Megahertz Frequencies Research Articles

Phononic Coupled-Resonator Waveguide Micro-Cavities

Phononic coupled-resonator waveguide cavities are formed by a finite chain of defects in a complete bandgap phononic crystal slab. The sample is machined in a fused silica plate by femtosecond printing to form an array of cross-shape holes. The collective resonance of the phononic cavities, in the Megahertz frequency range, are excited by a piezoelectric vibrator and imaged by laser Doppler vibrometry. It is found that well-defined resonant cavity modes can be efficiently excited, even though the phononic cavities are distant by a few lattice spacings and are only weakly coupled through evanescent elastic waves. The results suggest the possibility of engineering the dynamical response of a set of coupled phononic cavities by an adequate layout of defects in a phononic crystal slab.

Creating a Reference Plane Ultrasonic Wave in a Fluid Using a Plane Piezoelectric Transducer with a Large Wave Dimension

The article discusses the possibility of using a plane piezoelectric transducer with a large wave dimension as the source of a reference plane ultrasonic wave, which can be used to calibrate hydrophones in the megahertz frequency range. In the experiment, the source was a piezoceramic disk with a diameter of 100 mm and a thickness resonance frequency of about 1 MHz. A method was developed for determining the sensitivity of the transducer in the transmit mode by measuring its electrical impedance. A methodology is proposed for finding the parameters of the plane wave component of the emitted acoustic pulse from a known electrical signal on a generator. It is shown that the acoustic pulse profile measured by a calibrated hydrophone near the source agrees well with the theoretically predicted signal.

Determining the exact location of partial discharge in coil insulation of high-voltage machine using high-frequency model of the machine

The current study is aimed at investigating the exact location of partial discharge (PD) in coil insulation of a 6 KV/250 KW induction machine. Since measured PD signals have frequency components of kilohertz (KHz) to megahertz (MHz), it is necessary to employ fast and very fast transient models of electrical machines for locating PDs. In this study, a PD current pulse with MHz frequency component is injected to the multi-conductor transmission line model of each turn of the coil with frequency validity of >1 MHz. Considering coils as an interface between voltage source and finite element region, all circuit and field equations are rewritten to obtain the system final matrix, so that PD pulse is extracted. Experiment and simulation results show high accuracy of developed model to determine the exact location of PD.

Optical levitation using broadband light

The ability to create dynamic, tailored optical potentials has become important across fields ranging from biology to quantum science. We demonstrate a method for the creation of arbitrary optical tweezer potentials using the broadband spectral profile of a superluminescent diode combined with the chromatic aberration of a lens. A tunable filter, typically used for ultrafast laser pulse shaping, allows us to manipulate the broad spectral profile and therefore the optical tweezer potentials formed by focusing of this light. We characterize these potentials by measuring the Brownian motion of levitated nanoparticles in vacuum and also demonstrate interferometric detection and feedback cooling of the particle’s motion. This simple and cost-effective technique will enable wide application and allow rapid modulation of the optical potential landscape in excess of megahertz frequencies.

Dynamic mobility of surfactant-stabilized nano-drops: unifying equilibrium thermodynamics, electrokinetics and Marangoni effects

A theoretical analysis of the dynamic electrophoretic mobility of surfactant-stabilized nano-drops is undertaken. Whereas the theory for rigid spherical nanoparticles is well developed, its application to nano-drops is questionable due to fluid mobility of the interface and of the surfactant molecules adsorbed there. At zero frequency, small drops with surface impurities are well known to behave as rigid spheres due to concentration-gradient-induced Marangoni stresses. However, at the megahertz frequencies of electroacoustic (and other spectral-based) diagnostics, the interfacial concentration gradients are dynamic, coupling electromigration, advection and diffusion fluxes. This study addresses a parameter space that is relevant to anionic-surfactant-stabilized oil–water emulsions, using sodium-dodecylsulfate-stabilized hexadecane as a specific example. The drop size is several hundred nanometres, much larger than the diffuse-layer thickness, thus motivating thin-double-layer approximations. The theory demonstrates that fluid mobility and fluctuating Marangoni stresses can have a profound influence on the magnitude and phase of the dynamic mobility. We show that the drop interface transits from a rigid/immobile one at low frequency to a fluid one at high frequency. The model unifies electrokinetics and equilibrium interfacial thermodynamics. Therefore, with knowledge of how the interfacial tension varies with electrolyte composition (oil, surfactant and added salt concentrations), the particle radius might be adopted as the primary fitting parameter (rather than the customary -potential) from an experimental measure of the dynamic mobility. This theory is general enough that it might be applied to aerosols and bubbly dispersions (at sufficiently high frequencies).

(Keynote Paper) Mono-Domain Ferrites and Their Implications

Neutron depolarization experiments have shown that the intragranular magnetic domain structure of polycrystalline ferrites exhibits a marked grain size dependence. Below a grain size of around $3~\mu \text{m}$ , the magnetic domain structure changes from the two-domain to the mono-domain state. Ferrites composed of mono-domain grains exhibit low dissipation at megahertz frequencies. This is attributed to the absence of intragranular domain wall movement, i.e., a microscopic origin for dissipation in ferrites has been identified. The implications of this clear observation and evidence for a transition from the mono- to two-domain state in ferrites for micromagnetic theory when a single particle becomes monodomain, as well as for the initial permeability mechanisms in polycrystalline ferrites, are discussed.

Dielectric properties of Ag/paper-based metacomposite with sandwich-structure forward low dielectric loss in megahertz frequency range

In this paper, Ag/paper-based composites with low dielectric loss were obtained by introduce the concept of metamaterial into the fabrication process. In this kind of metacomposite, single-layer paper was used as basic block to build up a matrix with sandwich structure, in which Ag particles are embedded and silver particles are evenly distributed. The microstructure and phase composition were characterized by scanning electron microscope and X-ray diffractometer. The impedance spectrum of the composites in 20 Hz–1 MHz region was measured by using LCR Meter, which can be well fitted by equivalent circuit method. The results show that, compared to the composite in which Ag particles disorder distributed, the dielectric loss of the layer-by-layer structured metacomposite can be effectively reduced (tan θ ≤ 0.1), without negative effect on the dielectric constant. Moreover, the dielectric properties of metacomposites show obvious anisotropy in the horizontal and vertical directions, which might give a feasible way for flexible smart material in its design.

Opto-Mechanical Interactions in Multi-Core Optical Fibers and Their Applications

Optical fibers containing multiple cores are being developed towards capacity enhancement in space-division multiplexed optical communication networks. In many cases, the fibers are designed for negligible direct coupling of optical power among the cores. The cores remain, however, embedded in a single, mechanically-unified cladding. Elastic (or acoustic) modes supported by the fiber cladding geometry are in overlap with multiple cores. Acoustic waves may be stimulated by light in any core through electrostriction. Once excited, the acoustic waves may induce photo-elastic perturbations to optical waves in other cores as well. Such opto-mechanical coupling gives rise to inter-core cross-phase modulation effects, even when direct optical crosstalk is very weak. The cross-phase modulation spectrum reaches hundreds of megahertz frequencies. It may consist of discrete and narrow peaks, or may become quasi-continuous, depending on the geometric layout. The magnitude of the effect at the resonance frequencies is comparable with that of intra-core cross-phase modulation due to Kerr nonlinearity. Two potential applications are demonstrated: single-frequency opto-electronic oscillators that do not require radio-frequency electrical filters, and point-sensing of liquids outside the cladding of multi-core fibers, where light cannot reach.

Electrochemically Induced Changes in TiO2 and Carbon Films Studied with QCM-D.

Semi-solid fluid electrode-based battery (SSFB) and supercapacitor technologies are seen as very promising candidates for grid energy storage. However, unlike for traditional batteries, their performance can quickly get compromised by the formation of a poorly conducting solid–electrolyte interphase (SEI) on the particle surfaces. In this work we examine SEI film formation in relation to typical electrochemical conditions by combining cyclic voltammetry (CV) with quartz crystal microbalance dissipation monitoring (QCM-D). Sputtered layers of typical SSFB materials like titanium dioxide (TiO2) and carbon, immersed in alkyl carbonate solvents, are cycled to potentials of relevance to both traditional and flow systems. Mass changes due to lithium intercalation and SEI formation are distinguished by measuring the electrochemical current simultaneously with the damped mechanical oscillation. Both the TiO2 and amorphous carbon layers show a significant irreversible mass increase on continued exposure to (even mildly) reducing electrochemical conditions. Studying the small changes within individual charge–discharge cycles, TiO2 shows mass oscillations, indicating a partial reversibility due to lithium intercalation (not found for carbon). Viscoelastic signatures in the megahertz frequency regime confirm the formation and growth of a soft layer, again with oscillations for TiO2 but not for carbon. All these observations are consistent with irreversible SEI formation for both materials and reversible Li intercalation for TiO2. Our results highlight the need for careful choices of the materials chemistry and a sensitive electrochemical screening for fluid electrode systems.

Collective Resonances of a Chain of Coupled Phononic Microresonators

We study experimentally a chain of defect resonators in a phononic crystal slab and observe its collective resonances at ultrasonic frequencies of a few megahertz. A phononic crystal of cross holes is fabricated in a thin fused-silica plate by femtosecond-laser writing followed by $\mathrm{KOH}$ etching. A chain of 17 coupled resonators is defined with no definite spatial periodicity but similar coupling strength between nearest-neighbor resonators. The full phononic band gap ensures that only evanescent waves in the crystal can tunnel between adjacent resonators in the plane. The resulting evanescent-coupling strength decreases exponentially with distance. Collective resonances are excited by a frequency-driven piezoelectric vibrator attached at one end of the chain and imaged along the chain with a laser Doppler vibrometer. A discrete spectrum of resonances is observed and explained by a model representing the chain as a phononic polymer. The theoretical analysis is supported by finite-element simulations that agree well with experimental results. Chains of evanescently coupled microresonators forming phononic polymers could find applications in ultrasonic sensing, for implementation in topological phononics, and for the design of optomechanical resonator chains.

Synthesis and simulation study of non-restoring cell architecture layout in perpendicular nano-magnetic logic

Nano-magnetic logic is one of the most competitive technologies of conventional metal oxide semiconductor-based designs. There is no current flow, and information is transferred via the magnetic force between the magnets. Because of this phenomenon, it has low power dissipation and operating in the megahertz frequency range. In this paper, perpendicular nano-magnetic logic (pNML) technology is used for designing a non-restoring divider circuit. First, a new three-dimensional layout of the Exclusive-OR (XOR) gate is proposed. This layout is extended to a new 1-bit full adder circuit, 1-bit non-restoring cell and a 4-bit non-restoring divider. According to our insight, the proposed non-restoring divider circuit in the pNML layout and use of the multilayer is the first time in the literature. The areas of the proposed layouts are 9.72 µm2, 45.9 µm2, 61.2 µm2 and 3955.95 µm2 for the XOR gate, full adder, non-restoring cell and non-restoring divider designs, respectively. The proposed FA consumes 44.25% less latency and 60% fewer layers than the existing state-of-the-art work. All the proposed layouts are implemented using the MagCAD tool.

Electromagnetic 0.1-100 kHz noise does not disrupt orientation in a night-migrating songbird implying a spin coherence lifetime of less than 10 µs.

According to the currently prevailing theory, the magnetic compass sense in night-migrating birds relies on a light-dependent radical-pair-based mechanism. It has been shown that radio waves at megahertz frequencies disrupt magnetic orientation in migratory birds, providing evidence for a quantum-mechanical origin of the magnetic compass. Still, many crucial properties, e.g. the lifetime of the proposed magnetically sensitive radical pair, remain unknown. The current study aims to estimate the spin coherence time of the radical pair, based on the behavioural responses of migratory birds to broadband electromagnetic fields covering the frequency band 0.1-100 kHz. A finding that the birds were unable to use their magnetic compass under these conditions would imply surprisingly long-lived (greater than 10 µs) spin coherence. However, we observed no effect of 0.1-100 kHz radiofrequency (RF) fields on the orientation of night-migratory Eurasian blackcaps (Sylvia atricapilla). This suggests that the lifetime of the spin coherence involved in magnetoreception is shorter than the period of the highest frequency RF fields used in this experiment (i.e. approx. 10 µs). This result, in combination with an earlier study showing that 20-450 kHz electromagnetic fields disrupt magnetic compass orientation, suggests that the spin coherence lifetime of the magnetically sensitive radical pair is in the range 2-10 µs.

Three‐Dimensional Mapping of Lightning‐Produced Ionospheric Reflections

AbstractThe powerful high‐frequency/very high frequency radio emissions that occur during lightning flashes can be used as a signal of opportunity to study the bottom side ionosphere. The lightning emission is bright, broad spectrum, and short in duration, providing an ideal signal of opportunity for making ionograms. This study continues previous work in Obenberger et al. (2018), where the direct line of sight signal from lightning can be cross correlated with megahertz frequency radio telescope observations to reveal ionogram traces created from the reflected lightning signals. This process was further developed to automate production of ionograms made from individual lightning flashes over the course of several hours, as well as create new techniques to detect the lightning signal using the all‐sky‐imaging mode. By using the Long Wavelength Array Sevilleta radio telescope as an interferometer, the point of reflection of the lightning signal for each frequency of the ionogram can be located in the ionosphere, instantaneously revealing density gradients within the ionosphere on minute time scales. We also explore the minimum size stations required for the application of this technique, which we found to be at least 32 antennas.

Adding dynamics to x-ray microscopy

X-Ray Imaging The development of third-generation x-ray sources, such as synchrotrons and free-electron lasers, over the past two decades has provided light-beam brilliance and coherence that can be used to reveal structural details with exquisite spatial resolution. More recent efforts have focused on developing a dynamic capability that would reveal detailed snapshots of ultrafast structural and electronic processes occurring within solid-state and biological samples. By modulating the x-ray beam at megahertz frequencies, Vagovic et al. introduce a technique implemented at the European X-ray Free-Electron Laser (EuXFEL) facility that provides microsecond temporal resolution along with micrometer spatial resolution and a better signal-to-noise ratio in the images. The results illustrate a promising route for the further development of temporally resolved x-ray imaging. Optica 6 , 1106 (2019).

Optimal Design of Megahertz $LLC$ Converter for 48-V Bus Converter Application

Intermediate bus architecture employing 48-V bus converters is widely used in power supply applications. With the rapid increase of demanded power by these loads, higher efficiency and power density are driving better performance power management solutions. In this article, a Gallium Nitride (GaN) based design of a two-stage solution is proposed. The first stage is a multi-phase Buck for regulation. The second stage is an LLC converter with a fixed switching frequency for isolation. The detailed design and optimization of the LLC converter are studied. To achieve high power density and high efficiency, the transformer design becomes critical at megahertz frequency. The matrix transformer concept is applied and a merged winding structure is used for flux cancellation, which effectively reduces the ac winding losses. A novel primary termination and via structure is proposed, resulting in a great loss reduction. In addition, to study the current sharing of parallel winding layers, a 1-D analytic model is proposed, and a symmetrical winding layer scheme is used to balance the current distribution. Finally, the prototype for the two-stage bus converter is developed, with a peak efficiency of 96% and a power density of 615 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> .

(Invited) Investigation of Megahertz Frequency Modulation Effects on Charge and Discharge Behavior of Lithium Ion Batteries

Over the past several years, reports have emerged of ultrafast charging algorithms for lithium ion batteries [1,2] that in at least one instance [3] is attributed to megahertz frequency modulation of charging/discharging currents. Such algorithms are claimed to dramatically improve active material utilization as well as cycle life, but no mechanisms have been proposed in published work, nor, to our knowledge, have mechanistic studies have been conducted. In this work, we use a recently developed technique [4] in which single electrode particles having capacities of a few nAh can be interrogated with electrodynamic measurements (EIS, GITT, polarization-depolarization) while varying the state-of-charge. This capability allows clean separation of interfacial vs bulk transport limitations at the individual particle level, without complicating factors from typical composite electrode microstructures. To this capability, we have added AC pulse charging and discharging at ultra-low currents (down to picoA) with high-frequency waveforms (up to ~ MHz). With this capability, we aim to answer a number of questions such as: Are these effects real? Are they significant? And, what are possible mechanisms by which ion transport kinetics in a lithium ion battery are responsive to megahertz frequency excitation? This work was supported as part of the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012583.

Calvin F. Quate (1923-2019).

Nanoscience pioneer who invented advanced microscopes

Active Power Device Selection in High- and Very-High-Frequency Power Converters

This paper aims to provide a road map for selecting power devices in soft-switched, megahertz (MHz) frequency power converters. Minimizing ${C_{\text{OSS}}}$ losses, which occur when charging and discharging the parasitic output capacitor of power semiconductors, is critical to efficient operation. These losses are excluded from manufacturer-provided information, and measurements are either sparse or not reported at all in the existing literature. We report the first high-frequency ${C_{\text{OSS}}}$ loss data from silicon carbide (SiC) power mosfet s, with a range of devices tested from 1 to 35 MHz and up to 800 V. In contrast to GaN HEMTs, ${C_{\text{OSS}}}$ losses in SiC mosfet s do not increase with ${dV/dt}$ at these frequencies. A total of 3%–10% of the stored energy is dissipated in the measured SiC mosfet s. We report new ${C_{\text{OSS}}}$ loss measurements for vertical silicon mosfet s and expand on existing measurements for superjunctions, finding high variance in ${C_{\text{OSS}}}$ losses between devices for both constructions. High ${C_{\text{OSS}}}$ losses preclude the tested silicon mosfet s from efficient operation at MHz frequencies. Lastly, we compare devices in soft-switched applications using a loss calculation that includes these ${C_{\text{OSS}}}$ losses, and demonstrate a 100 W, 17 MHz dc-RF inverter using a custom-packaged SiC mosfet .

Wireless Power Transfer Utilizing a High-$Q$ Self-Resonant Structure

The range and efficiency of a wireless power transfer (WPT) system is limited by the quality factor of the resonant coils. Conventional resonant coils are made from solid or Litz wire. At megahertz frequencies solid wire is not utilized well due to skin effect, and Litz wire is very lossy due to proximity effect. We present a multilayer self-resonant structure as a low-cost method for creating high- $Q$ coils. This structure uses thin foil layers that are separated by a dielectric material in order to form an LC resonator, while also forcing equal current sharing between conductors. The self-resonant structure makes it feasible to achieve advantages similar to Litz wire, but at multi-megahertz frequencies where effective Litz wire is not commercially available. These structures are made with foil layers much thinner than a skin depth, which can make handling these thin layers a challenge. To solve this problem, we also present a modified self-resonant structure in which the layered conductors are made with flex-PCB substrates with no vias. The PCB substrates provide a relatively inexpensive way to handle thin conductive layers, and the modified self-resonant structure ensures that the poor dielectric properties of the PCB substrates do not impact the quality factor of the structure. A prototype of the modified self-resonant structure has a quality factor of 1183 at 7.09 MHz, despite only being 6.6 cm in diameter, which is more than 6.5x larger than other coils presented in the literature with a similar diameter. An experimental WPT setup utilizing two self-resonant structures achieves 94 $\%$ efficiency at a distance of 5.0 cm, which is more than twice the distance as similarly sized conventional coils can achieve while maintaining the same efficiency.

Circuit Design Considerations for Reducing Parasitic Effects on GaN-Based 1-MHz High-Power-Density High-Step-Up/Down Isolated Resonant Converters

With the availability of wide bandgap devices, the power converters can now potentially operate at megahertz (MHz) or higher frequencies while achieving ultrahigh efficiency. Such a high-frequency switching is particularly important for isolated converters to reduce the size of the passive components, achieve high power-density, and reduce costs. However, unlike the conventional kilohertz switching, in MHz switching, the parasitic components will significantly affect the circuit operation. These effects are compounded by high-ratio step-up or step-down isolated converters because the parasitic capacitances/inductances are squared times the turns’ ratio of the transformer when reflecting from high-voltage (HV) to low-voltage (LV) side to LV/HV side. In this paper, a high-step-up series resonant converter is used as a design example to explore the effects of parasitic inductances induced from the LV side and parasitic capacitances induced from the HV side on the circuit operation under MHz switching conditions. Afterward, the printed circuit board (PCB) layout and the planar transformer are optimized through the finite-element method to minimize the parasitic effects. Finally, a 1-MHz, 38-V/380-V, 300-W resonant converter prototypes are built and compared to verify the design optimization.