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Simultaneous observation of Tamm plasmon (TP) and surface plasmon (SP) resonances has attracted much attention to improve the sensing performance and broaden the application field of plasmonic biosensors. Currently, enormous effort is devoted to simultaneously excite both SP and TP modes by exploiting total internal reflection. However, this method requires bulky and complicated prism or microscopy setup to fulfill the phase-matching condition, limiting the miniaturization and integration of sensors. Herein, we propose and theoretically demonstrate a nanograting-coupled plasmonic-photonic multilayer structure potentially integrated on the optical fiber tip, comprising a multilayer of photonic crystal inserted between a flat gold film and a gold nanograting. The proposed plasmonic-photonic multilayer structure enables the simultaneous excitation of TP and SP modes at the normal incidence by using zeroth and first-order diffraction light of nanograting, respectively. The resonant positions of both SP and TP modes can be independently adjusted by changing the nanograting period and the center wavelength of photonic crystal bandgap. Due to the insensitivity of TP mode to ambient surroundings, the proposed structure can be served as a self-reference SP resonance sensor for robust measurement where the sensitivity of the SP resonance sensor is determined by the wavelength shift of SP mode relative to TP mode. Moreover, the spectral overlapping of both TP and SP modes resulting in the abrupt change of reflection intensity, enables a rapid judgment to the refractive index of ambient surroundings by observing the red or blue shifts of SP resonant position relative to the case of TP. The simultaneous excitation of both TP and SP modes in the proposed structure opens a new path for improving detection accuracy and broadening application scenarios of fiber-optic SP resonance sensors.

A new magnetic ionic liquid (MIL) based on 1-methylethyl ether-3-butylimidazole and [FeCl4]− is synthesized for application in electrolyte. It is found that the electrochemical performance of magnetic ionic liquid based mixed electrolyte can be improved by applying magnetic field. The enhanced electrochemical performance is attributed to the formation of microdomain in the mixed electrolyte under magnetic field. The ions of MIL can align along the direction of the magnetic field, providing the efficient transmission path for the migration of Li+ and Cl−. Under magnetic control, the MI-based electrolyte does not only retain its wide electrochemical window characteristics, but also solves the problem of limiting the high viscosity of IL as electrolyte. The work provides a new method to design and prepare high-performance electrolyte for various electrochemical applications.

Ring modes with large wave vectors cannot be easily excited on a single disk by the plane wave illumination with the polarization parallel to the disk interface. In this work, we show that special antisymmetric ring gap modes on the surface of the disk in close proximity to the metallic thin film can be excited in the visible light region of the electromagnetic spectrum. In the presence of the film, the strong plasmon interaction between disk and film causes ring gap modes to have lower energies and be more easily excited. We apply the plasmon hybridization method to illustrate the ring gap modes arising from the interaction between the localized disk plasmons and the continuum surface plasmons. The calculated hybridization data show good agreement with the results of finite element simulations. The excitation of ring gap modes provides further insight into the strong coupling of plasmons and the design of novel nanostructures.

All-inorganic halide perovskite nanowires (NWs) are promising materials due to they have broad application prospects in the field of optoelectronics, with mixed-halide perovskite nanowires can change the optoelectronic properties by adjusting the halide ratio. Here, we experimentally investigated the two-process governed anion-exchange reaction in single-crystalline CsPbX3 micro- and nanowires. The critical parameters affecting the outcome of the reaction are identified as the reaction temperature, reaction time, and precursor concentrations. Upon examining the photoluminescence and morphology of the NWs, high-quality NWs were obtained by optimizing these critical parameters. The bandgap of the NWs can be tuned over the entire visible spectra (430–700 nm). In addition, photodetectors incorporating single NWs were fabricated, which demonstrated excellent responsivity under illumination. Our results expand the validity of liquid-phase anion exchange to the microscale, and lay the basis for liquid-processed optoelectronics and displays.

The enhancement factor of surface-enhanced Raman scattering (SERS) is usually approximated by the fourth power of local electric field-enhanced strength. However, the applicability of the plane wave approximation used in this method remains a major problem in the theoretical study of SERS and the comparison with experiment results. In this work, we simultaneously calculated the local incident field enhancement factor under plane wave excitation and emission enhancement factor of a dipole in common SERS structures. For isolated particles and dimers, the local field enhancement factors are in good agreement with the emission enhancement factors. Nevertheless, the fourth power approximation method fails to predict the enhancement factors for structures on the substrate. The results show that when the SERS structures locate on the substrate, the local field method under-estimates the enhancement factor. The Purcell effect was used and compared with the emission enhancement. It indicates that the Purcell factor matches the emission enhancement quite well for the particles in homogeneous medium, while the Purcell factor over-estimates the emission enhancement for the particles on the substrate. These results are helpful for the SERS studies and the enhancement factor calculations.

Heterostructures play an irreplaceable role in high-performance optoelectronic devices. However, the preparation of robust perovskite heterostructures is challenging due to spontaneous interdiffusion of halogen anions. Herein, a vapor-phase anion exchange method universally suitable for the preparation of robust 2D Ruddlesden-Popper perovskite (RPP) heterostructures is developed. A variety of heterostructures are fabricated based on exfoliated RPP microplates (MPs). Depending on the specific organic cations, the heterostructures can be either sharp and uniform, or broad and gradient, suggesting a new anion diffusion behavior different from that in 3D perovskites. Further experimental studies reveal that the lateral transport of anions follows a threshold-dominating mechanism, while the vertical transport can be partially or completely suppressed by organic cations. Subsequently, quantitative investigation of anion diffusion in 2D perovskites is conducted. The lateral diffusion coefficient of halogen anions is calculated to be 6 to 7 orders of magnitude larger than the vertical coefficient, consistent with the observed highly anisotropic anion diffusion. In addition, it is shown that the anion exchange threshold can also enhance the thermodynamic stability of the heterostructures at elevated temperature. These results provide a general method to fabricate robust lateral RPP heterostructures, and offer important insights into anion behavior in low-dimensional perovskites.

Relationship of plasmonic properties of multiple clusters to molecular interactions and properties of a single cluster or molecule have become increasingly important due to the continuous emergence of molecular and cluster devices or systems. A hybrid phenomenon similar to plasmonic nanoparticle hybridization exists between two molecules with plasmon excitation modes. We use linear-response time-dependent density functional theory, real-time propagation time-dependent density functional theory, the plasmonicity index, and transition contribution maps (TCMs) to identify the plasmon excitation modes for the linear polyenes octatetraene with –OH and –NH2 groups and analyze the hybridization characteristics using charge transitions. The results show that molecular plasmon hybridization exists when the two molecules are coupled. The TCM analysis shows that the plasmon modes and hybridization result from collective and single-particle excitation. The plasmon mode is stronger, and the individual properties of the molecules are maintained after coupling when there is extra charge depose in the molecules because the electrons are moving in the molecules. This study provides new insights into the molecular plasmon hybridization of coupled molecules.

Fabry-Perot (FP) interference cavity for which the interference property is controlled by the tuning of the cavity length is widely used in many fields. However, in nanophotonic device applications...

Anion-exchange in halide perovskites provides a unique pathway of bandgap engineering for fabricating heterojunctions in low-cost photovoltaics and optoelectronics. However, it remains challenging to achieve robust and sharp perovskite heterojunctions, due to the spontaneous anion interdiffusion across the heterojunction in 3D perovskites. Here, it is shown that the anionic behavior in 1D perovskites is fundamentally different, that the anion exchange can readily drive an indirect-to-direct bandgap phase transition in CsPbI

Plasmonic chirality has attracted more and more attention recently due to the enhanced chiroptical response and its potential applications in biosensing. Plasmonic Fano resonance arises from the interference between a dark narrow resonance and a bright broad resonance, and it provides a new paradigm to control the plasmon mode interactions. Even though a strong circular dichroism (CD) effect has been predicted in chiral nanostructures with a Fano resonance, there are few experimental studies, and the correlation between the two effects is unclear. In this research, we investigate these two effects in plasmonic heterodimer nanorods in the same spectral range. We find that the heterodimer nanostructure exhibits a Fano-like resonance and Fano-like chiroptical response, both of which are correlated with the coupling between a super-radiant electric dipole and a sub-radiant magnetic dipole mode. Due to the interference nature of the Fano resonance, the Fano-like chiroptical response exhibits distinctively sharp features in a narrow spectral range. This Fano-like chiroptical response can be explained by a modified chiral molecule theory and a simplified coupled electric-magnetic dipole model. This research may provide new insight into the physics picture of plasmonic chirality and paves the way for the development of sensitive plasmonic sensors.

Plasmonic coupling between two or more metal nanostructures has attracted significant interest, since this fascinating near-field interaction is more effective for the aggregation/magnification of the incident light intensity compared to normal surface-plasmon-resonance (SPR) of single nanostructures. Herein, we report, for the first time, an IR-driven strong plasmonic coupling in an unusual metal/non-metal heterostructure system. This system is fabricated through random assemblage of Ag nanorices (NRs) onto W18O49 nanowires (NWs) film with F-doped SnO2 glass as substrate. Through the 3D-finite element simulation, we demonstrate that the plasmonic coupling between Ag NRs and W18O49 NWs significantly enhances the localized electric fields at the “hot spots”. This achieved magnitudes of 101-104 times of that of incident light, thus leading to the promoted generation of plasmonic “hot-electrons”. Moreover, the resonance excitation of plasmonic coupling on the heterostructures not only induces ultrafast electron transfer from W18O49 to Ag hetero-components, but also the photothermal effect to increase the localized temperature. Upon SPR-coupling excitation, the Ag/W18O49 heterostructures film exhibits a remarkable enhancement of the photo/thermal catalytic activity for H2 evolution from ammonia borane compared to that of either the Ag NRs (˜8.2 × ) or W18O49 NWs (˜9.0 × ) films. Notably, we observed an obvious H2 evolution over the Ag/W18O49 heterostructures film under natural sunlight irradiation.

In this review, the development context and scientific research results of chiral surface plasmons (SPs) in recent years are classified and described in detail. First, the principle of chiral SPs is introduced through classical and quantum theory. Following this, the classification and properties of different chiral structures, as well as the superchiral near-field, are introduced in detail. Second, we describe the excitation and propagation properties of chiral SPs, which lays a good foundation for the application of chiral SPs and their chiral spectra in various fields. After that, we have summarized the recent research results of chiral SPs and their applications in the areas of biology, two-dimensional materials, topological materials, analytical chemistry, chiral sensing, chiral optical force, and chiral light detection. Chiral SPs are a new type of optical phenomenon that have useful application potential in many fields and are worth exploring.

Microspectrometer features remarkable portability and integration, and is prospected to provide quick detection tools to daily life and revolutionary techniques to researchers. For the extremely finite footprint, microspectrometer can hardly work to analyze the polarization feature by placing polarizer in the optical path like conventional spectrometers. Here, we demonstrate a novel 2D wavelength-polarization dispersive microspectroscope based on carbon nanocoil with plasmonic Au nanopariticles (Au/CNC) as a dispersive component. Explored by the microspectrum and Fourier-space microscopy, a unique 2D dispersive characteristic of the Au/CNC is revealed. Along the axis of the coil, Au/CNC disperses light as wavelength to bands of different diffraction orders like a grating. Wavelength of the incident light can be obtained from the position of the signal lines in a quite large visible-near-infrared wavelength range with an acceptable resolution. In the direction perpendicular to the axis of the coil, incident light is dispersed as polarization with bright and dark areas. We can distinguish left- and right-circularly-polarized light, and also obtain the polarization orientation of linearly-polarized light. Based on this effect, a wonderful 2D wavelength-polarization microspectrometer can be built. It not only fulfills the wavelength analysis with a tiny dispersive component, but also simultaneously knows the polarization feature of the incident light in one shot. Moreover, this powerful tool can further evolve new revolutionary techniques via integrated with other systems., Comment: 30 pages including supporting information

Nanogaps are one of the most useful systems in nanooptics. The gap modes in a film coupled dielectric nanoparticle dimer system are influenced by both of the film and the electric and magnetic modes of the particles. In this work, strong confinement of gap modes of dielectric (Si) nanoparticle dimer on Au/Si film is investigated. The results show an abnormal electric field enhancement obtained between Si nanoparticle dimer on metal film, which is attributed to film coupled electric and magnetic dipole modes in dielectric nanoparticle dimer. The results are further analyzed with mode hybridization theory. Furthermore, the surface enhanced Raman spectroscopy (SERS) is performed to demonstrate these theoretical analyses. The film induced electromagnetic field redistribution in dielectric nanoparticle dimer not only extend the knowledge of dielectric gap modes but also has tremendous applications, e.g. light manipulating in subwavelength, light harvest, surface enhanced spectrum, etc.

Benefiting from the induced image charge on film surface, the nanoparticle aggregating on metal exhibits interesting optical properties. In this work, a linear metal nanoparticle trimer on metal film system has been investigated to explore the novel optical phenomenon. Both the electric field and surface charge distributions demonstrate the light is focused on film greatly by the nanoparticles at two sides, which could be strongly modulated by the wavelength of incident light. And the influence of nanoparticle in middle on this light focusing ability has also been studied here, which is explained by the plasmon hybridization theory. Our finding about light focusing in nanoparticle aggregating on metal film not only enlarges the novel phenomenon of surface plasmon but also has great application prospect in the field of surface-enhanced spectra, surface catalysis, solar cells, water splitting, etc.

Circular dichroism (CD) caused by the response of a chiral object to circularly polarized light has been well established, and the strong CD of plasmonic metamolecules has also become of interest in recent years; however, their response if the light also has orbital angular momentum is unclear. In this paper, the dichroism of a plasmonic cuboid-protuberance chiral structure under the illumination of a light beam with both orbital and spin angular momenta is numerically investigated. Distinguished spectra are observed under the different momenta. The circular dichroism under the combination of vortex beam and light spin is enhanced. This phenomenon is attributed to the partial spatial excitation of the nanoparticle, and the strong dichroism is simultaneously caused because of the interaction of the induced electric and magnetic modes and other higher-order modes caused by the partial excitation of the vortex beam. This research provides further insight into chiral light-matter interactions and the dichroism of light with orbital angular momentum.

By introducing Au-nanodisk antennas, we conveniently got hot carriers from decay of surface plasmons (SPs) on planar interface in an Au-antennas/ T i O 2 -spacer/Au-mirror (ASM) structure without an additional phase-matching process for SP generation. The presence of hot carriers from SPs is distinguished by opposite photocurrents compared with a similar structure without an Au mirror. Analyzed by extinction spectra and electrodynamics simulations, reflection between an Au nanodisk layer and an Au mirror induces an optical response of cavity mode, which excites SPs on an Au-mirror interface and significantly enhances the light harvesting, thus leading to a relatively high hot-carrier density from SP decay. The peak of incident photon-to-electron conversion efficiencies at different wavelength also well matches the optical response of the structure.

The single-molecule Raman detection has been realized for a long time because of the enhancement effect of surface plasmons. However, the small cross section of Raman optical activity (ROA) makes it so hard to detect the ROA of even a few molecules; and a normal surface-enhanced ROA (SE-ROA) is also very time consumable even with strong laser power. Detecting ROA in an economic way is an important issue. In this paper, we discuss the ultimate limit of the SE-ROA and provide the enhancement factor formula for SE-ROA. Following the formula, we proposed a structure with both huge Raman enhancement and ROA enhancement, which is helpful for single-molecule ROA detection.

Plasmon-driven diazo coupling reaction of p-nitroaniline (PNA), containing both an amine group (−NH2) and a nitro group (−NO2), in ambient atmosphere environments remains unknown in experiments, on both metal nanoparticles and graphene-mediated metal nanostructures. Using the surface-enhanced Raman scattering (SERS) technique, we presented the surface coupling reactions of PNA under atmospheric environment, adsorbed on the Ag nanoparticle (AgNP) arrays, graphene-coated Ag nanoparticle (G/AgNPs) hybrids, and graphene-covered Ag nanowire (G/AgNW) composites, respectively. It was found that PNAs were converted into 4,4′-diaminoazobenzene (DAAB) through a selective reduction reaction, rather than oxidized into 4,4′-dinitroazobenzene (DNAB). These results may be useful for understanding the reaction mechanism for plasmon-driven chemical reactions of the molecules containing both amine groups and nitro groups under ambient conditions.

Chirality is ubiquitous in nature and plays an important role in biochemistry because biological function is largely dependent on the handedness of chiral molecules. Methods for determining chirality essentially measure optical activity. Surface plasmons have been demonstrated to hugely enhance the optical activity of chiral molecules. As with surface-enhanced Raman spectroscopy, when chiral molecules are adsorbed on the surface of plasmonic particles, chemical enhancement of the Raman optical activity (ROA) also occurs. Herein, we studied the theoretical Raman optical activity and surface-enhanced Raman optical activity of the chiral molecule methyloxirane, compared its vibrational modes and elucidated the new vibrational modes resulting from chemical enhancement. We found that upon adsorption on the Ag20 cluster, three modes exhibited a weaker ROA (even none at all), whereas two modes exhibited a stronger ROA (which was zero without adsorption on Ag20). This was attributed to changes in the symmetry of methyloxirane upon adsorption on the Ag20 cluster. A chiral molecule adsorbed on metal particles leads to changes in the dipole moment such that the intensity of some Raman vibrational modes is enhanced, and the symmetry broken responses for the changing ROA.

Quantum theory of surface plasmons is very important for studying the interactions between light and different metal nanostructures in nanoplasmonics. In this work, using the canonical quantization method, the SPPs on nanowires and their orbital and spin angular momentum are investigated. The results show that the SPPs on nanowire carry both orbital and spin momentum during propagation. Later, the result is applied on the plasmonic nanowire waveguide to show the agreement of the theory. The study is helpful for the nano wire based plasmonic interactions and the quantum information based optical circuit in the future., Comment: 19 pages,4 figures. Corrected the commutation relations and other mistakes in this version

In plasmonic induced chirality, the circular dichroism of achiral nanoparticles (NPs) caused by Coulombic interactions between metal nanoparticles (NPs) and chiral molecules has been studied. Under resonance conditions, dye molecules and metal NPs produce large Rabi splitting due to strong coupling. If the resonance of chiral molecules is at the resonance of the plasmon, it is unclear what will happen to the strong interactions between the chiral plasmon and chiral molecules. In this paper, we investigate a coupled spherical core–shell model and analyze its spectra under excitation by circularly polarized light (CPL). Based on the Coulombic interactions between NPs and chiral molecules, we demonstrate how various factors affect this coupling. We have determined that three mechanisms are responsible for the interactions between plasmons and chiral molecules: strong coupling (Rabi splitting up to 243 meV), enhanced absorption, and induced transparency. The interactions between CPL and chiral molecules with same chiralities are stronger than those of the opposite chirality. The linewidth values of the splitted peaks are closer than those of the opposite chirality, which shows that for Rabi splitting with chiral molecules, there are more complicated mechanisms responsible for the interactions. This result will be helpful for further investigation of interactions between plasmons and chiral molecules.

Three-dimensional chiral plasmonic nanostructures have been shown to be able to dramatically boost photon-spin selective light-matter interactions, potentially leading to novel photonics, molecular spectroscopy, and light-harvesting applications based on circularly polarized light. Here, we show that chiral split-ring gold nanoresonators interfaced to a wide band gap semiconductor exhibit a contrast in hot-electron transfer rate between left-handed and right-handed visible light that essentially mimics the far-field circular dichroism of the structures. We trace down the origin of this effect to the differential excitation of the thinnest part of the split-ring structures using dichroic-sensitive cathodoluminescence imaging with nanometer spatial resolution. The results highlight the intricate interplay between the near-field and far-field chiral response of a nanostructure and establishes a clear link to the emerging field of hot carrier plasmonics with numerous potential applications in photocatalysis and solar light harvesting.

The plasmonic chirality has drawn a lot of attention because of the tunable circular dichroism (CD) and the enhancement for the signal of chiral molecules. Different mechanisms have been proposed for explaining the plasmonic CD, however, a quantitative one like ab initio mechanism in chiral molecules is still unavailable. In this work, a mechanism similar to the chiral molecules is analyzed. The giant extrinsic circular dichroism of plasmonic splitting rectangle ring is quantitatively investigated theoretically. The interplay of electric and magnetic modes of the meta-structure is proposed to explain the giant CD. The interplay is analyzed both in an analytical coupled electric-magnetic dipoles model and finite element method model. The surface charge distributions show that the circular current yielded in the splitting rectangle ring makes it behave like a magneton at some resonant modes, which interact with electric modes and results in a mixing of the two kinds of modes. The strong interplay of the two kinds of modes is mainly responsible for the giant CD.The analysis of the chiral near field of the structure shows potential applications in chiral molecule sensing., 21 pages, 8 figures, including supporting information

Metal nanoparticle-film system has been proved that it has the ability of focusing light in the gap between particle and film, which is useful for surface-enhanced Raman scattering and plasmon catalysis. The rapid developed plasmonic chirality can also be realized in such system. Here, we investigated an electromagnetic energy focusing effect and chiral near-field enhancement in a coupled chiral particle chain on gold film. It shows large electric field enhancement in the gap between particle and film, as well as chiral near field. The enhancement properties at resonant peaks for the system excited by left circularly polarized light and right circularly polarized light are obviously different. This difference resulted from the interaction of circularly polarized light and the chiral particle-film system is analyzed with plasmon hybridization. The enhanced optical activity can provide promising applications for the enhancement of chiral molecule sensor for this chiral particle chain-film system. Electronic supplementary material The online version of this article (10.1186/s11671-018-2600-8) contains supplementary material, which is available to authorized users.

Plasmonic metal nanostructures have been widely used to enhance the upconversion efficiency of the near-infrared (NIR) photons into the visible region via the localized surface plasmon resonance (LSPR) effect. However, the direct utilization of low-cost nonmetallic semiconductors to both concentrate and transfer the NIR-plasmonic energy in the upconversion system remains a significant challenge. Here, a fascinating process of NIR-plasmonic energy upconversion in Yb

Figure S1. The optical properties of the studied system with different gaps between nanoparticles; Figure S2. Chiral near-field enhancement distributions for different resonant peaks; Figure S3. The optical properties of chiral particle-film system with different R; Figure S4. The volume-averaged chiral enhancement spectra between NPs-film in chiral particle-film system with different R; Figure S5. The volume-averaged chiral enhancement spectra between particles in chiral particle-film systems with different R; Figure S6. Optical properties of chiral particle chain-film systems with different particle numbers. (DOCX 1098 kb)

Up-conversion luminescence (UCL) from rare-earth doped oxide (RE) films has great potential for application in fields such as solar cells, bioanalysis, or display technologies. However, the relatively high phonon energy of oxide matrices usually facilitates nonradiative relaxation leading to low UCL efficiency. Herein, we report a three-layer hierarchical structure of Ag/ZnO nanowires (nw-ZnO)/RE composite films, which enhances the UCL of rare-earth doped oxide films. An optimization of the geometric structure of the composite films demonstrated an increase of UCL by up to almost two orders of magnitude in Er3+ and Tm3+ doped YbMoO4 films. This UCL enhancement is attributed to the formation of a very strong electric field in the tips of the nw-ZnO creating a highly effective electric field at the composite films, combined with reflection at the silver layer. Furthermore, we use the UCL properties of these novel Ag/nw-ZnO/RE composite films to demonstrate their possible use in ZnO-based photocatalytic processes to enhance the utilization of near-infrared sunlight in these devices.

Emission of photoexcited hot electrons from plasmonic metal nanostructures to semiconductors is key to a number of proposed nanophotonics technologies for solar harvesting, water splitting, photocatalysis, and a variety of optical sensing and photodetector applications. Favorable materials and catalytic properties make systems based on gold and TiO2 particularly interesting, but the internal photoemission efficiency for visible light is low because of the wide bandgap of the semiconductor. We investigated the incident photon-to-electron conversion efficiency of thin TiO2 films decorated with Au nanodisk antennas in an electrochemical circuit and found that incorporation of a Au mirror beneath the semiconductor amplified the photoresponse for light with wavelength λ = 500–950 nm by a factor 2–10 compared to identical structures lacking the mirror component. Classical electrodynamics simulations showed that the enhancement effect is caused by a favorable interplay between localized surface plasmon excitatio...

Combining simulations and experiments, we demonstrate that the surface enhanced Raman scattering (SERS) system of a metal nanoparticle dimer on a dielectric substrate shows strong electromagnetic field enhancement in the particle/substrate gap because of the electromagnetic field redistribution. Besides, in some configurations, the presence of the substrate makes the strongest field enhancement in the gap between the two nanoparticles (i.e. the dimer) occur at the quadruple resonance, not the dipole resonance as usual. The result is very instructive in the difference between the ideal theoretical systems and the real experimental configuration.

Near field gradient effects in high vacuum tip-enhanced Raman spectroscopy (HV-TERS) are a recent developing ultra-sensitive optical and spectral analysis technology on the nanoscale, based on the plasmons and plasmonic gradient enhancement in the near field and under high vacuum. HV-TERS can not only be used to detect ultra-sensitive Raman spectra enhanced by surface plasmon, but also to detect clear molecular IR-active modes enhanced by strongly plasmonic gradient. Furthermore, the molecular overtone modes and combinational modes can also be experimentally measured, where the Fermi resonance and Darling-Dennison resonance were successfully observed in HV-TERS. Theoretical calculations using electromagnetic field theory firmly supported experimental observation. The intensity ratio of the plasmon gradient term over the linear plasmon term can reach values greater than 1. Theoretical calculations also revealed that with the increase in gap distance between tip and substrate, the decrease in the plasmon gradient was more significant than the decrease in plasmon intensity, which is the reason that the gradient Raman can be only observed in the near field. Recent experimental results of near field gradient effects on HV-TERS were summarized, following the section of the theoretical analysis.

In this review, we provide a brief overview of the recent progress achieved on the plasmon-enhanced fluorescence technique for the study of exciton-plasmon interactions using time-resolved photoluminescence spectroscopy. Quantum emitters situated in regions of localized field enhancement are excited by the enhanced electromagnetic field intensity. During the emission process, both the radiative decay rate and nonradiative decay rate can be increased by the enhanced electromagnetic field. These can be distinguished using two methods, either by neglecting the excitation rate enhancement when the excitation wavelength is far from the surface plasmon resonance or by assuming that the enhancement of the excitation is the same as the radiative decay rate. This review aims to provide a comprehensive understanding of the time-resolved photoluminescence spectroscopy of exciton-plasmon coupling dynamics.

We propose a simple structure of a silver nanorice heterodimer to achieve both Fano resonances and strong optical activity. Due to the ellipsoid shape of nanorice, the dimer shows a unique plasmon hybridization picture, being different from those in previously studied two-nanoparticle coupling, which benefits the generation of Fano resonances here. Under oblique incidence, this nanorice dimer exhibits not only multipolar Fano resonances but also very strong optical activity in the Fano resonant ranges. A chiral molecule model is proposed to explain the generation of chirality. It is also shown that Fano resonance has an assisting effect on optical activity (especially for subradiant modes), which can be ascribed to the energy transfer from the super-radiant modes to the subradiant modes during Fano interference.