluorescence, absorption, Rayleigh scattering, and hyper Raman scattering. Plasmon-enhanced spectroscopy implies the spectroscopy of enhanced optical responses of molecules in close proximity to plasmonic nanostructures, resulting in a strong enhancement in sensitivity. In this review, we explain the enhancement in plasmon-enhanced spectroscopy as an optical response of a molecule interacting with an optical resonator, which represents a plasmonic nanostructure, in analogy to cavity quantum optics to easily understand all types of plasmon-enhanced spectroscopy in the same manner. The keys to understanding the enhancement factor of each plasmon-enhanced spectroscopy are a quality factor and a mode volume of plasmonic resonators, which are well-known parameters in the Purcell effect of standard optical cavity resonators.

Recent topics on single-molecule fluctuation analysis using blinking in surface-enhanced resonance Raman scattering: clarification by the electromagnetic mechanism.

Surface-enhanced Raman scattering (SERS) spectroscopy has become an ultrasensitive tool for clarifying molecular functions on plasmonic metal nanoparticles (NPs). SERS has been used for in situ probing of detailed behaviors of few or single molecules (SMs) at plasmonic NP junctions. SM SERS signals are commonly observed with temporal and spectral changes known as "blinking", which are related to various physical and chemical interactions between molecules and NP junctions. These temporal and spectral changes simultaneously take place, therefore resulting in serious complexities in interpretations of the SM SERS results. Dual contributions of Raman enhancement mechanisms in SERS (i.e., electromagnetic (EM) and chemical enhancements) also make interpretations more difficult. To resolve these issues and reduce the degree of complexities in SM SERS analyses, the present review is focused on the recent studies of probing SM behaviors using SERS exclusively within the framework of the EM mechanism. The EM mechanism is briefly introduced, and several recent topics on SM SERS blinking analysis are discussed in light of the EM mechanism. This review will provide a basis for clarification of complex SERS fluctuations of various molecules.

Different behaviour of molecules in dark SERS state on colloidal Ag nanoparticles estimated by truncated power law analysis of blinking SERS.

For single colloidal Ag nanoaggregates, covered with either large or small amounts of citrate anions, blinking surface-enhanced Raman scattering (SERS) of anionic thiacyanine was measured and analyzed by a truncated power law. The power law without and with an exponential function reproduces a probability distribution for bright and dark SERS events versus their duration times, respectively. On the Ag surface, except for junctions of the nanoaggregate with a large or small amount of the citrate anions, two-dimensional fast or one-dimensional slow random walk of the anionic thiacyanine, respectively, was estimated by the exponents and the truncation times in the power law for the dark SERS events. In addition, the power law exponents for the bright SERS events were derived to be of similar values, indicating a similar molecular random walk near the junction, which may be dominated evenly by a surface-plasmon-enhanced electromagnetic field on the same-sized Ag nanoaggregate. Thus, not only the bright SERS, but also the dark SERS molecular behaviour on the Ag surface was investigated by the truncated power law analysis.

Plasmonic imaging of brownian motion of single DNA molecules spontaneously binding to Ag nanoparticles.

We find the spontaneous binding of single DNA molecules to uncoated silver nanoparticles (AgNPs) in aqueous solution with Mn(2+) (3 mM). From dark-field optical microscopic imaging of AgNPs bound to DNA molecules, we demonstrate analysis of the Brownian motion of single DNA molecules via plasmon resonance elastic light scattering. Our results provide that the plasmonic imaging technique is free from photobleaching and blinking and thus is useful in long-time observations of single-molecule DNA dynamics.

Plasmonic staining of DNA molecules with photo-induced Ag nanoparticles monitored using dark-field microscopy.

We demonstrate a dark-field microscopic technique for real-time monitoring of DNA metallization. The growth of silver nanoparticles on DNA molecules was observed using plasmon resonance light scattering in the presence of a weak catalyst triggering photo-reduction of Ag(+). This simple strategy could facilitate the controlled fabrication of DNA-templated nanoelectronic devices.

Direct conversion of silver complexes to nanoscale hexagonal columns on a copper alloy for plasmonic applications.

We introduced a novel method for the rapid synthesis of silver nanohexagonal thin columns from an aqueous mixture of sodium thiosulfate (Na2S2O3) and silver chloride (AgCl) simply added to a phosphor bronze substrate. The reaction is based on galvanic displacement and the products are potentially useful for plasmonic applications.

Correlated Polarization Dependences between Surface-Enhanced Resonant Raman Scattering and Plasmon Resonance Elastic Scattering Showing Spectral Uncorrelation to Each Other.

We investigated the origin of the identical polarization angle dependence between surface-enhanced resonant Raman scattering (SERRS) and plasmon resonance elastic scattering (PRES) for two types of single silver nanoparticle aggregates. The first type (Type I), in which the SERRS spectral envelopes are similar to the PRES spectra, shows the identical polarization dependence between the SERRS and PRES. The second type (Type II), in which the SERRS envelopes largely deviate from the PRES spectra, also exhibits identical polarization dependence. Scanning electron microscopy observations indicated that the aggregates were dimers. This unintuitive result was examined by calculating the electromagnetic enhancement by changing the morphology of the dimers. The calculations revealed that the Type I dimer generates SERRS directly by superradiant plasmons. The Type II dimer generates SERRS indirectly via subradiant plasmons, which receive light energy from superradiant plasmons. This indirect SERRS process clarifies that the interaction between the superradiant and subradiant plasmons results in an identical polarization dependence between SERRS and PRES for Type II dimers.

Between plasmonics and surface-enhanced resonant Raman spectroscopy: toward single-molecule strong coupling at a hotspot.

The purpose of this minireview is to build a bridge between two research fields: surface-enhanced resonant Raman spectroscopy (SERRS) under near-single-molecule conditions and the branch of plasmonics treating strong coupling between plasmons and molecular excitons. SERRS enables single-molecule spectroscopy owing to its significant enhancement at SERRS hotspots (HSs), localized at gaps or junctions between plasmonic nanoparticle aggregates. SERRS is SERS (surface enhanced Raman spectroscopy) under a resonant Raman excitation condition. The origin of the Raman enhancement in SERRS is electromagnetic coupling between plasmons and molecular excitons at HSs. It has been reported that the coupling energy at HSs reaches the strong coupling region, meaning that they are potential platforms for applications of single molecular excitons modified by strong coupling. In this review, we discuss recent progress related to electronic strong coupling in near-single-molecule SERRS: collective (e.g., vibrational) strong coupling is out of the scope of this minireview. First, we explain the relationship between the electromagnetic enhancement factor and coupling energy. Second, we introduce three theoretical methods for obtaining evidence of strong coupling at HSs. Third, we discuss a method for reproducing enhanced and modified molecular Raman and fluorescence spectra at HSs using the coupling energy. Finally, we propose the use of two experimental methods of absorption spectroscopy at HSs for modifying molecular electronic dynamics by strong coupling and comment on future applications of SERRS HSs to photophysics and photochemistry.

High axial resolution Raman probe made of a single hollow optical fiber.

A ball lens mounted hollow optical fiber Raman probe (BHRP) consisting of a single hollow optical fiber (HOF) and a micro-ball lens was developed for performing a high axial resolution and high-sensitivity remote Raman analysis of biomedical tissues. The total diameter of the probe head is 640 microm. The BHRP is useful in the measurement of thin-layered tissues that are in contact with the probe's surface because the probe has a limited depth-of-field optical property. An optical calculation study suggested that it is possible to vary the probe's working distance by selecting different materials and diameters for the ball lens. Empirical studies revealed that this probe has a higher axial resolution and a higher sensitivity than an HOF Raman probe without the ball lens. The spectrum of a mouse stomach measured with the BHRP had better quality and considerably lower noise than that measured with a conventional Raman microscope. These results strongly suggest that the BHRP can be used effectively in biomedical applications.

Noninvasive subsurface analysis using multiple miniaturized Raman probes, part I: basic study of thin-layered transparent models of biomedical tissues.

This study describes a basic theory for reconstructing pure Raman signals of materials composing a multilayer sample from Raman spectra obtained using two types of miniaturized Raman probes. An illustrative example is demonstrated using a multilayer system of samples composed of the transparent plastics polymethylmethacrylate (PMMA) and polyethylene (PE) as a model of thin-layered biomedical tissues. When the same region of an object is measured using Raman probes with different focal properties, the Raman spectra provide different depth profile information depending on the level of light penetration. Thus, a detailed comparison of the spectra can provide an interesting opportunity to probe the differences between the layers. A simple analytic form is presented for reconstructing the pure Raman spectra of the embedded layer. The method applies an understanding of the Raman sampling volume in layered transparent materials to the interpretation of Raman spectra experimentally measured by multiple probes. The basic theory described here is necessary for the expansion of the technique to turbid media, such as biological samples, where light-scattering effects must be considered. The potential applications of the proposed method include material and catalyst subsurface probing through different embedded materials, such as assessment of silicon wafers, effective noninvasive screening for catalyst synthesis, and biomedical tissue research.

Subsurface sensing of biomedical tissues using a miniaturized Raman probe: study of thin-layered model samples.

A ball lens hollow-fiber Raman probe (BHRP) is a powerful tool for in vivo nondestructive subsurface analysis of biomedical tissues in a living body. It has confocal-like optical properties, but its collection volume is rather large in comparison with that of a conventional confocal Raman system. Therefore, the obtained Raman spectra have contributions from the upper and lower layers at different rates depending on the thickness of the upper layer when the measurement point is close to the boundary surface of the two layers. In the present study, we describe a methodology to extract quantitative information about the thickness of the subsurface layer structure by using a BHRP combined with the partial least-square regression (PLSR) analysis. The simulation study indicates that distribution of the collection efficiency in the collection volume of the BHRP is similar to a Gaussian distribution. The empirical study suggests that the PLSR model built with only a principal component (PC) 1 based on the linearized depth data gives good prediction.