Large Electromagnetic Field Enhancement Research Articles

Tuning strong coupling towards the deep ultraviolet region realized through Tamm-plasmon exciton-polaritons

Light-matter interactions at the nanoscale have attracted considerable attention due to its features of high optical-field confinement and large electromagnetic-field enhancement. Herein, we theoretically demonstrate strong coupling between Tamm plasmon (TP) and exciton polaritons towards the deep ultraviolet (DUV) region in metal Al/distributed Bragg reflector (DBR)-molecular structures. By adjusting the thickness of the spacer layer and the angle of incident light, the obvious anti-crossing phenomena can be observed with a Rabi splitting of 38.4 meV. Moreover, through varying the period number of the DBR structure, the coupling strength can be effectively manipulated from weak to strong. These findings may extend the operating wavelength of strong coupling to the DUV region, and benefit the development of new-style optical filters.

Efficient multiplexed label-free detection by flexible MXene/graphene oxide fibers with enhanced charge transfer and hot spots effect

Sensors integrated with sampling and analysis processes are the key in analytical chemistry, environmental protection and national security. However, achieving reliable and sensitive large-scale rapid screening of target analytes is still challenging. Herein, we demonstrate a robust surface-enhanced Raman scattering (SERS) sensor by constructing dense and uniform plasmonic hotspots on the flexible Ti3C2Tx MXene/graphene oxide (MG) fibers through a self-assemble strategy at the oil-water interface. The produced SERS substrates exhibited an ultralow limit of detection (LOD, 1 ×10−15 M for R6G), outstanding sensitivity (EF = 1.53 ×1012) and high stability for R6G molecules (RSD = 9.47%, over 60 days storage). SERS experiments and theoretical simulations suggested that acidified Ti3C2Tx MXene further excited charge-transfer (CT) resonance of the system, while the assembled Ag nanostructures produced a large electromagnetic (EM) field enhancement. Thus, the fiber sensor not only realizes the multiplexed and powerful detection of pesticide residues (LOD of 10−11 M) with an error < 7.3% via an established concentration-dependent standard color barcodes, but also allows the recognization of target molecules in a wide range of fields, such as methylene blue, crystal violet and nikethamide. This work provides new insights in the development of versatile label-free sensors for the rapid multiplexed analysis of target molecules in real samples.

Molybdenum Oxide Functional Passivation of Aluminum Dimers for Enhancing Optical-Field and Environmental Stability

In this contribution, we present an experimental and numerical study on the coating of Al plasmonic nanostructures through a conformal layer of high-refractive-index molybdenum oxide. The investigated structures are closely coupled nanodisks where we observe that the effect of the thin coating is to help gap narrowing down to the sub-5-nm range, where a large electromagnetic field enhancement and confinement can be achieved. The solution represents an alternative to more complex and challenging lithographic approaches, and results are also advantageous for enhancing the long-term stability of aluminum nanostructures.

A polarization-resolved ECL strategy based on the surface plasmon coupling effect of orientational Au nanobipyramids patterned structures

Here, we developed a polarization-resolved surface plasmon coupling electrochemiluminescence (ECL) strategy based on orientational Au nanobipyramids (Au NBPs) patterned structures. As the surface plasmonic materials, the Au NBPs with large localized electromagnetic field enhancement were used to construct the different patterned structures through the droplet evaporation induced assembly. The distinct morphology of the horizontal and vertical Au NBPs patterned structures provided the different plasmonic properties reflecting on resonance peak positions and polarized absorbance. As a result, the variations of the polarized ECL emission of SnS2 QDs have been regulated by the Au NBPs patterned structures via the surface plasmon coupling progress. Because of the superior polarization-resolved ECL performances, the sensing system was used to analyze the miRNA-21 and miRNA-205 expression level in tumor tissues of triple negative breast cancer patients. The satisfactory results indicated this polarization-resolved sensing strategy possessed great potential for clinical analysis.

Large electromagnetic field enhancement in plasmonic nanoellipse for tunable spaser based applications

We theoretically demonstrated a class of plasmonic coupled elliptical nanostructure for achieving a spaser or a nanolaser with high intensity. The plasmonic ellipse is made up of gold film substrate. The proposed structure is then trialed for various light polarizations, moreover, a simple elliptical nanostructure has been chosen primarily from which different cases have been formed by geometry alteration. The structure supports strong coupled resonance mode i.e. localized surface plasmon (LSP). The localized surface plasmon resonance (LSPR) of the investigated system is numerically examined using the finite-element method (FEM). The calculations showed that the LSPR peaks and the local field intensity or near field enhancement (NFE) of the active nanosystem can be amplified to higher values by introducing symmetry-breaking techniques in the proposed ellipse and its variants. The coupled nanostructure having different stages of wavelengths can be excited with different plasmonic resonance modes by the selection of suitable gain media. In addition, a small-sized nanolaser with high tunability range can be developed using this nanostructure. The spaser phenomena are achieved at several wavelengths by changing light polarization and structure alteration methods. Giant localized field enhancement and high LSPR values enable the proposed model to be highly appealing for sensing applications, surface-enhanced Raman spectroscopy, and much more.

Light Scattering by Noble Metallic Nanoparticles for Performance of Compound Solar Cells Enhancement

Light scattering by noble metallic nanoparticles are of interest for a variety of applications due to the large electromagnetic field enhancement that occurs in the vicinity of the metal surface, and the dependence of the resonance photon energy on the nanoparticle size, shape, local dielectric environment, and material. Here, the influences of electromagnetic scattering by Au and Ag nanoparticles placed atop compound solar cells on optical absorption and photocurrent generation were investigated based on the variation in the noble nanoparticle densities. The results indicated that the short-circuit current and power conversion efficiency were strongly affected by the density and material of the noble nanoparticles. The great improvement of 28% in power conversion efficiency can be obtained with Au nanoparticle density of 2\(\times\)108 cm-2. This improvement can be attributed to light scattering, light trapping, and surface roughness by noble nanoparticles. Furthermore, Au nanoparticles showed more efficient in solar cell power conversion efficiency improvement than Ag nanoparticles did although density of Au nanoparticle was lower than that of Ag nanoparticles.

Optimizing SERS performance through aggregation of gold nanorods in Langmuir-Blodgett films

Nanoparticles with a high degree of anisotropy are characterized by large electromagnetic field enhancements at the single-particle level; however, their arrangement into aggregated structures does not lead to significantly better performance. Surprisingly, for nanorods a superior accumulation of hot spots is present at surface coverages above 60%, which ensures a higher efficiency for surface-enhanced Raman spectroscopy (SERS). In this study, we produced pegylated gold nanorods in Langmuir-Blodgett layers in order to optimize SERS performance for the ultrasensitive detection of molecules, by tuning plasmonic coupling and nanoparticle surface coverage. To understand the arrangement of the nanorods in Langmuir and Langmuir-Blodgett layers, we performed spectroscopic and microscopic analysis of the obtained films. Based on the compression isotherms of the nanorods, we proposed the best conditions for nanoplatform design. The Raman enhancement factor was around 2·105 with a deposition surface pressure equal to 12 mN·m−1. Due to the polymer’s presence at the surface of the nanorods and the dewetting process, specific dendritic-like aggregates were observed on the solid substrate. The results obtained indicate the significance of further studies on the arrangement of metallic nanoparticles in monolayers, taking into account the size and shape of the nanoparticles, as well as the type of stabilizing ligands at their surface.

Engineering gallium phosphide nanostructures for efficient nonlinear photonics and enhanced spectroscopies

Abstract Optical resonances arising from quasi-bound states in the continuum (QBICs) have been recently identified in nanostructured dielectrics, showing ultrahigh quality factors accompanied by very large electromagnetic field enhancements. In this work, we design a periodic array of gallium phosphide (GaP) elliptical cylinders supporting, concurrently, three spectrally separated QBIC resonances with in-plane magnetic dipole, out-of-plane magnetic dipole, and electric quadrupole characters. We numerically explore this system for second-harmonic generation and degenerate four-wave mixing, demonstrating giant per unit cell conversion efficiencies of up to ∼ 2 W−1 and ∼ 60 W−2, respectively, when considering realistic introduced asymmetries in the metasurface, compatible with current fabrication limitations. We find that this configuration outperforms by up to more than four orders of magnitude the response of low-Q Mie or anapole resonances in individual GaP nanoantennas with engineered nonlinear mode-matching conditions. Benefiting from the straight-oriented electric field of one of the examined high-Q resonances, we further propose a novel nanocavity design for enhanced spectroscopies by slotting the meta-atoms of the periodic array. We discover that the optical cavity sustains high-intensity fields homogeneously distributed inside the slot, delivering its best performance when the elliptical cylinders are cut from end to end forming a gap, which represents a convenient model for experimental investigations. When placing an electric point dipole inside the added aperture, we find that the metasurface offers ultrahigh radiative enhancements, exceeding the previously reported slotted dielectric nanodisk at the anapole excitation by more than two orders of magnitude.

Substrate‐Modulated Electromagnetic Resonances in Colloidal Cu2O Nanospheres

AbstractDielectric nanoparticles are expected to complement or even replace plasmonic nanoparticles in many optical and optoelectronic applications, because they exhibit small absorption losses and support strong electric and magnetic resonances simultaneously. Dielectric nanoparticles need to be deposited on various substrates in many applications. Understanding the substrate effect on the electromagnetic resonances of dielectric nanoparticles is of great importance for engineering their resonance properties and designing optical devices. In this study, moderate‐refractive‐index cuprous oxide nanospheres with uniform sizes and shapes are synthesized. The scattering spectra and images of the nanospheres deposited on three types of substrates are analyzed experimentally and theoretically. When supported on indium tin oxide–coated glass slides and Si wafers, the color of the nanospheres varies from blue, cyan, green, yellow, orange and red, covering almost the entire visible region. When deposited on gold films, the electromagnetic resonances of the nanospheres redshift intensively and a new effective magnetic resonance mode appears. The enhanced Raman scattering reveals that large electromagnetic field enhancements are produced in the gap region between the nanosphere and the substrate. The results shed light on the manipulation of the electromagnetic responses of dielectric nanoparticles and the design of dielectric metamaterials in the presence of various substrates.

Localized Surface Plasmon Resonance Spectroscopy for the Detection of Microtubule Nucleation

Localized surface plasmon resonance (LSPR) is a collective oscillation of conduction electrons in metal nanoparticles. The excitation of LSPR induces a large electromagnetic field enhancement near the metal surfaces, making it highly sensitive to the local refractive index surrounding the nanoparticles. LSPR spectroscopy uses this localized sensitivity to achieve label-free detection of biological molecules. Previously, LSPR spectroscopy has been employed to study various biomolecular interactions. However, it has not been used to detect biopolymer nucleation. Here, we present a LSPR spectroscopy approach for the detection of microtubule nucleation. Using a combination of an extended Mie theory and computational simulations, we investigate theoretically how the optical responses of spherical and nonspherical gold nanoparticles change when microtubules form around them. Our results indicate that the extinction peak wavelength is sensitive to the formation of short microtubules around the nanoparticles, but is insensitive to their elongation. We also demonstrate that LSPR spectroscopy can detect microtubule nucleation experimentally by inducting spontaneous microtubule formation around spherical gold nanoparticles. Consistent with our theoretical predictions, our experiments show that the peak wavelength changes significantly when short microtubules form around the nanoparticles, but not when they are elongated. We also show that the peak wavelength is sensitive to the formation of microtubule nuclei even when they are too small to be detectable with a turbidity measurement.

Surface-enhanced infrared absorption

Infrared spectroscopy is a powerful tool for the detection and identification of molecules. However, its application is limited by the low infrared absorption cross-sections of molecules. Localized surface plasmon resonances in metal and highly-doped semiconductor nanomaterials can produce large localized electromagnetic field enhancements. When molecular vibrations are in resonance with localized surface plasmon resonances, molecular vibrations are able to be largely amplified. The detection of minute amounts of target molecules can be realized in this way. Surface-enhanced infrared absorption (SEIRA) spectroscopy has recently attracted extensive attention owing to its enormous potential in molecular detection in chemistry, biology and medicine. In this review article, the enhancement mechanism and theoretical models of SEIRA are first introduced. The preparation methods and enhancement effects of various infrared-responsive nanomaterials are then discussed. The applications of SEIRA in diverse areas, such as hyperspectral infrared chemical imaging, detection of biomolecules, monitoring of pollutants in the environment, and on-chip gas sensing, are finally summarized.

Plasmonic Effects in Noble Metal-liquid Metal Based Nanoparticles

In the era of flexible and foldable devices, liquid metals emerge as a champion because they are being liquid at or near room temperature in addition to having high electrical and thermal conductivities. Plasmonic resonance occurs when conduction band electrons on metal nanoparticle surface collectively oscillates with same frequency as that irradiated light. This plasmonic resonance has attracted great attention because of large electromagnetic field enhancements near metal nanoparticle and the regulating resonance wavelength with change in material, size, shape and surrounding medium of metallic nanoparticle. Incorporation of liquid metal nanoparticles in plasmonics provides unique properties towards sensing (heart rate monitors etc.) which can become wearable. So, developing liquid metal based low-cost and large-scale plasmonic nanostructures may provide more optical efficiencies, fast kinetics, low temperature processing, versatility, easy embedding in structures and stretchy devices. Present work focuses on literature review highlighting the study of optical properties (absorption and scattering efficiencies, LSPR tunability, Figure of Merit (FOM) and Refractive Index Sensitivity (RIS)) of noble metal-liquid metal nanostructures and future scope of the field. Simulations can be performed on the basis of Mie Theory for spherical nanoparticles and by DDA/FDTD method for non-spherical particles or arrays. The results can help to optimize the plasmonic nanostructures of suitable material, size and shape according to the need of application in particular region of EM spectrum.

Theoretical investigation of a plasmonic substrate with multi-resonance for surface enhanced hyper-Raman scattering

Because of the unique selection rule, hyper-Raman scattering (HRS) can provide spectral information that linear Raman and infrared spectroscopy cannot obtain. However, the weak signal is the key bottleneck that restricts the application of HRS technique in study of the molecular structure, surface or interface behavior. Here, we theoretically design and investigate a kind of plasmonic substrate consisting of Ag nanorices for enhancing the HRS signal based on the electromagnetic enhancement mechanism. The Ag nanorice can excite multiple resonances at optical and near-infrared frequencies. By properly designing the structure parameters of Ag nanorice, multi- plasmon resonances with large electromagnetic field enhancements can be excited, when the “hot spots” locate on the same spatial positions and the resonance wavelengths match with the pump and the second-order Stokes beams, respectively. Assisted by the field enhancements resulting from the first- and second-longitudinal plasmon resonance of Ag nanorice, the enhancement factor of surface enhanced hyper-Raman scattering can reach as high as 5.08 × 109, meaning 9 orders of magnitude enhancement over the conventional HRS without the plasmonic substrate.

Surface-Enhanced Raman and Fluorescence Spectroscopy with an All-Dielectric Metasurface

Plasmonic substrates play a crucial role in the confinement and manipulation of localized electromagnetic fields at the nanoscale. The large electromagnetic field enhancement at metal/dielectric interfaces is widely exploited in surface-enhanced fluorescence (SEF) and surface-enhanced Raman scattering (SERS) spectroscopies. Despite the advantage of near-field enhancement, unfortunately, in metals, the large absorption at optical frequencies induces local heating of the analyte fluid with possible damage of the biological material. In addition, in SEF plasmonic substrates, spacer layers are necessary to minimize undesired fluorescence quenching due to nonradiative decay, which strongly depends on the distance between molecules and metallic substrates. Therefore, the possibility of managing surface electromagnetic states mimicking surface-plasmon resonances in terms of spatial localization, high-field intensity, and dispersion characteristics, while avoiding metallic losses is of great interest. However, di...

Concave Gold Bipyramid Saturable Absorber Based 1018 nm Passively Q-Switched Fiber Laser

Concave gold bipyramids (CAuBPs) exhibit enhanced saturable absorption characteristics due to their large electromagnetic-field enhancement, large third order nonlinearity and fast recovery time. In this paper, the CAuBPs are synthetized through a seed-mediated wet-chemical growth method, and a 1018 nm passively Q-switched fiber laser exploiting CAuBPs saturable absorber (SA) is demonstrated in a ring cavity. The maximum average output power of 9.61 mW is obtained under the pump power of 270 mW in Q-switched regime. The minimum pulse width is 1.83 μ s at the pulse repetition rate of 97.47 kHz. The results prove that CAuBP is a promising SA with potential for important applications in the field of pulsed lasers.

Efficient Excitation of Higher Order Modes in the Plasmonic Response of Individual Concave Gold Nanocubes

Recently, concave nanocube (CNC) shaped metal nanoparticles (MNPs) with high index facets have drawn special attention due to their high chemical activity and large electromagnetic (EM) field enhancements, making them good candidates for multifunctional platforms. However, most of the previously published works focused on the plasmonic properties of silver simple nanocubes of smaller dimension, i.e., within the quasi-static limit, hardly supporting efficient excitation of high-order plasmonic modes. Site-selective electron beam excitation of individual Au CNC particles gives rise to simultaneous excitation of edge and corner localized surface plasmon (LSP) modes. We show that spatial variation of the radiative modes is strongly localized at the corners and extended along the edges of the top surface of the CNCs. Extensive finite-difference time-domain (FDTD) numerical analysis reveals that the substrate-induced plasmon hybridization leads to the activation of corner octupolar and corner quadrupolar LSP mo...

Electromagnetically Induced Transparency and Sharp Asymmetric Fano Line Shapes in All-Dielectric Nanodimer

Low-loss electromagnetically induced transparency (EIT) and asymmetric Fano line shapes are investigated in a simple planar silicon dimer resonator. The EIT and Fano effects emerge due to near-field coupling of the modes supported by both the nanoparticles in a dimer structure. Different configurations of the dimer nanostructure are analyzed, which provide distinct EIT and Fano resonances. Furthermore, the tunability of EIT and Fano resonant modes are incorporated by changing the structural parameters. It is also found that the dimer resonator exhibits high Q factor and large electromagnetic field enhancement at Fano resonance and EIT window due to extremely low absorption loss. Such values and narrow resonances are supposed to be useful highly sensitive sensors and slow-light applications.

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.

Free-standing gold elliptical nanoantenna with tunable wavelength in near-infrared region for enhanced Raman spectroscopy

The purpose of this work is to present a surface-enhanced Raman scattering (SERS) amplifying antenna for the possible usage in the near-infrared region. Instead of the visible-light range amplifying antenna such as a bowtie, the finite-difference time-domain (FDTD) simulation results indicate that elliptical antenna could provide large electromagnetic field enhancement at near-infrared wavelength by combining the free-standing enhancement property with large aspect ratios of the ellipse geometry. The simulation results consist with the enhancement factors characterized by SERS measurements at the excited wavelength of 785 nm for different aspect ratios and periodicities. In addition to the redshift of the resonance wavelength as the aspect ratio of ellipse increases, the free-standing structure modifies the resonance behavior and the dielectric environment of antenna by elevating the elliptical disk from the substrate. To interpret the simulation results, the analytical solution of resonance wavelength for ellipsoid dimmer is derived based on Lorentz–Mie theory, and comparisons are made between the analytical solution and simulation results. The quasi-static analytical solution provides a way to characterize the resonance behavior of two ellipsoid particles as a function of the gap distance, aspect ratio, and dielectric environment. The electrodynamic analysis for the periodic structure was performed in our FDTD simulations.

Boosting Hot Electron-Driven Photocatalysis through Anisotropic Plasmonic Nanoparticles with Hot Spots in Au–TiO2 Nanoarchitectures

The use of plasmonic metal nanoparticles as photosensitizers has undergone a strong development in the past few years given their ability to increase the activity of semiconductors into the visible and near-infrared regions. The present work reports an experimental and theoretical study on the critical influence that shape anisotropy of gold nanoparticles exerts on the photocatalytic performance of Au–TiO2 nanoarchitectures. The obtained results show that for a given amount of metallic material, Au nanostars endow titania with a strongly enhanced catalytic efficiency compared to that found in the presence of Au nanospheres or nanorods. This is ascribed to the ability of nanostars to locally create extremely large electromagnetic field enhancements around their spikes, which ensures an increased population of hot electrons close to the interface between the metal and the semiconductor. Therefore, these nanostructures exhibit a novel regime of photocatalytic activity that could be described as plasmonic hot...