Rigorous Numerical Simulations Research Articles

Optical and Electrical Analyses of Solar Cells with a Radial PN Junction and Incorporating an Innovative NW Design That Mimics ARC Layers.

The implementation of a texturing pattern on the surface of a solar cell is well known for reducing reflection, thus increasing the absorption of sunlight by the solar cell. Nanowires (NWs) that are large in their height have been widely used for this purpose. Through rigorous numerical simulations, this work explores the benefits of short but index-matched NWs and how these designs are also affected by surface recombination. Additionally, this work further optimized power conversion efficiency (PCE) by placing two or three NWs of different heights and diameters on top of each other to mimic the performance of two-NW and three-NW ARC designs with PCEs of 16.8% and 17.55%, respectively, when a radial pn junction is considered. These are the highest reported so far for such a thin silicon solar cell. Furthermore, we also show how these designs were impacted by surface recombination velocity and compare these findings to simple NWs of different heights and diameters.

Optical computation of the Laplace operator at oblique incidence using a multilayer metal-dielectric structure.

We investigate the possibility of the optical computation of the Laplace operator in the oblique incidence geometry using a layered structure consisting of a set of homogeneous thin films. For this, we develop a general description of the diffraction of a three-dimensional linearly polarized optical beam by a layered structure at oblique incidence. Using this description, we derive the transfer function of a multilayer structure consisting of two three-layer metal-dielectric-metal structures and possessing a second-order reflection zero with respect to the tangential component of the wave vector of the incident wave. We show that under a certain condition, this transfer function can coincide up to a constant multiplier with the transfer function of a linear system performing the computation of the Laplace operator. Using rigorous numerical simulations based on the enhanced transmittance matrix approach, we demonstrate that the considered metal-dielectric structure can optically compute the Laplacian of the incident Gaussian beam with the normalized root-mean-square error of the order of 1%. We also show that this structure can be effectively utilized for optical edge detection of the incident signal.

Plasmon Enhanced Quantum Properties of Single Photon Emitters with Hybrid Hexagonal Boron Nitride Silver Nanocube Systems

AbstractHexagonal boron nitride (hBN) has emerged as a promising ultrathin host of single photon emitters (SPEs) with favorable quantum properties at room temperature, making it a highly desirable element for integrated quantum photonic networks. One major challenge of using these SPEs in such applications is their low quantum efficiency. Recent studies have reported an improvement in quantum efficiency by up to two orders of magnitude when integrating an ensemble of emitters such as boron vacancy defects in multilayered hBN flakes embedded within metallic nanocavities. However, these experiments have not been extended to SPEs and are mainly focused on multiphoton effects. Here, the quantum single‐photon properties of hybrid nanophotonic structures composed of SPEs created in ultrathin hBN flakes coupled with plasmonic silver nanocubes (SNCs) are studied. The authors demonstrate 200% plasmonic enhancement of the SPE properties, manifested by a strong increase in the SPE fluorescence. Such enhancement is explained by rigorous numerical simulations where the hBN flake is in direct contact with the SNCs that cause the plasmonic effects. The presented strong and fast single photon emission obtained at room temperature with a compact hybrid nanophotonic platform can be very useful to various emerging applications in quantum optical communications and computing.

Some Jerk Systems with Hidden Chaotic Dynamics

Hidden chaotic attractors is a fascinating subject of study in the field of nonlinear dynamics. Jerk systems with a stable equilibrium may produce hidden chaotic attractors. This paper seeks to enhance our understanding of hidden chaotic dynamics in jerk systems of three variables [Formula: see text] with nonlinear terms from a predefined set: [Formula: see text], where [Formula: see text] is a real parameter. The behavior of the systems is analyzed using rigorous Hopf bifurcation analysis and numerical simulations, including phase portraits, bifurcation diagrams, Lyapunov spectra, and basins of attraction. For certain jerk systems with a subcritical Hopf bifurcation, adjusting the coefficient of a linear term can lead to hidden chaotic behavior. The adjustment modifies the subcritical Hopf equilibrium, transforming it from an unstable state to a stable one. One such jerk system, while maintaining its equilibrium stability, experiences a sudden transition from a point attractor to a stable limit cycle. The latter undergoes a period-doubling route to chaos, which may be followed by a reverse route. Therefore, by perturbing certain jerk systems with a subcritical Hopf equilibrium, we can gain insights into the formation of hidden chaotic attractors. Furthermore, adjusting the coefficient of the nonlinear term [Formula: see text] in certain systems with a stable equilibrium can also lead to period-doubling routes or reverse period-doubling routes to hidden chaotic dynamics. Both findings are significant for our understanding of the hidden chaotic dynamics that can emerge from nonlinear systems with a stable equilibrium.

Plasmonic Generation of Spatiotemporal Optical Vortices

We investigate the transformation of spatiotemporal optical signals using the Kretschmann configuration with an additional dielectric layer, which can be referred to as the “generalized Kretschmann setup”. We demonstrate that in the considered structure, it is possible to achieve the condition of generating a reflected optical pulse containing a spatiotemporal optical vortex, which appears to be impossible in the conventional Kretschmann configuration. High-quality generation of spatiotemporal optical vortices using the investigated structure was confirmed by the results of rigorous numerical simulations. The obtained results are promising for applications in analog optical computing and optical information processing systems.

Cold-formed steel elliptical hollow section stub columns under combined compression and biaxial bending

The paper presents a numerical study on the cross-section behavior of cold-formed (CF) steel elliptical hollow sections (EHS) under combined compression and biaxial bending. In the present study, a finite element (FE) model for CF steel tubular stub columns under combined compression and biaxial bending was first developed, in which the local geometric imperfections, residual stresses and enhanced material behavior were predicted by a rigorous numerical simulation of roll forming. The developed FE model for CF steel tubular stub columns under combined compression and biaxial bending was validated against test results of various tubular sections (including EHS). Then, the validated FE model was employed to carry out a parametric study for CF steel EHS with a large range of steel grades (including both high strength and normal grade steels), section sizes, loading eccentricities and angles. Based on the generated FE data, the traditional design specifications for doubly symmetric cross-sections under combined loading in EN 1993-1-1: 2005 (CEN, 2005) and ANSI/AISC 360-16 (AISC, 2016), as well as the current design equations for hot-finished (HF) normal grade steel EHS suggested by Gardner et al. (2011) under combined loading were evaluated for the design of CF steel EHS under combined loading. New slenderness limits of Class 2 and Class 3 sections, as well as new design equations of cross-section resistances, for CF steel EHS stub columns under combined compression and biaxial bending are proposed.

Plasmonic Slot Waveguide Propagation Analysis

Plasmonic slot waveguides provide extreme light confinement with the benefits of having naturally present electrodes for switching and high thermal conductivity of the metal layers to remove excess heat. Past works relied on numerical computation for these structures, which is time-consuming and lacks physical insight. Here, we present an analytical model of plasmonic slot waveguides to determine the modal properties based on single-mode matching to continuum. The model is accurate to within 3% of rigorous numerical simulations. The theory allows for rapid design and provides physical insight into mode propagation in plasmonic slot waveguides for information processing, optical manipulation, and sensing applications.

A noise tolerant parameter-variable zeroing neural network and its applications

Time-varying problems frequently arise in the territories of science and engineering, and most of the time-varying problems can be described by dynamic matrix equations. As a powerful tool for solving dynamic matrix equations, the zeroing neural network (ZNN) develops fast in recent years. Convergence and robustness are two main performance indicators of the ZNN model. However, the development of the ZNN is focused on the improvement of its convergence in the past, and its robustness to noises is rarely considered. In order to achieve fast convergence and robustness of the ZNN model, a novel activation function (NAF) is presented in this paper. Based on the NAF, a noise-tolerant parameter-variable ZNN (NTPVZNN) model for solving dynamic Sylvester matrix equations (DSME) is realized, and its fixed-time convergence and robustness to noises are verified by rigorous mathematical analysis and numerical simulation results. Besides, two examples of electrical circuit currents computing and robotic manipulator trajectory tracking using the proposed NTPVZNN model in noisy environment further demonstrates its practical application ability.

Adaptive Control of Quadrotor Unmanned Aerial Vehicle With Time-Varying Uncertainties

This paper studies the real-time parameter estimation and adaptive tracking control problem for a six degrees of freedom (6-DOF) of quadrotor unmanned aerial vehicle (UAV) as an under-actuated system. A virtual proportional derivative (PD) is proposed to maintain position dynamics. Two adaptive control schemes are designed and compared to maintain the attitude dynamics of UAV while several parameters of UAV are unknown. In the first scheme, a classical adaptive scheme using the certainty equivalence principle is extended and designed for tracing control of the systems with unknown time-varying parameters. To improve the performance of the first scheme, a new control scheme is designed in the second scheme by proposing additional continuous function to handle the unknown parameters. An additional robust term is designed in both schemes to handle the perturbation caused by unknown time-varying parameters. The rigorous analytical proof and numerical simulation analysis are provided to support the efficacy of the proposed controller.

Bifurcation analysis of a two–dimensional p53 gene regulatory network without and with time delay

<abstract><p>In this paper, the stability and bifurcation of a two–dimensional p53 gene regulatory network without and with time delay are taken into account by rigorous theoretical analyses and numerical simulations. In the absence of time delay, the existence and local stability of the positive equilibrium are considered through the Descartes' rule of signs, the determinant and trace of the Jacobian matrix, respectively. Then, the conditions for the occurrence of codimension–1 saddle–node and Hopf bifurcation are obtained with the help of Sotomayor's theorem and the Hopf bifurcation theorem, respectively, and the stability of the limit cycle induced by hopf bifurcation is analyzed through the calculation of the first Lyapunov number. Furthermore, codimension-2 Bogdanov–Takens bifurcation is investigated by calculating a universal unfolding near the cusp. In the presence of time delay, we prove that time delay can destabilize a stable equilibrium. All theoretical analyses are supported by numerical simulations. These results will expand our understanding of the complex dynamics of p53 and provide several potential biological applications.</p></abstract>

A magnetic field coupling fractional step lattice Boltzmann model for the complex interfacial behavior in magnetic multiphase flows

In the present study, a magnetic field coupling fractional step lattice Boltzmann (FSLB) model is proposed to simulate the complex interfacial behaviors in magnetic multiphase flows with severe interface deformation and large density ratio, which can be recovered to the corresponding macroscopic governing equations with fully second-order accuracy. Since the equilibrium and the non-equilibrium terms are obtained by the Chapman-Enskog (C-E) expansion analysis, the basic idea of the present method is to predict the intermediate variables by updating the various moments of equilibrium distribution functions first, and then correct the physical variables by updating the various moments of non-equilibrium distribution functions. Besides, the present magnetic field coupling FSLB method does not need the evolution of distribution functions compared to the existing LB methods. The present method inherits the excellent performance from the conventional LB method and the relatively better numerical stability from the matured fractional-step method, which is more stable than the original LB method and more efficiency and easy-to-implement than the LB flux solver. Owing to those good properties, we successfully perform the first rigorous numerical simulation on the Rosensweig instability, which accurately captures the spikes forming at the surface of a ferrofluid under an applied magnetic field. Moreover, for the first time we numerically reproduce the falling ferrofluid droplet impacted on the solid surface. Intriguingly, our simulation results reveal that the falling velocity of the ferrofluid droplet is slightly accelerated by the elongation, although the direction of the magnetic field is opposite to that of the gravity. All the results for the interfacial behavior and the magnetic interaction highlights the comparative capability, stability, and accuracy of the present magnetic field coupling FSLB method for simulating the problems of complex fluid dynamics associated with a severe interface deformation and large density ratio in magnetic multiphase flows.

Online optimization of intelligent reflecting surface-aided energy-efficient IoT-edge computing

With the tremendous developments of Internet of Things (IoT), IoT devices and applications are imposing progressively higher requirements on computing, communication, and power services. Edge computing in combination with wireless power transmission (WPT) presents a viable solution. By empowering wireless access points (APs) with the capability of computation and WPT, APs can act as both edge computing servers and power stations. To improve the efficiency of data transmission and WPT, the Intelligent Reflective Surface (IRS) technique is adopted to passively beamforming thereby improving the channel condition of the wireless links. In such an IoT-edge system, efficiency and stability are the main concerns in designing a strategy of workload scheduling and resource allocation. In this paper, we propose an online algorithm named LSDR (Lyapunov optimization in combination with semi-definite relaxation) to minimize the total energy consumption while maintaining the system’s stable operation. The system’s stability implies that unprocessed data on the AP is limited and the battery level of each IoT sensor should not be depleted. Without requiring prior knowledge of the statistics of the IoT-edge system, the algorithm can balance the trade-off between system stability and total energy consumption. Through rigorous theoretical analysis and extensive numerical simulations, we demonstrate the effectiveness and efficiency of the algorithm.

Soliton dynamics in a piecewise gas-filled hollow-core fiber for efficient dispersive wave emission in the vacuum-ultraviolet range

Table-top coherent vacuum-ultraviolet (VUV) sources are important in many researches. Dispersive wave (DW) emission in a gas-filled hollow-core fiber (HCF) is an efficient way to obtain tunable VUV source with microjoule-level pulse energy. However, the shortest VUV wavelength is limited above 110 nm. This work investigated a method using a piecewise HCF in order to push the shortest VUV DW emission below 110 nm efficiently. The rich and complex spatiotemporal soliton dynamics in the HCF is revealed through rigorous numerical simulations and quantitative analysis. It is found that both the generation of VUV light below 110 nm and the overall conversion efficiencies below 150 nm can be significantly improved if the piecewise gas-filled HCF system is properly designed. As a result, the whole VUV spectrum (200--100 nm) can now be accessed efficiently, with single pulse energy of microjoule level. Different from other widely used intense VUV sources, this source has broad spectrum along with femtosecond time width. These unique characteristics in the VUV spectrum can lead to new research opportunities in various areas.

Optical properties of guided-mode resonant gratings with linearly varying period

Guided-mode resonant gratings with spatially varying parameters are widely used as linear variable optical filters, and their behavior is often described using the so-called local periodic approximation, in which the structure is locally replaced by a strictly periodic grating with the period coinciding with the ``local'' period at the considered point. In this work, we investigate the optical properties of guided-mode resonant gratings with the period linearly varying along the periodicity direction. Using full-wave numerical simulations, we show that when the period change rate is relatively high (about 0.5--1 $\ensuremath{\mu}\mathrm{m}/\mathrm{mm}$ for the considered structures), the local periodic approximation becomes inapplicable, and the linewidth and the line shape of the resonances depend significantly on the period change rate. We qualitatively explain the appearance of an asymmetric non-Fano line shape with secondary maxima by analyzing the local photonic band structure of the studied varying-period gratings. For a more accurate description of such gratings, we develop a spatiotemporal coupled-mode theory, the predictions of which are in good agreement with the rigorous numerical simulation results. The validity of the derived theoretical model is additionally confirmed by a proof-of-concept experiment with a varying-period guided-mode resonant grating. The obtained results may find application in the design of compact linear variable filters based on resonant gratings with spatially varying parameters.

Promoting Light Extraction Efficiency of Ultraviolet Light Emitting Diodes by Nanostructure Optimization

Ultraviolet (UV) light-emitting diodes (LEDs), as one of the more promising optoelectronic devices, are intrinsically limited by poor light extraction efficiencies (LEEs). To unlock the full potential of UV-LEDs, we propose a simple and effective strategy to promote the LEEs of UV-LEDs by screening and tailoring suitable optical structures/designs through rigorous numerical simulations. The photonic crystals (PCs) and/or nano-patterned sapphire substrates (NPSSs) equipped with the nano-pillar, nano-cone, nano-oval, and their derivates, are particularly investigated. The simulated results show that individual PC with an average transmittance of 28% is more efficient than that of individual NPSS (24.8%). By coupling PC and NPSS structures, a higher LEE with an average transmittance approaching 29% is obtained, much higher than that of the flat one (23.6%). The involved mechanisms are clarified and confirm that the promotion of optical performance of the nanostructured devices should be attributed to the widened response angles (from 0 to 60°), rather than the enhanced transmittances in the small angles within 30°.

Investigation of bevel-ended large-span soil-steel structures

Soil-steel structures (SSS) are increasingly used as a solution for roadway and waterway overpasses. The inlet and outlet of the buried steel structure may encounter different end treatments and bevelling slopes to accommodate different design aspects. Current design codes require precise numerical analysis of SSS considering soil-structure interaction to evaluate the stability of the steel structure and unbalanced surrounding backfill. The current study evaluates the performance of large-span SSS considering different end treatment conditions. The numerical modeling approach was validated by conducting three-dimensional nonlinear finite element model for the world’s largest-span soil-steel structure and comparing the model predictions with field observations. The steel structure has a span of 32.40 m and was constructed using a corrugation profile of 237 mm depth and 500 mm pitch. The ends of the structure were strengthened by attaching 400 mm X 1090 mm circumferential reinforced concrete collars. The numerical analysis predicted the same deflection trends as the field measurements at different construction stages. Furthermore, the validated numerical approach was employed to evaluate the performance of bevel-ended SSS. The benefits of stiffening SSS by using circumferential concrete collars and steel mesh reinforcements buried in the surrounding soil are evaluated in terms of the stability of steel structure and surrounding soil. The results indicated significant longitudinal bending moment induced in the steel plates that were affected by the slope of the beveled ends. It was also found that the steel mesh reinforcement and concrete collars reduced the induced straining actions in the steel structure by 30–50%. The obtained results demonstrate the necessity of rigorous numerical simulation in controlling the design and cost of SSS.

Influence of predator incited fear and interference competition in the dynamics of prey-predator system where the prey species are protected in a reserved area

In this article, we contemplated a prey-predator model where the dwelling place of prey species is partitioned into two areas, namely reserved area and unreserved area. Both the prey and predator species are allowed in unreserved area to access their living resources; whereas reserved area is protected only for prey species and predators are not allowed by any means in this area. At the unreserved zone, predation is allowed and prey species are in fear of predation by the predators. As a consequence, it is assumed that prey species in unreserved zone shows many anti-predator behaviour and defense mechanism (due to fear of predation), which create the shortage of food and compels the predators to engage into a clash among themselves. After completion of model construction, the persistence of all the species and well-posedness of the model system have been studied. The existence and stability criterion of all the equilibrium points have been found. Furthermore, bifurcation analysis is done and direction of Hopf-bifurcation is detected using center manifold theory. Through model analysis and rigorous numerical simulation, it is observed that cost of fear and interference competition among the predators cooperatively determine the eco-dynamics of the model system in a complex manner. Finally, we have plotted various 2-parameter bifurcation diagrams near the hopf-point to explore the role of different parameters involved in the model system.

Structure of an artificial-intelligence-assisted reservoir characterization and field development protocol

Characterization of oil reservoirs for effective field development strategies is an arduous task as it requires the integration of the field data collected in various forms (for instance, seismic survey, well log and field historical pressure/production/injection data, etc.) which requires challenging workflows. The implementation of high-fidelity numerical simulators plays a vital role in quantitively identifying the sweet spots for infill drilling and optimizing the field development strategies. However, the structuring of a reliable numerical simulation model requires gridding, property upscaling and history matching, which all could be considered as laborious and time-intensive processes. Moreover, when the availability of the field data is limited, the use of high-fidelity numerical model could be prohibitively difficult, which may necessitate the development of an alternative approach to effectively characterize, integrate and design the field development plans. This paper presents a class of artificial-neural-network based expert systems trained and tested using field data collected from an oil field in North America. The proposed expert system is capable of generating artificial well logs and assessing the hydrocarbon productivity at hypothetical infill drilling locations covered by the seismic survey. Taking advantages of the robust computational efficacies of the expert systems, high-resolution heat maps indicating the productivity sweet spots can be generated to assist the decision makers in selecting the most prolific infill drilling locations when the available field data is insufficient to structure a rigorous numerical simulation model. The expert systems are trained to adapt to the data structure as exhibited by the field exploration and production data and generate realistic predictions.

Reducing bend-induced loss and crosstalk in a two-mode ridge waveguide by steplike thickness structuring.

This study demonstrated that two-step structuring of the waveguide thickness in a bent section significantly reduces bend-related inter-mode crosstalk, excess loss, and pure bending loss. A simple analytical formula was derived linking the thickness change required to compensate for bend-induced effects with geometrical parameters of the waveguide and bending radius. The effectiveness of the proposed approach was verified through rigorous numerical simulations of the loss and inter-mode crosstalk based on transformation optics formalism. The numerical results obtained for a TiO2:SiO2 medium index contrast waveguide exhibited significant improvement of all relevant parameters for TE00 and TE01 modes.

A Better Than Alamouti OSTBC for MIMO Backscatter Communications

Backscatter communications have received increasingly attention in Internet of Things (IoT) due to its advantages of requiring no internal battery and almost zero maintenance. However, the drawback of backscatter communications is that, its channel fades deeper than conventional channel. To improve the performance, multiple-input multiple-output (MIMO) techniques have been investigated for backscatter communications. By considering duty cycle, a particular behavior of backscatter communications, as a new performance criterion, we propose a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2 \times 2$ </tex-math></inline-formula> orthogonal space-time block code for MIMO backscatter communications. The new code is a rotation of the classical Alamouti code, and in MIMO backscatter channels, this simple rotation will incur an improved duty cycle for both linear and nonlinear energy harvester models, as well as a same, or even better symbol error rate performance. We provide both rigorous mathematical proofs and numerical simulations to show the above results. An interesting thing is that, this new code does not outperform Alamouti code in conventional MIMO channels, which indicates that the space-time code design criteria for conventional channels is not the optimal one for backscatter channels, and existing space-time codes should be reconsidered and/or redesigned to achieve a higher performance in MIMO backscatter communications.