Local Electromagnetic Field Enhancement Research Articles

Transverse and longitudinal coupling of LSPPs in isolated triangular Al–SiO2–Al hybrid nanoplates for generation of local electromagnetic fields with enhanced intensity and increased decay time

Achieving a large enhancement of local electromagnetic fields in the ultraviolet waveband is desirable for some applications such as surface-enhanced Raman scattering and surface-enhanced fluorescence. In addition, it is more significant for some applications such as plasmon-enhanced harmonic generation to enhance the intensity of local electromagnetic fields and increase their decay time at the same time. In this paper, using the finite-difference time-domain method, we numerically demonstrate that using the linearly polarized light with a wavelength of 325 nm as the illumination light, an isolated triangular Al–SiO2–Al hybrid nanoplate with optimized geometric parameters can produce a local electric field enhanced by a factor of about 108 at one of its top apexes, and produce two local electric fields enhanced by a factor of about 150 at two transverse dielectric/metal interfaces of one of its longitudinal side edges. Moreover, we also numerically demonstrate that the decay time of enhanced local electric fields produced by the isolated triangular Al–SiO2–Al hybrid nanoplate is about 1.6 times as large as that of enhanced local electric fields produced by an isolated triangular Al nanoplate. These unique properties of the isolated triangular Al–SiO2–Al hybrid nanoplate arise because of both the transverse coupling and the longitudinal coupling of localized surface plasmon polaritons in this structure. Our findings make triangular Al–SiO2–Al hybrid nanoplates very promising for application in many fields such as surface-enhanced Raman scattering and plasmon-enhanced harmonic generation.

Enhanced photocatalytic water splitting of TiO2 by decorating with facet-controlled Au nanocrystals

Localized surface plasmon resonance (LSPR) has drawn much interest in photocatalysis due to the advantages in extending light-absorption and improving charge-separation efficiency. In this work, Au nanocubes with exposed {100} facets and Au nanotrioctahedrons with exposed {221} facets are synthesized via the seed-growth method with the aim of demonstrating the photocatalytic performance under the LSPR effect using a typical TiO2 photocatalyst. The modification of Au nanocrystals results in accelerated charge separation in the TiO2 photocatalyst, which is attributed to the synergetic effect of local electromagnetic field enhancement from the LSPR effect and the Schottky junction established in Au and TiO2. Consequently, the photocatalytic hydrogen-production from the TiO2 photocatalyst is greatly improved. Moreover, Au nanotrioctahedrons modified TiO2 exhibits superior photocatalytic performance due to the stronger local electromagnetic field of the LSPR and the more efficient electron trapping originated from the unique morphology with exposed {221} facets.

Enhancement effect of silver nanoparticles decorated titania nanotube array acting as active SERS substrate

Silver nanoparticles decorated titania nanotube array (Ag-TiO2 NTA) had been designed as active SERS substrate for a sensitive molecule detection. The Ag nanoparticles are deposited on the gaps and nanotube walls of the tubewall-separated TiO2 NTA to form the independent Ag-TiO2 NTA, alternatively on the nanotube wall surface of the tubewall-connected TiO2 NTA to form the interconnected Ag-TiO2 NTA. The microstructure of TiO2 NTA is more critical to affect the SERS performance rather than the deposition cycles. The analytical enhancement factor was accordingly enhanced from 1.67 × 105 for the interconnected Ag-TiO2 NTA up to 9.44 × 105 for the independent Ag-TiO2 NTA. The microstructure-tailored and Ag deposition level-promoted Ag-TiO2 NTA substrate achieved sensitive detection of 4-mercaptobenzoic acid. The superior SERS performance is caused by synergizing the localized electromagnetic field enhancement of Ag nanoparticles and the charge transfer chemical enhancement of TiO2 NTA along with the light scattering enhancement of vertically aligned nanoarray architecture.

Resonance-enhanced second harmonic generation via quantum dots integrated with Ag nanoarrays

The metal nanoarray can couple the excitation light energy to the surface, resulting in local electromagnetic field enhancement due to the resonance effect. This is beneficial to the generation of nonlinear optical processes which depend on electromagnetic resonance enhancement, taking advantage of the field enhancement properties of metal nanoarray. Here, silver nanoperiodic arrays are integrated with AgInS2 quantum dots (QDs) to enhance the second harmonic generation (SHG) process of QDs. The experimental results show that the enhancement factor of SHG is 8.8-fold in the condition of surface plasmon resonance. In contrast, the second harmonic emission from pure quantum dots is very weak. The simulation reveals that the second harmonic enhancement is caused by the resonance between the incident laser and the Ag nanoarray. The experimental results show that it is feasible to generate a highly efficient nonlinear optical process of QDs assisted by metal nanoarray. This is beneficial for extending the nonlinear applications of quantum dots.

Polarization-independent anapole response of a trimer-based dielectric metasurface

Abstract The phenomenon of anapole has attracted considerable attention in the field of metamaterials as a possible realization of radiationless objects. We comprehensively study this phenomenon in the cluster-based systems of dielectric particles by considering conditions of anapole manifestation in both single trimers of disk-shaped particles and metamaterial composed on such trimers. Our analytical approach is based on the multipole decomposition method and the secondary multipole decomposition technique. They allow us to associate the anapole with the multipole moments of the trimer and the separate multipole moments of its constitutive particles. The manifestation of anapole in a two-dimensional metamaterial (metasurface) is confirmed by checking the resonant states in the reflected field as well as from the electromagnetic near-field patterns obtained from the full-wave numerical simulation. It is demonstrated that the anapole excitation in trimers results in the polarization-independent suppression of reflection with the resonant enhancement of local electromagnetic fields in the metasurface. Finally, experimental verification of the theoretical results is presented and discussed.

Fluorescence Enhancement via Dual Coupling of Dye Molecules with Silver Nanostructures

We demonstrate the enhancement of fluorescence emitted from dye molecules coupled with two surface plasmons, i.e., silver nanoparticles (AgNPs)-induced localized surface plasmons (LSP) and thin silver (Ag) film supported surface plasmons. Excitation light is illuminated to a SiO2 layer that contains both rhodamine 110 molecules and AgNPs. AgNPs enhances excitation rates of dye molecules in their close proximity due to LSP-induced enhancement of local electromagnetic fields at dye excitation wavelengths. Moreover, the SiO2 layer on one surface of which a 50 nm-thick Ag film is coated for metal cladding (air on the other surface), acts as a waveguide core at the dye emission wavelengths. The Ag film induces the surface plasmons which couple with the waveguide modes, resulting in a waveguide-modulated version of surface plasmon coupled emission (SPCE) for different SiO2 thicknesses in a reverse Kretschmann configuration. We find that varying the SiO2 thickness modulates the fluorescent signal of SPCE, its modulation behavior being in agreement with the theoretical simulation of thickness dependent properties of the coupled plasmon waveguide resonance. This enables optimization engineering of the waveguide structure for enhancement of fluorescent signals. The combination of LSP enhanced dye excitation and the waveguide-modulated version of SPCE may offer chances of enhancing fluorescent signals for a highly sensitive fluorescent assay of biomedical and chemical substances.

Solution-Based SERS Detection of Weak Surficial Affinity Molecules Using Cysteamine-Modified Au Bipyramids.

To achieve ultrasensitive detection of trace targets through solution-based surface-enhanced Raman spectroscopy (SERS), direct adsorption of the target molecules on a SERS-active surface is vital. In this work, cetyltrimethylammonium bromide (CTAB)-capped gold nano-bipyramids (Au BPs) with different aspect ratios (ARs) are prepared and the surface is successfully modified by a simple ligand exchange method. Cysteamine-capped gold nano-bipyramids (cyst-Au BPs) are obtained by means of replacement of CTAB by cysteamine using Au-S covalent bonding and applied in the solution-based SERS detection of different pigment molecules, which always have weak affinity to the gold surface. The hydrogen bonding between the pigment molecule and cysteamine causes the aggregation of Au BPs to generate local electromagnetic field enhancement. The influence of the AR and concentration of Au BPs on SERS properties is investigated. The SERS detection of weak-affinity molecules to an extremely low limit shows that the cyst-Au BPs are highly sensitive compared to CTAB-capped Au BPs. The limit of detection (LOD) of allura red as low as 0.1 ppb and that of sunset yellow as low as 1 ppb show that the proposed strategy has many advantages due to its simplicity and fast and rapid detection for the sensitivity analysis of weak-affinity molecules.

Toroidal dipole response in the individual silicon hollow cylinder under radially polarized beam excitation

The toroidal dipole (TD) has attracted growing attention due to its unique properties. Here, we propose and demonstrate almost pure TD resonance in the visible region in a silicon hollow cylinder. The enhanced optical coupling to TD resonance is implemented using a focused radially polarized beam illumination matching, well-designed individual silicon nanostructure resonator. The polarization of the longitudinal electric field in the silicon hollow cylinder that breaks space-inversion symmetry is critical to the formation of enhanced TD resonance. Additionally, the pure TD resonance can be achieved in a wide spectral range by tuning the geometrical parameters of the structure. The proposed pure TD resonator may provide potential applications in the local enhancement of electromagnetic fields and the design of all-dielectric nanoantennas.

Hot carrier-mediated avalanche multiphoton photoluminescence from coupled Au-Al nanoantennas.

Avalanche multiphoton photoluminescence (AMPL) is observed from coupled Au-Al nanoantennas under intense laser pumping, which shows more than one order of magnitude emission intensity enhancement and distinct spectral features compared with ordinary metallic photoluminescence. The experiments are conducted by altering the incident laser intensity and polarization using a home-built scanning confocal optical microscope. The results show that AMPL originates from the recombination of avalanche hot carriers that are seeded by multiphoton ionization. Notably, at the excitation stage, multiphoton ionization is shown to be assisted by the local electromagnetic field enhancement produced by coupled plasmonic modes. At the emission step, the giant AMPL intensity can be evaluated as a function of the local field environment and the thermal factor for hot carriers, in accordance with a linear relationship between the power law exponent coefficient and the emitted photon energy. The dramatic change in the spectral profile is explained by spectral linewidth broadening mechanisms. This study offers nanospectroscopic evidence of both the potential optical damages for plasmonic nanostructures and the underlying physical nature of light-matter interactions under a strong laser field; it illustrates the significance of the emerging topics of plasmonic-enhanced spectroscopy and laser-induced breakdown spectroscopy.

A Programmable DNA-Silicification-Based Nanocavity for Single-Molecule Plasmonic Sensing.

Plasmonic nanocavities are highly desirable for optical sensing because of their singular ability to confine light into deep subwavelength volumes. Yet, it remains profoundly challenging to fabricate structurally resilient nanocavities with high fidelity, and to obtain direct, noninvasive visualization of the plasmonic hotspots within such constructs. Herein, highly precise and robust nanocavities, entitled DNA-silicified template for Raman optical beacon (DNA-STROBE), are engineered by using silicified DNA scaffolds for spatial organization of discrete plasmonic nanoparticles. In addition to substantially enhancing structural stability and chemical inertness, DNA silicification significantly improves nanogap control, resulting simultaneously in large and controlled local electromagnetic field enhancement. The ultrasmall mode volume of the DNA-STROBE constructs promotes single-molecule occupancy enabling surface-enhanced Raman spectroscopy (SERS) observations of single-molecule activity even at elevated background concentration, significantly relaxing the restrictive pico- to nanomolar molecular concentration condition typically required for such investigations. Additionally, leveraging super-resolution SERS measurements allows noninvasive and diffraction-unlimited spatial profiling of otherwise unresolvable plasmonic hotspots. The highly programmable and reproducible nature of the DNA-STROBE, coupled with its quantitative label-free molecular readouts, provides a versatile platform with applications across the spectrum of nanophotonics and biomedical sciences.

Facile fabrication of three-dimensional gold nanodendrites decorated by silver nanoparticles as hybrid SERS-active substrate for the detection of food contaminants

Applicable surface enhanced Raman scattering (SERS) active substrate require not only high enhancement factor (EF), but also excellent reproducibility and facile fabrication process. Although some colloidal SERS substrates have advantages of high EF, random assembly causing the unstable signal remains a challenge. This study presented a simple, effective and reproducible SERS substrate for sensitive detection of melamine and thiram in food samples. A three-dimensional gold nanodendrites (AuNDs) decorated by silver nanoparticles (AgNPs) via electrodeposition has been synthesized as hybrid SERS-active substrate. The synthesis conditions were optimized with voltage power of −1.5 V and deposition time of 300 s, consequently the gold nanodendrites were obtained. The developed AgNPs decorated AuNDs (Ag-AuNDs) substrate showed excellent Raman enhancement effect because of high local electromagnetic (EM) field enhancement. Under the optimal synthesis condition, the SERS substrate was utilized for quantitative detection of melamine and thiram, and the linear relationship were in the range of 0.01–5 mg/L for melamine and 0.12–24 mg/L for thiram. The limit of detection were 7.38 μg/L and 86.1 μg/L for melamine and thiram, respectively. Finally, the substrate was successfully applied to detect melamine in milk and thiram in apple juice, indicating that Ag-AuNDs substrate could be a promising SERS detection platform for food safety assurance.

Boosting a sub-10 nm nanogap array by plasmon-triggered waveguide resonance

Gap-type metallic nanostructures are widely used in catalytic reactions, sensors, and photonics because the hotspot effect on these nanostructures supports giant local electromagnetic field enhancement. To achieve hotspots, researchers devote themselves to reducing gap distances, even to 1 nm. However, current techniques to fabricate such narrow gaps in large areas are still challenging. Herein, a new coupling way to boost the sub-10 nm plasmonic nanogap array is developed, based on the plasmon-triggered optical waveguide resonance via near-field coupling. This effect leads to an amplified local electromagnetic field within the gap regions equivalent to narrower gaps, which is evidenced experimentally by the surface-enhanced Raman scattering intensity of probed molecules located in the gap and the finite-difference time-domain numerical simulation results. This study provides a universal strategy to promote the performance of the existing hotspot configurations without changing their geometries.

Plasma-Enabled Amorphous TiO2 Nanotubes as Hydrophobic Support for Molecular Sensing by SERS.

We devise a unique heteronanostructure array to overcome a persistent issue of simultaneously utilizing the surface-enhanced Raman scattering, inexpensive, Earth-abundant materials, large surface areas, and multifunctionality to demonstrate near single-molecule detection. Room-temperature plasma-enhanced chemical vapor deposition and thermal evaporation provide high-density arrays of vertical TiO2 nanotubes decorated with Ag nanoparticles. The role of the TiO2 nanotubes is 3-fold: (i) providing a high surface area for the homogeneous distribution of supported Ag nanoparticles, (ii) increasing the water contact angle to achieve superhydrophobic limits, and (iii) enhancing the Raman signal by synergizing the localized electromagnetic field enhancement (Ag plasmons) and charge transfer chemical enhancement mechanisms (amorphous TiO2) and by increasing the light scattering because of the formation of vertically aligned nanoarchitectures. As a result, we reach a Raman enhancement factor of up to 9.4 × 107, satisfying the key practical device requirements. The enhancement mechanism is optimized through the interplay of the optimum microstructure, nanotube/shell thickness, Ag nanoparticles size distribution, and density. Vertically aligned amorphous TiO2 nanotubes decorated with Ag nanoparticles with a mean diameter of 10-12 nm provide enough sensitivity for near-instant concentration analysis with an ultralow few-molecule detection limit of 10-12 M (Rh6G in water) and the possibility to scale up device fabrication.

Effect of additive acid on seeded growth of gold nanobipyramids

The presence of sharp tips and edges of gold nanobipyramids (GNBPs) lead to strong local electromagnetic field enhancement. Herein for the first time we present a study on efficacy of additive acid on the formation of GNBPs. The effect of additive acid type and its concentration in growth solution were investigated to obtain various yield and aspect ratio of GNBPs. Morphological characterization shows the nanoseeds that were grown in growth solution prepared using HCl, H2SO4, and HF at concentration from 0 to 1 mL produced GNBPs with yield ranges from 5.21 ± 0.44 to 91.46 ± 3.32% and aspect ratio from 2.00 ± 0.02 to 2.76 ± 0.05. The optical response of GNBPs exhibit dual plasmon band at wavelength around 550 nm–570 nm, corresponding to transverse surface plasmon resonance (t-SPR) and at wavelength around 700 nm–800 nm, corresponding to longitudinal surface plasmon resonance (l-SPR). The presented approach may be used to produce tunable morphological GNBPs nanostructures which potentially used in sensors, SERS, catalysis, and photonics.

Plasmonic photocatalysis and SERS sensing using ellipsometrically modeled Ag nanoisland substrates

Silver nanoislands are key platforms for plasmonic photocatalysis, SERS sensing and optical metamaterials due to their localized surface plasmon resonances. The low intrinsic loss in Ag enables high local electromagnetic field enhancements. Solution-based fabrication techniques, while cheap, result in highly non-reproducible plasmonic substrates with wide sample-to-sample variability in geometry, optical resonances and Q-factors. Herein, we present a non-lithographic method of forming silver nanoislands based on sputter deposition of Ag films followed by elevated temperature annealing to induce spontaneous dewetting. The resulting plasmonic substrates show reproducible, well-defined LSPR resonances with high ensemble Q-factors whose optical properties could be modeled using spectroscopic ellipsometry to yield n and k values across the visible range. UV–Vis-NIR, and XRD analyses define the optical and crystallographic characteristics of the Ag nanoisland samples. FESEM was utilized to discern the geometry and architecture of the Ag nanoisland as well as their uniformity and monodispersity. Our vacuum deposited Ag nanoislands demonstrated excellent photocatalytic activity for the transformation of 4-nitrobenzenethiol (4-NBT) and 4-aminothiophenol (PATP) into p,p’-dimercaptoazobenzene (DMAB).

Ultrabright Fluorescence Readout of an Inkjet-Printed Immunoassay Using Plasmonic Nanogap Cavities.

Fluorescence-based microarrays are promising diagnostic tools due to their high throughput, small sample volume requirements, and multiplexing capabilities. However, their low fluorescence output has limited their implementation for in vitro diagnostics applications in point-of-care (POC) settings. Here, by integration of a sandwich immunoassay microarray within a plasmonic nanogap cavity, we demonstrate strongly enhanced fluorescence which is critical for readout by inexpensive POC detectors. The immunoassay consists of inkjet-printed antibodies on a polymer brush which is grown on a gold film. Colloidally synthesized silver nanocubes are placed on top and interact with the underlying gold film creating high local electromagnetic field enhancements. By varying the thickness of the brush from 5 to 20 nm, up to a 151-fold increase in fluorescence and 14-fold improvement in the limit-of-detection is observed for the cardiac biomarker B-type natriuretic peptide (BNP) compared to the unenhanced assay, paving the way for a new generation of POC clinical diagnostics.

MoS2 Monolayers on Au Nanodot Arrays: Surface Plasmon, Local Strain, and Interfacial Electronic Interaction.

Metal and transition-metal dichalcogenide (TMD) hybrid systems have been attracting growing research attention because exciton-plasmon coupling is a desirable means of tuning the physical properties of TMD materials. Competing effects of metal nanostructures, such as the local electromagnetic field enhancement and luminescence quenching, affect the photoluminescence (PL) characteristics of metal/TMD nanostructures. In this study, we prepared TMD MoS2 monolayers on hexagonal arrays of Au nanodots and investigated their physical properties by micro-PL and surface photovoltage (SPV) measurements. MoS2 monolayers on bare Au nanodots exhibited higher PL intensities than those of MoS2 monolayers on 5-nm-thick Al2O3-coated Au nanodots. The Al2O3 spacer layer blocked charge transfer at the Au/MoS2 interface but allowed the transfer of mechanical strain to the MoS2 monolayers on the nanodots. The SPV mapping results revealed not only the electron-transfer behavior at the Au/MoS2 contacts but also the lateral drift of charge carriers at the MoS2 surface under light illumination, which corresponds to nonradiative relaxation processes of the photogenerated excitons.

Deep-Ultraviolet Plasmon Resonances in Al-Al2O3@C Core–Shell Nanoparticles Prepared via Laser Ablation in Liquid

We developed a convenient, facile, low-cost, and “green” method to synthesize nanoparticles (NPs) with deep-ultraviolet localized surface plasmon resonances (LSPRs) based on the laser ablation of an aluminum target in liquid. The nanoparticles had an Al-Al2O3@C core–shell structure, and the LSPR peak ranged from 240 to 250 nm with the increasing laser radiation time. It is found that the LSPR peak of the NPs is related to the presence of Al2O3 based on experimental characterization and theoretical simulation. The carbon shell can reduce the oxidation of Al nanoparticles and enhance the stability, which is significant for achieving the deep-ultraviolet LSPR. Moreover, we demonstrated the enhancement of the blue fluorescence intensity from CsPbBr3–xClx by Al-Al2O3@C NPs due to the stronger excitations for CsPbBr3–xClx by the enhancement of localized electromagnetic field from LSPR.

A Tunable Triple-Band Near-Infrared Metamaterial Absorber Based on Au Nano-Cuboids Array.

In this article, we present a design for a triple-band tunable metamaterial absorber with an Au nano-cuboids array, and undertake numerical research about its optical properties and local electromagnetic field enhancement. The proposed structure is investigated by the finite-difference time domain (FDTD) method, and we find that it has triple-band tunable perfect absorption peaks in the near infrared band (1000–2500 nm). We investigate some of structure parameters that influence the fields of surface plasmons (SP) resonances of the nano array structure. By adjusting the relevant structural parameters, we can accomplish the regulation of the surface plasmons resonance (SPR) peaks. In addition, the triple-band resonant wavelength of the absorber has good operational angle-polarization-tolerance. We believe that the excellent properties of our designed absorber have promising applications in plasma-enhanced photovoltaic, optical absorption switching and infrared modulator optical communication.

Ag loaded TiO2 nanotube photonic crystals self-doped Ti3+ periodically by anodization process and their performance of surface enhanced Raman scattering

Here, the preparation of TiO2 nanotube photonic crystals (PCs) and their periodical Ti3+ self-doping were synchronously accomplished for forming Ti3+-TiO2 PCs by introducing an appropriate reduction pulse voltage (−4 V) into the periodical anodic oxidation voltage waveform. Subsequently, some Ag nanoparticles were deposited on the surface of the Ti3+-TiO2 PCs by vacuum evaporation for getting Ag/Ti3+-TiO2 PCs nanostructures, which were ideally suited for the surface-enhanced Raman scatting (SERS) substrates. Compared to the Ag loaded TiO2 nanotubes, the SERS signal of the Ag/Ti3+-TiO2 PCs for RhB can be improved 8.4 times. The desirable SERS property would be attributed to the synergistic effects of the localized electromagnetic (EM) field enhancement and charge-transfer (CT) chemical enhancement mechanisms, which was also discussed in detail.