Cutoff Condition Research Articles

Microstructured optical fibers for terahertz waveguiding regime by using an analytical field model

Abstract Microstructured optical fibres (MOFs) are seen as novel optical waveguide for the potential applications in the terahertz (THz) band as they provide a flexible route towards THz waveguiding. Using the analytical field model (Sharma et al., 2014) developed for index-guiding MOFs with hexagonal lattice of circular air-holes in the photonic crystal cladding; we aim to study the propagation characteristics such as effective index, near and the far-field radiation patterns and its evolution from near-to-far-field domain, spot size, effective mode area, and the numerical aperture at the THz regime. Further, we present an analytical field expression for the next higher-order mode of the MOF for studying the modal properties at terahertz frequencies. Also, we investigate the mode cut-off conditions for identifying the single-mode operation range at THz frequencies. Emphasis is put on studying the coupling characteristics of MOF geometries for efficient mode coupling. Comparisons with available experimental and numerical simulation results, e.g., those based on the full-vector finite element method (FEM) and the finite-difference frequency-domain (FDFD) method have been included.

Transverse electric modes in planar slot waveguides

Transverse electric (TE) modes in symmetrical planar slot waveguides are analytically solved, and a comprehensive knowledge about them is provided. It has been shown that there are four types of TE modes in planar slot waveguides just as the TM modes. Their field solutions, characteristic equations and cut-off conditions are derived. The cut-off conditions and the power confinement factors of the TE and TM modes are compared. It is demonstrated that there may be degeneration points between TE and TM modes. Because a three-dimensional waveguide has similar modal characteristics as its planar counterpart, our work presents a foundation of recognizing and utilizing the three-dimensional slot waveguides.

Classification of a Subclass of Two-Dimensional Lattices via Characteristic Lie Rings

The main goal of the article is testing a new classification algorithm. To this end we apply it to a relevant problem of describing the integrable cases of a subclass of two-dimensional lattices. By imposing the cut-off conditions $u_{-1}=c_0$ and $u_{N+1}=c_1$ we reduce the lattice $u_{n,xy}=\alpha(u_{n+1},u_n,u_{n-1})u_{n,x}u_{n,y}$ to a finite system of hyperbolic type PDE. Assuming that for each natural $N$ the obtained system is integrable in the sense of Darboux we look for $\alpha$. To detect the Darboux integrability of the hyperbolic type system we use an algebraic criterion of Darboux integrability which claims that the characteristic Lie rings of such a system must be of finite dimension. We prove that up to the point transformations only one lattice in the studied class passes the test. The lattice coincides with the earlier found Ferapontov-Shabat-Yamilov equation. The one-dimensional reduction $x=y$ of this lattice passes also the symmetry integrability test.

“Less Is More” - Decreasing Silicon Particle Degradation By Limiting the Delithiation Cut-Off Potential

The electrochemical behavior of silicon-based anodes in lithium-ion batteries can be largely influenced by the cut-off potentials. Due to the strongly sloped voltage profile of silicon, changes in the cut-off potentials highly impact the degree of lithiation, and thus the associated volume expansion of the Li-Si alloy.1 While the lithiation cut-off potential of silicon has been addressed in several studies by the groups of Obrovac and Dahn, who highlighted the relevance of the crystalline Li15Si4 phase that forms at potentials below 0.07 V vs. Li/Li+, the de-lithiation cut-off potential of silicon is less understood.2–4 Recently, Klett et al. discussed the beneficial effect of a limited delithiation cut-off potential on the capacity retention of NCM523/silicon-graphite full-cells.5 In the present study, we evaluate the impact of the delithiation cut-off potential on the silicon particle degradation. Building up on previous studies,5,6 our characterization is concerned with the underlying morphological changes the silicon particles undergo during repeated cycling, which ultimately affect the entire electrode integrity. To test this hypothesis, silicon-graphite composite electrodes with a practical areal capacity of ~1.8 mAh cm-2 were prepared, consisting of 35 wt% nanometer-sized silicon (~200 nm) and 45 wt% graphite (~20 µm). Vapor grown carbon fibers and lithiated poly(acrylic acid) binder accounted for the remaining 20 wt%. Utilizing SiG//LiFePO4 Swagelok T-cells with a capacitively oversized positive electrode (~3.5 mAh cm-2) and a stable reference potential of ~3.45 V vs. Li/Li+, three cut-off conditions of the silicon-graphite electrodes, shown in Figure 1, were investigated. First, the silicon-graphite electrodes were fully lithiated to 0.01 V vs. Li/Li+ and completely delithiated to 1.25 V vs. Li/Li+ (brown curve). Second, we limited either the lithiation cut-off potential to 0.05 V vs. Li/Li+ (red curve) or, alternatively, the delithiation cut-off potential to 0.65 V vs. Li/Li+ (blue curve), with the latter two resulting in a capacity utilization of ~80%. By differential capacity analysis of selected galvano-static cycles we demonstrate that silicon-graphite electrodes with a limited delithiation cut-off potential of 0.65 V vs. Li/Li+ show less overpotential growth and significant improvement in capacity retention upon cycling. These observations are further supported by impedance spectroscopy at different states-of-charge using a gold-wire reference electrode to obtain individual impedance spectra from the positive and the negative electrode.7 Figure 1 . Voltage profiles of silicon-graphite electrodes (5th cycles) operated at different cut-off conditions. The data were obtained from SiG//LiFePO4 Swagelok-T cells with a gold-wire reference electrode that was sandwiched between two glass fiber separators soaked with an electrolyte of LP57 + 5 wt% FEC. Areal SiG capacity: ~1.8 mAh cm-2. The electrochemical results are complemented by transmission and scanning electron microscopy of the silicon particles and composite electrodes. The structural analysis indicates that the repeated delithiation of silicon particles to highly oxidative potentials (1.25 V vs. Li/Li+) causes dealloying reactions, which result in a roughening of the particle surface and the formation of highly porous structures, leading to significantly increased irreversible capacity losses and electrode polarization. Finally, a comparison of the different cut-off conditions and a proposal for an improved cycling protocol for silicon-graphite electrodes are discussed.

Ground state entanglement entropy for discrete-time two coupled harmonic oscillators

The ground state entanglement of the system, both in discrete-time and continuous-time cases, is quantified through the linear entropy. The result shows that the entanglement increases as the interaction between the particles increases in both time scales. It is also found that the strength of the harmonic potential affects the formation rate of the entanglement of the system. The different feature of the entanglement between continuous-time and discrete-time scales is that, for discrete-time entanglement, there is a cut-off condition. This condition implies that the system can never be in a maximally entangled state.

Electrochemical Peformance of Lithium-Ion Capacitors Evaluated Under High Temperature and High Voltage Stress Using Redox Stable Electrolytes and Additives

Lithium-ion capacitors (LICs) were investigated for high power, moderate energy density applications for operation in extreme environments with prolonged cycle life performance. The LICs were prepared as three-layer pouch cells with an asymmetric configuration employing Faradaic pre-lithiated hard carbon anodes and non-Faradaic anion adsorption−desorption activated carbon (AC) cathodes. The studies were conducted in order to develop and improve the LIC cell design, the cell formation procedure, the stability of the electrolytes and additives, and the long-term capacity retention under high stress conditions. The LIC cells have been evaluated using critical performance tests under the following conditions: long-term cycling stability at room temperature (2.2-3.8 V), high temperature holds at 3.8 V, and high upper cut-off potential conditions (2.2 V to 3.8-5.0 V). The rate performance of different electrolytes and additives were measured after the LIC cell formation by charging at a 1C rate and discharging at a 1C, 2C, 4C, 6C, 10C, and again 1C rate from 2.2 V to 3.8 V. The performance of the energy storage devices were evaluated in terms of the specific discharge capacity, operational voltage/temperature window, coulombic efficiency, and cycling stability under different conditions using various electrolytes and additives. The presence of electrolyte additives were found to be essential to the improved performance of the LIC cells at room temperature, at high temperature, and at higher cut-off potentials.

Thermal Runaway Characterization of Li-Ion Batteries Under External Heating Conditions

Recent Li-ion battery fire incidents in smartphones, hoverboards, e-cigarettes, electric vehicles, and aircrafts highlight the limited understanding of battery thermal runaway failure and inadequacy of current battery safety or protection mechanisms. Li-ion battery thermal runaway can occur due to the presence of material defects, operation under abusive/extreme conditions, or cumulative damage under normal operation. Electrical and thermal characterization of Li-ion batteries under thermal runaway conditions is necessary to improve the battery safety. In the past various studies have been conducted to investigate thermal runaway failures under different operating conditions [1-3]. This work studies the effects of additional parameters and operating conditions on Li-ion battery thermal runaway induced by external heating of the battery. External heating is a common loading condition for batteries located in close proximity to an electronic circuit or inside a battery pack. A detailed electrical and thermal characterization of battery has been performed to understand the failure mechanisms and to provide guidance on battery monitoring and safety management. Experiments were conducted on commercial 18650 Li-ion cells with 2600 mAh nominal capacity (at C/5) and nominal voltage of 3.6V. These cells have safety vents located below the positive terminal to allow release of gases. Cells in fully charged and fully discharged conditions were used for the experiments. A high temperature Nickel Chromium heating wire was wrapped around the cells and powered by a DC source to raise the cell temperature. The heating rate (current) of the wire was varied to study its effects on thermal runaway. Three different external heating cut-off conditions were investigated in this study: heating cut-off after the occurrence of thermal runaway, heating cut-off just after the trigger of cell venting, and heating cut-off at temperatures below cell venting trigger temperature. Two K-type thermocouples were mounted on cell near the two ends (terminals) to monitor the cell surface temperature. Cell voltage measurements were also recorded during the entire testing. All the experiments were conducted inside an in-house explosion chamber with a transparent glass window for the video recording. The experimental results identify the operating conditions with high susceptibility to thermal runaway failure. The electrical and thermal observations indicate the possible cell failure mechanisms, reveal different stages of cell failure, and correlate the electrical and thermal failure behavior of the cell. The results show that the heating of fully charged cell up to a critical temperature level can lead to electrical failure (voltage drop below the cell cut-off voltage) even if the cell does not undergo thermal runaway. Parameters such as cell venting trigger temperature, cell maximum temperature, cell self-heating rate, voltage drop, and time duration of the thermal runaway have been estimated from the measurements. The learnings from this study form the basis of recommendations for improvement in battery safety management. References H. Maleki, G. Deng, A. Anani, J. Howard, Thermal stability studies of Li-ion cells and components, J. Electrochem. Soc. 146 (9) (1999) 3224-3229.X. Liu, S.I. Stoliarov, M. Denlinger, A. Masias, K. Snyder, Comprehensive calorimetry of the thermally-induced failure of a lithium ion battery, J. Power Sources 280 (2015) 516-525.A.W. Golubkov, S. Scheikl, R. Planteu, G. Voitic, H. Wiltsche, C. Stangl, G. Fauler, A. Thaler, V. Hacker, Thermal runaway of commercial 18650 Li-ion batteries with LFP and NCA cathodes – impact of state of charge and overcharge, RSC Adv. 5 (2015) 57171-57186.

An improved method for calculating slope length (λ) and the LS parameters of the Revised Universal Soil Loss Equation for large watersheds

The Universal Soil Loss Equation (USLE) and its revised version (RUSLE) are often used to estimate soil erosion at regional landscape scales. USLE/RUSLE contain parameters for slope length factor (L) and slope steepness factor (S), usually combined as LS. However a major limitation is the difficulty in extracting the LS factor. Methods to estimate LS based on geographic information systems have been developed in the last two decades. L can be calculated for large watersheds using the unit contributing area (UCA) or the slope length (λ) as input parameters. Due to the absence of an estimation of slope length, the UCA method is insufficiently accurate. Improvement of the spatial accuracy of slope length and LS factor is still necessary for estimating soil erosion. The purpose of this study was to develop an improved method to estimate the slope length and LS factor. We combined the algorithm for multiple-flow direction (MFD) used in the UCA method with the LS-TOOL (LS-TOOLSFD) algorithms, taking into account the calculation errors and cutoff conditions for distance, to obtain slope length (λ) and the LS factor. The new method, LS-TOOLMFD, was applied and validated in a catchment with complexly variable slopes. The slope length and LS calculated by LS-TOOLMFD both agreed better with field data than with the calculations using the LS-TOOLSFD and UCA methods, respectively. We then integrated the LS-TOOLMFD algorithm into LS-TOOL developed in Microsoft's .NET environment using C# with a user-friendly interface. The method can automatically calculate slope length, slope steepness, L, S, and LS factor, providing the results as ASCII files that can be easily used in GIS software and erosion models. This study is an important step forward in conducting accurate large-scale erosion evaluation.

Magnetically-tunable cutoff in asymmetric thin metal film plasmonic waveguide

We theoretically investigated the magnetically tunable cutoff of long-range surface plasmon polariton along thin metal film surrounded by a magneto-optic material on one side and by a nonmagnetic dielectric on the other. The analytically derived cutoff condition predicts that a magnetic field bias can induce a novel degenerate cutoff-state near which the beyond-cutoff radiation on one side can be switched to that on the other by a minor variation of the magnetic field from the bias. The magnetization bias needed for the degeneracy is proportional to the metal film thickness and inversely proportional to the wavelength.

Necessary conditions for nonlinear excitation of a high-order mode in a single-mode optical fiber

The necessary conditions for nonlinear excitation of higher-order modes in a single-mode step-index optical fiber are analyzed. The cutoff conditions for such an optical waveguide are investigated taking into account Kerr nonlinearity. The minimal power of optical pulses required for fulfilling the necessary conditions for nonlinear excitation of higher-order modes in a single-mode step-index fiber is calculated as a function of the normalized frequency. The allowed ranges of variation of the normalized frequency and optical-radiation power are estimated. It is demonstrated that the conditions necessary for nonlinear excitation of a higher-order mode in a step-index single-mode optical fiber can be created for optical pulses shorter than 500 fs.

International Civil Aviation Organization Supersonic Task Group overview and status

A resurgence of interest in supersonic flight has emerged due to recent technological advances. Given the global impact of such aircraft, environmental standards and recommended practices (SARP) are being developed under the International Civil Aviation Organization (ICAO) to ensure the mitigation of accompanying sonic booms prior to any re-introduction of civil supersonic flight operations. In 2004, ICAO’s Committee of Environmental Protection (CAEP) established the supersonics work program under Working Group 1 (WG1) to develop SARPs. Current proposed concepts include (1) traditional sonic boom aircraft (like the Concorde), which operate supersonically over water; (2) low-boom designs with overland operation capabilities; and (3) traditional aircraft flying under Mach cut-off conditions. To-date, WG1/SSTG continues to formulate an enroute sonic boom SARP for civil supersonic airplanes. Metrics, test procedures, and a data framework continue to be investigated for suitability and efficiency. A newly formed Landing and Take-Off (LTO) noise subgroup for supersonic aircraft has been tasked to define a second SARP for the terminal environment (i.e., subsonic operations). An overview of ICAO’s hierarchy supporting SARPs development, WG1/SSTG technical program, and a current status update is presented.

Sensitivity analysis of supersonic Mach cut-off flight

Supersonic aircraft designers are pursuing various methods to help facilitate the re-introduction of overland supersonic flight operations. A substantial amount of research has been invested over recent years to demonstrate its feasibility. An alternative method for satisfying the noise standards for supersonic aircraft is more operation-oriented. Under Mach cut-off conditions, the vehicle still generates sonic booms but the acoustic waves refract in such a way that it does not reach the ground. To better understand the propagation of sonic booms during Mach cut-off flight, Georgia Tech (GT) has conducted research under the FAA’s Aviation Sustainability Center (ASCENT). An acoustical model for Mach cut-off flight was developed—GT leveraged this model for sensitivity analysis. The Mach cut-off model allowed GT to vary both atmospheric and flight conditions to study how these dynamic parameters impact sonic boom signatures through the atmosphere. The results of these analyses provide greater insight on how Mach cut-off flight can be achieved and highlights potential technologies to facilitate its re-introduction. [This work was supported by the FAA. The opinions, findings, conclusions, and recommendations expressed in this material are those of the authors and do not necessarily reflect the views of ASCENT FAA Center of Excellence sponsor organizations.]

Dr. Kenneth Plotkin’s myriad contributions to the National Aeronautics and Space Administration’s supersonic mission

The world as a whole, and NASA, in particular, owes a large debt of gratitude to Dr. Kenneth Plotkin for his decades of service in the field of sonic boom research and advancement of quiet supersonic transportation. This presentation will highlight the contributions of Dr. Plotkin to a myriad of NASA projects. One of the largest efforts was the assembly and continual improvement of sonic boom propagation software tools, collectively called PCBoom, which allowed the analysis of real and imagined vehicles from Mach cutoff conditions to the hypersonic. He was a driving force behind reshaping aircraft to demonstrate quieter sonic booms, first with the plans for a modified Firebee drone and SR-71, and then with the highly successful Shaped Sonic Boom Demonstrator series of flights. Dr. Plotkin’s partnership with NASA Armstrong resulted in the development of the low boom dive maneuver to allow quiet sonic boom testing on structures and people using existing aircraft, as well as a sonic boom cockpit display that has recently been tested in flight. Dr. Plotkin was also instrumental in such research campaigns as SCAMP, WSPR, and FaINT. Throughout all, Dr. Plotkin’s phenomenal intellect, tireless dedication, and irreverent humor made working with him a joy.

The Aerion AS2 and Mach cut-off

The Aerion supersonic business jet is intended to fly at Mach cut-off conditions over land. We will outline the history of investigations into Mach cut-off flight. Aerion’s concept of operations for Mach cut-off will be discussed. We will discuss the current state of the technology and identify areas where further research is required.

Ordered Mesoporous Titanium Nitride as a Promising Carbon-Free Cathode for Aprotic Lithium-Oxygen Batteries.

Despite the extraordinary gravimetric energy densities, lithium-oxygen (Li-O2) batteries are still facing a technological challenge; limited round trip efficiency leading to insufficient cycle life. Recently, carbonaceous electrode materials were found to be one of the primary origins of the limited cycle life, as they produce irreversible side products during discharge. A few investigations based on noncarbonaceous materials have demonstrated largely suppressed accumulation of irreversible side products, but such studies have focused mainly on the materials themselves rather than delicate morphology control. As such, here, we report the synthesis of mesoporous titanium nitride (m-TiN) with a 2D hexagonal structure and large pores (>30 nm), which was templated by a block copolymer with tunable chain lengths, and introduce it as a stable air-cathode backbone. Due to the well-aligned pore structure and decent electric conductivity of TiN, the battery reaction was quite reversible, resulting in robust cycling performance for over 100 cycles under a potential cutoff condition. Furthermore, by protecting the Li metal with a poreless polyurethane separator and engaging a lithium iodide redox mediator, the original capacity was retained for 280 cycles under a consistent capacity condition (430 mAh g-1). This study reveals that when the appropriate structure and material choice of the air-cathode are coupled with an advanced separator and an effective solution-phase redox mediator, the cycle lives of Li-O2 batteries can be enhanced dramatically.

Acoustic phonon modes in asymmetric AlxGa1−xN/GaN/AlyGa1−yN quantum wells

Using an elastic continuum model, the dispersion relations and phonon modes of propagating, confined, half space and interface acoustic phonons in asymmetric AlxGa1−xN/GaN/Al1−yGayN quantum wells (QWs) have been solved theoretically with the varieties of Al components x and y. Contrary to the previous conclusions, some regulations for the existence of the above different acoustic phonons are revealed as well as the transition conditions among these modes are also discussed. With increase of wave vectors, the dispersion relations split into several groups. Because the classification of these groups is related to the eigen frequencies of bulk materials forming QWs, phonon modes in these groups will be confined or propagating in different layers of QWs. Furthermore, the gradients of the dispersion relations' asymptotes are the velocities of longitudinal and the transverse acoustic phonons propagating in bulk materials in turns. The properties of the dispersion relations and their phonon modes are also analyzed in depth based on the cut-off conditions. By the changing of Al components x and y, the bottom of these groups will be modified to adjust eigen frequencies of AlxGa1−xN or Al1−yGayN layers. But the propagation properties of acoustic phonon modes will remain unchanged in each section.

Magnetic field control and wavelength tunability of SPP excitations using Al2O3/SiO2/Fe structures

Here, we show the high wavelength tunability and magnetic field modulation of surface plasmon polaritons (SPPs) of a waveguide mode that Double-layer Dielectrics and Ferromagnetic Metal, Al2O3/SiO2/Fe, trilayer structures exhibit when excited in the Otto configuration of attenuated total reflection setup. First by modeling, and then experimentally, we demonstrate that it is possible to tune the wavelength at which the angular dependent reflectance of these structures reaches its absolute minimum by simply adjusting the SiO2 intermediate dielectric layer thickness. This precise wavelength corresponds to the cut-off condition of SPPs' waveguide mode supported by the proposed structure, and it can be then switched between two values upon magnetization reversal of the Fe layer. In this specific situation, a large enhancement of the transverse magneto-optical effect is also obtained.

Effects of low-pass filtering on dialect and gender perception

In addition to linguistic (message-related) information, spoken language includes indexical information related to the speaker characteristics (e.g., gender, social status, and regional identity). This study is an extension of Jacewicz et al. (2015, JASA, 137, 2417–2418) which explored the nature of acoustic cues signaling indexical information. That study demonstrated that listeners were quite accurate in making decisions regarding the regional dialect and gender of a speaker when responding to short unprocessed phrases from 40 speakers (male and female) from two different dialects spoken in central Ohio and western North Carolina. However, when the signal was low-pass filtered at 400 Hz, sensitivity to both dropped significantly. The current study examined performance on the same phrases when the signal was low-pass filtered at progressively higher cutoff frequencies (500, 700, 900, and 1100 Hz). These stimuli were played to 20 listeners (10 males and 10 females) who identified the sex and dialect of the speaker. As expected, listener sensitivity improved with each wider low-pass filter condition. Performance in the 1100-Hz cutoff condition was very similar to that in the unprocessed condition. Discussion will focus on the nature of the acoustic cues utilized by listeners across the four low-pass filter conditions.

Calculation of Dispersion Relation and Single Mode Operation in Surface Plasmon Resonance Based Fiber Optic Refractive Index Sensors

The dispersion relations of two lowest-order surface plasmon (SP) modes are derived for a structure consisting of a thin silver layer around a reduced cladding fiber. Mode splitting is formed by reducing metal thickness. Fulfillment of phase matching condition is investigated by inspection of the SPs’ and fiber modes’ dispersion diagrams. Propagation lengths, field patterns, and resonance wavelengths are determined with high accuracy. The sensitivity of 7.13 × 10−6 RIU is obtained for ambient refractive index range of 1.333–1.375. Short-range SP modes cannot be excited in this structure even in IR region. Dependence of SP cutoff conditions on the core radii, metal thickness, ambient refractive index, and operating wavelength are explored. It is proved that the proposed method is going to be useful for proper structural design of single plasmonic mode operation in SP resonance based fiber optic refractive index sensors.

A New Method and a New Scaling for Deriving Fermionic Mean-Field Dynamics

We introduce a new method for deriving the time-dependent Hartree or Hartree-Fock equations as an effective mean-field dynamics from the microscopic Schroedinger equation for fermionic many-particle systems in quantum mechanics. The method is an adaption of the method used in [Pickl, Lett. Math. Phys., 97(2):151-164, 2011] for bosonic systems to fermionic systems. It is based on a Gronwall type estimate for a suitable measure of distance between the microscopic solution and an antisymmetrized product state. We use this method to treat a new mean-field limit for fermions with long-range interactions in a large volume. Some of our results hold for singular attractive or repulsive interactions. We can also treat Coulomb interaction assuming either a mild singularity cutoff or certain regularity conditions on the solutions to the Hartree(-Fock) equations. In the considered limit, the kinetic and interaction energy are of the same order, while the average force is subleading. For some interactions, we prove that the Hartree(-Fock) dynamics is a more accurate approximation than a simpler dynamics that one would expect from the subleading force. With our method we also treat the mean-field limit coupled to a semiclassical limit, which was discussed in the literature before, and we recover some of the previous results. All results hold for initial data close (but not necessarily equal) to antisymmetrized product states and we always provide explicit rates of convergence.