E-field Intensity Research Articles

Investigation of the 3D Printing Roughness Effect on the Performance of a Dielectric Rod Antenna

In this letter, the roughness effect of a three-dimensional (3-D) printed dielectric rod antenna is investigated. One of the issues of 3-D printing in microwave components and antennas is the limited printing resolution, which creates a surface roughness on printed devices and deteriorates the performance. The effect of surface roughness on the radiation characteristics and impedance matching of a dielectric rod antenna is studied. The roughness as a perturbation brings the antenna out of its optimum design by changing the E-field intensity alongside the rod. The whole antenna consisted of a coaxial to waveguide adapter and the dielectric rod is 3-D printed and the adaptor is dipped into a low viscosity solution of silver epoxy and isopropanol to coat a conductor layer on the inner surfaces of the waveguides. The fabrication method is cost effective and much easier than conventional methods.

Measurement of Low-level radiofrequency electromagnetic fields in the human environment

Abstract In recent years there has been an increase in development of electromagnetic (EM) technology in the telecommunication industry, resulting in an increase in human non-ionizing exposure. This fact has initiated a number of scientific studies on possible health effects of EM fields on human organism. Totally four representative microenvironments were investigated for RF EM fields distribution, namely: city center, residential area, rural area, and extra-village area. Each microenvironment was measured 20 times in accordance with the International Commission for Non-Ionizing Radiation Protection (ICNIRP) guidelines. The extra-village measurements were taken as the base values that reflect the E-field intensities with the lowest amplitudes. The statistical analysis revealed notable statistical significance (p < 0.001) in almost all measured frequency bands except the Wi-Fi where the p-values were less than 0.05 for the city center and residential area but not significant for rural area. The highest total E-field intensity was measured in the residential area (approximately 1.85 V/m). All measured values were below the legal limits of the Slovak Republic and ICNIRP safety guidelines. However, the ICNIRP safety limits were written in 1998 considering only the thermal effects of RF radiation. They were updated in 2009 without any changes in the limits and still recommend 27.5 – 61 V/m (2 – 10 W/m2) for the RF frequency band of 400–2,000 MHz. The BioInitiative Report of 2012 established the scientific benchmark for possible health risks as 30–60 μW/m2 (approximately 0.1 – 0.15 V/m). Thus, all measured values were above the scientifically derived limits.

Impact of non-brain anatomy and coil orientation on inter- and intra-subject variability in TMS at midline

ObjectiveTo investigate inter-subject variability with respect to cerebrospinal fluid thickness and brain-scalp distance, and to investigate intra-subject variability with different coil orientations. MethodsSimulations of the induced electric field (E-Field) using a figure-8 coil over the vertex were conducted on 50 unique head models and varying orientations on 25 models. Metrics exploring stimulation intensity, spread, and localization were used to describe inter-subject variability and effects of non-brain anatomy. ResultsBoth brain-scalp distance and CSF thickness were correlated with weaker stimulation intensity and greater spread. Coil rotations show that for the dorsal portion of the stimulated brain, E-Field intensities are highest when the anterior-posterior axis of the coil is perpendicular to the longitudinal fissure, but highest for the medial portion of the stimulated brain when the coil is oriented parallel to the longitudinal fissure. ConclusionsNormal anatomical variation in healthy individuals leads to significant differences in the site of TMS, the intensity, and the spread. These variables are generally neglected but could explain significant variability in basic and clinical studies. SignificanceThis is the first work to show how brain-scalp distance and cerebrospinal fluid thickness influence focality, and to show the disassociation between dorsal and medial TMS.

Investigation on picosecond laser-induced damage in HfO2/SiO2 high-reflective coatings

We investigate the influence of a protective layer on the picosecond laser-induced damage in HfO2/SiO2 high-reflective (HR) coatings. Two types of 1064-nm HR coatings, with and without a SiO2 protective layer, are deposited using electron beam evaporation. Laser-induced damage tests are operated with 1064-nm, 30-ps laser pulses in one-on-one mode. Five different angles of incidence (AOIs) are considered in the tests for both coatings, in order to modulate the electric field (E-field) intensity in the HR coatings. The damage morphologies and cross-sections of the damage sites are characterized using scanning electron microscope (SEM) and focused ion beam (FIB), respectively. Experimental results show that the SiO2 protective layer slightly improves the laser-induced damage threshold (LIDT) of the HR coatings for all AOI values. The damage morphology contains high-density micrometer-scale pits and layer delaminations, at low and high fluences, respectively. The cross-sections of the damage sites and LIDTs for different AOIs are compared with corresponding E-field distributions. For both coatings, the damage is initiated near the peak of the E-field. A negative linear relationship is observed between the LIDT and corresponding maximum E-field intensity for both coatings.

Performance Enhancement of Nanoscale VO2 Modulators Using Hybrid Plasmonics

Vanadium dioxide (${\rm VO}_2$) is a phase change material (PCM) that exhibits a large change in complex refractive index on the order of unity upon switching from its dielectric to its metallic phase. Although this property is key for the design of ultracompact optical modulators of only a few microns in footprint, the high absorption of ${\rm VO}_2$ leads to appreciable insertion loss (IL) that limits the modulator performance. In this paper, through theory and numerical modeling, we report on a new paradigm, which demonstrates how the use of a hybrid plasmonic waveguide to construct a ${\rm VO}_2$ based modulator can improve the performance by minimizing its IL while achieving high extinction ratio (ER) in comparison to a purely dielectric waveguide. The hybrid plasmonic waveguide that contains an additional metal layer with even higher loss than ${\rm VO}_2$ enables unique approaches to engineer the electric field (E-field) intensity distribution within the cross section of the modulator. The resulting figure-of-merit, FoM = ER/IL, is much higher than what is possible by simply incorporating ${\rm VO}_2$ into a silicon wire waveguide. A practical modulator design using this new approach, which also includes input and output couplers yields ER = 3.8 dB/{\mu}m and IL = 1.4 dB/{\mu}m (FoM = 2.7), with a 3-dB optical bandwidth >500 nm, in a device length = 2 {\mu}m, and cross-sectional dimensions = 200 nm $\times$ 450 nm. To the best of our knowledge, this is one of the smallest modulator designs proposed to date that also exhibits amongst the highest ER, FoM, and optical bandwidth, in comparison to existing designs. In addition to ${\rm VO}_2$, we investigate two other PCMs incorporated within the waveguide structure. The improvements obtained for ${\rm VO}_2$ modulators do not extend to other PCMs.

Novel GaN-based nanocomposites: Effective band structure and optical property tuning by tensile strain or external field

The electronic and optical properties of the GaN monolayer hybrid with ZnO, MoS2 and MoSe2 are studied by means of density functional theory. The lattice mismatches of these three heterobilayers are less than 2.5% and they are stable with the binding energies larger than the ones of bilayer graphene. Despite of the indirect band gap of GaN monolayer, the three heterostructures exhibit direct band gap characteristics and their optical spectrums are broader than those of the isolated constituent layers because of interlayer coupling. Besides, both of biaxial tensile strain and external electric field (E-field) can effectively tune the band gaps of these heterostructures. The band gaps will decrease with the increase of tensile strain or the positive E-field strength while the ones will increase with the increase of the negative E-field intensity. Meanwhile, the adsorption edges of their imaginary parts of the dielectric function display corresponding redshift or blueshift characteristics and the band gaps may undergo direct to indirect transition beyond a critical strain or E-field intensity. The results shows that these GaN heterostructures can be good candidate materials for further investigations on the photo-related applications such as light-emitting diodes (LEDs), photodetection or solar energy conversion.

Evaluation of the electrical properties of dust storms by multi-parameter observations and theoretical calculations

Dusty phenomena, such as wind-blown sand, dust devils, and dust storms, play key roles in Earth's climate and geological processes. Dust electrification considerably affects the lifting and transport of dust particles. However, the electrical properties of dust storms remain poorly understood. Here, we conducted multi-parameter measurements and theoretical calculations to investigate the electrical properties of dust storms and their application to dust storm prediction. The results show that the vertical electric field (E-field) decreases first, then increases, and finally decreases with the height above the ground, reversing its direction at two heights, ∼8–12 and ∼24 m. This suggests that the charge polarity of dust particles changes from negative to positive and back to negative again as the height increases. By carefully analyzing the E-field and dust concentration data, we further found that there is a significant positive linear relationship between the measured E-field intensity and dust concentration at the given ambient conditions. In addition, measurements and calculations demonstrate that a substantial enhancement in the vertical E-field can be observed several hours before the arrival of the external-source dust storms, indicating that the E-field can be used to provide an early warning of external-source dust storms.

Laser-resistance sensitivity to substrate pit size of multilayer coatings.

Nanosecond laser-resistance to dielectric multilayer coatings on substrate pits was examined with respect to the electric-field (E-field) enhancement and mechanical properties. The laser-induced damage sensitivity to the shape of the substrate pits has not been directly investigated through experiments, thus preventing clear understanding of the damage mechanism of substrate pits. We performed a systematic and comparative study to reveal the effects of the E-field distributions and localized stress concentration on the damage behaviour of coatings on substrates with pits. To obtain reliable results, substrate pits with different geometries were fabricated using a 520-nm femtosecond laser-processing platform. By using the finite element method, the E-field distribution and localized stress of the pitted region were well simulated. The 1064-nm damage morphologies of the coated pit were directly compared with simulated E-field intensity profiles and stress distributions. To enable further understanding, a simplified geometrical model was established, and the damage mechanism was introduced.

Nanopatterned bulk-heterojunction photovoltaic cells using polyurethane acrylate (PUA) film replica of colloidal crystal arrays via stamping transfer process

Bulk heterojunction (BHJ) films with nanopattern structure have been fabricated using a stamping transfer process with polyurethane acrylate (PUA) replica film based on 3-dimensional colloidal crystals. The uniformly arrayed nanostructure of the PUA film was investigated by SEM analysis. Compared to the organic photovoltaic cells wtih plain BHJ of poly(3-hexylthiophene-2,5-diyl) (P3HT) and indene-C60 bisadduct (ICBA), the device from the nanopatterned BHJ exhibit improved E-field distribution intensity, as assessed using a finite-difference time-domain (FDTD) method. The current density (J SC ) and power conversion efficiency (PCE) of the device were respectively increased by up to 27% and 20% due to the enhanced interfacial contact area and plasmonic effect, which can brought about effective light harvesting between the BHJ active layer and cathode. Through a simple low-temperature transfer process using a PUA mold, we can fabricate reproducible nanopatterned BHJ devices that have improved light absorption within the active layer of the devices.

Metal nano-particles sizing by thermal annealing for the enhancement of surface plasmon effects in thin-film solar cells application

The optical properties and surface plasmon effects of different thickness of silver film grown on a silicon substrate were experimentally and numerically investigated. By modifying the film thickness and the annealing temperature, the size of metal nanoparticles (MNPs) can be manipulated. The reflectance of silver film deposited onto silicon wafers were adjusted by controlling the film thickness and annealing temperature. The experimental results show the MNPs grew larger with thinner deposition of silver film, and this has a correlation to the reflectance. We found that the thermal annealing temperature affects the optical properties of the MNPs, and this enhances the E-field intensity on the MNPs, and contributes to the improvement in the conversion efficiency of solar cells. This paper describes our attempts to develop a simple method that can replace the previously reported processes of lithography and thermal vacuum evaporation of the silver film and MNPs preparation.

Tunable and amplified Raman gold nanoprobes for effective tracking (TARGET): in vivo sensing and imaging.

We describe the development of a highly tunable, physiologically stable, and ultra-bright Raman probe, named as TARGET (Tunable and Amplified Raman Gold Nanoprobes for Effective Tracking), for in vitro and in vivo surface-enhanced Raman scattering (SERS) applications. The TARGET structure consists of a gold core inside a larger gold shell with a tunable interstitial gap similar to a "nanorattle" structure. The combination of galvanic replacement and the seed mediated growth method was employed to load Raman reporter molecules and subsequently close the pores to prevent leaking and degradation of reporters under physiologically extreme conditions. Precise tuning of the core-shell gap width, core size, and shell thickness allows us to modulate the plasmonic effect and achieve a maximum electric-field (E-field) intensity. The interstitial gap of TARGET nanoprobes can be designed to exhibit a plasmon absorption band at 785 nm, which is in resonance with the dye absorption maximum and lies in the "tissue optical window", resulting in ultra-bright SERS signals for in vivo studies. The results of in vivo measurements of TARGETs in laboratory mice illustrated the usefulness of these nanoprobes for medical sensing and imaging.

Interfacial structure and wetting properties of water droplets on graphene under a static electric field.

The behavior of water droplets located on graphene in the presence of various external electric fields (E-fields) is investigated using classical molecular dynamics (MD) simulations. We explore the effect of E-field on mass density distribution, water polarization as well as hydrogen bonds (H-bonds) to gain insight into the wetting properties of water droplets on graphene and their interfacial structure under uniform E-fields. The MD simulation results reveal that the equilibrium water droplets present a hemispherical, a conical and an ordered cylindrical shape with the increase of external E-field intensity. Accompanied by the shape variation of water droplets, the dipole orientation of water molecules experiences a remarkable change from a disordered state to an ordered state because of the polarization of water molecules induced by static E-field. The distinct two peaks in mass density and H-bond distribution profiles demonstrate that water has a layering structure in the interfacial region, which sensitively depends on the strong E-field (>0.8 V nm(-1)). In addition, when the external E-field is parallel to the substrate, the E-field would make the contact angle of the water droplets become small and increase its wettability. Our findings provide the possibility to control the structure and wetting properties of water on graphene by tuning the direction and intensity of external E-field which is of importance for relevant industrial processes on the solid surface.

Collective photonic-plasmonic resonances in noble metal - dielectric nanoparticle hybrid arrays

Coherent scattering of gold and silver nanoparticles (NPs) in regular arrays can generate Surface Lattice Resonances (SLRs) with characteristically sharp spectral features. Herein, we investigate collective resonances in compositionally more complex arrays comprising NP clusters and NPs with different chemical compositions at pre-defined lattice sites. We first characterize the impact of NP clustering by exchanging individual gold NPs in the array through dimers of electromagnetically strongly coupled gold NPs. Then, we analyze hybrid arrays that contain both gold metal NP dimers and high refractive index dielectric NPs as building blocks. We demonstrate that the integration of gold NP clusters and dielectric NPs into one array enhances E-field intensities not only in the vicinity of the NPs but also in the ambient medium of the entire array. In addition, this work shows vividly that the ability to integrate multiple building blocks with different resonance conditions in one array provides new degrees of freedom for engineering optical fields in the array plane with variable amplitude and phase.

Tailoring of the plasmonic and waveguide effect in bulk-heterojunction photovoltaic devices with ordered, nanopatterned structures

Various nano-patterned bulk-heterojunction (BHJ) films with different diameters and pitches were fabricated by a stamping method to tailor the plasmonic effect. The nanopatterned BHJ active layers exhibit regular-ordered embossing structures, which were confirmed by the surface morphological analysis with SEM and AFM. The simulation results confirm that devices with nanopatterned BHJ film with a diameter/pitch of 265/400nm exhibit a strong improvement in E-field distribution intensity due to the combination of the plasmonic and waveguide modes compared to devices without a nanopattern, with 150/400nm, or with 265/800nm, which led to increased JSC and cell efficiency in J–V curves under solar light illumination. The optimized plasmonic effect plays an important role in the light harvesting of BHJ devices.

Deposition of Ta2O5 upon silver nanorods as an ultra-thin light absorber

We report the plasmon-assisted absorption enhancement of an ultra-thin light absorber composed of an Ag nanorod array with Ta2O5 deposited upon each rod. A significant increase in the absorptance efficiency of the plasmonic Ta2O5-capped Ag nanorod array was experimentally observed both in s- and p-polarization states. The causes of the enhanced local electric field intensity were systematically investigated using three-dimensional finite difference time domain. The simulations show that the maximum E-field intensities measured at the end surface of the Ta2O5-capped Ag rod are higher than those obtained from the bare Ag nanorod.

Microwave assisted in situ synthesis of Ag–NaCMC films and their reproducible surface-enhanced Raman scattering signals

Two kinds of Ag–NaCMC films for surface-enhanced Raman scattering (SERS) were prepared by conventional heating and microwave assisted in situ reduction methods without any additional capping or reducing agents. A relatively narrow and symmetric surface plasmon resonance band was observed in the absorption spectra of the films fabricated by the microwave assisted in situ reduction method. More uniform silver nanoparticles (NPs) implied by the symmetric absorption spectrum were further confirmed by the scanning electron microscopy images. After the simulation of the E-field intensity distribution around the silver NPs in NaCMC film, the Raman scattering enhancement factors (EFs) of these films were then investigated with 4-mercaptobenzoic acid molecule as a SERS reporter. Improved reproducibility of SERS signal was obtained in the microwave assisted synthesized Ag–NaCMC film, although it maintained an EF as only 1.11×108. The reproducible SERS signal of the Ag–NaCMC film is particularly attractive and this microwave assisted in situ reduction method is suitable for the production of excellent substrate for biosensor application.

Rational Assembly of Optoplasmonic Hetero-nanoparticle Arrays with Tunable Photonic-Plasmonic Resonances.

Metallic and dielectric nanoparticles (NPs) have synergistic electromagnetic properties but their positioning into morphologically defined hybrid arrays with novel optical properties still poses significant challenges. A template-guided self-assembly strategy is introduced for the positioning of metallic and dielectric NPs at pre-defined lattice sites. The chemical assembly approach facilitates the fabrication of clusters of metallic NPs with interparticle separations of only a few nanometers in a landscape of dielectric NPs positioned hundreds of nanometers apart. This approach is used to generate two-dimensional interdigitated arrays of 250 nm diameter TiO2 NPs and clusters of electromagnetically strongly coupled 60 nm Au NPs. The morphologydependent near- and far-field responses of the resulting multiscale optoplasmonic arrays are analyzed in detail. Elastic and inelastic scattering spectroscopy in combination with electromagnetic simulations reveal that optoplasmonic arrays sustain delocalized photonic-plasmonic modes that achieve a cascaded E-field enhancement in the gap junctions of the Au NP clusters and simultaneously increase the E-field intensity throughout the entire array.

Effects of nonlocal plasmons in gapped graphene micro-ribbon array and two-dimensional electron gas on near-field electromagnetic response in the deep subwavelength regime

A self-consistent theory involving Maxwell's equations and a density-matrix linear-response theory is solved for an electromagnetically coupled doped graphene micro-ribbon array (GMRA) and a quantum well (QW) electron gas sitting at an interface between a half-space of air and another half-space of a doped semiconductor substrate, which supports a surface-plasmon mode in our system. The coupling between a spatially modulated total electromagnetic (EM) field and the electron dynamics in a Dirac-cone of a graphene ribbon, as well as the coupling of the far-field specular and near-field higher-order diffraction modes, are included in the derived electron optical-response function. Full analytical expressions are obtained with nonlocality for the optical-response functions of a two-dimensional electron gas and a graphene layer with an induced bandgap, and are employed in our numerical calculations beyond the long-wavelength limit (Drude model). Both the near-field transmissivity and reflectivity spectra, as well as their dependence on different configurations of our system and on the array period, ribbon width, graphene chemical potential of QW electron gas and bandgap in graphene, are studied. Moreover, the transmitted E-field intensity distribution is calculated to demonstrate its connection to the mixing of specular and diffraction modes of the total EM field. An externally tunable EM coupling among the surface, conventional electron-gas and massless graphene intraband plasmon excitations is discovered and explained. Furthermore, a comparison is made between the dependence of the graphene-plasmon energy on the ribbon's width and chemical potential in this paper and the recent experimental observation given by [Nat. Nanotechnol.6, 630-634 (2011)] for a GMRA in the terahertz-frequency range.

E-field tuning microwave frequency performance of Co2FeSi/lead zinc niobate–lead titanate magnetoelectric coupling composites

Nanocrystalline Co2FeSi Heusler alloy films were deposited on single crystal lead zinc niobate–lead titanate substrates. It is revealed that this multiferroic composite exhibits very strong continuously electric field (E-field) tunable microwave frequency characteristics. With an increase of the E-field intensity from 0 to 6 kV/cm, the ferromagnetic resonance field Hr shifts by 348 Oe along the easy axis direction, being equivalent to 58 Oe cm/kV, and the ferromagnetic resonance frequency fFMR dramatically increases from 2.2 to 6.1 GHz with an increment of 3.9 GHz or an increment ratio of 177%; moreover, the damping constant α decreases from 0.0150 to 0.0085. These features demonstrate that this multiferroic structure is promising in the fabrication of E-field tunable microwave components.

Photonic–Plasmonic Mode Coupling in On-Chip Integrated Optoplasmonic Molecules

We investigate photonic-plasmonic mode coupling in a new class of optoplasmonic materials that comprise dielectric microspheres and noble metal nanostructures in a morphologically well-defined on-chip platform. Discrete networks of optoplasmonic elements, referred to as optoplasmonic molecules, were generated through a combination of top-down fabrication and template-guided self-assembly. This approach facilitated a precise and controllable vertical and horizontal positioning of the plasmonic elements relative to the whispering gallery mode (WGM) microspheres. The plasmonic nanostructures were positioned in or close to the equatorial plane of the dielectric microspheres where the fields associated with the plasmonic modes can synergistically interact with the evanescent fields of the WGMs. We characterized the far-field scattering spectra of discrete optoplasmonic molecules that comprised two coupled 2.048 μm diameter polystyrene microspheres each encircled by four 148 nm diameter Au nanoparticles (NPs), through far-field scattering spectroscopy. We observed a broadening of the TE and TM modes in the scattering spectra of the optoplasmonic dimers indicative of an efficient photonic-plasmonic mode coupling between the coupled photonic modes of the WGM resonators and the localized surface plasmon modes of the NPs. Our experimental findings are supported by generalized multiple particle Mie theory simulations, which provide additional information about the spatial distributions of the near fields associated with the photonic-plasmonic hybrid modes in the investigated optoplasmonic molecules. The simulations reveal partial localization of the spectrally sharp hybrid modes outside of the WGM microspheres on the Au NPs where the local E-field intensity is enhanced by approximately 2 orders of magnitude over that of an individual Au NP.