High Electric Field Enhancement Research Articles

Au Nanoparticles@Si Nanowire Oligomer Arrays for SERS: Dimers Are Best.

We report the synthesis of vertically aligned silicon nanowire (VA-SiNW) oligomer arrays coated with Au nanoparticle (NP) monolayers via a combination of colloidal lithography, metal-assisted chemical etching, and directed NP assembly. Arrays of SiNW monomers (i.e., isolated NWs), dimers, and tetramers are synthesized, decorated with AuNPs, and tested for their performance in surface-enhanced Raman spectroscopy. The ∼20 nm AuNPs easily enter within the ca. 40 nm gaps of the SiNW oligomers, thus reaching the hot spot region. At 785 nm excitation, the AuNPs@SiNW dimer arrays provide the highest Raman signal, in agreement with electromagnetic simulations showing a high electric field enhancement at the Au/Si interface within the dimer gap region.

Amplified linear and nonlinear chiral sensing assisted by anapole modes in hybrid metasurfaces

The interaction between chiral molecules and circularly polarized light is largely influenced by the local optical chirality density. This interaction prompts substantial demand of the design of nanophotonic platforms capable of enhancing such effects across large and accessible volumes. Such a magnification requires nanostructures that provide strong electric and magnetic field enhancements while preserving the phase relation of circular light. Dielectric nanostructures, particularly those able to support resonances, are ideal candidates for this task due to their capacity for high electric and magnetic field enhancements. On the other hand, efficient third harmonic generation requires strong electric field resonances within dielectric materials, a feature often boosted by incorporating plasmonic materials into hybrid systems. In this work, we numerically propose a coupled silicon disk-gold ring system that can exploit the anapole-induced field confinement to provide a broadband magnified circular dichroism under realistic conditions, reaching values up to a 230-fold enhancement. We also demonstrate that this structure can be employed as an efficient third harmonic generator, which, when integrated with chiral media, enables an 800-fold enhancement in circular dichroism. Furthermore, we show that pulsed illumination at intensities up to 10 GW/cm2 does not induce temperature increments that could potentially damage the samples. These findings suggest that this system can be a promising and versatile approach toward ultrasensitive chiral sensing.

Optical gain-tunable wideband metasurface device with high electric field enhancement

The metasurface devices with high electric field enhancement factor values have been widely discussed in recent years. However, these types of metasurface devices usually have very strict minimum spacing limitations, which seriously affect the difficulty of device fabrication and the reproducibility of experiments. In this work, we proposed a metasurface based on a metal-insulator-metal configuration that significantly enhances the electric field enhancement factor of the surface plasmon resonance structure on the upper surface by utilizing a bottom metal-SiO2 spiral lens. The results indicated that the gain bandwidth of this proposed device is more than 100 nm with a maximum electric field gain of 377 within the orange-red wavelength range. Furthermore, the electric field enhancement factor can be adjusted based on the number of rings in the spiral lens. Moreover, the minimum fabrication scale of this device is 130 nm, which facilitates easier fabrication and experimental replication. The focal pattern of the electric field generated inside the spiral lens with different polarization states of light was also examined.

Cross-shaped nanoaperture nanoantennas inside plasmonic nanorings for large SERS enhancement and multiple hotspots

We have designed a novel nanostructure consisting of a cross-shaped nanoaperture nanoantenna inside plasmonic nanorings for achieving very large values of electric field enhancement, as well as large theoretical surface-enhanced Raman scattering (SERS) enhancement factor, towards the center of the nanostructure. In this work, we employed Finite-difference-time-domain (FDTD) numerical modeling to simulate the plasmonic (gold) nanostructures present on silica substrates. We found that the nanostructures being proposed by us show very high localized electric field enhancements as well as multiple hotspots in which the electric field is enhanced and localized. We observed that these hotspots have large electric field enhancements (and therefore large theoretical SERS enhancement factors) at more than one wavelength. Thus, the proposed nanostructure can be used to achieve a multiple wavelength SERS response. The electric field enhancements and the resonance wavelengths of nanostructures can be tuned in the visible and the NIR region by modifying the nanostructure dimensions like the gap between the tips in the central nanoaperture structure, height of nanostructure, and tip angle variation. It is observed that as the number of gold nanorings increase, the electric field enhancement (as well as the theoretical SERS enhancement factor) also increase due to the focusing of light towards the center of nanostructure, and after the addition of a few rings, the electric field enhancement becomes almost constant. We also studied the polarization dependence of the nanostructure by varying the angle of polarization of the incident light to check the variation of the electric field of the nanostructure, and observed that the proposed nanostructures did not have much polarization dependence. Moreover, due to the symmetric nature of the plasmonic nanostructure, the position of the hotspot region shifts to the adjacent corner on rotating the incident field polarization. We optimized all the dimensional parameters to get the best possible theoretical SERS enhancement factor of ∼ 1010. Moreover, we simulated a periodic array of these plasmonic nanostructures on the silica substrates, having equal periodicity in X and Y directions, and achieved a theoretical SERS enhancement factor of ∼ 1011.

High Electric Field Enhancement Induced by Modal Coupling for a Plasmonic Dimer Array on a Metallic Film

A giant electric field on a subwavelength scale is highly beneficial for boosting the light–matter interaction. In this paper, we investigated a hybrid structure consisting of a hemispheric dimer array and a gold film and realized resonant mode coupling of the surface lattice resonance (SLR) and surface plasmon polariton (SPP). Mode coupling is demonstrated by observing anti-crossing in reflection spectra, which corresponds to Rabi splitting. Although the resonance coupling does not enter the strong coupling regime, an improved quality factor (Q~350) and stronger electric field enhancement in the gap region of the dimer (i.e., hot spot) in our hybrid structure are obtained compared to those of the single dimer or dimer array only. Remarkably, the magnitude of electric field enhancement over 500 can be accessible. Such high field enhancement makes our hybridized structure a versatile platform for the realization of ultra-sensitive biosensing, low-threshold nanolasing, low-power nonlinear optical devices, etc.

Self-assembly of hard anions around cationic gold nanorods: potential structures for SERS.

The placement of polyoxometalates next to the surface of noble metallic nanoparticles has been found to enhance the surface-enhanced Raman scattering (SERS) effect. The enhancement is believed to stem from either charge (electrostatic attraction) or chemical effects. Anisotropic gold nanorods are recognized as useful nanostructures for SERS, mainly due to the high electric field enhancement at their ends. The presented work examines the use of a polyoxometalate encapsulated gold nanorod for SERS, to assess whether the two enhancement pathways would be synergetic. For this, a gold nanorod-polyoxometalate composite was synthesized by coating cetyltrimethylammonium bromide-stabilized gold nanorods with a silicotungstic Keggin anion through electrostatic attraction. The structure was characterized, confirming that the nanorods have been fully encapsulated by the polyoxometalate. The SERS performance of the composite was assessed in solution using crystal violet as a SERS indicator, finding an analytical enhancement factor of 1.8 × 104 in colloidal solution. The enhancement mechanism was examined first by comparison to gold nanorods stabilized by a cetyltriethylammonium bromide bilayer, cationic thiol bound polyoxometalate, and polyelectrolyte coating. Next, composites made using polyoxometalates of different atomic composition and charge were examined. It was concluded that the polyoxometalate charge had a noticeable effect on the enhancement while the atomic composition did not. Furthermore, high enhancement is observed mainly in cases where the nanorod monolayer allows the sequestration of the dye molecule into the nanoparticle's ligand layer. The proposed mechanism therefore involves the negative charge of the polyoxometalate attracting the positively charged dye, and facilitating the sequestration of the dye within the ligand bilayer, closer to the nanorod's surface.

Asymmetric L-shaped resonant optical antennas with plasmon length tuning and high-electric field enhancement

L-shaped resonant optical antennas (ROAs) are low-symmetry plasmonic nanostructures with the unique ability to show two tunable resonances in the optical and near-infrared wavelength regions. The plasmon length of the so-called longitudinal dipolar fundamental plasmon mode of these asymmetric L-shaped ROAs can be used as plasmon resonance building blocks to design polarization-sensitive devices. This paper introduces and numerically analyzes a novel design of asymmetric L-shape ROAs. The reported design offers two resonance modes, i.e., bimodal longitudinal antenna resonance behavior with a high enhancement factor. These two resonances can be selectively excited by changing the linear polarization angle. It is found that the coupled L-shaped ROA with a very small 2 nm gap width exhibits field enhancements 40 and 147.3 on scale left|{E}_{TOT}right|/left|{E}_{IN}right| for the high energy and low energy resonance, respectively. The obtained results and the analysis open a new route for multiple plasmon resonance devices with ultra-high field enhancement that can be easily integrated with future nano-optical circuits with multiple operational frequencies.

Revealing Adsorption Mechanism of p-Mercaptobenzoic Acid with TiO2 Surfaces Using Electric Field-Enhanced Semiconductor SERS

p-Mercaptobenzoic acid (4-MBA) is a typical molecular probe for a surface-enhanced Raman scattering (SERS) study of the enhancement performance of semiconductor nanoparticles. Understanding the molecular adsorption mechanism of 4-MBA on a semiconductor surface is crucial to reveal the enhancement mechanism of semiconductor SERS. Herein, two types of submicrometer-sized TiO2 particles with amorphous (denoted as a-TiO2) and anatase structures (denoted as c-TiO2) were fabricated, and their potential as SERS-active substrates with high electric-field enhancement was explored based on the near-field scattering theory and finite-element method simulation. The electric field-enhanced semiconductor SERS provide a better vision for us to study the adsorption modes of molecules on the TiO2 surface. On this basis, adsorption behaviors of 4-MBA on a-TiO2 and c-TiO2 particles were systematically studied by the semiconductor SERS and density functional theory. The results demonstrated that the adsorption mechanism of 4-MBA with TiO2 surfaces is highly dependent on the exposure of acid sites of TiO2 surfaces. 4-MBA adsorbs preferentially on Brønsted acid sites of a-TiO2 through a carboxyl group, in contrast on Lewis acid sites of c-TiO2 through a sulfhydryl group. Furthermore, 4-MBA molecules may form multilayer adsorption on TiO2 surfaces through the hydrogen bond and/or π–π stacking interaction. Research results not only provide a new insight to re-evaluate the chemical enhancement mechanism for TiO2–4-MBA systems but also provide a theoretical guidance for the modification of TiO2 surface with organic molecules containing carboxyl and sulfhydryl groups.

Giant localized electromagnetic field of highly doped silicon plasmonic nanoantennas

In this work, we present the analysis and design of an efficient nanoantenna sensor based on localized surface plasmon resonance (LSPR). A high refractive index dielectric nanostructure can exhibit strong radiation resonances with high electric field enhancement inside the gap. The use of silicon instead of metals as the material of choice in the design of such nanoantennas is advantageous since it allows the integration of nanoantenna-based structures into integrated-optoelectronics circuits manufactured using common fabrication methods in the electronic industry. It also allows the suggested devices to be mass-produced at a low cost. The proposed nanoantenna consists of a highly doped silicon nanorod and is placed on a dielectric substrate. Different shapes and different concentrations of doping for the nanoantenna structures that are resonant in the mid-infrared region are investigated and numerically analyzed. The wavelength of the enhancement peak as well as the enhancement level itself vary as the surrounding material changes. As a result, sensors may be designed to detect molecules via their characteristic vibrational transitions. The 3D FDTD approach via Lumerical software is used to obtain the numerical results. The suggested nanoantennas exhibit ultra-high local field enhancement inside the gap of the dipole structure.

High-speed nanoscale optical trapping with plasmonic double nanohole aperture.

Optical trapping with plasmonic double nanohole (DNH) apertures has proven to be an efficient method for trapping sub-50 nm particles due to their suppressed plasmonic heating effect and very high electric field enhancement in the gap region of the aperture. However, plasmonic tweezers are generally diffusion-limited, requiring particles to diffuse down to a few tens of nanometres from the high field enhancement regions before they can be trapped. The loading of target particles to the plasmonic hotspots can take several minutes for diluted samples. In this work, rapid particle transport and trapping of a 25 nm polystyrene sphere is demonstrated, leveraging an electrothermoplasmonic flow induced upon application of an AC field in the presence of a laser-induced temperature gradient. Using this approach, we demonstrate the rapid transport of a 25 nm polystyrene particle across a distance of 63 μm and trapping at the DNH under 16 s. This platform shows great potential for applications involving simultaneous trapping and plasmon-enhanced spectroscopies, such as Raman enhancement via the intense electric field enhancement in the DNH gap.

Numerical investigation of gold nano-polygonal structures as SERS-active substrates using near and far-sided excitation

Surface-enhanced Raman scattering (SERS), an offshoot of Raman spectroscopy, has emerged as a popular detection/diagnostic technique that enhances Raman signal significantly when the analyte is placed in the vicinity of plasmonic structures. The design of a resilient SERS-active substrate constituted of gold polygonal nano-patterns with varying number of corners on SiO2 substrate has been proposed in this work. Different polygonal structures (n = 3 to n = 10) have been optimized in this study, yielding high electric field enhancement; ≈ 109in near sidedmode and≈1010 in far sided mode. In terms of design parameters, the proposed SERS substrates are robust with a high degree of tolerance. In addition, it has been established that the proposed SERS-active substrates are reproducible, such that experimental findings can be replicated owing to the precise positioning of hot-spots. The proposed SERS substrate responds to several analytes, such as rhodamine B, nicotine with a high degree of sensitivity. The suggested SERS substrate can be fabricated employing contemporary nano-lithography techniques such as e-beam lithography (EBL), focused ion beam (FIB) lithography, etc. The proposed structures can be used in various applications including detection of explosive, biomolecules, narco-analysis, detection of food and water adulterants etc.

Design of locally enhanced electric field in dielectric loaded rectangular resonator for quantum microwave measurements

Rydberg-atom electrometers have the remarkable advantages of self-calibration and high sensitivity. Based on the classical electromagnetic theory, a localized electric field enhancement structure of a hybrid rectangular resonator (RR) is proposed to improve the sensitivity of quantum microwave measurement. It should be noted that the prototype of the hybrid RR is fabricated and measured at 9.925 GHz. The results of full-wave simulations show that the uniform and high electric field enhancement in the TE101 fundamental mode is realized. The transient process of resonance is simultaneously simulated, and the time to settle steady state is given as about 104 ns. As indicated through experimental results that the structure can reach 24 dB (enhancement factor of 15.8). As a result, the method proposed in this study, based on atomic measurement capabilities, enables us to improve the measurement sensitivity further and promotes the practical development of quantum microwave measurement technology.

Parallel trapping of multiple nanoparticles using a quasi-bound state in the continuum mode

In this work, we put forward an all-dielectric nanotweezer using a quasi-bound state in the continuum (quasi-BIC) mode to trap nanoparticles with a radius of 10 nm. The quasi-BIC mode provides not only a very high electric field enhancement but also a high quality factor ( Q -factor), which gives it potential for the trapping of nanoparticles with low laser power and high stability. The simulation results show that when the input intensity is 1 m W / µ m 2 , the maximum optical trapping force of the 10 nm particles is 2.24 pN, and the maximum trapping potential is 29.08 k B T . Furthermore, the proposed nanotweezer array provides multiple optical hotspots with high field confinement and enhancement, resulting in multiple trapping sites for the parallel trapping of multiple nanoparticles. The high-throughput trapping of nanoparticles provides a good foundation for studying biological cells and protein molecules, especially for the heterogeneity of cells and the large-scale parallel analyses of basic drugs.

Analysis of a multi-Fano plasmonic split-ring structure using characteristic mode theory for optical applications

Multi-Fano plasmonic structures are attractive for sensing and energy harvesting due to their ability to adjust resonances at several wavelengths and the high electric field enhancement. In this paper, the Characteristic Mode Theory (CMT) is used to get a multi-Fano structure, for the first time. For this purpose, the results of CMT in the Heptamer consisting of nanorings, are used. It has been shown that modes can resonate and couple together at the same time if the bending frequency of their eigenvalues is between their resonant frequencies (1470 nm for bending, 1382 and 1600 nm for the two modes) which has not been reviewed anywhere before. Then, to create more couplings between the modes and to produce a multi-Fano structure, the modes with coupling ability and their characteristic current distribution are considered. Some gaps have been created in the location of the smallest currents for increasing the coupling and intensifying the electric field (unlike trial and error methods previously used to improve the performance of the structures), and so, two split-ring Heptamer structures are created as multi-Fano structures. It has been shown that the proposed structure has a significant electric field enhancement of |450|2 which is a good choice for energy harvesting. In addition, these structures can be used for sensors with the most sensitivity of 840 nm / RIU, which has a significant sensitivity than similar structures.

3D particle simulations of positive air–methane streamers for combustion

Streamer discharges can be used as a primary source of reactive species for plasma-assisted combustion. In this research we investigate positive streamers in a stoichiometric air–methane mixture at 1 bar and 300 K with a three-dimensional particle-in-cell model for the electrons. We first discuss suitable electron scattering cross sections and an extension of the photoionization mechanism to air–methane mixtures. We discuss that the addition of 9.5% methane leaves electron transport and reaction coefficients essentially unchanged, but it largely suppresses photoionization and shortens the photon mean free path. This leads to (1) accelerated streamer branching, (2) higher electric field enhancement at the streamer head, (3) lower internal electric fields, and (4) higher electron densities in the streamer channel. We also calculate the time-integrated energy density deposited during the evolution of positive streamers in background electric fields of 12.5 and 20 kV cm−1. We find typical values of the deposited energy density in the range of 0.5–2.5 kJ m−3 within the ionized interior of streamers with a length of 5 mm; this value is rather independent of the electric fields applied here. Finally we find that the energy deposited in the inelastic electron scattering processes mainly produces reactive nitrogen species: N2 triplet states and N, but also O and H radicals. The production of H2 and O2 singlet states also occurs albeit less pronounced. Our calculation of the primary production of reactive species can for example be used in global chemistry models.

Magnetic polaritons assisted effective excitation of multi-order anisotropic borophene surface plasmons in the infrared region

Anisotropic borophene surface plasmons with extraordinary features have triggered considerable research interest since its discovery. However, due to the atomically thin thickness of monolayer borophene, it is difficult to achieve borophene surface plasmons with high absorption, especially for high-order plasmonic resonant modes. In this paper, we theoretically proposed a hybrid plasmonic system consisting of a borophene monolayer and a periodic metallic grating separated by a dielectric spacer, where multi-order anisotropic borophene surface plasmons and magnetic polaritons can be simultaneously excited. Benefitting from the high electric field enhancement and localization of the magnetic polaritons, the intrinsic absorption of the multi-order borophene surface plasmons can be improved by at least 3 times when the resonant wavelengths of borophene plasmons and magnetic polaritons are tailored to match each other, either by passively tuning the height of grating slits or by actively adjusting the borophene electron density. By further designing compound grating with multiple slits in one period, the overall absorption of the system can be significantly improved within a wide infrared region. Such hybrid system with may find applications in the design of boron-based optoelectronics devices in the infrared region.

Herbicide/pesticide sensing with metamaterial absorber in THz regime

Terahertz (THz) technology has been attracted great interest in many research areas over the years, especially THz plasmonics for sensing applications since intra- and inter-molecular vibrations are within the THz range. The sensitivity of free-space THz detection can be boosted up by use of metamaterials (MM), which are artificial structures in subwavelength scale of the incident light. These artificial materials present high electric field enhancement and high sensitivity to the change in their surroundings. In this article, residual herbicides/pesticides have been investigated both theoretically and experimentally with a polarization-insensitive THz metamaterial absorber (MMA) composed of dielectric-metal disk antennas. Our THz MMA can detect as small as 5 ppM of commonly used herbicide/pesticide, namely paraquat and glyphosate. The results show that sensitivity is greatly improved by using THz MMA, with the limit of detection (LOD) of these herbicides/pesticides reaching to 5 ppM. These results indicate that THz MMA platform could be a valuable method for highly sensitive THz applications in food quality and safety control. We believe, our sensor platform based on general c-mos technology fabrication could be a potential detection tool for herbicides/ pesticides residues in agriculture and food products.

Strong field enhancement in individual Φ-shaped dielectric nanostructures based on anapole mode resonances.

Due to their ability to produce high electric field enhancements in relatively large nanoscale volumes with minimum absorption and nonradiating properties, anapole modes excited in high index dielectric nanostructures have attracted considerable attentions in these years. We propose a design strategy to simultaneously excite the anapole mode efficiently and maintain its resonant wavelength, which has been remained as a challenge in the conventional dielectric nanostructures. Based on analyzing the relationship between the field enhancement factor and scattering intensity of the electric and toroidal dipoles, we introduce two and four nanocuboids into the nil field intensity areas in the silicon disk system, respectively. The geometric volume of the system can be increased effectively and the electric field enhancement is boosted to be 190% and 250% while the resonant wavelength of the anapole mode is almost maintained constant. The systems combined with a slot in the strongest field intensity area also follow the same law, revealing that the design strategy can be easily extended to other geometric, material and frequency systems. Different from the design strategy to add new components into the areas with strong field intensity, the incorporations occurring at the minimum intensity area is another design scheme to engineer the properties of the resonant systems and can find broad applications in nano-device designs.

Plasmon mediated decomposition of brominated nucleobases on silver nanoparticles \u2013 A surface enhanced Raman scattering (SERS) study

The localized surface plasmon resonances (LSPRs) of silver nanoparticles (AgNPs) give rise to the generation of so called hot electrons and a high local electric field enhancement, which enable an application of AgNPs in different fields ranging from catalysis to sensing. Hot electrons generated upon the decay of LSPRs are transferred to molecules adsorbed on the surface of the NPs and trigger chemical reactions via dissociative electron attachment (DEA). Herein, we report on the hot electron induced decomposition of the brominated nucleobases – 8-bromoadenine, 8-bromoguanine, 5-bromocytosine and 5-bromouracil on laser illuminated AgNP surfaces. Surface enhanced Raman scattering (SERS) spectra of all canonical nucleobases and their brominated analogues have been recorded at different laser illumination times, and for the very first time we present SERS measurements of 8-bromoguanine and 5-bromocytosine. Reaction products have been identified by their vibrational fingerprint revealing the cleavage of the carbon bromide bond in all cases even under mild illumination conditions. These results indicate that the well-known reactions from DEA experiments in the gas phase (i) are also taking place on nanoparticle surfaces under ambient conditions, (ii) can be monitored by SERS, and (iii) are also of importance in analytical SERS applications involving electrophilic molecules, as the bands originating from reaction products need to be identified.Graphical abstract

Synthesis of Au–Ag Alloy Nanoparticle-Incorporated AgBr Crystals

Nanoscale composites consisting of silver and silver halide (Ag–AgX, X = Cl, Br, I) have attracted much attention as a novel type of visible-light photocatalyst (the so-called plasmonic photocatalysts), for solar-to-chemical transformations. Support-free Au–Ag alloy nanoparticle-incorporated AgBr crystals (Au–Ag@AgBr) were synthesized by a photochemical method. At the initial step, Au ion-doped AgBr particles were prepared by adding an aqueous solution of AgNO3 to a mixed aqueous solution of KBr and HAuBr4. At the next step, UV-light illumination (λ = 365 nm) of a methanol suspension of the resulting solids yielded Au–Ag alloy nanoparticles with a mean size of approximately 5 nm in the micrometer-sized AgBr crystals. The mole percent of Au to all the Ag in Au–Ag@AgBr was controlled below < 0.16 mol% by the HAuBr4 concentration in the first step. Finite-difference time-domain calculations indicated that the local electric field enhancement factor for the alloy nanoparticle drastically decreases with an increase in the Au content. Also, the peak of the localized surface plasmon resonance shifts towards longer wavelengths with increasing Au content. Au–Ag@AgBr is a highly promising plasmonic photocatalyst for sunlight-driven chemical transformations due to the compatibility of the high local electric field enhancement and sunlight harvesting efficiency.