Local Electromagnetic Field Enhancement Research Articles

Optical property of X-two ring structure

In this paper, the metal periodic array structure of X-two ring based on the principle of Fano resonance is proposed, which is composed of two concentric rings around the center X. The optical properties of the structure are investigated by using the finite difference time domain method. According to the simulated transmission spectra, electric field distribution and charge distribution, when linearly polarized light is incident to the metal surface, Fano resonance can be excited and the interaction between resonance modes can be produced in the structure of X-two ring, which can make resonance valleys generated at different positions. Fano resonance is mainly formed by the coherent interference between a bright mode with the larger radiation broadening and a dark mode with the weak radiation broadening, thus the structural resonance valley of X-two ring based on Fano resonance is strongly dependent on the relative parameters of the structure (the arm length of X, the distance between the inner ring and outer ring, the width of the inner ring and outer ring, the period, the number of ring, and the angle of X). In other words, over the wavelength range of 450 nm to 3000 nm, the intensity and position of the structural resonance valley are adjustable as the change of the relative geometric parameters of the structure. In addition, due to weak radiation damping and strong local electromagnetic field enhancement of Fano resonance, the resonance frequency and line type can significantly shift with the change of the environmental refractive index. Therefore, the further analysis of the variation of the structural resonance valley under the conditions of different refractive indices can be concluded that the structure of X-two ring has a higher sensitivity to the refractive index of surrounding environment, up to 1300 nm/RIU. The above results show that the structure of X-two ring not only is simple, economical, compact and efficient, but also has great potential applications in refractive index sensors and some photonic devices.

Tailoring Optical Properties of a Large-Area Plasmonic Gold Nanoring Array Pattern

A new fabrication route, which combines nanosphere lithography with silicon-based clean-room microfabrication processes, has been developed to produce large-area long-range ordered gold nanoring array patterns in a controllable fashion. Both the experimentation and the finite-difference time-domain (FDTD) simulation show that the surface plasmon resonance peak (SPR) of the nanoring array pattern can be tuned systematically in a large spectral range by varying the geometry parameters such as the ring thickness, the ring height, the ringer outer diameter, and the gap between neighboring rings. For the Au nanoring arrays with a large gap in the absence of plasmon coupling between neighboring rings, the local electromagnetic (EM) field enhancement occurs at both the outer and inner surfaces of individual nanorings; and the periodicity of Au nanoring array has no any effect on the plasmonic properties. For the Au nanoring arrays with a small gap, plasmon coupling takes place between neighboring rings. As a result, the characteristic plasmonic band is split into two new peaks corresponding to a bonding SPR mode and an antibonding SPR mode. The local EM field enhancement becomes stronger with a decrease in the gap between neighboring rings, but the SPR peaks shift away. Therefore, to maximize the surface-enhanced Raman scattering signal, the geometry parameters of the Au nanoring array need to be tuned to balance the contributions from the resonance excitation (spectral overlap) and the local EM field enhancement.

In situ decoration of plasmonic Au nanoparticles on graphene quantum dots-graphitic carbon nitride hybrid and evaluation of its visible light photocatalytic performance

This work spotlights the development of a plasmonic photocatalyst showing surface plasmon induced enhanced visible light photocatalytic (PC) performance. Plasmonic Au nanoparticles (NPs) are decorated over the hybrid nanosystem of graphitic carbon nitride (GCN) and graphene quantum dots (GQD) by citrate reduction method. Surface plasmon resonance (SPR) induced enhancement of Raman G and 2D band intensity is encountered on excitation of the Plasmonic hybrid at 514.5 nm, which is near to the 532 nm absorption band of Au NPs. Time-resolved photoluminescence and XPS studies show charge transfer interaction between GQD-GCN and Au NPs. Plasmonic hybrid exhibits an enhanced PC activity over the other catalysts in the photodegradation of methylene blue (MB) under visible light illumination. Plasmonic photocatalyst displays more than 6 fold enhancement in the photodecomposition rate of MB over GQD and nearly 2 fold improvement over GCN and GQD-GCN. GQD-GCN absorbs mostly in the near visible region and can be photoexcited by visible light of wavelength () < 460 nm. Plasmon activation in Au NPs decorated GQD-GCN could exploit the entire UV–visible light for photocatalysis. Furthermore, plasmonic Au act as antennas for accumulation and enhancement of localized electromagnetic field at the interface with GQD-GCN, and thereby promotes photogeneration of large numbers of carriers on GQD-GCN. The carriers are separated by charge transfer migration from hybrid to Au NPs. Finally, the carriers on the plasmonic Au nanostructures initiate MB degradation under visible light. Our results have shown that plasmon decoration is a suitable strategy to design a carbon based hybrid photocatalyst for solar energy conversion.

Assessing Interparticle Spatial Characteristics of DNA-Linked Core–Shell Nanoparticles with or without Magnetic Cores in Surface Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) of plasmonic nanoparticles enables their use as nanoprobes for the detection of biomolecules in solutions, which exploits the “hot-spot” arisen from small aggregates of the biomolecule-linked nanoprobes for effective harnessing of the interparticle plasmonic coupling of gold nanoparticles. While a “squeezed” interparticle spatial characteristic has been revealed from the duplex DNA-linked gold nanoparticles as dimers in solution, how this interparticle spatial characteristic is operative for plasmonic nanoparticles containing magnetic components remains unknown. We describe herein new findings of an investigation of the interparticle spatial characteristics of DNA-linked core–shell type nanoparticles consisting of magnetic cores and plasmonic gold or silver shells, focusing on theoretical–experimental correlation in terms of localized surface plasmon resonance and electromagnetic field enhancement. While the simulated enhancement for the DNA-linked dimers of plasmo...

Ultrafast carrier transfer evidencing graphene electromagnetically enhanced ultrasensitive SERS in graphene/Ag-nanoparticles hybrid

Graphene-metal hybrid systems are highly promising as a very important building block in plasmonic structure for biosensing and catalyzing applications. However, the potential and mechanism of graphene in plasmon enhancement for such hybrids are not clear enough, although the corresponding enhancement of light-matter interaction has been reported. Here, we report an ultrafast optical spectroscopy investigation of a graphene/Ag-nanoparticles (NP) hybrid structure and evidence photo-carrier transfer from graphene to the Ag-NP. More importantly, we show that the graphene layer acts as an electron reservoir, and the electrons that transfer from it to the Ag-NP greatly enhance plasmon excitations therein, thereby generating giant local electromagnetic field enhancement in its hybrid. Further, we show that this graphene-derived local electromagnetic enhancement contributes greatly to ultrasensitive (10−13 M) surface-enhanced Raman scattering (SERS) in this hybrid. Our investigation reveals graphene's electromagnetic enhancement of SERS by uncovering an ultrafast carrier transfer mechanism in the graphene-plasmonic hybrid, and these results pave the way for the development of ultrasensitive plasmonic devices.

Self-Assembly of Large Gold Nanoparticles for Surface-Enhanced Raman Spectroscopy.

Performance of portable technologies from mobile phones to electric vehicles is currently limited by the energy density and lifetime of lithium batteries. Expanding the limits of battery technology requires in situ detection of trace components at electrode-electrolyte interphases. Surface-enhance Raman spectroscopy could satisfy this need if a robust and reproducible substrate were available. Gold nanoparticles (Au NPs) larger than 20 nm diameter are expected to greatly enhance Raman intensity if they can be assembled into ordered monolayers. A three-phase self-assembly method is presented that successfully results in ordered Au NP monolayers for particle diameters ranging from 13 to 90 nm. The monolayer structure and Raman enhancement factors (EFs) are reported for a model analyte, rhodamine, as well as the best performing polymer electrolyte salt, lithium bis(trifluoromethane)sulfonimide. Experimental EFs for the most part correlate with predictions based on monolayer geometry and with numerical simulations that identify local electromagnetic field enhancements. The EFs for the best performing Au NP monolayer are between 106 and 108 and give quantitative signal response when analyte concentration is changed.

Tailored Fano resonance and localized electromagnetic field enhancement in Ag gratings

Metallic gratings can support Fano resonances when illuminated with EM radiation, and their characteristic reflectivity versus incident angle lineshape can be greatly affected by the surrounding dielectric environment and the grating geometry. By using conformal oblique incidence thin film deposition onto an optical grating substrate, it is possible to increase the grating amplitude due to shadowing effects, thereby enabling tailoring of the damping processes and electromagnetic field couplings of the Fano resonances, hence optimizing the associated localized electric field intensity. To investigate these effects we compare the optical reflectivity under resonance excitation in samples prepared by oblique angle deposition (OAD) and under normal deposition (ND) onto the same patterned surfaces. We observe that by applying OAD method, the sample exhibits a deeper and narrower reflectivity dip at resonance than that obtained under ND. This can be explained in terms of a lower damping of Fano resonance on obliquely deposited sample and leads to a stronger localized electric field. This approach opens a fabrication path for applications where tailoring the electromagnetic field induced by Fano resonance can improve the figure of merit of specific device characteristics, e.g. quantum efficiency (QE) in grating-based metallic photocathodes.

Efficient light harvesting in dye sensitized solar cells using broadband surface plasmon resonance of silver nanoparticles with varied shapes and sizes

Plasmonic silver nanoparticles of varied shapes and sizes (v-AgNPs) were synthesized via facile one-pot chemical reduction method and investigated their applicability in dye sensitized solar cells (DSSCs). The synthesized v-AgNPs exhibit broadband absorption over entire visible region owing to their unique localized surface plasmon resonance (LSPR). The photon conversion efficiency (PCE) of DSSCs increased ∼50% by incorporating ∼1wt% of v-AgNPs in ∼4μm thin reference photoanode. The enhancement is mainly due to increase in light harvesting ability of plasmonic photoanode as a result of local electromagnetic field enhancement and strong near field scattering effect of v-AgNPs. In addition, the reduction in charge transfer resistance value (R2) further confirms the high charge carrier generation in plasmonic DSSCs.

Super-Resolution Imaging and Plasmonics.

This review describes the growing partnership between super-resolution imaging and plasmonics, by describing the various ways in which the two topics mutually benefit one another to enhance our understanding of the nanoscale world. First, localization-based super-resolution imaging strategies, where molecules are modulated between emissive and nonemissive states and their emission localized, are applied to plasmonic nanoparticle substrates, revealing the hidden shape of the nanoparticles while also mapping local electromagnetic field enhancements and reactivity patterns on their surface. However, these results must be interpreted carefully due to localization errors induced by the interaction between metallic substrates and single fluorophores. Second, plasmonic nanoparticles are explored as image contrast agents for both superlocalization and super-resolution imaging, offering benefits such as high photostability, large signal-to-noise, and distance-dependent spectral features but presenting challenges for localizing individual nanoparticles within a diffraction-limited spot. Finally, the use of plasmon-tailored excitation fields to achieve subdiffraction-limited spatial resolution is discussed, using localized surface plasmons and surface plasmon polaritons to create confined excitation volumes or image magnification to enhance spatial resolution.

Controllable optical activity of non-spherical Ag and Co SERS substrate with different magnetic field**Project supported by the Key Science and Technology Research Project of Henan Province, China (Grant No. 162102210164), the Natural Science Foundation of Henan Educational Committee, China (Grant No. 17A140002), the National Natural Science Foundations of China (Grant Nos. 11574276, 11404291, and 11604079), and the Program for Science

We experimentally fabricate a non-spherical Ag and Co surface-enhanced Raman scattering (SERS) substrate, which not only retains the metallic plasmon resonant effect, but also possesses the magnetic field controllable characteristics. Raman detections are carried out with the test crystal violet (CV) and rhodamine 6G (R6G) molecules with the initiation of different magnitudes of external magnetic field. Experimental results indicate that our prepared substrate shows a higher SERS activity and magnetic controllability, where non-spherical Ag nanoparticles are driven to aggregate effectively by the magnetized Co and plenty of hot-spots are built around the metallic Ag nanoparticles, thereby leading to the enhancement of local electromagnetic field. Moreover, when the external magnetic field is increased, our prepared substrate demonstrates excellent SERS enhancement. With the 2500 Gs and 3500 Gs (1 Gs = 10−4 T) magnetic fields, SERS signal can also be obtained with the detection limit lowering down to 10−9 M. These results indicate that our proposed magnetic field controlled substrate enables us to freely achieve the enhanced and controllable SERS effect, which can be widely used in the optical sensing, single molecule detection and bio-medical applications.

Multiple surface plasmon resonances of square lattice nanohole arrays in Au-SiO2-Au multilayer films

The optical properties and the local electromagnetic field enhancement of a multilayer structure with square lattice nanohole arrays in Au-SiO2-Au multilayer films are numerically studied using finite-difference time domain method. Simulation results demonstrate that the multiple surface plasmons (SP) resonances consist of SP on the air/Au interface, SiO2/Au interfaces (the middle layers), Au/SiO2 interface (the lower layer) and coupling modes on the Au film and Au film. We investigated some of structure parameters that influence the SP resonances of the multilayer nanostructure. Adjustment of the thickness of SiO2 film (H2), the diameter (R) of circular nanoholes, the periods (C) of square lattice and the thickness of Au film (H1) could change the absorption intensity and the SP resonances. The simulation of the electromagnetic field distributions shows that the location of the local electromagnetic field enhancement can specify the different SP resonances patterns. Dipole, quadrupole, and twelve-pole SP resonances modes can be found in the multilayer nanostructure. These studies are important for applications using multiple SP resonances.

Investigation on the Enhanced Fluorescence Emission from Self-Assembled Au Nanorod Film

A two-dimension distributed metallic nanostructure substrate containing two-dimensional array of Au nanorods with different configurations is prepared via a self-assembly method. Enhanced fluorescence emission with assembled substrate is investigated by employing Rhodamine 6G as probe molecule. The experimental observation shows that the prepared nanostructure substrate has strong fluorescence enhancement effect and the enhancement effect critically depends on the configuration of the particle distribution. The study demonstrates that the local electromagnetic field enhancement around the gold nanorod array plays an important role in achieving fluorescence enhancement with the assembled substrate array. This work suggests that the assembled substrate can be considered as a potential candidate for effective enhancement application.

Optical Properties and Local Electromagnetic Field Enhancement of Periodic Rectangular Nanohole Arrays in Au-Interlayer-Au Multilayer Films

The optical properties and the local electromagnetic field enhancement of a compound structure with rectangular nanohole arrays in Au-interlayer-Au multilayer films are numerically studied using finite-difference time domain method. We investigated some of structure parameters that influence the optical properties of the compound nanostructure. Adjustment of the length of rectangular nanoholes (L), the polarization direction (θ), the thickness of interlayer (SiO2, H2), and the materials in the interlayer can change the absorption intensity and the resonance peaks. An interlayer is located between the double Au films (Au-interlayer-Au multilayer films) with the function of confining local electromagnetic field. The simulation of the electromagnetic field distribution shows that the location of the local electromagnetic field enhancement can specify the different resonance patterns. The proposed compound nanostructure will be promising for application in portable nanoplasmonic multisensing and imaging.

Engineering a Large Scale Indium Nanodot Array for Refractive Index Sensing.

In this work, we developed a simple method to fabricate 12 × 4 mm2 large scale nanostructure arrays and investigated the feasibility of indium nanodot (ND) array with different diameters and periods for refractive index sensing. Absorption resonances at multiple wavelengths from the visible to the near-infrared range were observed for various incident angles in a variety of media. Engineering the ND array with a centered square lattice, we successfully enhanced the sensitivity by 60% and improved the figure of merit (FOM) by 190%. The evolution of the resonance dips in the reflection spectra, of square lattice and centered square lattice, from air to water, matches well with the results of Lumerical FDTD simulation. The improvement of sensitivity is due to the enhancement of local electromagnetic field (E-field) near the NDs with centered square lattice, as revealed by E-field simulation at resonance wavelengths. The E-field is enhanced due to coupling between the two square ND arrays with [Formula: see text]x period at phase matching. This work illustrates an effective way to engineer and fabricate a refractive index sensor at a large scale. This is the first experimental demonstration of poor-metal (indium) nanostructure array for refractive index sensing. It also demonstrates a centered square lattice for higher sensitivity and as a better basic platform for more complex sensor designs.

Absorption Spectroscopy of an Individual Fano Cluster.

Plasmonic clusters can exhibit Fano resonances with unique and tunable asymmetric line shapes, which arise due to the coupling of bright and dark plasmon modes within each multiparticle structure. These structures are capable of generating remarkably large local electromagnetic field enhancements and should give rise to high hot carrier yields relative to other plasmonic nanostructures. While the scattering properties of individual plasmonic Fano resonances have been characterized extensively both experimentally and theoretically, their absorption properties, critical for hot carrier generation, have not yet been measured. Here, we utilize single-particle absorption spectroscopy based on photothermal imaging to distinguish between the radiative and nonradiative properties of an individual Fano cluster. In observing the absorption spectrum of individual Fano clusters, we directly verify the theoretical prediction that while Fano interference may be prominent in scattering, it is completely absent in absorption. Our results provide microscopic insight into the nature of Fano interference in systems of coupled plasmonic nanoparticles and should pave the way for the optimization of hot carrier production using plasmonic Fano clusters.

Controlled plasmon enhanced fluorescence by silver nanoparticles deposited onto nanotube arrays

Three-dimensional (3D) plasmonic nanostructures of porous alumina array (PAA) with silver nanoparticles (AgNPs) were prepared for enhancing fluorescence emission. In order to avoid fluorescence quenching effects and obtain clear fluorescence enhancement, the molecules were separated by using such 3D substrates, and the mean distance between the molecules and nanoparticles’ surface can be easily controlled by changing the diameters of the PAA tube. It was found that the PAA tube with smaller size provides better fluorescence enhancement. Enhanced cross section, a new fluorescence enhanced factor, combined with the simulation of localized electromagnetic field enhancement was presented to understand the experimental results.

Origin of Dispersive Line Shapes in Plasmonically Enhanced Femtosecond Stimulated Raman Spectra

A theoretical model is described to explain the observed dispersive-like vibrational line shapes reported in previous studies of plasmonically enhanced (PE) femtosecond stimulated Raman spectroscopy (FSRS). These Raman line shapes are rationalized in terms of interference or heterodyne terms between the PE stimulated Raman response from the molecules located in the plasmonically enhancing regions of Au nanoparticles and the nonlinear emission/scattering due to the resonant surface plasmon resonance (SPR) of these nanoparticles. The treatment of the FSRS signal response follows the standard perturbative polarization approach apart from the familiar g4 field enhancement factor found in spontaneous SERS description, where g represents the local electromagnetic field enhancement due to proximity with the plasmonic nanoparticles. The SPR contribution is modeled as a PE resonant two-level system. The heterodyne cross-term between these two complex third-order responses is large because both of the components ar...

Plasmon-enhanced energy transfer between quantum dots and tunable film-coupled nanoparticles

The radiative processes associated with quantum dots (QDs), fluorophores and other radiating systems can be profoundly modified by their interaction with plasmonic nanostructures. This interaction is associated with the localized electromagnetic field enhancement and the resonance absorption band. Extreme electromagnetic environments can be created in well-designed plasmonic nanostructures, such as metal film-dielectric layer-metal nanoparticles (NPs). Recently, so called film-coupled NPs have received a significant amount of attentions due to the strong surface plasmon resonances (SPRs). In this paper, we experimentally study the plasmon-enhanced energy transfer between CdSe/ZnS QDs and tunable film-coupled NPs. A facile route of seeding growth of self-assembled particles in situ is primarily presented for the large-area uniform fabrication of film-coupled NPs. The size-dependent energy transfer of Au NPs is demonstrated and the influence of the distribution of top Au NPs is described in detail. The experimental results demonstrate that the metallic nanostructures interact with the CdSe/ZnS QDs with obviously enhanced Förster resonance energy transfer (FRET) probability, and the fluorescence lifetime of QD materials is dramatically shortened. FRET is considered to be responsible for the PL quenching and the nonradiative decay acts as the dominant decay channel. The experimental results are also supported by finite-difference time-domain (FDTD) simulations which intend to collect design paths for the plasmon-enhanced energy transfer.

Photonic Crystal Hydrogel Enhanced Plasmonic Staining for Multiplexed Protein Analysis.

Plasmonic nanoparticles are commonly used as optical transducers in sensing applications. The optical signals resulting from the interaction of analytes and plamsonic nanoparticles are influenced by surrounding physical structures where the nanoparticles are located. This paper proposes inverse opal photonic crystal hydrogel as 3D structure to improve Raman signals from plasmonic staining. By hybridization of the plasmonic nanoparticles and photonic crystal, surface-enhanced Raman spectroscopy (SERS) analysis of multiplexed protein is realized. It benefits the Raman analysis by providing high-density "hot spots" in 3D and extra enhancement of local electromagnetic field at the band edge of PhC with periodic refractive index distribution. The strong interaction of light and the hybrid 3D nanostructure offers new insights into plasmonic nanoparticle applications and biosensor design.

Broadband Optical Response in Ternary Tree Fractal Plasmonic Nanoantenna

The ability to precisely tailor lineshapes, operational bandwidth, and localized electromagnetic field enhancements (“hot spots”) in nanostructures is currently of interest in advancing the performance of plasmonics-based chemical and biological sensing techniques. Fractal geometries are an intriguing alternative in the design of plasmonic nanostructures as they offer tunable multiband response spanning the visible and infrared spectral regions. A numerical study of the optical behavior of ternary tree fractal plasmonic nanoantenna is presented. Self-similar features are seen to emerge in the extinction spectra with the increase in fractal order N of the tree structure. Plasmon oscillations occurring at different length scales are shown to correspond to the multiple peaks and are compared with the spatial maps of electric field enhancement at the surface of the nanoantenna. The multiple peaks are shown to be independently tunable by structural variation. The robustness of the spectral response and polarization dependence arising due to various asymmetries is discussed.