Strong Local Enhancement Research Articles

The Ionospheric Leg of the Substorm Current Wedge: Combining Iridium and Ground Magnetometers

AbstractUtilizing magnetic field measurements made by the Iridium satellites and by ground magnetometers in North America we calculate the full ionospheric current system and investigate the substorm current wedge. The current estimates are independent of ionospheric conductance, and are based on estimates of the divergence‐free (DF) ionospheric current from ground magnetometers and curl‐free (CF) ionospheric currents from Iridium. The DF and CF currents are represented using spherical elementary current systems (SECS), derived using a new inversion scheme that ensures the current systems' spatial scales are consistent. We present 18 substorm events and find a typical substorm current wedge (SCW) in 12 events. Our investigation of these substorms shows that during substorm expansion, equivalent field‐aligned currents (EFACs) derived with ground magnetometers are a poor proxy of the actual FAC. We also find that the intensification of the westward electrojet can occur without an intensification of the FACs. We present theoretical investigations that show that the observed deviation between FACs estimated with satellite measurements and ground‐based EFACs are consistent with the presence of a strong local enhancement of the ionospheric conductance, similar to the substorm bulge. Such enhancements of the auroral conductance can also change the ionospheric closure of pre‐existing FACs such that the ground magnetic field, and in particular the westward electrojet, changes significantly. These results demonstrate that attributing intensification of the westward electrojet to SCW current closure can yield false understanding of the ionospheric and magnetospheric state.

Phenylboronic acid modification-based novel dumbbell-shaped Au-Ag nanorod SERS substrates for ultrasensitive detection of SO42.

Based on the coordination principle of Lewis acids, a 4-mercaptophenylboronic acid (4-MPBA)-modified novel dumbbell-shaped Au-Ag nanorod (4-MPBA@DS Au-AgNR) substrate was developed, which could be combined with the surface-enhanced Raman scattering (SERS) technique to detect SO42- with high sensitivity and specificity. DS Au-AgNRs synthesized in this study with a dumbbell-shaped structure were verified by finite-difference time domain (FDTD) simulation to be capable of stimulating strong localized electromagnetic enhancement (EM) at nano-edge and gap, generating a large number of "hot spots" exhibiting excellent SERS performance. The 4-MPBA modified on its surface could specifically recognize SO42-, producing a change in the spectral peak at 1382 cm-1, thus realizing highly sensitive and specific sensing of SO42-. Under optimized conditions, this SERS sensor responded rapidly to SO42- within 2 minutes and demonstrated outstanding specificity. Calculation of the ratio of the characteristic peaks at 1382 and 1070 cm-1 (I1382/I1070) enabled the quantitative detection of SO42- in the range of 1 × 10-8-1 × 10-3 M, and the detection threshold was as low as 1 nM, which was superior to those of similar detection methods. Importantly, the utility and reliability of this SERS substrate for the determination of SO42- in actual samples were evaluated using ion chromatography as the gold standard, and there was no significant difference between the two protocols (P > 0.05), and the RSD was less than 6% with a satisfactory recovery rate (97.6-102.3%). Therefore, the present protocol has the advantages of simplicity and rapidity, high sensitivity, specificity, stability, and practicability in the determination of SO42- in aqueous solution, providing a reliable solution for tracing SO42- in the fields of food safety and environmental testing.

Platform for surface-enhanced Raman scattering in layered quantum materials

Raman spectroscopy has been used to study transition metal dichalcogenides (TMDs), quantum materials with promising properties. However, monolayer (1L) TMDs have limited observable Raman modes due to low light absorption. In this study, we propose a potential platform that can effectively enhance Raman scattering and increase the number of observable Raman modes in 1L TMDs. To enhance Raman scattering in 1L TMDs, a platform was fabricated by forming large-scale periodic arrays of gold micropillars (MPs) using conventional photolithography followed by gold film deposition. 1L WSe2 was transferred onto the Au MPs and Raman scattering was observed. The Raman intensity of 1L WSe2 on Au MPs was 19 times higher than that of 1L WSe2 on SiO2 plates, and Raman modes difficult to detect in typical 1L WSe2 are observed. Confocal Raman spectroscopic mapping revealed that the strong local Raman enhancement at the edge of the Au MP resulted in the amplification of Raman scattering in 1L WSe2 on Au MP. This local field enhancement was theoretically verified using finite difference time domain (FDTD). The platform has industrial advantages and wide applicability due to its low cost, simple process, large controllable area, and short process time.

FEM investigation of Cu@SiO2 core-shell nanoparticle for a sensitive and low-cost SERS and SEF substrate

We theoretically investigate the plasmon coupling properties of Cu@SiO2 core-shell system that supports the measurement of surface enhanced Raman spectroscopy (SERS) and surface enhanced fluorescence (SEF) technology. The roles of Cu core and SiO2 shell in tuning LSPR have been quantitatively discussed by calculating the localized electric field enhancement factor (EF), fluorescent quantum yield, SERS and SEF EF in the nanogap based on finite element simulations. In this work, we demonstrate that the SERS and SEF effect strongly depend on the core size, shell thickness and dimer distance. The larger size of Cu core, ultrathin thickness of SiO2 shell and shorter dimer distance could be considered as the ideal substrate. Meanwhile, the strong localized excitation enhancement is a vital contribution to fluorescence spectral. By comparison, it is found that the enhancement effect of the Cu, Au and Ag cores are very close in order of magnitude in the red and near infrared regions, and thus the low-cost Cu core may act as an active substrate in these regions. At the excitation wavelength of 630 nm, the optimized core size of 60 nm and shell thickness of 1 nm provide the maximum Raman and fluorescence EF as high as 9 and 4 orders of magnitude. Our theoretical results not only help understanding the underlying enhancement mechanism for the core-shell SERS and SEF effect, but also contribute to develop the low-cost and sensitive detection technology.

Correlative Nanoscale Imaging of Strained hBN Spin Defects.

Spin defects like the negatively charged boron vacancy color center (VB-) in hexagonal boron nitride (hBN) may enable new forms of quantum sensing with near-surface defects in layered van der Waals heterostructures. Here, the effect of strain on VB- color centers in hBN is revealed with correlative cathodoluminescence and photoluminescence microscopies. Strong localized enhancement and redshifting of the VB- luminescence is observed at creases, consistent with density functional theory calculations showing VB- migration toward regions with moderate uniaxial compressive strain. The ability to manipulate spin defects with highly localized strain is critical to the development of practical 2D quantum devices and quantum sensors.

Analysis of the relationship between turbulence characteristics and loss mechanism in the tip leakage flow of turbine blade

The tip leakage flow passed over the tip clearance makes the flow very complicated near the tip gap, and the interaction of the tip leakage vortex and endwall vortex enhances the instability of the flow. Accurately capturing detailed flow structures and investigating the relationship between the flow structures and loss are beneficial for understanding the flow physics and providing guidance on reducing the loss. Due to the conventional Reynolds Averaged Navier-Stokes (RANS) methods is limited to predict the complex turbulence structures of the tip clearance flow, high fidelity simulation approaches are needed. In this work, the hybrid RANS/Large Eddy Simulation (LES) is adopted to simulate the tip leakage flow in linear cascade and demonstrates its ability to capture the small-scale flow structures. With the POD method, the time-averaged flow field and the dominating modes are obtained. Based on the analysis of the POD modes, it is found that the induced vortex generated by the interaction between the leakage vortex and the endwall vortex has strong turbulence characteristics. Based on the entropy generation rates, viscous loss mechanism is further analyzed. It is found that the shear strain rates dominate the viscous dissipation losses, and the fluctuation dissipation has a strong local enhancement effect.

Combined cell and nanoparticle models for TOPAS to study radiation dose enhancement in cell organelles

Dose enhancement by gold nanoparticles (AuNP) increases the biological effectiveness of radiation damage in biomolecules and tissue. To apply them effectively during cancer therapy their influence on the locally delivered dose has to be determined. Hereby, the AuNP locations strongly influence the energy deposit in the nucleus, mitochondria, membrane and the cytosol of the targeted cells. To estimate these effects, particle scattering simulations are applied. In general, different approaches for modeling the AuNP and their distribution within the cell are possible. In this work, two newly developed continuous and discrete-geometric models for simulations of AuNP in cells are presented. These models are applicable to simulations of internal emitters and external radiation sources. Most of the current studies on AuNP focus on external beam therapy. In contrast, we apply the presented models in Monte-Carlo particle scattering simulations to characterize the energy deposit in cell organelles by radioactive 198AuNP. They emit beta and gamma rays and are therefore considered for applications with solid tumors. Differences in local dose enhancement between randomly distributed and nucleus targeted nanoparticles are compared. Hereby nucleus targeted nanoparticels showed a strong local dose enhancement in the radio sensitive nucleus. These results are the foundation for future experimental work which aims to obtain a mechanistic understanding of cell death induced by radioactive 198Au.

Seed-mediated Growth of Silver Nanoplates and Investigation on Their Nonlinear Optical Behaviors

Silver nanoplates have obvious advantages compared with other silver structural nanoparticles due to their wide tunability of the local surface plasmon resonance (LSPR) and significant local field enhancement around the sharp corners. Nonlinear optical absorption (NLA) and nonlinear optical refraction (NLR) are widely employed to characterize the nonlinear optical properties due to their simply testing requirement and comparable valuable results. Z-scan method is widely used in optical characterization of nonlinear properties of different materials. Here, a seed-mediated growth method of the silver nanoplates with well controlled size and local surface plasmon wavelength is presented. The nanoparticle size increases gradually and becomes more uniform distribution as the red-shift of the plasmon resonance wavelength from 650 nm to 950 nm. In particular, we show the dependence of NLA and NLR of the silver nanoplates on the LSPR. The absolute value of effective NLA coefficient increases from 1.38 to 4 cm/GW and that of the NLR index increases as large as 3 times when the LSPR wavelength changes from 728 nm to 898 nm, which is attributed to the strong plasmon absorption and local filed enhancement of larger size silver nanoplates. These findings have great potentials in the explorations of functional non-linear devices.

Pollution trace gas distributions and their transport in the Asian monsoon upper troposphere and lowermost stratosphere during the StratoClim campaign 2017

Abstract. We present the first high-resolution measurements of pollutant trace gases in the Asian summer monsoon upper troposphere and lowermost stratosphere (UTLS) from the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) during the StratoClim (Stratospheric and upper tropospheric processes for better climate predictions) campaign based in Kathmandu, Nepal, 2017. Measurements of peroxyacetyl nitrate (PAN), acetylene (C2H2), and formic acid (HCOOH) show strong local enhancements up to altitudes of 16 km. More than 500 pptv of PAN, more than 200 pptv of C2H2, and more than 200 pptv of HCOOH are observed. Air masses with increased volume mixing ratios of PAN and C2H2 at altitudes up to 18 km, reaching to the lowermost stratosphere, were present at these altitudes for more than 10 d, as indicated by trajectory analysis. A local minimum of HCOOH is correlated with a previously reported maximum of ammonia (NH3), which suggests different washout efficiencies of these species in the same air masses. A backward trajectory analysis based on the models Alfred Wegener InsTitute LAgrangian Chemistry/Transport System (ATLAS) and TRACZILLA, using advanced techniques for detection of convective events, and starting at geolocations of GLORIA measurements with enhanced pollution trace gas concentrations, has been performed. The analysis shows that convective events along trajectories leading to GLORIA measurements with enhanced pollutants are located close to regions where satellite measurements by the Ozone Monitoring Instrument (OMI) indicate enhanced tropospheric columns of nitrogen dioxide (NO2) in the days prior to the observation. A comparison to the global atmospheric models Copernicus Atmosphere Monitoring Service (CAMS) and ECHAM/MESSy Atmospheric Chemistry (EMAC) has been performed. It is shown that these models are able to reproduce large-scale structures of the pollution trace gas distributions for one part of the flight, while the other part of the flight reveals large discrepancies between models and measurement. These discrepancies possibly result from convective events that are not resolved or parameterized in the models, uncertainties in the emissions of source gases, and uncertainties in the rate constants of chemical reactions.

Plasmon modes characterization in hybrid noble/ ferromagnetic nanodisks using an effective medium approximation

Magneto-plasmonic systems composed by noble/ferromagnetic metals structures can be used to control light polarization. The extraordinary optical properties arising from combining strong local enhancements of electromagnetic fields in surface plasmon excitations with the magneto-optical activity inherent to ferromagnetic materials, can be controlled by external magnetic fields that have demonstrated the possibility to control and amplify the magneto-optical properties via plasmonic excitations. In this work, the anisotropic optical and ellipsometric sensitivity to dielectric environment of multi-layered hybrid gold/cobalt magnetoplasmonic nanodisks are studied in the framework of the effective-medium approximation, where the components of the dielectric tensor of the system are modelled using Lorentz-like oscillators. Furthermore, full electromagnetic simulations were performed using the standard Finite Element Method, which allow us to characterize the modes and explore the role of the dielectric environment of the nanodisks in the optical properties of the nanostructures via the variation of a coating silicon dioxide (SiO2) layer thickness. Our model and numerical results show a very good agreement with spectral ellipsometry measurements, where two orthogonal plasmonic modes one in-plane and one out-of-plane- are well characterized. These results lead us to conclude that the effective-medium approximation is good enough to accurately describe the plasmonic behavior of multi-layered confined nanostructures.

Cavity QED based on room temperature atoms interacting with a photonic crystal cavity: a feasibility study

The paradigm of cavity QED is a two-level emitter interacting with a high-quality factor single-mode optical resonator. The hybridization of the emitter and photon wave functions mandates large vacuum Rabi frequencies and long coherence times; features that so far have been successfully realized with trapped cold atoms and ions, and localized solid-state quantum emitters such as superconducting circuits, quantum dots, and color centers Reiserer and Rempe (Rev Modern Phys 87:1379, 2015), Faraon et al. (Phys Rev 81:033838, 2010). Thermal atoms, on the other hand, provide us with a dense emitter ensemble and in comparison to the cold systems are more compatible with integration, hence enabling large-scale quantum systems. However, their thermal motion and large transit-time broadening is a major bottleneck that has to be circumvented. A promising remedy could benefit from the highly controllable and tunable electromagnetic fields of a nano-photonic cavity with strong local electric-field enhancements. Utilizing this feature, here we investigate the interaction between fast moving thermal atoms and a nano-beam photonic crystal cavity (PCC) with large quality factor and small mode volume. Through fully quantum mechanical calculations, including Casimir–Polder potential (i.e. the effect of the surface on radiation properties of an atom), we show, when designed properly, the achievable coupling between the flying atom and the cavity photon would be strong enough to lead to quantum interference effects in spite of short interaction times. In addition, the time-resolved detection of different trajectories can be used to identify single and multiple atom counts. This probabilistic approach will find applications in cavity QED studies in dense atomic media and paves the way towards realizing large-scale, room-temperature macroscopic quantum systems aimed at out of the lab quantum devices.

Recent advances in nano-photonic techniques for pharmaceutical drug monitoring with emphasis on Raman spectroscopy

Abstract Innovations in Raman spectroscopic techniques provide a potential solution to current problems in pharmaceutical drug monitoring. This review aims to summarize the recent advances in the field. The developments of novel plasmonic nanoparticles continuously push the limits of Raman spectroscopic detection. In surface-enhanced Raman spectroscopy (SERS), these particles are used for the strong local enhancement of Raman signals from pharmaceutical drugs. SERS is increasingly applied for forensic trace detection and for therapeutic drug monitoring. In combination with spatially offset Raman spectroscopy, further application fields could be addressed, e.g. in situ pharmaceutical quality testing through the packaging. Raman optical activity, which enables the thorough analysis of specific chiral properties of drugs, can also be combined with SERS for signal enhancement. Besides SERS, micro- and nano-structured optical hollow fibers enable a versatile approach for Raman signal enhancement of pharmaceuticals. Within the fiber, the volume of interaction between drug molecules and laser light is increased compared with conventional methods. Advances in fiber-enhanced Raman spectroscopy point at the high potential for continuous online drug monitoring in clinical therapeutic diagnosis. Furthermore, fiber-array based non-invasive Raman spectroscopic chemical imaging of tablets might find application in the detection of substandard and counterfeit drugs. The discussed techniques are promising and might soon find widespread application for the detection and monitoring of drugs in various fields.

Luminescent Sensing via Photonic Nanojets

A new optical sensor system based on an array of dielectric aluminum oxide microspheres covered with a layer of chitosan polymer with molecules of a luminescent chemosensitive receptor was developed for determination of ammonia ion concentration in air. The photonic nanojet effect, i.e. strong electromagnetic field local enhancement, was implemented for most effective excitation of the sensitive layer. By using of chitosan swelling polymer the effect of refractive index variation on photonic nanojets excited sensor response was demonstrated. It was shown that under certain conditions such as microsphere diameter and refractive index contrast a detection limit of chemosensitive receptor increases in three times.

Highly-sensitive fluorescent detection of chemical compounds via photonic nanojet excitation

A photonic nanojet phenomenon that is strong electromagnetic field local enhancement is widely used for various optical tasks like surface-enhanced Raman scattering or surface-enhanced fluorescence. Here we present a novel implementation of this effect for boosting the sensory response of the fluorescent optical probe through its more efficient pumping via photonic nanojets. For this, a sensitive layer consisting of chitosan polymer impregnated with a fluorescent probe designed for the selective determination of Au3+ ions was deposited onto the surface of an array of dielectric alumina microspheres. The photonic nanojet effect was numerically predicted and experimentally observed in the developed system. It was demonstrated that the change in the refractive index of the matrix caused by polymer swelling has a significant effect on the characteristics of the sensor. Under certain conditions, such as the diameter of the microspheres and the contrast of the refractive index a four-fold decrease in the detection limit of the chemosensor was achieved.

Surface plasma discharge generated by spoof surface plasmon polariton excitation: a computational modeling study

We discuss a computational modeling study of surface plasma generation by spoof surface plasmon polariton (SSPP) mode excitation of a corrugated metal surface. The SSPP mode is a highly localized electromagnetic wave propagating along the interface between a conductor and dielectric. The corrugations are defined by an array of rectangular cavities (comb structures) in the metal at the location of the interface. Spoof plasmon resonances occurring at each cavity couple with one another at the interface to produce localized surface wave structures whose wavelength can be much smaller than the incident wave. The electromagnetic wave structure for a finite length of the interface (metasurface) is analytically estimated and its correlation to the resonance frequency of the infinite metasurface is studied numerically. Strong local enhancement of the electromagnetic fields at the metasurface is used to initiate the plasma breakdown of pure argon gas at 10 Torr. Confined microdischarges produced initially at each of the comb structure evolve rapidly to create an extended structure of high density ~1019 m−3 surface plasma whose properties are comparable to surface streamers generated in dielectric barrier discharges.

Flexoelectric and local heating effects on CdSe nanocrystals in amorphous As2Se3 films

Raman and photoluminescence (PL) spectra of amorphous As2Se3:Cd films show that laser irradiation can result in the local formation of CdSe nanocrystals (NCs). Due to a pronounced laser-induced drastic drop of the film viscosity, a strong local enhancement of diffusion of atoms in the laser spot area results in a material transfer from the laser spot (evidenced by SEM and AFM data) and simultaneously facilitates the formation of CdSe NCs. The difference of the PL maximum positions measured at a high laser power density Pexc and subsequently (from the same spot) at a lower Pexc value can be explained either by local heating of the sample by the high-Pexc illumination, or by the flexoelectric effect (additional strain in the CdSe NCs induced by the electric field gradient caused by the charging of the NC surface). Based on the spectral positions of the PL maximum, with the account of the Stokes shift, the average radii of the CdSe NCs formed are estimated.

Numerical simulation of dynamic fracture behavior in bulk superconductors with an electromagnetic-thermal model

Single-grain GdBCO bulk superconductors have significant applications potential due to their ability to trap stable and large magnetic fields. The internal fracture of these bulk superconductors caused by Lorentz force and thermal load is a key issue in their practical applications. The aim of this work presented here is to investigate the mechanical behavior of the bulk superconductor during pulsed field magnetization. The H-formulation and heat transfer equation are used to obtain the electromagnetic force and thermal load in the bulk with and without defects. Numerical simulations show strong local enhancement of the electromagnetic load at the crack tips. Moreover, dynamic stress intensity factors at the crack tips are presented based on the two-dimensional state-based peridynamic theory. In addition, the crack propagation path is predicted. Finally, different dynamic crack problems are considered to discuss the influence of crack, void, inclusion, and hole on the mechanical stability of the bulk. Results show that defects increase the risk of damage of superconducting bulks.

Fracture analysis of bulk superconductors under electromagnetic force

Bulk high-temperature superconductors exhibit priority magnetic properties compared to the conventional permanent magnets. Single-grain bulk GdBaCuO superconductors have significant application potential due to higher critical current density Jc and trapped fields. However, bulk superconductors are subject to larger Lorentz forces under high magnetic fields and their performance is limited by damage to the materials. In this paper, a finite element model based on the H-formulation is used to solve for the electromagnetic force in bulk GdBCO containing defects or inclusions under application of a pulsed field. Strong local electromagnetic force enhancement is observed at the crack tip. A bond-based peridynamic (PD) approach is proposed for analysis of the dynamic mechanical behavior and brittle damage of the sample. Crack initiation and propagation paths can be predicted using PD theory. PD is thus shown to be a suitable method for dynamic fracture problem analysis. As a further demonstration of the proposed model’s capabilities, the bulk fracture process is simulated. The effect of voids, cross cracks and multiple inclusions are presented. Results show that mechanical stability can be enhanced by the presence of silver particles.

Effects of Heterogeneity, Connectivity, and Density Variations on Mixing and Chemical Reactions Under Temporally Fluctuating Flow Conditions and the Formation of Reaction Patterns

AbstractSolute mixing, spreading, and fast chemical reactions in aquifers are strongly influenced by spatial variability of the hydraulic properties, temporal flow fluctuations, and fluid density differences. We study the coupling of heterogeneity, transient forcing, and density‐driven flow on mixing and chemical reactions between two fluids of different density under a stable stratification. We consider the reaction of the fast dissolution of calcite. We find that temporal fluctuations and heterogeneity cause strong local enhancement of the mixing and reaction rates and this impact increases with the degree of connectivity of hydraulic conductivity. The global mixing and reactivity, however, are on the order of or smaller than their homogeneous counterparts due to heterogeneity‐induced fluid segregation. The local maxima of the mixing and reaction rates are found to be located around strongly stretched regions corresponding to high velocity zones where dispersive mass transfer mechanisms are increased by dispersion. We also find that density variations compress the interface, which in turn emphasizes local maxima in mixing and reaction rates. Numerical results provide evidence that the stretching of the interface induced by spatial heterogeneity and transient effects coupled with density variations lead to the formation of complex patterns of reactive hotspots, zones of enhanced reaction efficiency, and that its distribution is directly linked to the deformation properties and topology of the flow field. These results provide new insights into the role of spatial and temporal variability on the mixing and reaction efficiency as well as the formation of reactive geochemical patterns in actual environmental systems.

Tip-enhanced bulk photovoltaic effect

Using the conventional macroscopic description of the bulk photovoltaic effect we analyze the light-induced currents and electric fields arising in the optical configuration with a continuous bottom electrode and a small circular top electrode. This scheme is relevant to recent experiments on the tip-enhanced photovoltaic effect in ferroelectrics. It is shown that a light-induced electric field remains nonzero inside the sample even in the short-circuit regime. Moreover, it is enhanced compared to the photovoltaic field in a large area and strongly enhanced near the top electrode. A field-assisted collection of charge carriers from the illuminated area produces a strong local enhancement of the current density near the top electrode. The tip-enhanced electric field is typically parallel to the photovoltaic current. It is sufficient to repolarize the crystal near the top electrode. The effect of the tip enhancement on the light-current transformation efficiency is considered, and predictions for the tip radius and sample thickness dependencies of the total light-induced current are made.