Large Electric Field Enhancement Research Articles

Remarkable Plasmonic Enhanced Luminescence of Ce3+doped Lanthanide Downconversion Nanoparticles in NIR‐II Window by Silver Hole‐Cap Nanoarrays

AbstractLanthanide downconversion nanoparticles (DCNPs) have huge potential in biosensing and imaging applications in the NIR‐II window. However, DCNPs inherently suffer from low quantum efficiency, due to low absorption cross‐section and the restricted doping concentration of lanthanide ions. In this work, a combined strategy for downconversion luminescence in the NIR‐II window is investigated by the integration of Ce3+ ions into the conventional NaYF4: Yb3+, Er3+ DCNPs and incorporation of periodic silver hole‐cap coupled Nanoarrays (Ag‐HCNAs) simultaneously. Over two orders of magnitude, luminescence enhancement is achieved by the combination of optimized Ce3+ doping and plasmonic effects, compared to NaYF4: Yb3+, Er3+ DCNPs immobilized on the glass substrate. Moreover, 3D Finite‐Difference Time‐Domain (FDTD) simulations and time‐resolved luminescence measurements are combined to gain important insights into the mechanism of downconversion luminescence enhancement. The results show that there is a large electric field enhancement between the Ag nanoholes and the Ag hemisphere cap at 980 nm (excitation enhancement), while the lifetime shortening at 1525 nm revealed an increased radiative decay rate and enhanced quantum yield (emission rate enhancement). The strategy for downconversion luminescence enhancement demonstrated in this work holds a significant potential for advancing the next generation biosensing and bioimaging based on DCNPs in the NIR‐II window.

Optical Anapole Modes of the Hybrid Ring-Disk Nanoantenna for Electric Field Enhancement

A promising design combining low loss, high refractive index dielectric materials with metal nanoparticles, and local plasma resonance is described for ring-disk nanoantenna. A hybrid Au ring Si disk structure with chain gaps is constructed and the distributions of the electric field and near-field enhancement of the electric fields are analyzed by numerical simulation utilizing the multipole decomposition method. The electric field strength can be enhanced efficiently using metals to improve the anapole mode response of dielectric materials with large refractive indexes. Additional enhancement of the electric field is achieved by opening the chain gap to create high-intensity hot spots inside the disk. The chain comprises 14 normal octahedrons and a few tips are created by intersecting octahedrons. The electric field is greatly enhanced at the tips where there are high-intensity hot spots. The effectiveness of this structure as a refractive index sensor is assessed. The scattering cross-section (SCS) and wavelength shifts result in a maximum sensitivity of 210[Formula: see text]nm/RIU. The results validate the design concept and the hybrid structure boasting large electric field enhancement and excellent optical sensing properties have large potential in nonlinear photonics.

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.

Deep‐Subwavelength Resonant Meta‐Optics Enabled by Ultra‐High Index Topological Insulators

AbstractIn nanophotonics, small mode volumes, high‐quality factor resonances, and large field enhancements without metals fundamentally scale with the refractive index and are key for many implementations involving light‐matter interactions. Topological insulators (TIs) are a class of insulating materials that host topologically protected surface states, some of which exhibit extraordinarily high permittivity values. Here, the optical properties of TI bismuth telluride (Bi2Te3) single crystals are studied. It is found that both the bulk and surface states contribute to the extremely large optical constants, with the real part of the refractive index peaking at n ≈ 11. Utilizing these ultra‐high index values, it is demonstrated that Bi2Te3 metasurfaces are capable of squeezing light in deep‐subwavelength structures, with the fundamental magnetic dipole (MD) resonance confined in unit cell sizes smaller than λ/10. It is further shown that dense ultrathin metasurface arrays can simultaneously provide large magnetic and electric field enhancements arising from the high index of the bulk and the surface metallic states. These findings demonstrate the potential of chalcogenide TIs as a platform leveraging the unique combination of ultra‐high‐index dielectric response with surface metallic states for metamaterial design and nanophotonic applications in sensing, non‐linear generation, and quantum information.

Design of optical anapole modes of all-dielectric nanoantennas for SERS applications.

To obtain large electric field enhancement while mitigating material losses, an all-dielectric nanoantenna composed of a heptamer and nanocubes is designed and analyzed. A numerical simulation by the finite element method reveals that the nanoantenna achieves the optical electric anapole modes, thereby significantly enhancing the coupling between different dielectrics to further improve the near-field enhancement and spontaneous radiation. Field enhancement factors |E/E 0|2 of 3,563 and 5,395 (AM1 and AM2) and a Purcell factor of 3,872 are observed in the wavelength range between 350 and 800nm. This nanoantenna has promising potential in applications involving surface-enhanced Raman scattering and nonlinearities due to its low cost and excellent compatibility.

Morphology Engineering for High-Q Plasmonic Surface Lattice Resonances with Large Field Enhancement

Plasmonic surface lattice resonances (SLRs) have endowed plasmonic systems with unprecedently high quality (Q) factors, giving rise to great advantages for light–matter interactions and boosting the developments of nanolaser, photodetector, biosensor and so on. However, it still lacks exploration to develop a strategy for achieving large electric field enhancements (FEs) while maintaining high Q factors of SLRs. Here, we investigate and verify such a strategy by engineering morphologies of plasmonic lattice, in which the influences of geometrical shapes, cross-section areas and structural compositions of particles are investigated. Firstly, we found that the Q factor of a plasmonic SLR is inversely proportional to the square of the cross-section area of the cell particles in the studied cases. Secondly, larger FEs of SLRs appear when the separated cell particles support stronger FEs. By combining these effects of particle morphology, we achieve a plasmonic SLR with Q factor and FE up to 2100 and 592 times, respectively. Additionally, supported by the derived connections between the Q factors and FEs of SLRs and the properties of cell particles, the property optimizations of SLRs can be done by optimizing the separated particles, which are distinctly time-saving in simulations. These results provide a guideline for the design of high-performance optical nanocavities, and can benefit a variety of fields including biosensing, nonlinear optics and quantum information processing.

Electrochemical Synthesis of Plasmonic Nanostructures.

Thanks to their tunable and strong interaction with light, plasmonic nanostructures have been investigated for a wide range of applications. In most cases, controlling the electric field enhancement at the metal surface is crucial. This can be achieved by controlling the metal nanostructure size, shape, and location in three dimensions, which is synthetically challenging. Electrochemical methods can provide a reliable, simple, and cost-effective approach to nanostructure metals with a high degree of geometrical freedom. Herein, we review the use of electrochemistry to synthesize metal nanostructures in the context of plasmonics. Both template-free and templated electrochemical syntheses are presented, along with their strengths and limitations. While template-free techniques can be used for the mass production of low-cost but efficient plasmonic substrates, templated approaches offer an unprecedented synthetic control. Thus, a special emphasis is given to templated electrochemical lithographies, which can be used to synthesize complex metal architectures with defined dimensions and compositions in one, two and three dimensions. These techniques provide a spatial resolution down to the sub-10 nanometer range and are particularly successful at synthesizing well-defined metal nanoscale gaps that provide very large electric field enhancements, which are relevant for both fundamental and applied research in plasmonics.

Noble Metal Nanoparticle Biosensors: From Fundamental Studies toward Point-of-Care Diagnostics.

Noble metal nanoparticles (NMNPs) have become firmly established as effective agents to detect various biomolecules with extremely high sensitivity. This ability stems from the collective oscillation of free electrons and extremely large electric field enhancement under exposure to light, leading to various light-matter interactions such as localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering. A remarkable feature of NMNPs is their customizability by mechanisms such as particle etching, growth, and aggregation/dispersion, yielding distinct color changes and excellent opportunities for colorimetric biosensing in user-friendly assays and devices. They are readily functionalized with a large variety of capping agents and biomolecules, with resultant bioconjugates often possessing excellent biocompatibility, which can be used to quantitatively detect analytes from physiological fluids. Furthermore, they can possess excellent catalytic properties that can achieve significant signal amplification through mechanisms such as the catalytic transformation of colorless substrates to colored reporters. The various excellent attributes of NMNP biosensors have put them in the spotlight for developing high-performance in vitro diagnostic (IVD) devices that are particularly well-suited to mitigate the societal threat that infectious diseases pose. This threat continues to dominate the global health care landscape, claiming millions of lives annually. NMNP IVDs possess the potential to sensitively detect infections even at very early stages with affordable and field-deployable devices, which will be key to strengthening infectious disease management. This has been the major focal point of current research, with a view to new avenues for early multiplexed detection of infectious diseases with portable devices such as smartphones, especially in resource-limited settings.In this Account, we provide an overview of our original inspiration and efforts in NMNP-based assay development, together with some more sophisticated IVD assays by ourselves and many others. Our work in the area has led to our recent efforts in developing IVDs for high-profile infectious diseases, including Ebola and HIV. We emphasize that integration with digital platforms represents an opportunity to establish and efficiently manage widespread testing, tracking, epidemiological intelligence, and data sharing backed by community participation. We highlight how digital technologies can address the limitations of conventional diagnostic technologies at the point of care (POC) and how they may be used to abate and contain the spread of infectious diseases. Finally, we focus on more recent integrations of noble metal nanoparticles with Raman spectroscopy for accurate, noninvasive POC diagnostics with improved sensitivity and specificity.

Electric field distribution of Ag@CuO and Ag@ZnO core–shell dimer

The effect of shape, material and shell thickness on electric field distribution of both Ag@CuO and Ag@ZnO core–shell dimer is investigated. First, three core–shell structures, including plane@circle, triangle@circle and quadrilateral@circle are designed. The field enhancement advantages of the quadrilateral@circle structures are summarized by analyzing the electric field distribution diagrams. Then the influence of materials on the field enhancement effect of core–shell dimer is discussed. The results show that Ag@CuO structure has better stability, while Ag@ZnO structure have larger electric field enhancement. Finally, the shell thickness is changed from 0.1 nm to 1.5 nm. We found that the X-component electric field of both the Ag@ZnO and Ag@CuO core–shell dimer structure is decreased with the increase in the shell thickness. Also, the X-component electric field of Ag@ZnO core–shell dimer is about five times than that of the Ag@CuO. The increase of electric field indicated that Ag@ZnO core–shell dimer has strong local surface plasma resonance characteristics.

Giant enhancements of high-order upconversion luminescence enabled by multiresonant hyperbolic metamaterials

Photonic nanostructures with resonant modes that can generate large electric field (EF) enhancements are applied to enhance light-matter interactions in nanoscale, bringing about great advances in both fundamental and applied science. However, a small hot spot (i.e., the regions with strong EF enhancements) and highly inhomogeneous EF distribution of the resonant modes usually hinder the enhancements of light-matter interactions in a large spatial scale. Additionally, it is a severe challenge to simultaneously generate multiple resonant modes with strong EF enhancements in a broadband spectral range, which greatly limits the capacity of a photonic nanostructure in boosting optical responses including nonlinear conversion, photoluminescence, etc. In order to overcome these challenges, we presented an arrayed hyperbolic metamaterial (AHMM). This AHMM structure is applied to simultaneously enhance the three-photon and four-photon luminescence of upconversion nanoparticles. Excitingly, the enhancement of the three-photon process is 1 order of magnitude larger than previous records, and for the enhancing four-photon process, we achieve an enhancement of 3350 times, greatly beneficial for overcoming the crucial problem of low efficiency in near infrared light upconversion. Our results demonstrated a promising platform for realizing giant enhancements of light-matter interactions, holding potential in constructing various photonics applications such as the nonlinear light sources.

Enhanced and tunable double Fano resonances in plasmonic metasurfaces with nanoring dimers

The appearance of the double-resonance substrate has promoted the application of surface-enhanced Raman scattering (SERS). By controlling the frequencies of the double resonances to match the excitation and Raman scattering frequencies, the detection of the object to be measured can be more effective. For the double-resonance substrate, while the resonance frequencies can be highly controllable, the electric field enhancement is also one of the important factors affecting the application in SERS. In this paper, we designed a metasurface composed of a nanoring dimer array, silica dielectric and gold film. The nanoring dimer array and gold film are separated by the silica dielectric to form a resonant cavity. The localized surface plasmon resonance generated in the nanoring dimer array is coupled with the cavity mode of the resonant cavity. Double Fano resonance with strong electric field enhancement is generated at the gap of the nanoring dimer. The electric field enhancement value can reach 100, which is an order of magnitude larger than that of the nanoring metasurface without the gap structure. The double Fano resonance peaks can be flexibly adjusted while maintaining large electric field enhancements by changing the following parameters: the period of the nanoring dimer array along the direction of the short axis, the ratio of the inner and outer radius of the nanoring and the length of the resonant cavity. Therefore, the proposed metasurface-enhanced Raman scattering substrate provides both the enhanced and tunable double Fano resonances necessary for high-sensitivity, high-selectivity and high-throughput detection. In addition, we proved that the length of the cavity can be determined by theoretical calculation, which avoids a lot of simulation processes.

All-optical self-switching with ultralow incident laser intensity assisted by a bound state in the continuum.

We present a new, to the best of our knowledge, scheme of realizing all-optical switching by exploiting the phenomenon of the bound state in the continuum (BIC) supported by an array of slotted silicon disks. The air slot in an individual silicon disk works as a void antenna and can then help to excite the quasi-BIC (QBIC) mode by a linearly polarized plane wave. Thanks to the high quality factor of the QBIC resonance and the associated large local electric field enhancement, the self-switching effect is quite pronounced, and a laser power intensity level less than 1KW/cm2 is required to achieve a change of transmittance from 0 to above 65%.

Huge field enhancement and high transmittance enabled by terahertz bow-tie aperture arrays: a simulation study.

Sub-wavelength aperture arrays featuring small gaps have an extraordinary significance in enhancing the interactions of terahertz (THz) waves with matters. But it is difficult to obtain large light-substance interaction enhancement and high optical response signal detection capabilities at the same time. Here, we propose a simple terahertz bow-tie aperture arrays structure with a large electric field enhancement factor and high transmittance at the same time. The field enhancement factor can reach a high value of 1.9×104 and the transmission coefficient of around 0.8 (the corresponding normalized-to-area transmittance is about 14.3) at 0.04 µm feature gap simultaneously. The systematic simulation results show that the designed structure can enhance the intensity of electromagnetic hotspot by continuously reducing the feature gap size without affecting the intensity of the transmittance. We also visually displayed the significant advantages of extremely strong electromagnetic hot spots in local terahertz refractive index detection, which provides a potential platform and simple strategy for enhanced THz spectral detection.

High‐Curvature Transition‐Metal Chalcogenide Nanostructures with a Pronounced Proximity Effect Enable Fast and Selective CO2 Electroreduction

AbstractA considerable challenge in the conversion of carbon dioxide into useful fuels comes from the activation of CO2 to CO2.− or other intermediates, which often requires precious‐metal catalysts, high overpotentials, and/or electrolyte additives (e.g., ionic liquids). We report a microwave heating strategy for synthesizing a transition‐metal chalcogenide nanostructure that efficiently catalyzes CO2 electroreduction to carbon monoxide (CO). We found that the cadmium sulfide (CdS) nanoneedle arrays exhibit an unprecedented current density of 212 mA cm−2 with 95.5±4.0 % CO Faraday efficiency at −1.2 V versus a reversible hydrogen electrode (RHE; without iR correction). Experimental and computational studies show that the high‐curvature CdS nanostructured catalyst has a pronounced proximity effect which gives rise to large electric field enhancement, which can concentrate alkali‐metal cations resulting in the enhanced CO2 electroreduction efficiency.

High-Curvature Transition-Metal Chalcogenide Nanostructures with a Pronounced Proximity Effect Enable Fast and Selective CO2 Electroreduction.

A considerable challenge in the conversion of carbon dioxide into useful fuels comes from the activation of CO2 to CO2 .- or other intermediates, which often requires precious-metal catalysts, high overpotentials, and/or electrolyte additives (e.g., ionic liquids). We report a microwave heating strategy for synthesizing a transition-metal chalcogenide nanostructure that efficiently catalyzes CO2 electroreduction to carbon monoxide (CO). We found that the cadmium sulfide (CdS) nanoneedle arrays exhibit an unprecedented current density of 212 mA cm-2 with 95.5±4.0 % CO Faraday efficiency at -1.2 V versus a reversible hydrogen electrode (RHE; without iR correction). Experimental and computational studies show that the high-curvature CdS nanostructured catalyst has a pronounced proximity effect which gives rise to large electric field enhancement, which can concentrate alkali-metal cations resulting in the enhanced CO2 electroreduction efficiency.

Gold nanonails for surface-enhanced infrared absorption.

Surface-enhanced infrared absorption (SEIRA) can dramatically enhance the vibrational signals of analyte molecules owing to the interaction between plasmons and molecular vibrations. It has huge potential for applications in various detection and diagnostic fields. High-aspect-ratio rod-like metal nanostructures have been the most widely studied nanomaterials for SEIRA. However, nearly all of the rod-like nanostructures reported previously are fabricated using physical methods. They suffer from damping and low areal number densities. In this work, high-aspect-ratio Au nanorods are synthesized, and Au nanonails are prepared through Au overgrowth on the as-prepared Au nanorods. The aspect ratios of the Au nanorods and nanonails can be varied in the range of ∼10 to ∼60, and their longitudinal dipolar plasmon resonance wavelengths can be correspondingly tailored from ∼1.6 to ∼8.3 μm. The Au nanonails exhibit superior SEIRA performance with 4-aminothiophenol used as the probe molecules. They are further used to detect the common biomolecule l-cysteine. Numerical simulations are further performed to understand the experimental results. They match well with the experimental observations, revealing the mechanism of the SEIRA enhancement. Our study demonstrates that colloidal high-aspect-ratio Au nanonails and nanorods can function as SEIRA nanoantennas for highly sensitive molecular detection in various situations.

Localized ZnO Growth on a Gold Nanoantenna by Plasmon-Assisted Hydrothermal Synthesis.

The excitation of localized surface plasmon resonances (LSPRs) in metal nanostructures enables subwavelength photon localization and large electric field enhancement, which can be advantageously used to strongly enhance light-matter interactions at the nanoscale. For this purpose, efficient methods for deterministically handling and arranging nanomaterials at the exact position of the localized electric field are required. In this Letter, we propose a novel method based on a hydrothermal synthesis reaction to locally and selectively synthesize zinc oxide in a plasmonic nanoantenna. We first make evident the role of LSPR for achieving efficient heating of gold nanostructures. Then, by selectively addressing one of the LSPRs of a gold antenna, we demonstrate that localized zinc oxide formation at the targeted location of the antenna can be achieved due to the nanoscale confinement of the heat production.

Cascaded plasmonic nanorod antenna for large broadband local electric field enhancement**Project supported by the National Natural Science Foundation of China (Grant No. 11704416) and the Hunan Provincial Natural Science Foundation, China (Grant No. 2017JJ3408).

We propose a cascaded plasmonic nanorod antenna for large broadband electric near-field enhancement. The structure has one big gold nanorod on each side of a small two-wire antenna which consists of two small gold nanorods. For each small nanorod, the enhanced and broadened optical response can be obtained due to the efficient energy transfer from its adjacent big nanorod through strong plasmonic near-field coupling. Thus, the electric field intensity of the cascaded antenna is significantly larger and broader than that of the individual small two-wire antenna. The resonant position, field intensity enhancement, and spectral width of the cascaded antenna are highly tunable by varying the geometry of the system. The quantum efficiency of the cascaded antenna is also greatly enhanced compared with that of the small antenna. Our results are important for the applications in field-enhanced spectroscopy.

Optimization of electric field enhancement of Ag@SiO2 trimer nanospheres by finite difference time domain method

Plasmonic properties of trimer nanocluster depend on their geometric configuration. In this work, local electric field enhancement in the junction region of the trimer Ag@SiO2 core-shell is investigated using three-dimensional electromagnetic simulation based on finite-difference time domain (FDTD) method. Plasmonic wavelength dependence for the two orthogonal polarizations is analyzed for different geometric configurations by gradually breaking the symmetry of the trimer nanospheres. The highly symmetric trimer in the form of equilateral triangle pattern possesses highly intense hotspots with the resonance wavelengths that are degenerate in nature. Breaking of this mode degeneracy occurs when the symmetric triangular pattern of the trimer is gradually transformed to a linear chain arrangement. Trimer core shell with linear chain symmetry exhibits the highest electromagnetic field enhancement which is nearly four times greater than the bare metal trimer but is very sensitive to polarization direction. The localized surface plasmon resonance (LSPR) is red shifted and blue shifted under longitudinal and transverse polarization, respectively. The LSPR shift and the electric field enhancement of the trimer nanocluster upon symmetry breaking under different polarization is compared and is explained using plasmon hybridization theory. The large polarization insensitive electric field enhancement exhibited by the symmetry core-shell may be employed for surface enhanced Raman spectroscopy based chemical and biological molecule detection.

Large third-order optical nonlinearity and ultrafast optical response in thin Au nanodisks

Nanostructures with remarkable optical nonlinearity and ultrafast light response have enabled numbers of applications in the fields varying from physics to biochemistry, and even quantum science. Maximally increasing the optical nonlinearity and simultaneously reducing the light response time of nanostructure is regarded as a great challenge for the optics from both fundamental and applied. In this paper, we report unprecedentedly huge third-order nonlinearity and ultrafast property in the nanosystem of Au nanodisks with ∼7 nm thickness. Their thin thickness brings about large electric field enhancements, leading to the third-order nonlinearity susceptibility (χ(3)) reaching the order of 10−18 m2/V2 (∼10−10 esu), which is 1-3 orders of magnitude larger than that of other shapes of Au nanostructures in the same solution environment, such as nanospheres, nanorods and triangle nanoprisms. Furthermore, optical Kerr measurements demonstrate their optical response time is as fast as ∼100 fs. Our findings demonstrate the thin Au nanodisk can be a suitable candidate for ultrafast nonlinear optical devices, such as the all-optical switches and all-optical signal processing devices.