Field Amplitude Research Articles

Acoustically-switchable nonlinear frequency conversion in gas-filled hollow-core photonic crystal fibers

We found that an acoustically-induced perturbation can support a different type of quasi-phase matching for nonlinear frequency conversions in gas-filled hollow-core photonic crystal fibers. A gas pressure grating, a periodic spatial modulation of gas pressure, which is reversibly constructed by the acoustically-induced perturbation, compensates a nonzero phase mismatch and enhances the nonlinear optical process. The field amplitude oscillates about the averaged field amplitude which lineally grows with distance. While group velocity walk-off is suppressed by manipulating the gas pressure inside the fiber, a nonzero wavevector mismatch is inevitable, which can be compensated by the acoustically-induced perturbation. The frequency conversion is switchable by turning on/off the acoustically-induced perturbation. We exemplarily consider third-harmonic generation in gas-filled hollow-core photonic crystal fibers, however the conception of acoustically-induced quasi-phase matching is applicable to other kinds of nonlinear optical processes in gas-filled structures such including four-wave mixing and sum frequency generation and it would provide a new route for nonlinear optics and acoustooptics.

Generation of structured light beams by dual phase modulation with a single spatial light modulator

Precise control of amplitude and wavefront of optical fields are prerequisites for many applications, especially in singular optics. This has led to the increasing efforts for developing efficient techniques to control the shape of the light in different dimensions. A spatial light modulator (SLM) can be efficiently used for phase-only or amplitude-only modulation; but offers limitation in complex light field modulation. Hence, shaping the complex amplitude of optical beams is challenging mainly because there are no complex modulators. While there is ongoing research to develop complex amplitude modulating SLMs, a solution is still non-existent. In this study, to achieve complex light modulations, a simple experimental set-up employing single phase-only SLM has been proposed. The SLM has been used as operating in a split-screen-mode. The non-iterative approach of dual-pass modulation has been applied where two cascaded phase value distributions (PVD) are encoded side-by-side onto the SLM. The first PVD is designed to enable amplitude modulation in the second PVD plane which finally helps achieve wavefront shaping. Hence, both amplitude and phase modulation of light beam are possible in this configuration. Commonly known singular beams such as Laguerre–Gaussian and Bessel-Gaussian modes have been generated theoretically as well as experimentally to verify the feasibility of the proposed technique. The method used helps to achieve arbitrary shaped beams as well.

Incoherent Shallow-Water Seabed Logging Using Numerical-Model-Based Optimization for the Prediction of Unperturbed UEP Signatures

Underwater electric potential (UEP) ship signatures are evaluated at a test site that provides only incoherent data from electric field sensors placed on the seabed. This paper shows how these data sets can be used to extract significant seabed parameters, which are evaluated by a numerical model combined with an optimization process in the frequency range from 1 Hz up to 100 kHz. This is in contrast to the controlled electromagnetic source technology used by geophysicists, which has to track both phase and amplitude, and, therefore, cannot simply be applied to data from per se stationary systems. Using the aforementioned frequency range within COMSOL Multiphysics (v6.0), the finite-element-based simulation software, a sensitivity analysis of seabed parameter behavior within a seabed depth range from 1 m to 20 m is evaluated for the highest sensitivity for each of the measurable field amplitudes of the locally available sensors. From this, the electrical conductivities of two seabed layers and the depth of the first layer can be extracted using a combination of a genetic algorithm and the Nelder–Mead method. In summary, phase information is not required to characterize the stratified seabed, providing a cost-effective approach to obtaining effective environmental conditions for free-water signature calculations.

Magnetic field sensor based on a low-frequency-tail spintronic diode

Spin-torque diode have shown great potentials and performance in many applicative fields, from microwave detectors to energy harvesters. In this work, we use micromagnetic simulations to study, at room temperature, a state-of-the-art non-resonant low-frequency-tail spin-torque diode in terms of dc output voltage as a function of the amplitude of an in-plane external field applied along different directions. We find that there exists a threshold value of the injected ac current that promotes a linear behavior of the output voltage of field down to the pT range, and we suggest exploiting such a behavior for the design of a magnetic field sensor.

Predicting the phase distribution during multi-channel transcranial alternating current stimulation in silico and in vivo

BackgroundTranscranial alternating current stimulation (tACS) is a widely used noninvasive brain stimulation (NIBS) technique to affect neural activity. TACS experiments have been coupled with computational simulations to predict the electromagnetic fields within the brain. However, existing simulations are focused on the magnitude of the field. As the possibility of inducing the phase gradient in the brain using multiple tACS electrodes arises, a simulation framework is necessary to investigate and predict the phase gradient of electric fields during multi-channel tACS. ObjectiveHere, we develop such a framework for phasor simulation using phasor algebra and evaluate its accuracy using in vivo recordings in monkeys. MethodsWe extract the phase and amplitude of electric fields from intracranial recordings in two monkeys during multi-channel tACS and compare them to those calculated by phasor analysis using finite element models. ResultsOur findings demonstrate that simulated phases correspond well to measured phases (r = 0.9). Further, we systematically evaluated the impact of accurate electrode placement on modeling and data agreement. Finally, our framework can predict the amplitude distribution in measurements given calibrated tissues’ conductivity. ConclusionsOur validated general framework for simulating multi-phase, multi-electrode tACS provides a streamlined tool for principled planning of multi-channel tACS experiments.

Study of microwave quantum electrometric sensors via electromagnetically induced transparency in thermal Rydberg atoms

Abstract Quantum electrometric sensing properties of a four-level system are theoretically investigated for microwave (MW) field measurement in thermal Rydberg atoms. In the considered system, the phenomenon of electromagnetically induced transparency (EIT) is invoked to study the impact of microwave (MW) field strength on its optical response. The amplitude of the MW electric field is obtained directly by measuring the height and frequency of EIT peaks. Further, for MW frequency measurement, the probe spectra are examined for red and blue MW detuned cases. It is found that the responsiveness of 133Cs for MW electric field as well as frequency is relatively higher than 87Rb. The overall sensitivity of both these atoms can be enhanced by the convenient setting of the system and field parameters.

Heating efficiency of Gd- and Co-doped γ-Fe2O3 nanoparticles measured by AC magnetometer for magnetic-mediated hyperthermia

Most research groups, including us, utilize calorimetric methods to determine the heat dissipation by magnetic nanoparticles (MNPs) under an alternating magnetic field (AMF). Herein, we report the heating efficiencies of γ-Fe2O3 and doped γ-Fe2O3 NPs using AC magnetometry, which allows us to directly calculate the AC hysteresis loop area from which the heating abilities can be deduced. First, all NPs were prepared and thoroughly characterized both structurally (XRD, Rietveld, and TEM) and magnetically (DC and AC magnetization measurements). Structural analysis indicated the phase purity (γ-Fe2O3) and crystallite sizes (∼10 nm) of the as-prepared γ-Fe2O3 NPs. Both DC and AC measurements indicated the superparamagnetic behavior for γ-Fe2O3 and Gd-doped γ-Fe2O3(Gd-5%) NPs, while Co-doped γ-Fe2O3(Co-5%) NPs exhibited ferrimagnetic nature. The heating abilities and specific absorption rate (SAR) values were then analyzed at frequency, f = 132 kHz and several AC field amplitudes (µ0HAC) ranging from 0 to 88 mT. From AC magnetometry calculations, the SAR values were found to be 20 W/g and 17 W/g for γ-Fe2O3 and γ-Fe2O3(Gd-5%) NPs, respectively, while that of γ-Fe2O3(Co-5%) NPs reached SAR of 120 W/g, almost 6 times higher. This high heating efficiency of γ-Fe2O3(Co-5%) sample is attributed to their higher effective anisotropy and saturation magnetization where the heat release is mainly dominated by Neel relaxation. Finally, a viability assay against metastatic breast cancer cells was conducted, indicating the biocompatibility and low toxicity of the as-synthesized γ-Fe2O3 and doped γ-Fe2O3 NPs. These results strongly suggest the promising utilization of γ-Fe2O3 NPs, particularly Co-doped, as a potential candidate for magnetic-mediated hyperthermia.

Farklı malzeme özelliklerine sahip gömülü nesnelerin yer radarı (GPR) yöntemiyle tespit edilmesi

GPR, which permits the capture of high-resolution subterranean data, has developed into a key geophysical technique for determining the depth, geometry, boundaries, and volumes of buried shallow objects. In this study, the detectability of the location, size, and physical property parameters of buried objects with the ground radar method was revealed by creating a laboratory environment in real field conditions. For this purpose, a realistic laboratory environment with a depth and length of 5 m was created on the filled soil material at the test site, and buried objects with different material properties were placed. In addition, by adding sand material to the middle part of the soil fill material in the test area, its situation in different layers was tried to be examined. GPR data were collected on the models using the RAMAC CU II system and a 500 MHz center frequency shield antenna. After processing the data, reflected/scattered electromagnetic (EM) wave fields on the radargram of a profile perpendicular to the buried objects were examined. In this way, the positions, sizes, physical properties (types) of buried objects along with their depths and their situations in different layer environments have been revealed. According to the results, the peak width of hyperbolas and the sizes of buried objects were determined on the processed radargrams. The types of buried objects and their situations in different layer environments are clearly revealed. The scattered wave field amplitudes of the plastic pipe (A and C regions) from the reflection coefficients are substantially lower than the scattered wave field amplitudes of the iron pipe (B region) from the buried objects. It is thought that the strong reflections extending from the C region to the deep on the radar are caused by the lead blocks in the plastic pipe.

Effect of a dynamic axial magnetic field on a preconditioned single-wire Z-pinch

In this study, the effect and mechanism of a dynamic axial magnetic field on a preconditioned single-wire Z-pinch were investigated experimentally and theoretically. Optical diagnostic methods, including shadowgraphy, interferometry, Faraday rotation, and Thomson scattering, have been used to measure the parameters of magnetized plasmas. Compression of the azimuthal and axial magnetic fields was observed, and the suppression of the plasma instability was recorded and analyzed. The results showed that an external axial magnetic field could reduce the plasma instability and non-uniformity, but prolong the implosion time and weaken the compression ratio. In the implosion process with an axial magnetic field, the plasma rotated at a speed similar to that of imploding, which could be regarded as a stabilization method. A simplified model of the diffusion and compression processes of a dynamic axial magnetic field was developed to investigate the conditions for maximizing the amplitude of the axial magnetic field. Subsequently, the snowplow model was used to calculate the effect of axial magnetic fields on the implosion process and energy conversion.

Influence of plasma inhomogeneity and ohmic heating on the nonlinear absorption of intense laser pulse in collisional magnetized plasma

AbstractThe nonlinear absorption in the interaction of an intense laser pulse with a collisional magnetized plasma is studied by considering the effects of plasma inhomogeneity, ohmic heating, and ponderomotive force. For this purpose, we obtain the plasma electron density, the effective dielectric permittivity, and the wave equation using the Maxwell and hydrodynamic equations and solve this equation by the Runge–Kutta numerical method. The results are shown that by increasing the plasma inhomogeneity, the inverse bremsstrahlung absorption coefficient is increased, and also the density ramp parameter and its sign can affect more the absorption coefficient than the temperature ramp parameter . However, when the initial electron density and temperature increase, the laser field amplitude and the absorption coefficient are increased and the spatial damping rate of the laser pulse becomes highly peaked inside the plasma. It is shown by increasing laser pulse energy, the nonlinear bremsstrahlung absorption coefficient is decreased significantly. The results also indicate that by increasing the external magnetic field, the dielectric permittivity is decreased while the laser energy spatial damping, and the absorption coefficient are increased. Moreover, it is found that by considering the ohmic heating effect, the electrons absorb further energy from the fields and consequently the nonlinear absorption is increased.

High-quality tunable optical vortex arrays with multiple states of orbital angular momentum

We demonstrate the generation of high-quality optical vortex arrays (OVAs) with multiple states of orbital angular momentum (OAM) by uploading computer-generated holograms that are specially designed onto a liquid crystal spatial light modulator. The method of encoding both the amplitude and phase of an optical field into a phase-only holograms enables precise control of spatial transverse vortex modes. By combining the spiral, checkerboard and blazed-grating phase, the zero-order diffracted light can be efficiently suppressed while energy is redistributed onto higher orders to improve the quality of vortex array. Furthermore, the intensity distribution and spatial position can be flexibly modulated as well as OAM state of each vortex laser beam. It significantly increases the versatility of OAM, broadening its application in high-capacity optical communication and manipulation of multiple particles.

A self-loading suction cup driven by a resonant dielectric elastomer actuator

Suction cups are widely utilized in industries and robotics fields for object manipulation and robotic positioning. Conventional vacuum pumps can remove fluid from the suction cup continuously, enabling reliable adhesion. However, the bulky and rigid nature limits their integration with soft robotics. On the other hand, suction cups driven by soft smart materials offer better integration with soft robots but face challenges in achieving continuous fluid removal, resulting in potential suction failures in case of seal breaks. Aiming to address this limitation, a novel self-loading suction cup driven by a resonant dielectric elastomer actuator is proposed. This mechanism allows for continuous and efficient removal of the enclosed fluid in the sucker, thereby achieving successful and sustained adhesion. The structure design is presented and its fundamental working principles are revealed through theoretical analysis and experiments. The effects of several key design parameters (i.e., actuation electric field amplitude, moving mass, substrate roughness) on the performance of the suction cup are experimentally characterized to achieve performance optimization. This design demonstrates a maximum net suction force of 24.9 N (12.7 kPa), which is equivalent to 80 times its body weight. The suction cup design holds potential application values in soft robots, surveillance and environmental monitoring.

Inflation and electroweak symmetry breaking

We present a model of Cosmological Electroweak Symmetry Breaking (CEWSB), where a Higgs-like field and a cosmological background of weak boson gauge fields interact with gravity to realize the epoch of cosmic inflation, which is then followed by a Higgs resonance preheating. As a result, the scale of electroweak symmetry breaking is linked with the end of inflation. The theory is equipped with a shift symmetry that can protect the Higgs mass, and it has close semblance to natural inflation and its variants. As the Higgs field's amplitude decays at the end of inflation, its mass emerges. The model has a built in Higgs self-resonance preheating mechanism which leads to the possible emergence of the cosmic microwave background (CMB) due to resonant Higgs, quark and lepton production after inflation. We provide a pathway to implement a similar mechanism with the realistic Higgs-doublet of the standard electroweak theory and discuss phenomenological considerations.

Precession of magnetic moments with change of their orientations

In the work the precession of the magnetization vector in nano-sized magnets is induced by a femtosecond laser pulse, the frequency of which lies in the transparency region of the magnet material. It is shown that an alternating magnetic field applied to the XOY plane generates the magnetic equilibrium position vectors and elliptical precession occurs if a constant magnetic field acts along the Z axis. In this case the precession angle changes periodically in time and the precession frequency is determined by the amplitude of the total magnetic field.

Influence of External Magnetic Field on 3D Thermocapillary Convective Flow in Various Thin Annular Pools Filled with Silicon Melt

Thermocapillary convection flows can have an impact on the homogeneity of floating zone semiconductor crystals. An external magnetic field can also help to reduce this non-homogeneity. The goal of this research is to minimize thermocapillary convection in various thin annular pools filled with silicon melt. A three-dimensional (3D) numerical technique is proposed that employs an implicit finite volume formulation. The steady-state thermocapillary flow in six thin annular pools (R=0.3, 0.4, 0.5, 0.6, 0.7, and 0.8) subjected to an externally induced magnetic field was observed. Under magnetic field influence, the effects of increasing annular gap, R on the hydrothermal wave number and azimuthal pattern are obtained. The results reveal that hydrothermal waves m=14, m=11, m=8, m=6, m=4, and m=3 are observed in steady flow for R=0.3; 0.4; 0.5; 0.6; 0.7, and R=0.8, respectively. The maximum temperature occurs in the intermediate zone between the inner and outer walls when there is no magnetic field. Under a strong enough magnetic field, isothermal lines change form and become concentric circles. As the amplitude of the magnetic field (Ha) grows, the azimuthal velocity and temperature at the free surface reduce, and the asymmetric 3D flow becomes axisymmetric steady when Ha surpasses a threshold value.

Engineered asymmetric diffractions of diagonal-line odd-symmetric phase gratings

A two-dimensional multi-element phase grating has been designed in terms of the offset refractive index to exhibit the spatially odd symmetry (antisymmetry) along one transparent diagonal line with an even number of rectangular elements while leaving other elements in a unit cell opaque. This grating can be engineered to attain a few intriguing phenomena of asymmetric diffraction, including the elimination of equally spaced oblique diffraction lines, the elimination of alternately crossed oblique diffraction lines, and the selection of equally spaced oblique diffraction lines. These phenomena of engineered asymmetric diffraction are well explained via destructive interference between transmitted field amplitudes from paired, dual-paired, and successive elements along the transparent diagonal line.

PONDEROMOTIVE FORCES IN THE INTERACTION OF A CHARGED PARTICLE WITH AN INHOMOGENEOUS ELECTROMAGNETIC WAVE

In the article, based on the analysis of the features of the trajectory of a charged particle in an inhomogeneous electromagnetic wave, the physical processes that determine the nature of pondermotive forces are specified. In particular, the reason for changing the accelerated motion of a particle when passing through zero of the electric field amplitude in the direction perpendicular to the electric field to deceleration with increasing electric field amplitude is detailed. The influence of the inertial mass on the trajectory of a charged particle in an inhomogeneous electromagnetic wave is determined.

Electric field and temperature dependent ferroelectric behavior of Bi0.5Na0.5TiO3–Bi0.5K0·5TiO3–BiMg0.5Ti0.5O3 piezoceramics

The ferroelectric (FE) hysteresis behavior of solid state synthesized 72.5Bi0.5Na0.5TiO3-22.5Bi0.5K0.5TiO3–5BiMg0.5Ti0.5O3 ceramics were studied at different electric field amplitudes (E0) and temperatures (T). The three-stage dynamic behavior was identified on the basis of hysteresis shape in different E0 regions (stage I: 0 <E0 < 8.7 kV/cm, stage II: 8.7 <E0 < 39.2 kV/cm, stage III: 39.2 <E0 < 68 kV/cm). The variation in electro-strain (S) with E0 also support the same and indicates a correlation between S and polarization (P), which was further confirmed by dS/dP vs E0 and S vs P2 plot. The T dependent FE and electro-strain loops was investigated at 55 kV/cm in 25 °C–180 °C. On the basis of its hysteresis shape, the FE loops were grouped into three T regimes, i.e., (i) 25 °C–70 °C (ii) 80 °C–120 °C and (iii) 130 °C–180 °C.

Numerical study for the impact of current sharing effect upon dynamic behaviour of DC-carrying HTS coils under alternating magnetic fields

Numerical characterization for the dynamic behaviour of DC-carrying HTS coils subjected to alternating magnetic fields is crucial for its design and protection. In conventional approaches, only superconducting layers are modeled. The missing current sharing effect in those models results in the overestimation of dynamic voltage/resistance. In reality, the transport current will migrate into the metal layers, and therefore the dynamic voltage/resistance is limited. In this paper, we present an equivalent modeling approach (EPM) instead of modeling the real geometry which will cost massive computation resources. The effectiveness of the approach is validated against a full-scale model containing all the layers. Then, this approach is applied to estimate the dynamic behaviour of a multiturn racetrack HTS coil. It is found that the current sharing effect mainly happens at the outer layers of the coil due to the shielding effect, and is enhanced with the increase of external field amplitude then gradually spreads to the inner layers. Moreover, the competition between the transport current and the induced current in the metal layers is observed.

(Invited) Towards a Mechanistic Understanding of the Oxygen Evolution Reaction on Hematite: Operando Characterization of Band Bending and Interfacial Chemical Speciation Using Sum Frequency Generation Spectroscopy

Hematite is an attractive photoanode for the oxygen evolution reaction (OER) due to its cheap price, relatively small band gap, stability in alkaline solution and a valence band energy suitable for OER catalysis. However, insight into the OER mechanism, and thus the capability to rationally modify this material to optimize performance, is limited by the challenge in relating band bending, surface state population and chemical reactivity operando. Conventionally these issues are addressed by some combination of electrochemical impedance spectroscopy (EIS), photocurrent/voltage kinetics, and linear optical spectroscopies in the visible. While important, all of these approaches suffer from significant limitations: e.g. EIS and related techniques integrate over all charge transfer processes in the device while only those at the hematite/water interface are of interest (and thus require a challenging to validate model to quantify) and UV/Vis absorption measurements (whether steady-state or time resolved) are bulk-sensitive and probe spectrally broad features that are challenging to relate to surface structure.Sum Frequency Generation (SFG) spectroscopy is interface-specific by its symmetry selection rules, can straightforwardly probe interfacial chemical speciation if one of the two incident fields is in the infrared, and is sensitive to the amplitude of interfacial electric fields. These attributes make the technique particularly suitable for characterization of interfacial chemical speciation, double layer and space charge layer structure at the working electrode/electrolyte interface within thin-film spectroelectrochemical cells. Here I describe our recent efforts to characterize chemical speciation and interfacial field at a hematite/electrolyte interface, in the presence and absence of external illumination, as one moves from the Hematite flat band potential through the onset of the OER. Using vibrationally resonant SFG spectroscopy, at infrared wavelengths > 10 micron, we quantify the population of surface holes, i.e. Fe(IV)=0 groups, as the OER turns on. Concurrently we quantify the development of the space charge potential using non-resonant SFG. Recent work has argued, largely from current or optical transients in the visible under chopped illumination, that the OER on Hematite proceeds by a multi-hole mechanism. Because our approach allows for the simultaneous quantification of interfacial field and chemical speciation we directly address this mechanism and the circumstances under which it is expected to apply. More generally, the role of interfacial field in oxidation catalysis has been an object of much current interest. The approach described here – in which interfacial speciation and field can be quantified operando – offers an ability to explore this relationship not possible via any other combination of experimental approaches.