Maximum Electric Field Enhancement Research Articles

Fast On-Site Speciation and High Spatial Resolution Imaging of Labile Arsenic in Freshwater and Sediment Using the DGT-SERS Sensor.

Diffusive gradients in thin films (DGT) technique is renowned for in situ passive sampling but not for rapid on-site analysis, whereas surface-enhanced Raman spectroscopy (SERS) excels in ultrasensitive on-site detection but is limited by substrate contamination from complex matrices. Here, a hierarchical nanostructure of silver (Ag) mirror-supported large Ag nanoparticles (∼120 nm) was grown in situ in polyacrylamide hydrogel with a restricted pore size (PAM/Ag mirror/AgNPs) to serve as both the DGT binding phase and the SERS substrate. The substrate exhibited a maximum electric field enhancement factor of 9.9 × 108 and a signal relative standard error of 4.8%. Using the DGT-SERS sensor, As(III) and As(V) in freshwater were simultaneously detected at limits of 0.9 and 0.8 μg L-1, respectively, applicable across a wide range of environmental conditions. The DGT-SERS effectively mitigated the interfacial reduction of As(V) caused by humic acid by excluding it from plasmonic hotspots through size exclusion of the diffusive layer. The Raman analysis of a DGT sample in the field requires only 2 s using a portable spectrometer without DGT device disassembly. More importantly, the DGT-SERS captured the first two-dimensional image of As(III) and As(V) in one DGT at the micron scale resolution, revealing their spatially supplementary distribution patterns at the sediment-water interface. This study paves the way for next-generation speciation imaging DGT and the application of SERS in complex environments.

Corn-inspired high-density plasmonic metal-organic frameworks microneedles for enhanced SERS detection of acetaminophen

Effective monitoring of acetaminophen (APAP) dosage is crucial for preventing antipyretic abuse, ensuring therapeutic efficacy, and minimizing toxic effects. However, existing self-monitoring methods are limited. In this study, we designed a plasmonic microneedle (MN) sensor for real-time nondestructive monitoring of acetaminophen levels in dermal interstitial fluid (ISF) by employing a handheld Raman spectrometer. The fabricated MN sensor incorporated a high-density plasmonic MOFs known as HDPM, which unique structure of large specific surface area, specific pore structure as well as high density gold nanospheres packing enabled the excellent performance of selective ISF drug enrichment and surface-enhanced Raman scattering (SERS). The maximum electric field enhancement factor of the HDPM nanostructure could be calculated as 5.73 × 107. The developed HDPM@MNs was characterized with a core-shell type “soft on the outside and rigid on the inside” structure, which exhibited sufficient hardness and flexibility to penetrate the dermal tissue with little damage, and robust SERS enhancement effect in APAP detection without any interfering peaks. Through a hydrogel drug simulation experiment, the sensor demonstrated robust capabilities for acetaminophen enrichment and monitoring, exhibiting excellent stability and repeatability. The quantitative detection window spanned from 1 to 100 μM, with a low detection limit reaching 0.45 μM. Furthermore, by monitoring the concentration of acetaminophen in the interstitial fluid of rat skin at different doses and for different administration times, the HDPM@MNs can be used to determine the pharmacokinetics of acetaminophen in rats and the physiological characteristics associated with various dosage regimens. This work not only holds promise for drug monitoring but also provides a novel approach for nondestructive monitoring of other crucial low-abundance physiological markers.

Photonic hook propagation from eccentric microcylinder

The Photonic hook (PH) is an intricately curved photonic nanojet (PNJ) or a highly intense electromagnetic beam featuring a subwavelength waist, whose principal hallmark lies in its capacity to bend light at the nanoscale. According to existing literature, the origin of PH can be attributed to symmetry breaking, whereas symmetrical microstructures predominantly contribute to PNJ formation. This study presents the novel revelation of PH emergence from an isolated eccentric core–shell dielectric microcylinder, achieved through the illumination of a paraxial Gaussian beam (PGB). The eccentrically structured core–shell microscale geometry introduces an additional degree of freedom, influencing PH formation and directly shaping its characteristic parameters. Much like PNJ, the propagation of PH depends on different parameters such as core and shell refractive indices of the micro-structures, microstructure geometry, incident light type, and direction of propagation. A fascinating outcome from our numerical simulations is the switchable occurrence of PNJ and PH from an eccentric core–shell microcylinder by a simple adjustment of eccentricity, either parallel or perpendicular to the PGB’s propagation direction. This computational investigation emphasizes the impact of eccentricity and the incident wave’s beam waist, maintaining a consistent refractive index contrast between the core and shell. The outcomes are interpreted in terms of key parameters governing PH generation characteristics, encompassing FWHM, maximum electric field enhancement, and focal plane. Notably, we have observed the coexistence of whispering gallery modes (WGM) and PH within this system and these modes exhibit high sensitivity to the excitation wavelength. The potential applications of PH are believed to be far-reaching, including areas like optical trapping, sensing, and functioning as a versatile focusing element. This study contributes to the fundamental understanding of PH and illuminates its potential as a robust tool across diverse optical applications.

Enhanced upconversion luminescence using long-range surface plasmon resonance mode with prism coupler

In the study, we present the significant enhancement of the upconversion luminescence by taking advantage of long-range surface plasmon resonance mode. Compared with the reflection enhanced upconversion luminescence of Ag film, the intensity of upconversion luminescence enhanced by long-range surface plasmon resonance mode is improved 100 times. Additionally, the finite-difference time-domain simulation indicates that the incident light energy can concentrate in the upconversion nanoparticles doped dielectric layer at the long-range surface plasmon resonance mode, which the maximum electric field enhancement factor |E|2/|E0|2 can reach to 58. The experimental result shows that the upconversion luminescence enhancement is related to the thickness of doped Polymethylmethacrylate (PMMA) layer. Furthermore, due to the excited upconversion luminescence having high directionality, it is shown that the strong upconversion luminescence could be received at the specific angle.

Materials for Dielectric Nanotweezers in the Near-Visible Region

Nanotweezers made of dielectric nanoantennas have enabled optical trapping and manipulation of nanoscale objects. Commonly used dielectric materials with a high refractive index, notably silicon (Si), suffer from absorption at visible wavelengths and thus Joule heating of the structure and the trapped object. They are thus normally considered unsuitable for trapping applications in this wavelength range. In this work, dielectric materials with low absorption at 785 nm are studied to minimize Joule heating. We consider a bowtie configuration, which is simulated for four different dielectric materials and optimized for maximum electric field enhancement. The results show a significant electric field enhancement in the bowtie gap, which can act as a “hotspot” for optical trapping and enhancement of Raman scattering of a single nanosphere. We numerically investigate optical trapping of nanospheres and compare the results with an analytical expression. Three nanospheres with different refractive indices are used as examples. We further study the temperature increase and the thermally induced flow around the bowtie nanoantenna to understand if and how the trapping is perturbed by Joule heating. We find that nanoantennas made by Si on a Si substrate, although absorbing, can be used for trapping quantum dots (QDs) and polystyrene (PS) beads. The required input intensities for stable trapping are approximately 10 and 50 mW/μm2, for QDs and PS beads, respectively, inducing temperature increases of ∼1.4 K and ∼7 K, respectively. For transparent materials, the input intensity is the limiting factor, and gallium phosphide (GaP) requires the lowest input intensity due to its high refractive index. However, Si is a far more common and standard material and thus gives a significantly easier fabrication process.

Simultaneous, Label-Free and High-throughput SERS Detection of Multiple Pesticides on Ag@Three-Dimensional Silica Photonic Microsphere Array

Rapid identification and quantitative simultaneous analysis for multiple pesticide in real samples based on surface-enhanced Raman spectroscopy (SERS) is still a challenge because of sample complexity, reproducibility, and stability of SERS substrate. With use of colloidal silver nanoparticles loaded three-dimensional (3D) silica photonic microspheres (SPMs) array as the analytical platform, a SERS-based array assay for multiple pesticides was developed in this work. The silver nanoparticles were fixed into the gaps formed by the self-assembled nanospheres of the 3D SPMs to produce "hot spots", on which the Raman enhanced effect was up to 9.86 × 107 and the maximum electric field enhancement effect reached to 9.75 times, ensuring the target pesticides on the surface of the SERS-substrate integrated SPM can be detected sensitively. Using 2,4-dichlorophenoxyacetic acid (2,4-D), glyphosate, and imidacloprid as the testing pesticides, the label-free and high-throughput SERS assay for simultaneous detection of the pesticides was established, giving good linear detection ranges (0.1-204.8 μg/mL for 2,4-D, 0.3-247.9 μg/mL for glyphosate, and 0.2-204.8 μg/mL for imidacloprid) and low detection limits (3.03 ng/mL for 2,4-D, 3.14 ng/mL for glyphosate, and 8.82 ng/mL for imidacloprid). The spiked recovery rates in the real samples were measured in the range of 82-112%, which was consistent with that of the classical standard methods. The label-free 3D SERS array analytical platform provides a powerful tool for high-throughput and low-cost screening of multiple pesticide residues in real samples.

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Bowtie nanoantenna driven by a Yagi-Uda nanoantenna: a device for plasmon-enhanced light matter interactions

We propose a plasmonic device, based on the combination of a Yagi-Uda nanoantenna and a bowtie nanoantenna, that can enable on-chip implementation of plasmon-enhanced light-matter interaction processes such as surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption spectroscopy, and plasmon-enhanced fluorescence. In this device, a localized source is employed to excite the Yagi-Uda nanoantenna, which in turn drives the bowtie nanoantenna. We employ finite difference time domain (FDTD) method to perform numerical simulations to obtain radiation characteristics of the Yagi-Uda nanoantenna as well as the electric field enhancements in the vicinity of the bowtie nanoantenna excited by the Yagi-Uda nanoantenna. We find that for a wavelength of 785 nm, an electric field enhancement of ∼ 196 can be achieved in between the arms of the bow-tie nanoantenna even when the minimum gap between nanostructures is as large as 10 nm. It is found that this electric field enhancement is significantly large when compared with the maximum electric field enhancement (∼ 11) obtained for direct excitation of the bowtie nanoantenna by a point source or with the maximum electric field enhancement (∼ 34) obtained for plane wave excitation of the bowtie nanoantenna. As the electromagnetic enhancement of SERS can be approximated to be the fourth power of the electric field enhancement, SERS electromagnetic enhancement of ∼ 1.5 × 109 is achieved for the bow-tie nanoantennas excited by the Yagi-Uda nanoantennas, even when the minimum gap between the arms of the bow-tie nanoantenna is as large as 10 nm. We also analyze the effect of various geometrical parameters of the nanoantennas and show that the maximum electric field enhancement at a given wavelength can only be obtained when both the Yagi-Uda nanoantenna and the bowtie nanoantenna are resonant at that wavelength. Moreover, we calculate the electric field enhancements at different near-infrared wavelengths. Employing the proposed device, an electric field enhancement of ∼ 945 is obtained at a wavelength of 1500 nm resulting in a SERS electromagnetic enhancement factor as high as ∼ 8 × 1011, even when the minimum gap between nanostructures is as large as 10 nm.

Large improvement in DC electrical properties of EPDM with 2D platelet nanoclay

Space charge, electrical conductivity and breakdown strength of polymeric insulations are considered to be the most important characteristics for designing DC cable insulation. In this paper, we report the DC electrical properties of ethylene propylene diene polymethylene (EPDM) polymer with two different 2D inorganic clays, one of which shows large improvement of space charge distribution, controlled electrical conductivity and breakdown voltage. The results revealed that compared with the pure EPDM, the space charge accumulation for EPDM filled with Talc was suppressed significantly, especially at 50 °C with thermal gradient. The maximum local electric field enhancement in the volume decreased from 60% for the pure EPDM to 4% for the Talc-filled EPDM under 20 kV mm−1 at 25 °C, while the corresponding value dropped dramatically from 83% to 8% at 50 °C with thermal gradient. Compared with the EPDM, the Talc-filled EPDM showed higher conductivity but with less dependence to electric field and temperature. Microstructure studies suggest the platelet Talc particles distribute in the EPDM matrix homogeneously and make a uniform morphology. This condition consequently causes a desired trap distribution with high density of shallow traps due to the presence of high interfacial area between polymer chains and clay particles which prevents the charge accumulation during the charge carrier transport under DC electric field. Results of the thermally stimulated depolarization current spectra of the samples are in good agreement with the predicted trap distribution based on the experimental results and morphological study.

Study on surface enhanced Raman scattering of Au and Au@Al2O3 spherical dimers based on 3D finite element method

In this paper, the surface enhanced Raman scattering (SERS) characteristics of Au and Au@Al2O3 nanoparticle dimers were calculated and analyzed by using finite element method (3D-FEM). Firstly, the electric field enhancement factors of Au nanoparticles at the dimer gap were optimized from three aspects: the incident angle of the incident light, the radius of nanoparticle and the distance of the dimer. Then, aluminum oxide is wrapped on the Au dimer. What is different from the previous simulation is that Al2O3 shell and Au core are regarded as a whole and the total radius of Au@Al2O3 dimer is controlled to remain unchanged. By comparing the distance of Au nucleus between Au and Au@Al2O3 dimer, it is found that the electric field enhancement factor of Au@Al2O3 dimer is much greater than that of Au dimer with the increase of Al2O3 thickness. The peak of electric field of Au@Al2O3 dimer moves towards the middle of the resonance peak of the two materials, and it is more concentrated than that of the Au dimer. The maximum electric field enhancement factor 583 is reached at the shell thickness of 1 nm. Our results provide a theoretical reference for the design of SERS substrate and the extension of the research scope.

Bacterial inactivation via microfluidic electroporation device with insulating micropillars.

Electroporation is a promising method to inactivate cells and it has wide applications in medical science, biology and environmental health. Here, we investigate the bacteria inactivation performance of two different microfluidic electroporation devices with rhombus and circular micropillars used for generating locally enhanced electric field strength. Experiments are carried out to characterize the inactivation performance (i.e., the log removal efficiency) of two types of bacteria: Escherichia coli (E. coli, gram-negative) and Enterococcus faecalis (E. faecalis, gram-positive) in these two microfluidic devices. We find that under the same applied electric field, the device with rhombus micropillars performs better than the device with circular micropillars for both E. coli and E. faecalis. Numerical simulations show that due to the corner-induced singularity effect, the maximum electric field enhancement is higher in the device with rhombus micropillars than that in the device with circular micropillars. We also study the effects of DC and AC electric fields and flowrate. Our experiments demonstrate that the use of the DC field achieves higher log removal efficiencies than the use of AC field.

Short and elongated photonic nanojets emerged from single solid/hollow core-shell microparticles illuminated by focused Gaussian beams and plane wave

For the first time, herein, we report the theoretical investigation on short and elongated photonic nanojets (PNJs) generated by single solid/hollow core-shell microparticles illuminated with focused Gaussian beams (FGBs). This investigation is carried out using developed analytical equations based on Bromwich formalism. The effect of the beam waist, thickness of shell, size and refractive index of the microparticle on the characteristic parameters of the PNJs such as maximum electric field enhancement, spatial separation between the PNJs, and length of the PNJs is studied in detail. The electric field enhancement values of the PNJs upon FGB illumination are compared to those obtained from the plane wave illumination.

Photonic nanojets generated by single microspheres of various sizes illuminated by resonant and non-resonant focused Gaussian beams of different waists

We report theoretical simulations on the photonic nanojet (PNJ) of single dielectric microspheres of various sizes illuminated by focused Gaussian beams (FGBs), using analytical theory developed by Gouesbet et al. based on the Bromwich formalism. The radius of the microsphere ( R ) is varied from 1 to 45 µm. The effect of the beam waist ( ω 0 ) on characteristic parameters such as maximum electric field enhancement ( η m a x ) of the PNJ, the focal length of the microsphere ( f ), effective length, and width of the PNJ is studied in detail. The effect of the refractive index of the microsphere ( n p ) and R on the η m a x for different values of ω 0 is studied. The dependence of (i) the f value on R and (ii) the η m a x value on λ is investigated systematically.

Dual-polarization star-gap nano-antenna

Nano-antennas are used in optical data storage, near-field imaging, photovoltaics, sensing, and spectroscopy applications. In many cases, the antennas work with linear polarization, but some applications require circularly polarized (CP) light such as field-enhanced circular dichroism spectroscopy. Circular dichroism spectroscopy is a technique used to study organic molecules and is widely used in biology and medicine. In this article, we analyze a star-gap nano-antenna array. The antenna combines concentric circular gratings and a symmetric gap to provide a maximum electric field enhancement factor of around 48 at 780 nm with the average electric field enhancement factor of about 34.75. Due to its geometry, the antenna can work for both linearly polarized and CP light, having similar electric field enhancement factors. Due to its symmetry and capacity to work with different polarizations, the antenna can find applications in sensing and spectroscopy. As an example of application, we have characterized the antenna for surface enhanced Raman scattering (SERS) and obtained a SERS factor of 4.18×106 for the 880cm−1 Raman line of ethanol.

Combining Experiments and Theoretical Modeling To Interrogate the Anisotropic Growth and Structure-Plasmonic Property Relationships of Gold Nanostars.

We present a combined strategy of experiments and theoretical modeling for understanding the evolution of the morphology and plasmonic properties of gold nanostars (GNSs) in the seed-mediated synthesis by changing the poly(vinylpyrrolidone) (PVP) molecular weight, PVP concentration, and synthesis temperature. A dramatic change of the morphology of GNSs as a function of these synthesis parameters is observed that is related to variations of the plasmonic properties and thus surface-enhanced Raman spectroscopy (SERS) enhancement. We observe the favorable growth of anisotropic GNS structures with sharp protruding tips using PVP of low molecular weight and of rounded GNSs with short protruding tips using PVP of high molecular weight. The PVP concentration has less influence on the core size than on the tip length of GNSs. The high synthesis temperature causes the rounding of the GNS structure. Finite-difference time-domain (FDTD) simulations reveal a remarkable correlation of the GNS morphology with the plasmonic properties as well as the SERS enhancement. The maximum local electric field enhancement occurs at the apex of the sharp protruding tips of the GNSs. The weak plasmonic coupling is observed between the protruding tips of GNSs because of their large separation distance, and increasing the number of protruding tips beyond two only increases the extinction cross section without further red-shifting the plasmon peak. A resonance overlap of the plasmon band with the incident laser wavelength is responsible for the morphology-dependent plasmonic properties and SERS enhancement. The present work demonstrates that a mechanistic understanding of the structural evolution of GNSs along with their morphology-plasmonic property correlation can be achieved through the combination of experimental investigations and FDTD-based theoretical modeling.

Design of a Novel Compact Slotted Cavity With Shaped Beam for High Power Microwave Feed Antenna Using Asymmetric High Order Mode

In order to achieve a shaped beam on a compact aperture and reduce the cost and the volume, a conformal slotted cavity as a feed antenna for high-power microwave using the asymmetric high-order mode is investigated. The proposed antenna working in a high-order mode has merits on reducing the complexity of a power network and the volume of a mode converter. The alternate arrangement of the slot array is taken into account, and the asymmetric high-order mode TE 430 is selected to improve the symmetry of far-field pattern at the E-plane. A novel design of wide slot with the horn-cavity etched on the top surface of the upper cavity achieves the uniform aperture field and mitigates the maximum electric field enhancement, and calculated results verify that the proposed antenna is capable of high-power handling capability (above 1.29 GW in vacuum). The conformal slot array is introduced to obtain a shaped beam, which helps to illuminate reflect array with shaped aperture efficiently. The simulation and measured results show that the feed antenna achieves a low-profile structure of 2.4λ 0 × 2.8λ 0 × 0.8λ 0 , the 10-dB gain beamwidth of E-plane is 56.6° at the operating frequency of 9.1 GHz, the maximum difference of the gain, and the 10-dB beamwidth angle between the two sides of main beam is no more than 1 dB and 1°, respectively.

Quantifying and Comparing the Near-Field Enhancement, Photothermal Conversion, and Local Heating Performance of Plasmonic SiO2@Au Core-Shell Nanoparticles

In this work, the finite element method and two-temperature model were used to optimize the near electric field, photothermal conversion efficiency, and local heating of SiO2@Au core-shell nanoparticles (NPs) using water as the dispersion medium. The optimal core-shell sizes to achieve maximum near electric field enhancement, photothermal conversion efficiency, and local heating were 14 nm/6 nm, 8 nm/3 nm, and 24 nm/12 nm. The optimized core-shell ratio decreased with increasing the optimized wavelength for both near electric field and photothermal conversion optimizations. In the local heating or temperature increase, due to the existence of thermal boundary conductance, a temperature drop was observed between the electrons and the lattice, the electrons and the dielectrics, and the NP and the surrounding water. All these barriers and high absorption ability led to a temperature increase in the NP region. All optimized core-shell NPs achieved surface plasmon resonance for different targets. Comparison between different optimized targets with a normalized processing function indicated a smaller optimized core-shell size and core-shell ratio were obtained with a consideration of greater effect of the NP radius.

Engineering Plasmonic Lattice Structures for Lab-on-Chip Biosensing Platforms

The fabrication of plasmonic devices has often relied on low-throughput sub-micron to nanoscale fabrication techniques, making the production of transducers for experimentation and implementation as lab-on-chip components challenging. This paper aims to improve on those challenges, and discusses the design and fabrication of nanosphere-based plasmonic lattices that directly utilize nanosphere deposition to create resonant plasmonic transducers that operate in the visible spectrum. Parametric simulation of critical plasmonic lattice dimensions using finite-difference time-domain (FDTD) software to determine maximum electric field (E-field) enhancement for a range of commercially-available silica nanobeads are presented. An evaporative process that achieves fabrication of a sub-micron lattice consisting of a self-assembled monolayer of silica nanobeads sized in the 100 nm range is described, along with an evaluation of fabrication tolerance compared to as-drawn geometries used in simulations. Characterization results indicating greater than 300-fold enhancement of photonic emission from suspended labeled analytes is also presented.

Graphene Tunable Plasmon–Phonon Coupling in Mid‐IR Complementary Metamaterial

AbstractMetamaterial‐based plasmonics has become an overwhelming research field due to its enormous potential and versatility in molecular sensing, imaging, and nonlinear optics. This work presents a new tunable plasmonic platform on which the metamaterial resonance is coupled with infrared vibrational bond in the presence of graphene electrostatic modulation. The maximum electric field enhancement factor induced by mode coupling is 14 and the quality factor (Q‐factor) of phonon mode is increased approximately by fourfold. The graphene electrostatic modulation based on the parallel‐plate capacitor configuration enables a wavelength shift of 1.57 nm V−1, resonance intensity and Q‐factor modulation depth of 103.34% and 70%, respectively. Metamaterial based plasmon polariton perfectly matched with phonon mode yields the highest Q‐factor of 40. However, this perfectly matched resonance appears to be prohibitively “switched off” in the electrostatic tuning, which is reported for the first time. Mode splitting investigation reveals the largest coupling strength of 8.1 meV (1.96 THz) that results in the insensitivity to the perturbation caused by graphene modulation. Finally, an averaged sensitivity of 1.677 µm RIU−1 and a tunable figure of merit are reported, depicting the versatility of this platform for multiplexed sensing applications in various conditions.

High-efficiency polarization-independent wideband multilayer dielectric reflective bullet-alike cross-section fused-silica beam combining grating.

A high-efficiency polarization-independent wideband multilayer dielectric reflective bullet-alike cross-section (combination of trapezoidal-rectangular grating profile) fused-silica beam combining grating (BCG) used in the -1st order for spectral beam combining (SBC) is designed and fabricated. Exact grating profile parameters are optimized by using the rigorous coupled-wave analysis and simulated annealing algorithm. As a comparison, traditional pure trapezoidal and pure rectangular gratings are also designed. Numerical results show that such a bullet-alike cross-section BCG exhibits wide bandwidth with the lowest maximum electric field enhancement in the grating material, which is greatly beneficial for the promotion of the power scaling level of the grating-based SBC system. A two-step dry-etching procedure is developed to fabricate such a grating. The averaged diffraction efficiency of greater than 91% was experimentally demonstrated.