Fano Transparency in Rounded Nanocube Dimers Induced by Gap Plasmon Coupling

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.

Sensitivity of Localized Surface Plasmon Resonance and Acoustic Vibrations to Edge Rounding in Silver Nanocubes

Precise knowledge of the dependence of nano-object properties on their structural characteristics such as their size, shape, composition, or crystallinity, in turn, enables them to be finely characterized using appropriate techniques. Spectrophotometry and inelastic light scattering spectroscopy are noninvasive techniques that are proving highly robust and efficient for characterizing the optical response and vibrational properties of metal nano-objects. Here, we investigate the optical and vibrational properties of monodomain silver nanocubes synthesized by the chemical route, with edge length ranging from around 20 to 58 nm. The synthesized nanocrystals are not perfectly cubic and exhibit rounded edges and corners. This rounding was quantitatively taken into account by assimilating the shape of the nanocubes to superellipsoids. The effect of rounding on their optical response was clearly evidenced by localized surface plasmon resonance spectroscopy and supported by calculations based on the discrete dipole approximation method. The study of their acoustic vibrations by high-resolution low-frequency Raman scattering revealed a substructure of the T2g band, which was analyzed as a function of rounding. The measured frequencies are consistent with the existence of an anticrossing pattern of the two T2g branches. Such an avoided crossing in the T2g modes is clearly evidenced by calculating the vibrational frequencies of silver nanocubes using the Rayleigh-Ritz variational method that accounts for both their real size, shape, and cubic elasticity. These results show that it is possible to assess the rounding of nanocubes, including by means of ensemble spectroscopic measurements on well-calibrated particles.

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.

Selective high-order resonance in asymmetric plasmonic nanostructures stimulated by vortex beams

Orbital angular momentum (OAM) of light has the potential to induce high-order transitions of electrons in atoms by compensating for the OAM required. However, due to the dark spot situating at the focal center of the OAM beam, high-order transitions are typically weak. In this study, we demonstrate efficient and selective high-order resonances in symmetric and asymmetric plasmonic nanoparticles that are comparable in size to the waist radius of the OAM beam. In a symmetric nanoparticle configured with a complete nanoring lying on the focal center, there is a pure high-order resonance obeying the law of conservation of angular momentum during the interaction between OAM light and the nanosystem. In an asymmetric nanoparticle configured with an complete ring off the beam center or a splitting nanoring, there are multiple resonances whose resonance orders are influenced by the ring's geometry, position, orientation, and photon OAM. Thus, high-order resonances in the symmetric and asymmetric plasmonic nanostructures are selectively stimulated using vortex beams. Our results may help to understand and control OAM-involved light-material interactions of asymmetric nanosystems.

Plasmonic Dual-Gap Nanodumbbells for Label-Free On-Particle Raman DNA Assays

Metal nanostructures with a tunable plasmonic gap are useful for photonics, surface-enhanced spectroscopy, biosensing, and bioimaging applications. The use of these structures as chemical and biological sensing/imaging probes typically requires an ultra-precise synthesis of the targeted nanostructure in a high yield, with Raman dye-labeling and complex assay components and procedures. Here, a plasmonic nanostructure with tunable dual nanogaps, Au dual-gap nanodumbbells (AuDGNs), is designed and synthesized via the anisotropic adsorption of polyethyleneimine on Au nanorods to facilitate tip-selective Au growths on nanorod tips for forming mushroom-shaped dumbbell-head structures at both tips and results in dual gaps (intra-head and inter-head gaps) within a single particle. AuDGNs are synthesized in a high yield (>90%) while controlling the inter-head gap size, and the average surface-enhanced Raman scattering (SERS) enhancement factor (EF) value is 7.5 × 10⁸ with a very narrow EF distribution from 1.5 × 10⁸ to 1.5 × 10⁹ for >90% of analyzed particles. Importantly, AuDGNs enable label-free on-particle SERS detection assays through the diffusion of target molecules into the intraparticle gap for different DNA sequences with varying ATGC combinations in a highly specific and sensitive manner without a need for Raman dyes.

Multiband tunable exciton-induced transparencies: Exploiting both strong and intermediate coupling in a nanocube-hexagonal-nanoplate heterodimer J-aggregates hybrid

Understanding and mastering the light-light and light-matter interactions in coupled structures have become significant subjects, as they provide versatile tools for manipulating light in both classical and quantum regimes. Mimicking quantum interference effects in pure photonic nanostructures, from weak Fano dip to intense electromagnetically induced transparency, usually requires strong asymmetries in complex geometries and larger interactions between resonances, i.e., in the intermediate coupling regime. Here, we numerically demonstrate a simple and chemically feasible plasmonic nanocube-hexagonal-nanoplate heterodimer with a strong, tunable self-induced transparency window created by the intermediate coupling between the near-degenerate dark and bright hybridized modes. Further assisted by the strong coupling introduced by the J-aggregate excitons covering the heterodimer, three evident exciton-induced transparency windows were observed. These multiband transparencies in a single-particle-level subwavelength configuration, could on one hand enrich the toolbox of multi-frequency light filtering, slowing and switching beyond the diffraction limit, and on the other hand, work as a fundamental testbed for investigating multiscale light-matter interactions at the nanoscale.

SPP standing waves within plasmonic nanocavities

Surface plasmons usually take two forms: surface plasmon polaritons (SPP) and localized surface plasmons (LSP). Recent experiments demonstrate an interesting plasmon mode within plasmonic gaps, showing distinct characters from the two usual forms. In this investigation, by introducing a fundamental concept of SPP standing wave and an analytical model, we reveal the nature of the recently reported plasmon modes. The analytical model includes SPP propagating and SPP reflection within a metal-insulator-metal (MIM) cavity, which is rechecked and supplemented by numerical simulations. We systematically analyze SPP standing waves within various nanocavities. During the discussion, some unusual phenomena have been explained. For example, the hot spot of a nanodimer could be off-tip, depending on the order of standing wave mode; and that a nanocube on metal film can be viewed as a nanocube dimer with the same separation. And many other interesting phenomena have been discussed, such as dark mode of SPP standing wave and extraordinary optical transmission. The study gives a comprehensive understanding of SPP standing waves, and may promote the applications of cavity plasmons in ultrasensitive bio-sensings.

Nanoengineering of conductively coupled metallic nanoparticles towards selective resonance modes within the near-infrared regime

In this work, the mode transition effect of different plasmonic resonances in linked dimers by a conductive junction is numerically investigated.Without the junction, the dimer supports a single dipolar bonding plasmon mode, while two new resonance modes, a screened bonding dipolar mode and a low energy charge transfer plasmon mode, emerge when two nanoparticles are linked via a bridge. Such effect is proved to be unrelated to the shape of the nanoparticles, whether sphere, core-shell or nanoegg. However, it was found that the status of each specific resonance mode is profoundly influenced by the shape of nanoparticles. Furthermore, a detailed discussion of mechanisms of controlling plasmon modes, specially charge transfer mode, and tuning their corresponding spectra in bridged nanoparticles as functions of nanoparticle parameters and junction conductance is presented. These results show that the optical response of the dimer is highly sensitive to changes in the inter-particle gap. While the capacitive dimer provides a strong hotstop, the conductive dimer leads to highly controllable low energy plasmon mode at the mid-infrared region appropriate for novel applications. These findings may serve as an important guide for optical properties of linked nanoparticles as well as understanding the transition between the capacitive and conductive coupling.

Theoretical Study on the Ultrafast Selective Excitation of Surface-Enhanced Coherent Anti-Stokes Raman Scattering Based on Fano Resonance of Disk-Ring Nanostructures by Shaped Femtosecond Laser Pulses

The enhancement and selective excitation of coherent anti-Stokes Raman scattering (CARS) and the suppression of background noise are very important problems for real-time detection at the single-molecule level. Optimizing the plasmonic substrate to ensure that all the hot spots of the pump, probe, Stokes, and anti-Stokes light are at the same position is the key to increasing the CARS signal to reach the level of single-molecule detection. The selective excitation of the target CARS peak and the suppression of the other peaks are the key to improving the signal-to-noise ratio. In this paper, we present a theoretical study to control the selective excitation and enhancement of any one of the three CARS peaks using the Fano resonance of a disk-ring structure. By optimizing the modulation of the pump, Stokes, and probe pulse, one CARS peak is maximized, while the other two are suppressed to zero. Fano resonance is applied to simultaneously enhance the four surface plasmon modes of the pump, probe, Stokes, and anti-Stokes light and to ensure that all the hot spots are located at the same position by adjusting the size of the disk-ring structure. The hot spots of the four pulses are concentrated in the disk-ring gap with a deviation distance of less than 2 nm, and the intensity of the CARS is enhanced by 1.43 × 1012 times, which is much higher than the requirement of single-molecule detection. The time, frequency, and phase distribution of the input and the response of the four pulses are studied in detail. It was found that the selective excitation and the spectra of CARS are both well preserved.

Strong and weak polarization-dependent interactions in connected and disconnected plasmonic nanostructures

We explore numerically and experimentally the formation of hybridized modes between a bright mode displayed by a gold nanodisc and either dark or bright modes of a nanorod - both elements being either separated by a nanometer-size gap (disconnected system) or relied on a metal junction (connected system). In terms of modeling, we compare the scattering or absorption spectra and field distributions obtained under oblique-incidence plane wave illumination with quasi-normal mode computation and an analytical model based on a coupled oscillator model. Both connected and disconnected systems have very different plasmon properties in longitudinal polarization. The disconnected system can be consistently understood in terms of the nature of hybridized modes and coupling strength using either QNMs or coupled oscillator model; however the connected configuration presents intriguing peculiarities based on the strong redistribution of charges implied by the presence of the metal connection. In practice, the fabrication of disconnected or connected configurations depends on the mitigation of lithographic proximity effects inherent to top-down lithography methods, which can lead to the formation of small metal junctions, while careful lithographic dosing allows one to fabricate disconnected systems with a gap as low as 20 nm. We obtained a very good agreement between experimentally measured scattering spectra and numerical predictions. The methods and analyses presented in this work can be applied to a wide range of systems, for potential applications in light-matter interactions, biosensing or strain monitoring.

Synthesis, Assembly, Optical Properties, and Sensing Applications of Plasmonic Gap Nanostructures

Plasmonic gap nanostructures (PGNs) have been extensively investigated mainly because of their strongly enhanced optical responses, which stem from the high intensity of the localized field in the nanogap. The recently developed methods for the preparation of versatile nanogap structures open new avenues for the exploration of unprecedented optical properties and development of sensing applications relying on the amplification of various optical signals. However, the reproducible and controlled preparation of highly uniform plasmonic nanogaps and the prediction, understanding, and control of their optical properties, especially for nanogaps in the nanometer or sub-nanometer range, remain challenging. This is because subtle changes in the nanogap significantly affect the plasmonic response and are of paramount importance to the desired optical performance and further applications. Here, recent advances in the synthesis, assembly, and fabrication strategies, prediction and control of optical properties, and sensing applications of PGNs are discussed, and perspectives toward addressing these challenging issues and the future research directions are presented.

Resonant nano-dimer metasurface for ultra-thin a-Si:H solar cells

Low-cost hydrogenated amorphous silicon solar cells (a-Si:H) can perform better and be more competitive by including nanostructures. An optimized nano-dimer structure embedded in close contact with the back electrode of an aSi:H ultra-thin solar cells can enhance the deliverable short-circuit current up to 27.5 %. This enhancement is the result of an increase in the absorption at the active layer, that is the product of an efficient scattering from the nanostructure. From our calculations, the nano-dimer structure must be made out of a high-index of refraction material, like GaP. The evaluation of the scattering and absorption cross section of the structure supports the calculated enhancement in short-circuit current, that is always accompanied by a decrease in the total reflectance of the cell, which is reduced by about 50 %.

Soft Plasmonic Assemblies Exhibiting Unnaturally High Refractive Index

The increases in refractive indices (n) of materials are crucial for transformative optical technologies. With the progress of monolithic lithography, large advances have been achieved with several semiconductors, including silicon, germanium, and gallium arsenide, which generally provide higher n of ∼4.0 compared to those of other elements. Nevertheless, above this upper limit of naturally available n, the range of light-matter interactions could be unprecedentedly expanded, which in turn enriches the possible applications. Here, we present a soft self-assembly of polyhedral Au colloids as a promising method to achieve unnaturally high n values. The interfacial assembly of Au nanocubes provides n of 6.4 at the resonant wavelength (near-infrared) and 4.5 in the off-resonant regimes (mid-infrared), which have not been previously reached. The soft self-assembly of polyhedral Au colloids can be a versatile and highly effective route for the fabrication of optical metamaterials with unnaturally high n values.

Quantitative optical microspectroscopy, electron microscopy, and modelling of individual silver nanocubes reveal surface compositional changes at the nanoscale

The optical response of metal nanoparticles is governed by plasmonic resonances, which depend often intricately on the geometry and composition of the particle and its environment. In this work we describe a method and analysis pipeline unravelling these relations at the single nanoparticle level through a quantitative characterization of the optical and structural properties. It is based on correlating electron microscopy with microspectroscopy measurements of the same particle immersed in media of different refractive indices. The optical measurements quantify the magnitude of both the scattering and the absorption cross sections, while the geometry measured in electron microscopy is used for numerical simulations of the cross section spectra accounting for the experimental conditions. We showcase the method on silver nanocubes of nominal 75 nm edge size. The large amount of information afforded by the quantitative cross section spectra and measuring the same particle in two environments, allows us to identify a specific degradation of the cube surface. We find a layer of tarnish, only a few nanometers thick, a fine surface compositional change of the studied system which would be hardly quantifiable otherwise.

Tunable Fano Resonances in an Ultra-Small Gap

A Fano resonance is experimentally observed in a single silver nanocube separated from a supporting silver film by a thin aluminum oxide film. The resonance spectrum is modulated by changing the size of the silver nanocube and its distance from the silver film. The system is fabricated by a bottom-up process with an accurately controlled nanogap at the sub-6-nm scale. The simulation result shows that the destructive interference between the dipole mode and the quadrupole mode in this “nanocube on mirror” (NCoM) structure is responsible for the resonance. The spectra red-shifted as the size of the silver nanocube increased and its distance from the silver film decreased. In addition, a refractive index sensitivity of the spectrum of 140 meV/RIU (refractive index unit), with a 2.4 figure of merit, is obtained by changing the dielectric environment around the silver nanocube. This work will enable the development of high-performance tunable optical nanodevices based on NCoM structures.

Au@Ag Core-Shell Nanorods Support Plasmonic Fano Resonances

In this work, we investigated experimentally and theoretically the plasmonic Fano resonances (FRs) exhibited by core-shell nanorods composed of a gold core and a silver shell (Au@Ag NRs). The colloidal synthesis of these Au@Ag NRs produces nanostructures with rich plasmonic features, of which two different FRs are particularly interesting. The FR with spectral location at higher energies (3.7 eV) originates from the interaction between a plasmonic mode of the nanoparticle and the interband transitions of Au. In contrast, the tunable FR at lower energies (2.92-2.75 eV) is ascribed to the interaction between the dominant transversal LSPR mode of the Ag shell and the transversal plasmon mode of the Au@Ag nanostructure. The unique symmetrical morphology and FRs of these Au@Ag NRs make them promising candidates for plasmonic sensors and metamaterials components.

Simulation guided design of silver nanostructures for plasmon-enhanced fluorescence, singlet oxygen generation and SERS applications

Plasmonic nanostructures such as gold and silver could alter the intrinsic properties of fluorophores, photosensitizers or Raman reporters in their close vicinity. In this study, we have conducted systematic simulations to provide insight for the design of silver nanostructures with appropriate geometrical features for metal-enhanced fluorescence (MEF), metal-enhanced singlet oxygen generation (ME-SOG) and surface-enhanced Raman scattering (SERS) applications. The size-dependent optical properties and electric field enhancement of single and dimeric nanocubes were simulated. The extinction spectra of silver nanocubes were analysed by the multipole expansion method. Results show that a suitable size of Ag nanocubes for MEF and ME-SOG can be selected based on their maximum light scattering yield, the excitation and emission wavelengths of a particular fluorophore/photosensitizer and their maximum spectral overlap. Simulations of the 'hot-spot' or gap distance between two silver nanocubes with different configurations (i.e., face-to-face, edge-to-edge and corner-to-corner) were also performed. A direct correlation was found between the size and enhanced electric field around the Ag nanocubes simulated under 15 common Raman laser wavelengths from the UV to near-infrared region. The maximum SERS enhancement factor can be achieved by selecting the silver nanocubes with the right orientation, suitable edge length and gap distance that give the highest electric field at a specific Raman laser wavelength. It was also found that the higher order of silver nanostructures, e.g., trimer and tetramer, can lead to better enhancement effects. These simulation results can serve as generic guidelines to rationally design metal-enhancement systems including MEF, ME-SOG and SERS for different application needs without cumbersome optimization and tedious trial-and-error experimentation.

Multi-interfacial plasmon coupling in multigap (Au/AgAu)@CdS core-shell hybrids for efficient photocatalytic hydrogen generation

Plasmon coupling induced intense light absorption and near-field enhancement have vast potential for high-efficiency photocatalytic applications. Herein, (Au/AgAu)@CdS core-shell hybrids with strong multi-interfacial plasmon coupling were prepared through a convenient strategy for efficient photocatalytic hydrogen generation. Bimetallic Au/AgAu cores with an adjustable number of nanogaps (from one to four) were primarily synthesized by well-controlled multi-cycle galvanic replacement and overgrowth processes. Extinction tests and numerical simulations synergistically revealed that the multigap Au/AgAu hybrids possess a gap-dependent light absorption region and a local electric field owing to the multigap-induced multi-interfacial plasmon coupling. With these characteristics, hetero-photocatalysts prepared by further coating of CdS shells on multigap Au/AgAu cores exhibited a prominent gap-dependent photocatalytic hydrogen production activity from water splitting under light irradiation (λ > 420 nm). It is found that the hydrogen generation rates of multigap (Au/AgAu)@CdS have an exponential improvement compared with that of pure CdS as the number of nanogaps increases. In particular, four-gap (Au/AgAu)@CdS core-shell catalysts displayed the highest hydrogen generation rate, that is 96.1 and 47.2 times those of pure CdS and gapless Au@CdS core-shell hybrids. These improvements can be ascribed to the strong plasmon absorption and near-field enhancement induced by the multi-interfacial plasmon coupling, which can greatly improve the light-harvesting efficiency, offer more plasmonic energy, and boost the generation and separation of electron-hole pairs in the multigap catalysts.

Strengthening Fano resonance on gold nanoplates with gold nanospheres

Plasmonic Fano resonance has attracted extensive attention due to its many applications, including plasmonic sensing, electromagnetically induced transparency, light trapping and stopping, due to its narrow linewidth and asymmetric spectral shape. However, many metal nanostructures are designed with complex geometries to generate Fano resonance and few of them can support a deep Fano dip. Herein we report on the strengthening of the Fano resonance on silicon-supported Au nanoplates through the formation of (Au nanosphere)-(Au nanoplate) heterodimers. The deposition of the Au nanosphere on the top can greatly strengthen the substrate-induced Fano resonance of the Au nanoplate with a deep dip. We also observe that the replacement of the Au nanosphere with a Au nanocube can suppress the excitation of the Fano resonance in the heterodimer. When the sharp corners and edges of the nanocubes gradually become rounded, the Fano resonance appears again with increasing asymmetry. Both the dip depth and wavelength of the Fano resonance can be independently tailored by varying the nanosphere diameter and the nanoplate thickness, respectively. We believe that our results provide an attractive and facile platform for modulating Fano dips and constructing Fano resonance-based devices.

Optical properties of symmetry-breaking tetrahedral nanoparticles

Spectrally rich but geometrically simple plasmonic metallic nanoparticles are highly favored in nanophotonics. However, they remain elusive owing to the symmetry-induced mode degeneracy and interband transition-induced plasmonic damping. Hence, most nanoparticles exhibit a single major extinction peak originating from the lowest-order dipole resonance. In this study, we uncover that even a simple tetrahedral nanoparticle supports rich spectral features due to symmetry breaking. This discovery runs counter to the reported gold tetrahedral nanoparticles, where only a single extinction peak was observed. We find that, in the case of a tetrahedral nanoparticle, the plasmonic quadrupole vertex mode becomes a bright mode and hybridizes with the dipole vertex mode, which splits the extinction peak and contributes to spectral diversity and tunability. The peak splitting is also found to be sensitively dependent on the roundness of vertices and edges. Furthermore, the tetrahedral depolarization factors are determined using the previously generalized absorption coefficient. We envision that this work will not only help fill the gap in understanding the optical properties enriched by symmetry breaking but also guide the superior probe design by combining spectral tunability with geometric simplicity of the nanoparticle.

Symmetry-broken square silicon patches for ultra-narrowband light absorption

The effect of ultra-narrowband light absorption enhancement is presented by using metamaterials with symmetry-broken square silicon patches (SSPs). The symmetry of the SSP can be broken by introducing a narrow slit deviating from its center. By breaking the symmetry of the SSPs, slit resonance mode with standing wave patterns can be excited, and the locations of the absorption peaks can be well estimated by using the Fabry-Pérot (F-P) cavity model. Although there is no excitation of surface plasmon resonance, ultra-narrowband light absorption can be achieved by minimizing the reflectance through perfect impedance matching and simultaneously eliminating the transmittance by the metallic substrate. Good ultra-narrowband absorption features can be maintained as the parameters of the buffer layer and the SSPs are altered. When this type of symmetry-broken SSPs-based metamaterial is used in refractive-index sensors, it shows excellent sensing properties due to its stable ultra-narrowband absorption enhancement.

Aluminum Nanocubes Have Sharp Corners

Of the many plasmonic nanoparticle geometries that have been synthesized, nanocubes have been of particular interest for creating nanocavities, facilitating plasmon coupling, and enhancing phenomena dependent upon local electromagnetic fields. Here we report the straightforward colloidal synthesis of single-crystalline {100} terminated Al nanocubes by decomposing AlH₃ with Tebbe's reagent in tetrahydrofuran. The size and shape of the Al nanocubes is controlled by the reaction time and the ratio of AlH₃ to Tebbe's reagent, which, together with reaction temperature, establish kinetic control over Al nanocube growth. Al nanocubes possess strong localized field enhancements at their sharp corners and resonances highly amenable to coupling with metallic substrates. Their native oxide surface renders them extremely air stable. Chemically synthesized Al nanocubes provide an earth-abundant alternative to noble metal nanocubes for plasmonics and nanophotonics applications.

Quantitative Measurement of the Optical Cross Sections of Single Nano-objects by Correlative Transmission and Scattering Microspectroscopy

The scattering and absorption of light by nano-objects is a key physical property exploited in many applications, including biosensing and photovoltaics. Yet, its quantification at the single object level is challenging and often requires expensive and complicated techniques. We report a method based on a commercial transmission microscope to measure the optical scattering and absorption cross sections of individual nano-objects. The method applies to microspectroscopy and wide-field image analysis, offering fine spectral information and high throughput sample characterization. Accurate cross-section determination requires detailed modeling of the measurement, which we develop, accounting for the geometry of the illumination and detection as well as for the presence of a sample substrate. We demonstrate the method on three model systems (gold spheres, gold rods, and polystyrene spheres), which include metallic and dielectric particles, spherical and elongated, placed in a homogeneous medium or on a dielectric substrate. Furthermore, by comparing the measured cross sections with numerical simulations, we are able to determine structural parameters of the studied system, such as the particle diameter and aspect ratio. Our method therefore holds the potential to complement electron microscopy as a simpler and cost-effective tool for structural characterization of single nano-objects.

Hierarchic Interfacial Nanocube Assembly for Sensitive, Selective, and Quantitative DNA Detection with Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS)-based sensing is promising in that it has potential to allow for highly sensitive, selective, and multiplexed detection and imaging. However, the controlled assembly and gap formation between plasmonic particles for generating strong SERS signals in a quantitative manner is highly challenging, especially on biodetection platforms, and particle-to-particle variation in the signal enhancement can vary by several orders of magnitude in a single batch, largely limiting the reliable use of SERS for practical sensing applications. Here, a hierarchic-nanocube-assembly based SERS (H-Cube-SERS) bioassay to controllably amplify the electromagnetic field between gold nanocubes (AuNCs) is developed. Based on this strategy, H-Cube-SERS assay allows for detecting target DNA with a wide dynamic range from 100 aM to 10 pM concentrations in a stable and reproducible manner. It is also found that the uniformly formed AuNCs with flat surfaces are much more suitable for highly sensitive, reliable, and quantitative biodetection assays due to faster DNA binding kinetics, sharper DNA melting transition, wider hot spot regions, and less dependence on light polarization direction than spherical Au nanoparticles with curved interfaces. This work paves the pathways to the quantitative and sensitive biodetection on a SERS platform and can be extended to other particle assembly systems.

Analytical analysis of spectral sensitivity of plasmon resonances in a nanocavity

Metallic nanocavities exhibit extremely high spectral sensitivity to geometrical variations and are promising for sensing applications. Here, the sensitivity of a cubic dimer cavity, to picometer gap variation, is analysed in a model, which takes into account the phase shift of scattering at the boundaries and the quantum tunnelling effect in the small gap limit. The resonance wavelengths are expressed in terms of the plasmon frequency, the medium dielectric function, and the geometry of the gap. The sensitivity of the resonance wavelength to the gap width variation is found to be as high as 1 nm pm-1. While the resonance wavelengths depend on the materials' dielectric functions, the sensitivity is found to scale universally as a function of gap distance. In the sub-nanometer regime, electron tunnelling across the gap starts to suppress the plasmonic field, setting the limit of sensitivity of such a dimer cavity. The results given by the analytical model are complemented by numerical simulations using Comsol. Our model reveals the origin and universal behaviours of the sensitivity of the cavity plasmon and provides guidance for the design of new sensitive rulers at the picometer scale.

Plasmonic Nanocomposites Based on Silver Nanocube-Polymer Blends Displaying Nearly Perfect Absorption in the UV Region

Plasmonic nanocomposites based on well-dispersed silver nanocubes in poly(vinylpyrrolidone) are presented that are solution-processed into layers of varying volume fractions of nanocubes. We show that the high-energy modes of the nanocubes are almost insensitive to plasmonic coupling within the nanocube assemblies, leading to a linear increase in light absorption in the UV region with the nanocube densities. Concerning the main dipolar resonance mode at 450 nm, it is strongly affected by the formation of these assemblies, leading to an increased absorption in the UV region as well as a large absorption band in the visible region. Simulations of the optical response of the nanocube assemblies as a function of nanocube spacing and electric field polarization reveal that optical features in the visible region are due to intercube couplings at short intercube distances and parallel electric field orientation. In contrast, the additional plasmonic band in the UV region has its origin in residual dipolar oscillations of the nanocubes in combination with weak dipolar coupling for both parallel and transversal field polarizations. The combination of these effects leads to an enlarged absorption band in the UV region with nearly perfect light absorption of 98.8% at a high silver volume fraction of 8% that is accompanied by a very weak specular reflection of only 0.28%. Although such perfect absorption is usually observed only when nanocubes are assembled on a gold surface, nearly perfect absorption herein is achieved on a large palette of substrates including glass, plastic, and cheap metals such as aluminum, making it a promising approach for solution-processed robust and cheap quasi-perfect absorption coatings.

Manipulating the confinement of electromagnetic field in size-specific gold nanoparticles dimers and trimers

Confinement of the electromagnetic field in gold nanoparticle dimers and trimers with variations in the interparticle distances and angles has been calculated.

Tailoring Fano lineshapes using plasmonic nanobars for highly sensitive sensing and directional emission

Plasmonic oligomers are one class of the most promising nanoclusters for generating Fano resonances. This study reveals that a nanobar-based heptamer concurrently sustains triple polarization-dependent Fano resonances, in sharp contrast to traditional nanodisk or nanosphere-based counterparts. Benefiting from the enhanced near field and reduced spectral linewidth, the gold heptamer exhibits a high refractive index sensitivity (940 nm per RIU) together with a figure of merit (FoM) value as large as 20.9, which outperforms that of most other gold oligomers. On the other hand, it is found that the spectral positions of hybridized eigenmodes depend strongly on the spatial configurations of the constituent nanobars. As a proof of concept, we design a simple heterodimer comprising a nanocross and a nanobar, where plasmonic modes with opposite radiative decay characteristics are excellently overlapped both spectrally and spatially by elaborate tailoring. Double strong Fano resonances appear on opposite sides of the spectrum as expected. More interestingly, the radiation main lobes all point to one direction at these two Fano resonances due to the spatial charge distributions and mode interferences with the maximal directivity ratio (DR) as high as 22.4, in a similar manner to the radio frequency (RF) Yagi-Uda antenna. Furthermore, the emission directions can also be easily switched by adjusting the orientations of the individual nanobar in the heterodimer. Our study demonstrates that the nanobar-based oligomers with tailored Fano lineshapes could serve as versatile and delicate platforms for the label-free biochemical sensing and directional transmission of optical information at the nanometre scale.

Precisely Shaped, Uniformly Formed Gold Nanocubes with Ultrahigh Reproducibility in Single-Particle Scattering and Surface-Enhanced Raman Scattering

Synthesizing plasmonic nanostructures in an ultraprecise manner is of paramount importance because the nanometer-scale structural details can significantly affect their plasmonic properties. Au nanocubes (AuNCs) have been a highly promising, heavily studied nanostructure with high potential in various fields, but an ultraprecise synthesis from 10 to 100 nm in size over a large number of AuNCs has not been well established. Precisely structured AuNC-based studies for a highly reproducible, quantitative plasmonic signal generation [e.g., quantitative surface-enhanced Raman scattering (SERS)] are needed for reliable use and exploration in the beneficial properties of AuNCs. Here, we developed a strategy for AuNC synthesis with the desired size and shape, ranging from 17 to 78 nm particularly with highly controlled corner sharpness, by precisely controlling the growth rate of different facets and AuNC-specific flocculation which enabled ultrahigh yields (∼98-99%). Importantly, the precisely shaped AuNCs can scatter light in a spectrally reproducible manner, and the SERS enhancement factors (EFs) for the AuNC dimers are very narrowly distributed (the EFs of 72 nm sharp-cornered cube dimers have a distribution within 1 order of magnitude). Our results pave the paths to ultrahigh yield synthesis of metal nanocubes with a precise size and shape and offer single-particle-level spectral controllability and reproducibility over a large number of particles.

Single gold bipyramids on a silanized substrate as robust plasmonic sensors for liquid environments

Sensitive, robust and stable sensors are required to bring biosensing techniques from the forefront of research to clinical and commercial settings. To this end, we report on the development of new robust plasmonic sensors consisting of gold nano-bipyramids (BPs) grafted to a glass substrate via silanization, associated with a novel setup based on Spatial Modulation Spectroscopy allowing the measurement of the optical response of individual nano-objects in a liquid environment. We thereby show that changes in the refractive index of the medium around individual silanized BPs can be detected by measuring their plasmonic shift with sensitivities comparable to values reported elsewhere and in good agreement with theoretical calculations. The optical response is furthermore shown to be stable and robust allowing for repeated measurements in different media and storage over many months. This work opens up new perspectives in the field of plasmonic bio-sensing as our setup is readily adaptable to dynamic liquid measurements and a wide range of applications such as the detection of clinically important analytes or pollutants in water.

Theoretical study on narrow Fano resonance of nanocrescent for the label-free detection of single molecules and single nanoparticles

This paper reports a narrow Fano resonance of 3D nanocrescent and its application in the label-free detection of single molecules. The Fano resonance depends not only on the gap size but also on the height. The Fano resonance originates from the interference between the quadrupolar mode supported by the horizontal crescent and the dipolar mode along the nanotip. When the height of 3D nanocrescent is 30 nm, the width of Fano resonance is as narrow as 10 nm. The narrow linewidth is caused by the strong narrow resonant absorption coming from the dipolar mode of nanotip overlapping with the quadrupolar mode of nanocrescent, where the absorption spectra are calculated under a horizontal incident light. The narrow Fano resonance is highly sensitive to a single nanoparticle trapped by the nanocrescent. The wavelength shift increases linearly with the refractive index with the relation of Δλ = 22.10n - 28.80, and increases with the size of trapped nanoparticle following a relation of Δλ = 0.826 × r 1.672. These results indicate that if a protein nanoparticle with radius of 2.5 nm is trapped by the nanocrescent, the shift is as large as 4.03 nm.

Second harmonic generation enhancement from a nonlinear nanocrystal integrated hyperbolic metamaterial cavity

We theoretically investigate the dipolar whispering-gallery modes (WGMs) with different mode orders supported by spherical hyperbolic metamaterial (HMM) cavities consisting of alternating metal and dielectric layers. Associated with the excitations of the WGMs with the highest and the second highest mode orders, the HMM cavities are capable of creating highly enhanced and uniformly distributed local fields in the entire dielectric core region. Variation on the metal filling ratio allows for easily tuning the resonant wavelengths of WGMs over a wide spectral range. By integrating a nonlinear nanocrystal into the HMM cavities, we show enhancements of intensity of second harmonic generation up to a factor of 3.9 × 10¹⁰, which is two orders of magnitude higher than the largest enhancement achieved in the single-layer plasmonic core-shell cavities.