Permittivity Gradient Research Articles

Shadow Mask Molecular Beam Epitaxy for In‐Plane Gradient Permittivity Materials

AbstractInfrared spectroscopy currently requires the use of bulky, expensive, and/or fragile spectrometers. For gas sensing, environmental monitoring, or other applications, an inexpensive, compact, robust on‐chip spectrometer is needed. One way to achieve this is through gradient permittivity materials, in which the material permittivity changes as a function of position in the plane. Here, synthesis of infrared gradient permittivity materials is demonstrated using shadow mask molecular beam epitaxy. The permittivity of the material changes as a function of position in the lateral direction, confining varying wavelengths of infrared light at varying horizontal locations. An electric field enhancement corresponding to wavenumbers ranging from ≈650 to 900 cm−1 over an in‐plane width of ≈13 µm on the flat mesa of the sample is shown. An electric field enhancement corresponding to wavenumbers ranging from ≈900 to 1250 cm−1 over an in‐plane width of ≈13 µm on the slope of the sample is also shown. These two different regions of electric field enhancement develop on two opposite sides of the material. This demonstration of a scalable method of creating in‐plane gradient permittivity material can be leveraged for the creation of a variety of miniature infrared devices, such as an ultracompact spectrometer.

In-depth experimental search for a coupling between gravity and electromagnetism with steady fields

Any means to control gravity like electromagnetism is currently out of reach by many orders of magnitude even under extreme laboratory conditions. Some often poorly executed experiments or pseudoscience theories appear from time to time claiming for example anomalous forces from capacitors that suggest a connection between the two fields. We developed novel and high resolution horizontal-, vertical- and rotation-balances that allow to test electric devices completely shielded and remotely controlled under high vacuum conditions to perform the first in-depth search for such a coupling using steady fields. Our testing included a variety of capacitors of different shapes and compositions as well as for the first-time solenoids and tunneling currents from Zener diodes and varistors. A comprehensive coupling-scheme table was used to test almost all combinations including capacitors and solenoids with permittivity and permeability gradients as well as capacitors and varistors within crossed magnetic fields. We also tested a crossed-coil producing helical magnetic field lines as well as interactions between a pair of shielded toroidal coils to look for proposed extensions to Maxwell’s equations. No anomalous forces or torques down to the nano-Newton or nano-Newton-Meter range were found providing new limits many orders of magnitude below previous assessments ruling out claims or theories and providing a basis for future research on the topic.

Thermoelectrohydrodynamic convection in a finite cylindrical annulus under microgravity

Numerical simulations of thermoelectrohydrodynamic convection in a dielectric liquid inside a finite-length cylindrical annulus with a fixed temperature difference have been performed with increasing high-frequency electric tension under microgravity conditions. The electric field, coupled with the permittivity gradient, generates a dielectrophoretic buoyancy force whose non-conservative part can induce thermoelectric convection in the liquid. The liquid remains in a conductive state below a critical value of the applied electric voltage. At a critical value, a supercritical bifurcation occurs from the conductive state to a convective state made of stationary helicoidal vortices. A further increase of electric voltage leads to oscillatory helicoidal vortices and then to wavy patterns before spoke patterns dominate the convective flow. The dielectrophoretic force is shown to enhance the heat transfer from the hot to cold walls due to induced convective flows. Particularly, these results demonstrate that the dielectrophoretic buoyancy force holds promise to replace the gravitational force to induce efficient heat transfer in microgravity conditions, and they contribute to a better fundamental understanding of heat transfer in microgravity.

A permittivity imaging method in the polar coordinates: Electrical capacitance tomography based on the electromagnetic momentum

Electrical capacitance tomography (ECT) is widely used in petroleum, chemical, and other industrial detection. The ECT equations are nonlinear, and their corresponding inverse problems are ill-posed, so the imaging resolution is low. The ECT technique detects scalar data. The permittivity imaging method based on the electromagnetic momentum reciprocity theorem detects vector data, which carries more information than scalar data. To improve the ability of the imaging system to recognise permittivity boundaries, we propose a permittivity imaging method in the polar coordinate system, which is centred on electrical capacitance tomography based on the electromagnetic momentum (ECT-EMM). ECT-EMM principle with the electrodes-moving scheme and the object-moving scheme is proposed. We reconstruct the permittivity gradient from the capacitance gradient. We adopt the object-moving scheme and use the Tikhonov regularisation algorithm to reconstruct the permittivity through the permittivity gradient. The image reconstruction results and image evaluation metrics show that the proposed method clearly recognises the object boundaries compared to ECT. The proposed method performs better when using threshold filtering.

Dielectrophoretically-Assisted Electrohydrodynamic-Driven Liquid Film Flow Boiling in the Presence and Absence of Gravity

Abstract The ongoing development of modern electronic systems leads to smaller, more powerful devices that are expected to operate in complex environments. Due to this, advanced thermal management technologies are required to meet the growing demand, especially in space where two-phase thermal systems are limited by the absence of gravity. Electrohydrodynamic (EHD) and dielectrophoretic (DEP) forces can be used to sustain stable liquid film flow boiling in the absence of gravity, which is otherwise impractical due to the lack of a required buoyancy force to initiate bubble departure. EHD is a phenomenon that is represented by the interaction between electric fields and fluid flow. The DEP force is characterized by its ability to act on liquid/vapor interfaces due to a high gradient of electrical permittivity. This study investigates the heat transfer characteristics of EHD conduction pumping driven liquid film flow boiling coupled with DEP vapor extraction during a microgravity parabolic flight and on the ground. The results of this study show that EHD and DEP raise the critical heat flux, lower heater surface temperature, and successfully sustain boiling in both microgravity and on the ground with low power consumption. Additionally, the heat transfer data captured in terrestrial, microgravity, and 1.8 g conditions compare well, indicating that combining these mechanisms can provide thermal enhancement independent of gravity. This study provides fundamental understanding of electrically driven liquid film flow boiling in the presence of phase change, paving the way toward developing next-generation heat transport devices for space and terrestrial applications.

Ionic Signal Amplification of DNA in a Nanopore (Small Methods 11/2022)

Back Cover In article number 2200761, Tsutsui, Kawai, and co-workers demonstrated ionic signal amplifications of single-molecule DNA in a nanopore by applying a liquid permittivity gradient across membranes. This simple method can boost the ability of nanopore sensing to readout genome sequences at the single-molecule level.

Ionic Signal Amplification of DNA in a Nanopore.

Ionic signal amplification is a key challenge for single-molecule analyses by solid-state nanopore sensing. Here, a permittivity gradient approach for amplifying ionic blockade characteristics of DNA in a nanofluidic channel is reported. The transmembrane ionic current response is found to change substantially through modifying the liquid permittivity at one side of a pore with an organic solvent. Imposing positive liquid permittivity gradients with respect to the direction of DNA electrophoresis, this study observes the resistive ionic signals to become larger due to the varying contributions of molecular counterions. On the contrary, negative gradients render adverse effects causing conductive ionic current pulses upon polynucleotide translocations. Most importantly, both the positive and negative gradients are demonstrated to be capable of amplifying the ionic signals by an order of magnitude with a 1.3-fold difference in the transmembrane liquid dielectric constants. This phenomenon allows a novel way to enhance the single-molecule sensitivity of nanopore sensing that may be useful in analyzing secondary structures and genome sequence of DNA by ionic current measurements.

A 3‐D ‐printed Luneburg lens antenna with consistent multibeams based on quasi‐pyramid structure

A wide-scanning dual-polarized Luneburg lens antenna based on the quasi-pyramid structure is proposed. To achieve beam consistency, 18 identical quasi-pyramid sections are employed to approximate a spherical lens antenna, and each section is divided into 12 layers. The 6-stepped gradient permittivity index is achieved by the diced ring-type periodic unit cells in accordance with the Effective Medium Theory. Each quasi-pyramid is incorporated with the same connecting structure to form an integrity. And a dielectric rod is penetrated parallelly to the center axis of the lens, thus improving the robustness of the overall structure. The radius of the whole lens is merely 0.88λ0. The spherical Luneberg lens antenna is fabricated and measured, with an impedance bandwidth of 45.5%, a measured peak gain of 15.4/15.1 dBi and a cross-polarization level of higher than 17/17.5 dB. Placing the feed antenna at different locations during the measurement, results attest that those beams possess an excellent consistency. The multibeams are able to achieve an ultra-wide beam coverage of 165°, promising it a serviceable candidate for mobile communication base stations and wireless networking.

A bi-dimensional compressed Luneburg lens antenna for miniaturization based on transformation optics

Transformed Luneburg lens has been widely employed to provide aberration-free imaging and high-gain antenna system, but whose focal plane and beam scanning range decrease correspondingly. In this paper, a two-dimensional compressed elliptical cylindrical Luneburg lens is presented based on transformation optics (TO) to achieve miniaturization and wide-angle beam steering. The Jacobian matrix and the permittivity tensor are calculated after supposing formulas to compress the focal plane, while maintaining the lens’ inherent performance. The gradient permittivity is achieved by two ring-type periodic unit cells on the basis of the Equivalent Medium Theory. The lens is then attached between a pair of parallel metal plates to further improve its gain and lower the side lobe level (SLL). To demonstrate this assumption, a prototype of this Luneburg lens is manufactured by isotropic material and 3D printing technique. The antenna operates at 3.3–5 GHz with a peak gain of 16.1/15.9 dBi. A 2D beam scanning range of ±50° and ± 20° can be implemented by merely five feeds, the side lobe level keeping less than -16.3/-16 dB. Measured results coincide well with theoretical predictions, offering a beneficial transformation mapping to both microwaves and optics.

Improved convergence in planar nanophotonic topology optimization via the multigradient

Adjoint Topology Optimization is a physics-informed inverse-design strategy which has been widely applied in electromagnetics and nanophotonics to create highly performant meta-devices. By interfering the electric fields of both forward and reverse (i.e., adjoint) simulations within a device, it is possible to compute a permittivity gradient which will improve its performance. In the specific case of planar metasurface design, the permittivity gradient will vary depending on the z-height, which presents a problem for designing devices suitable for single-layer lithographic fabrication, since such products have uniform material properties in the z-direction. State-of-the-art applications of adjoint topology optimization address this issue by averaging together the gradients at multiple z-heights. This article proposes an alternative strategy for combining these same gradients based on the multigradient. This modification produces a notable speedup of as much as 2–3 times for a representative supercell design problem, indicating this new strategy may be preferable for planar nanophotonic design problems in general.

Nanofocusing performance of plasmonic probes based on gradient permittivity materials

Probe is the core component of an optical scanning probe microscope such as scattering-type scanning near-field optical microscopy (s-SNOM). Its ability of concentrating and localizing light determines the detection sensitivity of nanoscale spectroscopy. In this paper, a novel plasmonic probe made of a gradient permittivity material (GPM) is proposed and its nanofocusing performance is studied theoretically and numerically. Compared with conventional plasmonic probes, this probe has at least two outstanding advantages: first, it does not need extra structures for surface plasmon polaritons excitation or localized surface plasmon resonance, simplifying the probe system; second, the inherent nanofocusing effects of the conical probe structure can be further reinforced dramatically by designing the distribution of the probe permittivity. As a result, the strong near-field enhancement and localization at the tip apex improve both spectral sensitivity and spatial resolution of a s-SNOM. We also numerically demonstrate that a GPM probe as well as its enhanced nanofocusing effects can be realized by conventional semiconductor materials with designed doping distributions. The proposed novel plasmonic probe promises to facilitate subsequent nanoscale spectroscopy applications.

Enhancement of drying rate of moist porous media with dielectrophoresis mechanism

Drying of moist porous media, such as food or pulp and paper, is an energy-intensive process. Traditional drying technologies have disadvantages, including high energy consumption, thermal degradation of the samples, as well as high capital cost. In this work, a new technology, making use of the Dielectrophoresis (DEP) mechanism, is introduced, to enhance the drying process of a moist porous medium. DEP is a translational motion of neutral matter caused by polarization effects in a non-uniform electric field. In a typical drying process, due to the existence of an electric permittivity gradient between liquid and vapor interfaces, the DEP force can act as an external force to effectively extract the vapor phase away from the product. This in turn enhances the evaporation rate. This article experimentally investigates the influence of the DEP force on the drying of moist hand-sheet paper samples. Specifically, the temperature profile and sample weight are monitored under various applied electric potentials. The non-uniform electric field is generated from a unique electrode design. The experimental results show a significant impact of the DEP force on the temperature profile and drying rate. In addition, the results also show extremely low energy consumption and high energy efficiency associated with the application of the DEP force for drying. With the application of the electric field, up to 10 °C drop in surface temperature and a 33.6% reduction in drying time have been achieved. The experimental work provides a basic understanding of this novel technology that could enhance the drying process in various industry sectors such as forest products, foods, pharmaceuticals, and chemicals.

Super electro-optic modulation in bulk KTN:Cu based on electric-field-enhanced permittivity

The electric-field-enhanced effect of permittivity can improve the performance of electro-optic modulators and deflectors. A theoretical model of super electro-optic modulation based on the field-enhanced effect of the permittivity was proposed. Results showed that a strong field-enhanced effect can greatly reduce the half-wave voltage and increase the modulation depth as a result of increased relative dielectric permittivity and permittivity gradient to the electric field. For bulk paraelectric KTN:Cu near the Curie temperature, we found a novel phenomenon that the response of relative dielectric permittivity to the bias electric field was closely related to the frequency, including attenuation, invariance, and enhancement. We effectively selected the frequencies corresponding to the strong field-enhanced effect by measuring the dielectric-frequency spectrum under the bias voltage. At these frequencies, a phase retardation of π was achieved through 2Vpp AC modulation voltage, indicating that the half-wave voltage was reduced by one order of magnitude.

Fabry-Pérot Resonator Antenna in Equivalent-Medium Metamaterials

In this communication, a wideband Fabry–Perot resonator antenna (FPRA) is proposed. An optimized partially reflective surface (PRS) with a transverse permittivity gradient (TPG) composed of four nonrotationally symmetric sections is employed in the design of the antenna. The use of nonrotationally symmetric PRS results in more than 31% improvement of the 3 dB gain bandwidth compared with the traditional rotationally symmetric PRS. Furthermore, two types of nonresonant metamaterials (metallic-ring and etching-hole unit cells) are used to implement the equivalent permittivity of PRS. In this way, the equivalent permittivity value covers a broad range (3.5–9.5) for the same dielectric. As a result, the restrictions imposed by the use of only commercially available dielectrics and the errors that occur during the fabrication and assembly progress of different materials can be avoided. The primary radiator of FPRA is a double-ridge waveguide horn, which ensures wideband antenna operation. An FPRA prototype is fabricated and measured, which exhibits a broad bandwidth (5.2–11.5 GHz) with the return loss |S11| of less than −10 dB. The measured 3 dB gain bandwidth is 73.8% (in the frequency range of 5.3–11.5 GHz) with a peak gain of 17.1 dBi at 7.8 GHz.

Field Grading Composites Tailored by Electrophoresis — Part 1: Principle and Permittivity Gradient in Uniform Electric Field

A series of three articles present an innovative way to build advanced functionally graded materials (FGM) based on polymer/ceramic composites tailored by electrophoresis from the process principle to their field grading application in power electronics. In this Part 1, the process is presented and relies on applying a DC voltage on liquid composite compound before curing in order to `freeze' the particles where they have been accumulated. To exemplify this principle, FGM composites involving an epoxy resin with SrTiO3 high-k particles with a permittivity gradient are presented. The methodology to build and characterize them is carried out. An accumulated particle region is observed at the high voltage electrode while the depleted bulk region remains unmodified. This accumulated particle layer both increases in thickness and in filler densification with increasing the field and/or time allowing tuning its permittivity. This work paves the way to the development of more robust electronic systems where the electrical fringe field in critical regions can be mitigated.

Field Grading Composites Tailored by Electrophoresis—Part 2: Permittivity Gradient in Non-Uniform Electric Field

A series of three articles present an innovative way to build advanced functionally graded materials (FGM) based on polymer/ceramic composites tailored by electrophoresis from the process principle to their field grading application in power electronics. In this Part 2, it was studied the impact of a non-uniform electric field on the high-k SrTiO3 particle organization within an epoxy matrix. In that purpose, DBC substrates with sharp metallization tracks were used to generate a strong electric field divergence. This non-uniform field has been used to organize the particles around the field reinforcement regions during the electrophoresis process. It was discovered that the particles self-arrange into a conformal composite FGM layer that presents a local permittivity gradient: the highest ε being located around the electric field peak areas. This new process could be used to selectively `heal' the electric field-induced weaknesses of any high voltage electrical system by using its own design.

EHF Wave Propagation in the Plasma Sheath Enveloping Sharp-Coned Hypersonic Vehicle

The plasma sheath, which is the weakly ionized gas layer that envelopes hypersonic vehicles in near space, could shield communication signals and yield communication blackout. In the recent couple of decade, the extremely high frequency (EHF) communication is being considered as a potential solution to the blackout problem. On the other hand, previous studies used to focus on the EHF wave propagation in idealized plasma slabs or plasma sheaths covering blunt-coned vehicles. In the present letter, the plasma sheath enveloping sharp-coned hypersonic vehicles was modeled. The characteristics of EHF wave propagation in the sharp-coned plasma sheath were investigated. According to the study, the maximum electron densities and electron collision frequencies in sharp-coned plasma sheath is in the same magnitude of that in blunt-coned plasma sheath. However, the thickness of sharp-coned plasma sheath is much lower than that of blunt-coned plasma sheath. In addition, the power transmission rate at the frequencies of 140 and 225 GHz is high enough to afford communication tasks. Moreover, the reflection plays important roles in the energy loss of EHF wave propagation in sharp-coned plasma sheath, whereas the reflection in blunt-coned plasma sheath is ignorable. According to the analysis, the reason for such difference is that the gradient of relative permittivity in sharp-coned plasma sheath is larger than that of blunt-coned plasma sheath, which yields significant reflection to the EHF waves.

A metamaterial lens based on transformation optics for horizontal radiation of OAM vortex waves

Vortex electromagnetic waves carrying orbital angular momentum (OAM) have been widely discussed for potential applications in wireless communications. Belonging to the Laguerre–Gaussian beams family, such type of waves present a hollow conical shape and divergence characteristics along with a directional radiation. In this paper, an innovative method to produce omnidirectional OAM beams based on spatial transformation is proposed at microwave frequencies. As a proof-of-concept demonstration, a lens with omnidirectional radiation in the horizontal plane is designed and simulated with an incident vortex beam carrying the OAM mode l = +2. The designed lens can be potentially implemented with an all-dielectric medium showing a gradient permittivity distribution. Furthermore, the proposed lens presents good performances over a wide operational bandwidth spanning from 8 to 17 GHz. By converting the directional beam to an omnidirectional one, the proposed method opens the door to the potential development of microwave vortex antenna systems.

Analytical model for the spatiotemporal permittivity of uncured-composite devices in an AC electric field

Electric-field grading by dielectric permittivity gradient devices is an effective way of enhancing the insulation performance. In situ electric-field-driven assembly is an advanced method for the fabrication of insulating devices with adaptive permittivity gradients; however, there is no theoretical guidance for its use in design. We develop an analytical model for the spatiotemporal permittivity of an uncured-composite device in an AC electric field and investigate the coupling effects between the in situ assisted electric field and rod-like filler self-assembly in three devices: a pin-flat insulator, a basin insulator, and a silicone-gel-insulated gate bipolar transistor. Our model is based on optical images and dielectric permittivity monitoring, thus avoiding complicated electrodynamic calculations. The electric-field uniformity follows a U-shaped curve with assisted-voltage application time. We also find a combination of experimental parameters that constitutes an optimal tradeoff between internal and surface electric-field uniformities. This work establishes a theoretical design framework to optimize the performance (e.g. flashover voltage and breakdown strength) of a composite device.

Joule heating effects on electrokinetic flows with conductivity gradients.

Instability occurs in the electrokinetic flow of fluids with conductivity and/or permittivity gradients if the applied electric field is beyond a critical value. Understanding such an electrokinetic instability is significant for both improved transport (via the suppressed instability) and enhanced mixing (via the promoted instability) of liquid samples in microfluidic applications. This work presents the first study of Joule heating effects on electrokinetic microchannel flows with conductivity gradients using a combined experimental and numerical method. The experimentally observed flow patterns and measured critical electric fields under Joule heating effects to different extents are reasonably predicted by a depth-averaged numerical model. It is found that Joule heating increases the critical electric field for the onset of electrokinetic instability because the induced fluid temperature rise and in turn the fluid property change (primarily the decreased permittivity) lead to a smaller electric Rayleigh number.