Strong Field Gradients Research Articles

Enhanced trapping properties of coupled plasmonic tweezers via plasmon-exciton interaction

Excited plasmonic nanoantennas enable the manipulation of photons coupled with quantum emitters or the trapping of particles as plasmonic tweezers, leveraging the strong evanescent gradient fields at the nanoscale. However, the ohmic loss of metals presents a significant challenge for the stable and high-precision manipulation of nanoparticles without causing damage. In this study, we investigated the enhanced trapping properties induced by plasmon-exciton interaction for coupled plasmonic tweezers. Through the coupling between plasmons and excitons, dynamic particle trapping is achievable under low excitation power conditions of 0.45 mW, with the trapping stiffness increasing by nearly 20 times. Furthermore, the trapping stiffness can be fine-tuned by modulating the quantity of excitons to regulate the coupling strength. Coupled plasmonic tweezers offer an effective strategy to mitigate the influence of ohmic loss on trapping performance, by manipulating particles with minimal laser power. These findings provide insights into enhancing trapping performance through plasmon-exciton coupling, with potential applications in biomedicine and quantum information science.

Influence of Quadrupolar Molecular Transitions within Plasmonic Cavities.

Optical nanocavities have revolutionized the manipulation of radiative properties of molecular and semiconductor emitters. Here, we investigate the amplified photoluminescence arising from exciting a dark transition of β-carotene molecules embedded within plasmonic nanocavities. Integrating a molecular monolayer into nanoparticle-on-mirror nanostructures unveils enhancements surpassing 4 orders of magnitude in the initially light-forbidden excitation. Such pronounced enhancements transcend conventional dipolar mechanisms, underscoring the presence of alternative enhancement pathways. Notably, Fourier-plane scattering spectroscopy shows that the photoluminescence excitation resonance aligns with a higher-order plasmonic cavity mode, which supports strong field gradients. Combining quantum chemistry calculations with electromagnetic simulations reveals an important interplay between the Franck-Condon quadrupole and Herzberg-Teller dipole contributions in governing the absorption characteristics of this dark transition. In contrast to free space, the quadrupole moment plays a significant role in photoluminescence enhancement within nanoparticle-on-mirror cavities. These findings provide an approach to access optically inactive transitions, promising advancements in spectroscopy and sensing applications.

Simulation of Remote Forces Generated by High-Temperature Superconducting Bulks for Magnetic Drug Delivery

The strong background fields and field gradients required for the magnetic delivery of chemotherapeutic drugs in cancer treatments has proven to be challenging to achieve at the scale of the human body. Although several magnetic drug delivery (MDD) methods have been proposed to generate adequate forces in deep tissues, current technologies lack in either force strength, directional changes and/or duty cycle of the treatment. The current MDD system capable of generating the strongest forces, dipole field navigation, uses ferromagnetic (FM) cores in the strong uniform field of an MRI to steer the drug-loaded particles. Hence, considering the nearly ten-fold increase in magnetization of high-temperature superconducting (HTS) bulks in comparison to the strongest FM, the forces obtained in dipole field navigation could potentially be increased by replacing the FM cores with HTS bulks. In this work, we evaluate two different methods for generating magnetic forces using an HTS bulk in a magnetic resonance imaging (MRI) scanner by using finite element method simulations. First, the HTS pellet is zero field cooled and inserted in the uniform field of an MRI, such that the pellet is magnetized by the fringe field of the MRI. Secondly, a field cooled HTS pellet is rotated in the uniform field of an MRI in order to generate magnetic field gradients. The magnetic forces produced by both methods are compared with those obtained by dipole field navigation. In both methods evaluated, we found that the forces generated by the HTS pellet is either complementary to or stronger than those obtained with FM cores. Therefore, this work shows that HTS bulks could potentially be used to navigate magnetic particles in the deep tissues of the human vascular network more efficiently.

Predictive SOLPS-ITER simulations to study the role of divertor magnetic geometry in detachment control in the MAST-U Super-X configuration

The SOLPS-ITER code has been utilised to study the movement of the detachment front location from target towards the X-point for MAST-U Super-X plasmas. Two sets of detached steady state solutions are obtained by either varying the deuterium (D 2) fuelling rate or the nitrogen (N) seeding rate to scan the corresponding ‘control’ parameters of outboard midplane density, , and the divertor impurity concentration, . At seeding and fuelling rates ∼10× and ∼5× that required to start detachment at the divertor target, the detachment front only reaches ∼50% of the poloidal distance to the X-point, , corresponding to a region of strong parallel gradients in the total magnetic field B. The region of strong total field gradients correlates with where the detachment front location becomes less sensitive to control parameter variation. This result is qualitatively consistent with the predictions of a simple, analytic detachment location sensitivity (DLS) model (Lipschultz et al 2016 Nucl. Fusion 56 056007) which is based in a scaled parallel-to-B space, z. While the DLS model predictions are in agreement with SOLPS-ITER results in terms of where the front location becomes less sensitive to controls (i.e. in the region of strong parallel gradients in B), the DLS model predicts a higher sensitivity in the region of weak parallel gradients in B downstream as compared to the simulation results. Potential sources of differences between the SOLPS-ITER and DLS model predictions were explored: The DLS model does not include energy sinks beyond radiation from a single impurity nor cross-field energy transport. Momentum and particle balance are also not included in the DLS model. The tight opening into the divertor for flux surfaces could lead to variations in plasma-neutral pressure balance as the detachment front reaches that region, exactly how this affects the front movement needs further investigation.

A robust Freeman-Hill-inspired pulse protocol for ringdown-free T1 relaxation measurements

A new difference-spectroscopy method is introduced for measuring T1 relaxation times. It is inspired by the earlier work of Freeman and Hill and eliminates the need for recording signal intensities at thermodynamic equilibrium. The new method is termed SIP-R (Split-Inversion Pulse and Recovery) and reduces the number of refinable parameters in the curve fitting process of relaxation-delay-dependent signal intensities by using two instead of the three parameters typically used in the standard inversion-recovery sequence. The SIP-R method preserves the dynamic range of measurement of the standard inversion-recovery method but converts the rise-to-maximum mathematical functionality of the recorded data into a decay-to-zero functionality. The decay-to-zero functionality renders the SIP-R sequence advantageous for inverse Laplace transformation numerical optimizations. The new technique proves to be extremely robust with respect to pulse imperfections, pulse-power changes during the pulse sequence, pulse-width miscalibrations, resonance offsets, and radiofrequency field variations. It also compensates for acoustic ring-down effects and proves reliable for measurements with inhomogeneously broadened signals up to several kilohertz linewidth. 1H NMR experiments with methane gas at pressures up to 50 atm in toroid-cavity pressure vessel probes and in the presence of the methane-to-methanol conversion catalyst Cu-ZnO/Al2O3 are used to show the usefulness of the new method for relaxation time investigations under pressure, at strong radiofrequency field gradients, and in the presence of paramagnetic materials.

Atomically Local Electric Field Induced Interface Water Reorientation for Alkaline Hydrogen Evolution Reaction.

The slow water dissociation process in alkaline electrolyte severely limits the kinetics of HER. The orientation of H2 O is well known to affect the dissociation process, but H2 O orientation is hard to control because of its random distribution. Herein, an atomically asymmetric local electric field was designed by IrRu dizygotic single-atom sites (IrRu DSACs) to tune the H2 O adsorption configuration and orientation, thus optimizing its dissociation process. The electric field intensity of IrRu DSACs is over 4.00×1010 N/C. The ab initio molecular dynamics simulations combined with in situ Raman spectroscopy analysis on the adsorption behavior of H2 O show that the M-H bond length (M=active site) is shortened at the interface due to the strong local electric field gradient and the optimized water orientation promotes the dissociation process of interfacial water. This work provides a new way to explore the role of single atomic sites in alkaline hydrogen evolution reaction.

Atomically Local Electric Field Induced Interface Water Reorientation for Alkaline Hydrogen Evolution Reaction

AbstractThe slow water dissociation process in alkaline electrolyte severely limits the kinetics of HER. The orientation of H2O is well known to affect the dissociation process, but H2O orientation is hard to control because of its random distribution. Herein, an atomically asymmetric local electric field was designed by IrRu dizygotic single‐atom sites (IrRu DSACs) to tune the H2O adsorption configuration and orientation, thus optimizing its dissociation process. The electric field intensity of IrRu DSACs is over 4.00×1010 N/C. The ab initio molecular dynamics simulations combined with in situ Raman spectroscopy analysis on the adsorption behavior of H2O show that the M−H bond length (M=active site) is shortened at the interface due to the strong local electric field gradient and the optimized water orientation promotes the dissociation process of interfacial water. This work provides a new way to explore the role of single atomic sites in alkaline hydrogen evolution reaction.

External bioelectric control of tissue hydrostatic pressure and ion pumping.

Direct-Current (DC) currents exist throughout the body and regulate cellular responses spanning directed cell migration (electrotaxis) to transcriptional regulation. New approaches using electrical cues to control biological systems are, therefore, quite exciting, and we have previously demonstrated electrically programmable collective migration in epithelia. Here, we demonstrate those same cues reprogramming 3D growth in lumenized tissues such as hollow kidney spheroids and intestinal organoids. We found that electrical stimulation induces a rapid, powerful, and reversible swelling in these tissues, and we discuss how external electrical stimulation can control the inflation of lumenized structures and cellular mechanisms mediating the response. We first studied hollow kidney spheroids within a hydrogel integrated into our SCHEEPDOG electrobioreactor. This allowed 3D, timelapse imaging during electrical stimulation. Hollow spheroids inflated nearly 3X by volume relative to control spheroids within 4 hrs of stimulation. The rate and magnitude of inflation increased with increasing electric field strength, and overly strong field gradients induced ‘popping’. We hypothesized that the dense actomyosin network at the apical surface of the structures provided a physical limit to the inflation response, and showed that inhibiting actomyosin contractility prevented popping and increased the maximum swelling volume. We used ion channel inhibitors to determine that the majority of the response can be attributed to the CFTR ion channel (apical chloride channel) and NKCC (basal sodium/potassium channel). Our computational model relating direct-current fields, electrochemistry, and hydrostatic pressure capture many of these effects, as well as predicting asymmetric ion distributions around spheroids. Interestingly, 3D cross-sections of inflated tissues reveal asymmetric cell shapes on the anode/cathode cysts of the spheroids. We have recently confirmed that mouse intestinal organoids also inflate upon field stimulation in a CFTR-mediated fashion, and are exploring other lumenized tissues to assess the generality of this phenomenon.

Experimental study on levitation control of particles and liquid droplets by vortex acoustic field enhanced by subwavelength pipe

The nonlinear propagation of acoustic waves in a medium generates acoustic radiation force. Using acoustic radiation force, particles and liquid droplets in gases can be levitated and manipulated. Acoustic levitation techniques can manipulate larger objects in the medium without contact, and therefore have been widely used in chemical analysis, droplet dynamics, and bioreactors. The acoustic levitation researches mainly focus on manipulating particles and droplets in an open environment, which provides flexibility in its use. However, this approach has limitations in terms of its efficiency in utilizing acoustic field energy. In this work we propose a concept of subwavelength pipe-enhanced acoustic tweezers, in which the acoustic field is used to manipulate expanded polystyrene particles (EPS) and droplets inside an acoustic pipe with an inner diameter smaller than the wavelength. In this work, we use four small transducers to excite a single low-order mode of a circular waveguide and its simplex state, and we also use the vortex sound field generated inside the waveguide to levitate and manipulate expanded polystyrene particle and droplet in the air. Compared with previous work in an open environment, we significantly enhance the acoustic radiation force by means of the acoustic resonance effect of the subwavelength duct, with both radial and axial suspension force magnitude increasing considerably. Similar concepts of subwavelength optical waveguides and resonant cavities and their effectiveness were already well known and widely used in the field of optics. In this work we first explain theoretically the basis for the design of subwavelength pipe-enhanced acoustic tweezer dimensions. Then, we point out in simulation that the pipe-enhanced acoustic tweezers, compared with the open environment acoustic tweezers, have strong sound field gradient distribution and acoustic radiation force distribution in the pipe. This conclusion is demonstrated experimentally. Finally, the manipulation of droplet and particle levitation and rotation in subwavelength-pipe-enhanced acoustic tweezers is systematically carried out. In this work we introduce the concept of subwavelength acoustic pipe for acoustic manipulation, which is expected to deepen the physical understanding of the interaction between acoustic fields and matter, and to develop new miniaturized acoustic manipulation devices for levitating particles and droplets.

Ultra-compact Archimedes spiral plasmonic lens with a circular groove for low power optical trapping in the far-field region.

Particle levitation is crucial in optical trapping considering contamination and alteration of the character of the particle due to physical contact with the structure. A strong field gradient along the optical axis is required in this case. To manipulate the particle at a distance from the surface, we propose an Archimedes spiral plasmonic lens with a circular groove (CG-ASPL). The optical properties and parameters influencing the trapping performance of CG-ASPL are fully analyzed and discussed. By illuminating the structure with circular polarization and structure optimization, we can reduce the required optical power down to 2.4 mW for trapping particle of 1 µm in diameter with groove width and height of 100 and 125 nm, respectively. The particle can be stably trapped with trapping potential of 4138 kBT/W in the far-field region (1.1λ) owing to constructive interference of the scattered SPP waves. Furthermore, this structure is ultra-compact with a size of about 6.7 µm in diameter. We believe the results demonstrated in this work would be very useful for lab-on-a-chip applications and many others.

Dynamical l-bits and persistent oscillations in Stark many-body localization

Stark many-body localization (MBL) occurs when interacting quantum particles are placed in a strong linear field gradient. Localization of excitations, coherent Bloch oscillations, Hilbert-space fragmentation, and scarring have been observed numerically and experimentally, but their origin is not understood. The authors explain these features analytically by perturbatively constructing an algebraic structure of Stark MBL Hamiltonians they name ``dynamical l-bits''. These structures are exponentially stable in the field gradient strength. Thus, the work has potential implications for quantum information processing.

Spectroscopy of nanoantenna-covered Cu2O : Towards enhancing quadrupole transitions in Rydberg excitons

Plasmonic antennas channel incoming light into a confined near field that comes along with strong field gradients at the edges. Transverse field gradients are capable of driving quadrupole transitions in a more effective way than the weak longitudinal field gradient in dipolar light. Cuprous oxide Rydberg excitons with principal quantum number $n$ between 5 and 15 are several tens to a few hundreds of nanometers in diameter and can have different orbital angular momentum quantum numbers. Usually, only $P$- and $F$-excitons are electric dipole allowed, while $S$- and $D$-excitons remain dark. Here, we use 30-nm-wide and 60- to 110-nm-long plasmonic aluminum nanoantennas deposited on the cuprous oxide crystal surface to create transverse field gradients in similar spatial dimensions as the size of the Rydberg excitons. This way, light field and matter wave function overlap spatially and quadrupole transitions of $S$excitons are enhanced. The relative enhancement of the $S$-exciton oscillator strength with respect to the $P$-exciton oscillator accounts for a factor of up to 1.4.

Advancements in Gradient System Performance for Clinical and Research MRI.

In magnetic resonance imaging (MRI), spatial field gradients are applied along each axis to encode the location of the nuclear spin in the frequency domain. During recent years, the development of new gradient technologies has been focused on the generation of stronger and faster gradient fields for imaging with higher spatial and temporal resolution. This benefits imaging methods, such as brain diffusion and functional MRI, and enables human imaging at ultra-high field MRI. In addition to improving gradient performance, new technologies have been presented to minimize peripheral nerve stimulation and gradient-related acoustic noise, both generated by the rapid switching of strong gradient fields. This review will provide a general background on the gradient system and update on the state-of-the-art gradient technology. EVIDENCE LEVEL: 5 TECHNICAL EFFICACY: Stage 1.

NMR Relaxometry Using Outer Field of Single-Sided HTS Bulk Magnet Activated by Pulsed Field

We have succeeded in getting <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> H NMR signals at 47.8 MHz (1.12 T) from a single-sided HTS bulk magnet activated by pulse-field magnetization. Diffusion NMR using an inhomogeneous outer field was also demonstrated on the surface. This kind of NMR relaxometry can not get the information of molecular structures, which conventional NMR using a homogeneous magnetic field can acquire. However, the relaxation of nuclear spin magnetization is still observable. A higher magnetic field gradient generated by the small HTS bulk magnet, 67 T/m at a 5 mm distance from the surface, can be effectively used to observe the self-diffusion coefficient of liquids in higher spatial resolution, 44 μm. The easy accessibility to the strong field gradient is one of the unique properties ofcompact HTS bulk magnets. We believe that single-sided (unilateral) diffusion NMR will become the new application of HTS bulk magnets.

Temperature-Assisted Piezoresponse Force Microscopy: Probing Local Temperature-Induced Phase Transitions in Ferroics

The combination of local heating and biasing at the tip-surface junction in temperature-assisted piezoresponse force microscopy (TPFM) opens a pathway for probing local temperature-induced phase transitions in ferroics, exploring the temperature dependence of polarization dynamics in ferroelectrics and potentially discovering coupled phenomena driven by strong temperature and electric field gradients. Here, we analyze the signal-formation mechanism in TPFM and explore the interplay between thermal- and bias-induced switching in model ferroelectric materials. We further explore the contributions of the flexoelectric and thermopolarization effects to the local electromechanical response and demonstrate that the latter can be significant for ``soft'' ferroelectrics. These results establish a framework for the quantitative interpretation of TPFM observations, predict the emergence of nontrivial switching and relaxation phenomena driven by nonlocal thermal-gradient-induced polarization switching, and open a pathway for exploring the physics of thermopolarization effects in various noncentrosymmetric and centrosymmetric materials.

High-Precision Control Method for High-Power MRI Gradient Power Amplifiers

In magnetic resonance imaging (MRI), high-power gradient power amplifiers (GPAs) are required to drive the gradient coils to generate strong and high-fidelity gradient fields. High precision is an essential requirement for the GPAs since precision directly impacts MRI imaging quality. Various aspects of the GPA have been the subject of research in the past years; however, high-precision GPA control that meets the stringent requirements of MRI applications is still a challenge. This article proposes a novel multi-rational-delay variables state space control method and an efficient out-of-band signal injection method to achieve GPA control accuracy at the level of one part per million at MVA output power levels. First, a systematic modeling and design method of the state space controller for high-power GPA is introduced utilizing state vectors with multi-rational-delay variables. This method improves the GPA dynamic performance significantly. Second, an efficient out-of-band signal injection method is presented to further improve the control precision at low output current, enabling the fulfillment of the challenging high-precision MRI requirements over the full GPA output range. Finally, the high-precision control method is validated in a GPA demonstrator with 500 A/1000 V output. Key results are a 30% improvement in current pulse reproducibility with respect to the conventional control method and a factor of 2.5 less noise at low currents. These experimental results validate the proposed novel method for the high-precision control of GPAs in MRI applications and prove its capability to contribute to significantly improved MRI image quality.

Magnetic buoyancy instability and the anelastic approximation: regime of validity and relationship with compressible and Boussinesq descriptions

Magnetic buoyancy instability, which is of astrophysical importance, results from the influence of magnetic pressure variations on the density of a fluid in a gravitational field. It is inherently a compressible phenomenon and is, as such, fully described by the equations of compressible magnetohydrodynamics (MHD). For analytical and computational reasons, it is often convenient to study compressible MHD within simpler, asymptotically consistent reduced systems; the two most widely used result from the Boussinesq and anelastic approximations. Within the standard Boussinesq approximation of MHD, leading to the equations of Boussinesq magnetoconvection, magnetic buoyancy is excluded. It can, however, be included by a rescaling of the basic-state variables and by making further assumptions about the perturbation length scales. Within the anelastic approximation, no special measures are taken to incorporate magnetic buoyancy. It is, however, a priori unclear as to whether this neglect is justified, particularly in the light of the Boussinesq results. Our aims here are thus twofold. The first is to formulate the relationship between descriptions of magnetic buoyancy in the compressible, anelastic and Boussinesq systems. In so doing, we show that, under both the anelastic and Boussinesq approximations, magnetic buoyancy can be included either through a combination of a weak field and strong gradient, or, conversely, a strong field and weak gradient. Each has its own asymptotically consistent reduction, with dedicated governing equations. Our second aim is to address, through a linear stability analysis, under which conditions the standard anelastic system provides a faithful representation of magnetic buoyancy instability. For completeness, we also formulate the energy principle of ideal MHD within the anelastic framework, and demonstrate the relation with its fully compressible counterpart.

Analysis of flexochemical effect and its application in scanning probe microscopy

Electrochemical processes in solids are affected by the properties of various interfaces, where the flexoelectric effect manifests itself considerably due to the inevitable strong gradient fields. Thus, it is crucial to study the coupling between the electrochemical process and the flexoelectric effect. Based on the continuum theory, we conducted the finite element implementation for the flexochemical effect, being the coupling between flexoelectricity, Vegard effect and chemical reactions. Then, the developed method is employed to investigate the flexochemical effect arising in scanning probe microscopy (SPM), including evaluating the contributions from the flexoelectric effect and Vegard effect to the electromechanical response on material SrTiO3 (STO) in piezoresponse force microscopy (PFM) as well as to mechanical redistribution of oxygen vacancy in STO. It is found that at room temperature the nanoscale electromechanical response of the undoped STO in PFM imaging is mainly induced by the converse flexoelectricity while the contribution of direct Vegard effect is negligible. Furthermore, the contact force exerted by SPM tip in manipulating the redistribution of oxygen vacancies is multifunctional, including diminishing vacancies underneath the contact area and enriching the regions around the tip-surface contact edge and inside the sample below the tip, resulting from the synergy of the converse Vegard effect and the direct flexoelectricity. These analyses explain some experimental observations well. This paper provides a continuum framework for the analysis of electrochemomechanical systems with the flexoelectric effect.

Plasmonic nanostructures for shrinking structured light to access forbidden transitions

Abstract Plasmonic nanostructures have considerable applicability in light–matter interactions owing to their capacity for strong field confinement and enhancement. Nanogap structures allow us to tailor electric field distributions at the nanoscale, bridging the differences in size and shape of atomic and light structures. In this study, we demonstrated that a plasmonic tetramer structure can squeeze structured light into a nanoscale area, in which a strong field gradient allows access to forbidden transitions. Numerical simulations showed that the gold tetramer structure on a glass substrate possesses a plasmonic eigenmode, which forms structured light with a quadrupole profile in the nanogap region at the center of the tetramer. The top–down technique employed using electron-beam lithography allows us to produce a gap size of approximately 50 nm, which supports plasmonic resonance in the near-infrared regime. In addition, we demonstrated an array architecture in which a collective lattice resonance enhances the intensity of the quadrupole field in multiple lattice units. This study highlights the possibility of accessing multipolar transitions in a combined system of structured light and plasmonic nanostructures. Our findings may lead to new platforms for spectroscopy, sensing, and light sources that take advantage of the full electronic spectrum of an emitter.

Ignition and propagation of nanosecond pulsed plasmas in distilled water—Negative vs positive polarity applied to a pin electrode

Nanosecond plasmas in liquids are being used for water treatment, electrolysis, or biomedical applications. The exact nature of these very dynamic plasmas and, most importantly, their ignition physics are strongly debated. The ignition itself may be explained by two competing hypotheses: ignition in water may occur (i) via field effects at the tip of the electrode followed by tunneling of electrons in between water molecules causing field ionization or (ii) via gaseous processes of electron multiplication in nanovoids that are created from liquid ruptures due to the strong electric field gradients. Both hypotheses are supported by theory, but experimental data are very sparse due to the difficulty in monitoring the very fast processes in space and time. In this paper, we analyze nanosecond plasmas in water that are created by applying a positive and a negative polarity to a sharp tungsten electrode. The main diagnostics are fast camera measurements and fast emission spectroscopy. It is shown that plasma ignition is dominated by field effects at the electrode–liquid interface either as field ionization for positive polarity or as field emission for negative polarity. This leads to a hot tungsten surface at a temperature of 7000 K for positive polarity, whereas the surface temperature is much lower for negative polarity. At ignition, the electron density reaches 4×1025 m−3 for the positive and 2×1025 m−3 for the negative polarity. At the same time, the emission of the Hα light for the positive polarity is four times higher than that for the negative polarity. During plasma propagation, the electron densities are almost identical of the order of 1–2×1025 m−3 followed by a decay after the end of the pulse over 15 ns. It is concluded that plasma propagation is governed by field effects in a low density region that is created either by nanovoids or by density fluctuations in supercritical water surrounding the electrode that is created by the pressure and temperature at the moment of plasma ignition.