Magnetic Field Enhancement Research Articles

Independently tunable Fano resonance from quasibound state in the continuum in hybrid graphene–dielectric metasurface for magnetic field tunability

We study an independently tunable Fano resonance from quasibound state in the continuum and polarization-independent toroidal dipole resonance in hybrid graphene–dielectric metasurface consisting of a nanoring and a cross-shaped nanobar. The unique structural properties of the proposed metasurface can support directional toroidal dipole mode, where the asymmetric magnetic field enhancement can be dynamically switched on and off while maintaining polarization-independent Fano spectral response. Through reducing or increasing inner radius of nanoring, an additional quasi-BIC dominated by magnetic dipole moment can be excited and independently tuned by altering the polarization direction of the incident wave. In addition, the quasi-BIC can be effectively modulated via adjusting the Fermi energy and layer numbers of the graphene. Our results can be of practical interest for a variety of applications including optical modulator, filter, switches, and light trapping.

Aperture–patch sandwich metasurface for magnetic field enhancement in 1.5 T MRI

Magnetic resonance imaging (MRI) is an increasingly popular non-invasive technique for clinical diagnosis. Signal-to-noise ratio (SNR) is a crucial performance metric of MRI, improvement of which can be exchanged for increased image resolution or decreased scan time. Besides the progress in various hardware and software techniques for improving SNR in MRI scanners, use of metasurfaces as accessories has recently shown potential towards enhancing SNR by boosting local magnetic field in the scanned volume. Magnetic field enhancement over a larger depth from the skin is essential for imaging of deeper tissues, which can be facilitated by a specifically designed metasurface. Here we present such a metasurface with complementary-type resonant structures on the two sides of a high-permittivity dielectric, which substantially increases magnetic flux density on the skin (forty-five fold) that decays down to unity at a depth of 95 mm from the skin. This results in boosting of SNR up to forty-fold on the skin in 1.5 T MRI, while keeping tissue heating below the safety limit. An original analytical approach is formulated to readily estimate the SNR enhancement factor of this metasurface. Using the designed metasurface as an accessory for MRI scanners could help making MRI scans more efficient and affordable.

Selective excitation of hyperbolic phonon polaritons-induced broadband absorption via α-MoO3 square pyramid arrays

Optical anisotropy of α-MoO3 in its reststrahlen (RS) bands provides exciting opportunities for constructing the polarization-dependent devices. However, achieving broadband anisotropic absorptions through the same α-MoO3 arrays is still challenging. In this study, we demonstrate that selective broadband absorption can be achieved by using the same α-MoO3 square pyramid arrays (SPAs). For both the x and y polarizations, the absorption responses of the α-MoO3 SPAs calculated by using the effective medium theory (EMT) agreed well with those of the FDTD, indicating the excellent selective broadband absorption of the α-MoO3 SPAs are associated with the resonant hyperbolic phonon polaritons (HPhPs) modes assisted by the anisotropic gradient antireflection (AR) effect of the structure. The near-field distribution of the absorption wavelengths of the α-MoO3 SPAs shows that the magnetic-field enhancement of the lager absorption wavelength tends to shift to the bottom of the α-MoO3 SPAs due to the lateral Fabry–Pérot (F–P) resonance, and the electric-field distribution exhibits the ray-like light propagation trails due to the resonance nature of the HPhPs modes. In addition, broadband absorption of the α-MoO3 SPAs can be maintained if the width of the bottom edge of the α-MoO3 pyramid is large than 0.8 μm, and excellent anisotropic absorption performances are almost immune to the variations of the thickness of the spacer and the height of the α-MoO3 pyramid.

Pulsating magnetohydrodynamic flow of Fe[formula omitted]O[formula omitted]-blood based micropolar nanofluid between two vertical porous walls with Cattaneo–Christov heat flux and entropy generation

The present article is aimed to investigate the impact of entropy generation on pulsating hydromagnetic flow of a micropolar nanofluid between two vertical porous walls using the Cattaneo–Christov heat flux model. Here Fe3O4(magnetite) is taken as a nanoparticle and blood as micropolar fluid (base fluid). The significance of viscous dissipation, Ohmic heating, and thermal radiation are considered. This model is noteworthy in the field of magnetic bioseparation, pressure surges, magnetofection agent, biomedical engineering, cancer therapeutic, artificial kidney, brain tumors, and nano-drug delivery in the arteries. The governing partial differential equations are transformed into the system of ordinary differential equations by deploying the perturbation process and then solved numerically by employing the fourth-order Runge–Kutta scheme with the support of the shooting technique. The flow variables like velocity, microrotation, temperature, entropy generation, and Bejan number are depicted graphically and discussed in detail. The heat transfer rate is displayed through a table. The results depict that the velocity is diminishing with the enhancement of coupling parameter, Hartmann number, and nanoparticle volume fraction. The temperature of micropolar nanofluid is increasing with an increment of viscous dissipation, thermal radiation, and heat source while it is reducing with the enhancement of magnetic field, thermal relaxation time, and heat sink parameter. The entropy generation is diminished by increasing the values of Hartmann number, thermal relaxation, and coupling parameter. Further, the Bejan number is enhanced by varying thermal radiation and nanoparticle volume fraction.

Pure magnetic hotspots via hollow silicon nanoparticles illuminated by cylindrical vector beams

Overshadowed by the electric dipole ones, the magnetic dipole transitions of a quantum emitter are hard to probe. This can be remedied by using pure magnetic hotspots, which host strong magnetic but quite weak electric fields in a subwavelength region. Utilizing the multiparticle Mie theory, we demonstrate pure magnetic hotspots via hollow silicon nanoparticles arranged in a ring configuration and illuminated by Bessel cylindrical vector beams. Normalized local field admittances of about 105–107 and magnetic field enhancements of about 10–30 are achievable. Moreover, the peak of the magnetic field enhancement can be shifted about 370 nm via the hollow size. In other words, the pure magnetic hotspot can be optimized for a particular magnetic dipole transition.

Dynamical magnetic fields in heavy-ion collisions

The magnetic fields in heavy-ion collisions are important ingredients for many interesting phenomena, such as the Chiral Magnetic Effect, Chiral Magnetic Wave, the directed flow $v_1$ of $D^0$ mesons and the splitting of the spin polarization of the $\Lambda$/$\bar{\Lambda}$. Quantitative studies of these phenomena however suffer from limited understanding on the dynamical evolution of these fields in the medium created by the collisions, which remains a critical and challenging problem. The initial magnetic fields from the colliding nuclei decay very fast in the vacuum but their lifetime could be extended through medium response due to electrically conducting quarks and antiquarks. Here we perform a detailed analysis of such medium effect on the dynamical magnetic fields by numerically solving the Maxwell's equations concurrently with the expanding medium described by viscous hydrodynamics, under the assumption of negligible back reaction of the fields on the fluid evolution. Our results suggest a considerable enhancement of late time magnetic fields, the magnitude of which depends sensitively on the fireball expansion as well as the medium electric conductivity both before and during hydrodynamic stage.

Theory of magnetic turbulence and shock formation induced by a collisionless plasma instability

Magnetic fields are ubiquitous in universe, space, and laboratory plasmas. Especially, self-generated magnetic fields are important to know the mind of nature. The formation of Weibel-mediated collisionless shock is studied theoretically as a structure formation by the linear plasma wave growth, nonlinear saturation, and mode–mode coupling. Following a series of computer simulations and experimental studies of the physics, a simple model equation is proposed here to describe the time evolution of magnetic turbulence. Weibel instability is saturated by magnetic pressure, and thicker filaments continue to be generated by current coalescence (magnetic reconnection) mechanism. The model equation concludes the fact that the filament spacing increases linearly in time, and the magnetic energy power spectrum is given as Bk2 ∝ k−2. The time evolution of the turbulence is characterized with the cascade toward smaller k. Such inverse cascade is well-known in 2D hydrodynamic turbulence such as a typhoon or hurricane formation and is known to have Kolmogorov spectrum k−5/3. Although only a small difference in power, the physics of inverse cascades is very different as shown in the present paper. With use of Alfvén current limit condition, the criteria of collisionless shock formation are evaluated. The present theory is compared to corresponding experiments done with Omega and NIF lasers and a variety of PIC simulations. The theory is also applied to evaluate the strength of magnetic field near the shock front of the supernova remnant SN1006. The enhancement of magnetic field of about 25 μG is concluded in the present theory. Finally, a universality of the model equation is shown by applying the theory to the turbulent mixing due to Rayleigh–Taylor instability at the contact surface of two fluids in a gravitational or inertial force, which is very important in compressing plasma such as inertial confinement fusion by implosion. It is shown that the well-known evolution physics, mixing layer of the two fluids grows in proportion to (time)2, can be explained with the same model equation.

Radiation absorption on MHD free conduction flow through porous medium over an unbounded vertical plate with heat source

The aim of the present paper is to investigate the radiation absorption and diffusion thermo on unsteady magneto hydrodynamic flow past an infinite vertical permeable moving plate in the presence of thermal radiation, heat absorption and homogenous chemical reaction, subjected to variable suction. The plate is assumed to be embedded in a uniform porous medium and moves with a constant velocity in the flow direction in the presence of a transverse magnetic field. The equations governing the flow are transformed into a system of nonlinear ordinary differential equations by using perturbation technique. Graphical results for the velocity distribution, temperature distribution and concentration distribution based on the numerical solutions are presented and discussed. Also we discuss the effects of various parameters on the skin-friction coefficient and the rate of heat transfer in the form of Nusselt number and rate of mass transfer in the form of Sherwood number at the surface. Velocity and temperature distribution is observed to increase with an increase in radiation absorption and Dufour parameter, whereas it diminishes with the enhancement in magnetic field, heat absorption coefficient, radiation parameter and chemical reaction parameter with respect to velocity, temperature and concentration distributions.

The enhancement of magnetic field assisted water electrolysis hydrogen production from the compact disc recordable waste polycarbonate layer

E-waste is growing rapidly in today's data technology era while the need for green energy is critical. Current compact disc recordable (CD-R) e-waste utilization for hydrogen production by electrolysis is still classified as gray hydrogen production. This study aims to synthesize electrocatalyst for green hydrogen production from CD-R polycarbonate layer. The elemental and morphological characterizations were performed in this study along with molecular dynamics simulation and direct electrolysis experiment. The electrolysis test results shows EMF and high Bisphenol-A (BPA) content from 3 g polycarbonate produce 26000 ppm H2 almost tripled the EMF only with 10000 ppm H2 and doubled the EMF with 1 g polycarbonate 15000 ppm H2. EMF and BPA cooperatively reduces water ionization energy through diamagnetic response and aromatic resonance which vibrates water molecule. Following that, the EMF slowed down OH− ion movement causes the H+ movement towards electron become unrestricted. In conclusion, the EMF-BPA cooperation increases hydrogen evolution reaction (HER) through water ionization energy reduction and ion transfer modification.

Plasmonic bound states in the continuum for unpolarized weak spatially coherent light

Plasmonic resonances empowered by bound states in the continuum (BICs) offer unprecedented opportunities to tailor light–matter interaction. However, excitation of high quality-factor ( Q -factor) quasi-BICs is often limited to collimated light at specific polarization and incident directions, rendering challenges for unpolarized focused light. The major hurdle is the lack of robustness against weak spatial coherence and poor polarization of incident light. Here, addressing this limitation, we demonstrate sharp resonances in symmetric plasmonic metasurfaces by exploiting BICs in the parameter space, offering ultraweak angular dispersion effect and polarization-independent performance. Specifically, a high- Q ( ≈ 71 ) resonance with near-perfect absorption ( > 90 % ) is obtained for the input of unpolarized focused light covering wide incident angles (from 0° to 30°). Also, giant electric and magnetic field enhancement simultaneously occurs in quasi-BICs. These results provide a way to achieve efficient near-field enhancement using focused light produced by high numerical aperture objectives.

Enhanced Faraday rotation by a Fano resonance in substrate-free three-dimensional magnetoplasmonic structures.

Three-dimensional magnetoplasmonic nanostructures possess more novel and richer optical and magneto-optical (MO) behaviors compared with planar nanostructures, and exhibit attractive potential applications in micro-nano non-reciprocal photonic devices. However, fabrication of three-dimensional magnetoplasmonic nanostructures is difficult using the usual nanofabrication methods. This work constructs three-dimensional substrate-free Au/Co/Au structures prepared using focused ion beam (FIB) technology. In the three-dimensional split-ring structure, with y-polarized light normal incidence, a three-dimensional coupling current is formed between the vertical split-ring and the bottom square hole, which causes excitation of the Fano resonance. The Fano resonance causes a significant enhancement of the local magnetic field, resulting in a larger Faraday rotation (FR). The resonance also brings about a sign reversal of FR, which is related to the direction of the Lorentz force on electrons. Similar effects also exist in the three-dimensional nanopillar structure and the three-dimensional nanoring structure in the simulation results. Due to the high flexibility of FIB machining, the height and shape of the three-dimensional split-ring can be arbitrarily changed, which means the FR intensity and the position of the FR null point are tunable. The designed three-dimensional structures provide a new route to regulate the Faraday effect, and broaden the possibilities for the design and construction of MO devices.

Magnetic Field Enhancement of Water Evaporation in Confined Spaces

Water is studied in confined environments where it evaporates into its own vapor. Simultaneous experiments are conducted for 0.4 – 0.5 µL droplets confined at the center of 54 mm long microchannels with a cross section of 0.38 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in the presence and absence of a 300 mT magnetic field. Results are compared with those for water in half-filled 100 mL beakers. The magnetic enhancement of the evaporation rate is much greater in the microchannels where effects range up to 140% although the air is saturated with water vapor, compared to 12 ± 7% in a 500 mT field in the beakers. The average steady-state, no-field evaporation rate of 0.13 kgm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> h <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in the microchannels is roughly double that in the beakers, but less than the value expected at an open surface in still air. The magnetic enhancement is analyzed in terms of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ortho</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">para</i> nuclear isomers of water vapor, which behave as independent gasses. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ortho:para</i> ratio in fresh vapor is close to 2:3, and quite different from the 3:1 equilibrium ratio in ambient air. Evaporation is increased by the gradient of the applied magnetic field which de-phases the Larmor precession of the two proton spins of hydrogen in a water molecule and tends to equalize the isomeric populations in the vapor, thereby increasing the evaporation rate.

Magnetic field enhancement of the FeCoSe2 photoanode for the oxygen evolution reaction by adjusting the hole density to reduce competitive adsorption between Fe and Co in a photoelectrochemical water-splitting system

FeCoSe2 has fast oxygen evolution due to polarization electric field and d-band center. However, the large adsorption energy of Fe slows the reaction. Magnetic field reduces Fe's adsorption, enhancing OER performance.

High gradient off-axis coupled C-band Cu and CuAg accelerating structures

We report the high gradient testing results of two single-cell off-axis coupled standing wave accelerating structures. Two brazed standing wave off-axis coupled structures with the same geometry were tested: one made of pure copper (Cu) and one made of a copper–silver (CuAg) alloy with a silver concentration of 0.08%. A peak surface electric field of 450 MV/m was achieved in the CuAg structure for a klystron input power of 14.5 MW and a 1 μs pulse length, which was 25% higher than the peak surface electric field achieved in the Cu structure. The superb high gradient performance was achieved because of the two major optimizations in the cavity's geometry: (1) the shunt impedance of the cavity was maximized for a peak surface electric field to accelerating gradient ratio of ∼2 for a fully relativistic particle, and (2) the peak magnetic field enhancement due to the input coupler was minimized to limit pulse heating. These tests allow us to conclude that C-band accelerating structures can operate at peak fields similar to those at higher frequencies while providing a larger beam iris for improved beam transport.

Surface plasmon polaritons enhanced magnetic plasmon resonance for high-quality sensing

We investigate the metamaterials consisting of top layer U-shaped metallic split-ring resonators, the middle thin dielectric layer, and the bottom thick metal film, which can realize high-quality refractive index sensing. When propagating surface plasmon polaritons are introduced, two ultra-narrow-band hybridized modes will appear, whose full width at half maximum can reach up to 2.5 nm. Because of the ultra-narrow linewidth and huge magnetic field enhancement at hybridized modes, the metamaterials display very high sensitivity (S = 912 nm RIU−1, S* = 95.64 RIU−1) and large figure of merit (FOM = 364.8, FOM*= 160 740.34), which has great potential in label-free biomedical sensing.

Strong superradiance of coherently coupled magnetic dipole emitters mediated by whispering gallery modes of a subwavelength all-dielectric cavity

The interaction of magnetic dipole (MD) emitters and common photonic cavities is usually weak, which is partially due to the low magnetic near field enhancements of the cavities. Here, we show that whispering gallery modes (WGMs) of a subwavelength dielectric cavity can not only greatly boost the emission rate of a MD emitter but also bring efficient couplings between coherent MD emitters. In a WGM cavity, the maximal emission rate ({\gamma}max) of a single emitter occurs at an antinode of the field pattern. The emission of the MD emitter can also be greatly affected by another coherent one depending on the magnetic field response of the WGM. The maximal contribution can also reach {\gamma}max. Notably, the cooperative emission rate of the coherent MD emitters does not decay with distance in the considered range due to the high-quality feature of a WGM. In contrast to the emission, the absorption of an emitter is hardly affected by the coherent couplings between emitters mediated by a WGM. The difference between the performances of emission and absorption is highly related to the excitation behaviors of WGMs. Our results are important for enhanced magnetic light-matter interactions.

Asymmetric excitations of toroidal dipole resonance and magnetic dipole quasi-bound state in the continuum in hybrid graphene-dielectric metasurface

We study asymmetric excitations of toroidal dipole resonance and magnetic dipole quasi-bound state in the continuum (BIC) in hybrid graphene-dielectric metasurface consisting of a nanoring and a Y-shaped nanobar. Through reducing or increasing inner radius of nanoring, the quasi-BIC dominated by magnetic dipole moment can be excited and effectively modulated via adjusting the Fermi energy and layer numbers of the graphene. The proposed metasurface can not only produce a symmetric localized magnetic field distribution but also create two asymmetric localized magnetic field distributions in near-infrared wavelength, providing a new way of indirectly manipulating the localized magnetic field enhancement. Our results can be of practical interest for a variety of applications including optical modulator, filter, switches and light trapping.

Dynamics of magnetic microbubble transport in blood vessels

Magnetic microbubbles (MMBs) can be controlled and directed to the target site by a suitable external magnetic field, and thus have potential in therapeutic drug-delivery application. However, few studies focus on their dynamics in blood vessels under the action of magnetic and ultrasonic fields, giving little insight into the mechanism generated in diagnostic and therapeutic applications. In this study, equations of MMBs were established for simulating translation, radial pulsation and the coupled effect of both. Meanwhile, the acoustic streaming and shear stress on the vessel wall were also presented, which are associated with drug release. The results suggest that the magnetic pressure increases the bubble pulsation amplitude, and the translation coupled with pulsation is manipulated by the magnetic force, causing retention in the target area. As the bubbles approach the vessel wall, the acoustic streaming and shear stress increase with magnetic field enhancement. The responses of bubbles to a uniform and a gradient magnetic field were explored in this work. The mathematical models derived in this work could provide theoretical support for experimental phenomena in the literature and also agree with the reported models.

Change of Rankine–Hugoniot Relations during Postshock Relaxation of Anisotropic Distributions

Collisionless shocks channel the energy of the directed plasma flow into the heating of the plasma species and magnetic field enhancement. The kinetic processes at the shock transition cause the ion distributions just behind the shock to be nongyrotropic. Gyrotropization and subsequent isotropization occur at different spatial scales. Accordingly, for a given upstream plasma and magnetic field state, there would be different downstream states corresponding to the anisotropic and isotropic regions. Thus, at least two sets of Rankine–Hugoniot relations are needed, in general, to describe the connection of the downstream measurable parameters to the upstream ones. We establish the relation between the two sets.

Enhancement of static magnetic field on biological hydrogen production via photo-fermentation of giant reed

The effect of static magnetic field (SMF) on the system of photo-fermentation biological hydrogen production remains dimness. The goal of this study was to clarify the correlation between external SMF addition and hydrogen production via photo-fermentation from giant reed. SMF with 20 mT improved the cumulative H2 yield by 26.1% and reduced the lag time of hydrogen production by 56.7% compared with that of without external magnetic field. Moreover, 20 mT of SMF not only enhanced the activity of nitrogenase by 94.52%, but also obtained the maximum energy conversion efficiency of 27.27%. The distribution of volatile fatty acids proved that the concentration of acetic acid and butyric acid were 137% and 81% higher than that of without SMF, respectively. The results would help to trigger the positive interaction between SMF and microorganism and to avoid the possible negative interaction.