Time-dependent Pulse Research Articles

Resonance-like enhancement in high-order above-threshold ionization of formic acid

We have investigated the high-order above-threshold ionization (HATI) spectrum of formic acid by means of high-resolution electron spectroscopy using 800-nm femtosecond laser light. The resonance-like enhancement structures in the HATI spectrum are shown to be experimentally reproducible and three enhancement-suppression patterns are observed in different energy regimes with increasing laser intensity. We simulated these patterns successfully by computing the exact solution of time-dependent Schr\"odinger equation based on the simple hydrogen-like atom, subject to a time-dependent high-intensity laser pulse. The agreement between experiment and theory shows that ``channel closing'' effects play an important role in the observed resonance-like enhancements (RLEs). The existence of the RLEs for formic acid, which lacks any symmetry, shows that molecular symmetry does not affect the RLEs.

An effective shortcut to adiabatic passage for fast quantum state transfer in a cavity quantum electronic dynamics system

We propose an alternative scheme for constructing a shortcut to implement the quantum state transfer between two three-level atoms founded on the invariant-based inverse engineering in a cavity quantum electronic dynamics (QED) system. Quantum information can be quickly transferred between atoms by taking advantage of the cavity field as a medium. Through our design of the time-dependent laser pulse and atom–cavity coupling, we send atoms through the cavity within a short time interval, which involves the two processes of the invariant dynamics between each atom and the cavity field simultaneously. We redesign a reasonable Gaussian-type wave form in the atom–cavity coupling for a realistic experimental operation. Numerical simulation shows that the target state can be quickly populated with a high fidelity which is robust against both the parameter fluctuations and the dissipation.

Superadiabatic particle number in Schwinger and de Sitter particle production

We consider the time evolution of the adiabatic particle number in both time-dependent electric fields and in de Sitter spaces, and define a superadiabatic particle number in which the (divergent) adiabatic expansion is truncated at optimal order. In this superadiabatic basis, the particle number evolves smoothly in time, according to Berry's universal adiabatic smoothing of the Stokes phenomenon. This superadiabatic basis also illustrates clearly the quantum interference effects associated with particle production, in particular, for sequences of time-dependent electric field pulses, and in eternal de Sitter space where there is constructive interference in even dimensions and destructive interference in odd dimensions.

Influence of the time-dependent pulse spectrum on ionization and laser propagation in nonlinear optical materials

The interaction and the propagation of intense and short laser pulses in nonlinear optical materials are addressed. A consistent introduction of a time-dependent ionization model into the Maxwell's equations allows one to account for the influence of strong variations of the pulse spectrum on the ionization dynamics, which in turn affects the pulse propagation. The interaction between both ionization and propagation dynamics is demonstrated for two cases of practical interest: frequency conversion and propagation of chirped pulses.

Effects of autoionizing states on two-photon double ionization of the H2 molecule

Treating the effects of autoionizing intermediate states on two-photon double ionization (DI) of the H2 molecule using time-dependent laser pulses is a significant computational challenge. Relatively long exposure times are critical to understanding the dynamics. Using the fixed-nuclei approximation, we demonstrate how the doubly excited states enhance the angle-integrated generalized cross sections in H2, and how they affect the angular distribution pattern of the ejected electrons. As the energy approaches the threshold for sequential DI, there is a sharp rise in the cross section due to virtual sequential ionization.

Generating Highly Dark–Bright Solitons by Gaussian Beam Propagation in a PANDA Ring Resonator

A novel system for generating dark-bright solitons is proposed. The system uses a PANDA ring resonator incorporated with an add/drop filter system. With resonant conditions, chaotic signals are generated from the through and drop ports of the PANDA system and then propagated in the add/drop filter system. Simulation results show that inputting Gaussian pulses in the system generates and controls highly dark-bright signals. To increase system capacity and obtain the required channel signals, we select 20, 100, and 400 μm as the radii of the add/drop filter system. The central wavelength of input power is 1.55 μm, by which ultrashort dark-bright soliton pulses are generated. The simulation results for the wavelength-dependent soliton pulses show full widths at half maximum (FWHMs) of 2.1, 0.42, and 0.11 nm, as well as free spectral ranges (FSRs) of 5.75, 1.15, and 0.29 nm. These values correspond to 20, 100, and 400 μm, respectively. The simulated time-dependent soliton pulses have FWHMs of 8.1, 9.2, and 15.3 ps with a constant FSR of 25 ps. A maximum output power of 80 W can be obtained from the drop port of the PANDA system.

Performance of Electro-Thermally Driven ${\rm VO}_{2}$-Based MEMS Actuators

The integration of VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films in a MEMS actuator device is presented. The structural phase transition of VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> was induced electro-thermally by resistive heaters monolithically integrated in the MEMS actuator. The drastic mechanical displacements generated by the large stress induced during the VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin film phase transition have been characterized for static and time-dependent current pulses to the resistive heater, for air and vacuum environments. A comprehensive and simplified finite element model is developed and validated with experimental data. It was found that the cut-off frequency of the 300 μm-long VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based MEMS actuator operated in vacuum (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3dB</sub> =29 Hz) was mostly limited by conductive heat loss through the anchor, whereas convection losses were more dominant in air (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3dB</sub> =541 Hz). The cut-off frequency is found to be strongly dependent on the dimensions of the cantilever when operated in air but far less dependent when operated in vacuum. Total deflections of 68.7 and 28.5 μm were observed for 300 and 200 μm-long MEMS cantilevers, respectively. Full actuation in air required ~ 16 times more power than in vacuum.

Constant-Momentum Acceleration Time-of-Flight Mass Spectrometry with Energy Focusing

Fundamental aspects of constant-momentum acceleration time-of-flight mass spectrometry (CMA-TOFMS) are explored as a means to improve mass resolution. By accelerating all ions to the same momentum rather than to the same energy, the effects of the initial ion spatial and energy distributions upon the total ion flight time are decoupled. This decoupling permits the initial spatial distribution of ions in the acceleration region to be optimized independently, and energy focus, including ion turn-around-time error, to be accomplished with a linear-field reflectron. Constant-momentum acceleration also linearly disperses ions across time according to mass-to-charge (m/z) ratio, instead of the quadratic relationship between flight time and m/z found in conventional TOFMS. Here, CMA-TOFMS is shown to achieve simultaneous spatial and energy focusing over a selected portion of the mass spectrum. An orthogonal-acceleration time-of-flight system outfitted with a reduced-pressure DC glow discharge (GD) ionization source is used to demonstrate CMA-TOFMS with atomic ions. The influence of experimental parameters such as the amplitude and width of the time-dependent CMA pulse on mass resolution is investigated, and a useful CMA-TOFMS focusing window of 2 to 18Da is found for GD-CMA-TOFMS.

Time-spatial drift of decelerating electromagnetic pulses

A time dependent electromagnetic pulse generated by a current running laterally to the direction of the pulse propagation is considered in paraxial approximation. It is shown that the pulse envelope moves in the time-spatial coordinates on the surface of a parabolic cylinder for the Airy pulse and a hyperbolic cylinder for the Gaussian. These pulses propagate in time with deceleration along the dominant propagation direction and drift uniformly in the lateral direction. The Airy pulse stops at infinity while the asymptotic velocity of the Gaussian is nonzero.

A rate equation method for the sequential double ionisation, including autoionising state excitation, of a noble gas

AbstractA set of rate equations have been tested against a more robust set of Time-Dependent Density Matrix (TDDM) equations [D. P. W. Middleton, L. A. A. Nikolopoulos, J. Mod. Opti. 59, 1650 (2012)] by using them to determine the populations of ion species and autoionising states (AIS) in noble gas atoms when interacting with a strong external field. Two field shapes were tested here — sinusoidal and square — and a variety of pulse characteristics were examined, i.e. intensity, duration and photon energy, for the neon atomic system. It was found that the rate equations were sufficiently accurate only when the external field is way off-resonant with the AIS. Moreover, analytical solutions of the rate equations in the square pulse case agree with the numerical solutions for a time-dependent pulse containing many cycles. An attempt to model a stochastic field was also made and it was found that the use of such a field diminished and broadened the ion yield ratio due to the presence of an added bandwidth.

Studies in ultrasonic pulse velocity of concrete containing GGBFS

This paper presents the results of experimental investigations into the time-dependent ultrasonic pulse velocity (UPV) of concrete containing GGBFS. The UPV was determined at the age of 3, 7, 28, 56, 90, 150 and 180days for twelve concrete mixes using the prism specimens of plain and GGBFS concrete. The amount of cement replaced by GGBFS was varied from 20% to 60%. The UPV of concrete containing GGBFS has been found to be lower than that of plain concrete for all percentage replacements of cement at all ages and for all the mixes. Relationship between cube and cylinder compressive strength and UPV for GGBFS based concrete has been presented. A new model for time-dependent dynamic modulus of elasticity of concrete containing GGBFS has been proposed.

Suppression of Dendrite Formation via Pulse Charging in Rechargeable Lithium Metal Batteries

We introduce a coarse-grained simulation model for the reductive deposition of lithium cations in secondary lithium metal batteries. The model accounts for the heterogeneous and nonequilibrium nature of the electrodeposition dynamics, and it enables simulation of the long timescales and lengthscales associated with metal dendrite formation. We investigate the effects of applied overpotential and material properties on early-stage dendrite formation, as well as the molecular mechanisms that govern this process. The model confirms that dendrite formation propensity increases with the applied electrode overpotential, and it demonstrates that application of the electrode overpotential in time-dependent pulses leads to dramatic suppression of dendrite formation while reducing the accumulated electrode on-time by as much as 96%. Moreover, the model predicts that time dependence of the applied electrode overpotential can lead to positive, negative, or zero correlation between cation diffusivity in the solid–electrolyte interphase (SEI) and dendrite formation propensity. Analysis of the simulation trajectories reveals that dendrite formation emerges from a competition between the timescales for cation diffusion and reduction at the anode/SEI interface, with lower applied overpotentials and shorter electrode pulse durations shifting this competition in favor of lower dendrite formation propensity. This work provides a molecular basis for understanding and designing pulsing waveforms that mitigate dendrite formation while minimally affecting battery charging times.

Nonlinear dynamic analysis of a laminated hybrid composite plate subjected to time-dependent external pulses

Nonlinear dynamic responses of a laminated hybrid composite plate subjected to time-dependent pulses are investigated. Dynamic equations of the plate are derived by the use of the virtual work principle. The geometric nonlinearity effects are taken into account with the von Kármán large deflection theory of thin plates. Approximate solutions for a clamped plate are assumed for the space domain. The single term approximation functions are selected by considering the nonlinear static deformation of plate obtained using the finite element method. The Galerkin Method is used to obtain the nonlinear differential equations in the time domain and a MATLAB software code is written to solve nonlinear coupled equations by using the Newmark Method. The results of approximate-numerical analysis are obtained and compared with the finite element results. Transient loading conditions considered include blast, sine, rectangular, and triangular pulses. A parametric study is conducted considering the effects of peak pressure, aspect ratio, fiber orientation and thicknesses.

Population Evolution of Rydberg Rubidium Atoms by Half-Cycle Pulses

Using the B-spline basis set method combined with model potential, the Stark energy level of rubidium atoms in the vicinity of n = 30 is presented. By using a using time-dependent multilevel approach, we calculate the population redistribution of high Rydberg rubidium atoms under the interaction of external time-dependent half-cycle pulses. Our numerical results show that the population of rubidium atoms can be driven to lower or higher n levels with a train of half cycle pulses, the final population distribution of all the l states for the same n is observed after these interactions.

Evaluating the use of a continuous approximation for model-based quantification of pulsed chemical exchange saturation transfer (CEST)

Many potential clinical applications of chemical exchange saturation transfer (CEST) have been studied in recent years. However, due to various limitations such as specific absorption rate guidelines and scanner hardware constraints, most of the proposed applications have yet to be translated into routine diagnostic tools. Currently, pulsed CEST which uses multiple short pulses to perform the saturation is the only viable irradiation scheme for clinical translation. However, performing quantitative model-based analysis on pulsed CEST is time consuming because it is necessary to account for the time dependent amplitude of the saturation pulses. As a result, pulsed CEST is generally treated as continuous CEST by finding its equivalent average field or power. Nevertheless, theoretical analysis and simulations reveal that the resulting magnetization is different when the different irradiation schemes are applied. In this study, the quantification of important model parameters such as the amine proton exchange rate from a pulsed CEST experiment using quantitative model-based analyses were examined. Two model-based approaches were considered – discretized and continuous approximation to the time dependent RF irradiation pulses. The results showed that the discretized method was able to fit the experimental data substantially better than its continuous counterpart, but the smaller fitted error of the former did not translate to significantly better fit for the important model parameters. For quantification of the endogenous CEST effect, such as in amide proton transfer imaging, a model-based approach using the average power equivalent saturation can thus be used in place of the discretized approximation.

A modular method to handle multiple time-dependent quantities in Monte Carlo simulations

A general method for handling time-dependent quantities in Monte Carlo simulations was developed to make such simulations more accessible to the medical community for a wide range of applications in radiotherapy, including fluence and dose calculation. To describe time-dependent changes in the most general way, we developed a grammar of functions that we call ‘Time Features’. When a simulation quantity, such as the position of a geometrical object, an angle, a magnetic field, a current, etc, takes its value from a Time Feature, that quantity varies over time. The operation of time-dependent simulation was separated into distinct parts: the Sequence samples time values either sequentially at equal increments or randomly from a uniform distribution (allowing quantities to vary continuously in time), and then each time-dependent quantity is calculated according to its Time Feature. Due to this modular structure, time-dependent simulations, even in the presence of multiple time-dependent quantities, can be efficiently performed in a single simulation with any given time resolution. This approach has been implemented in TOPAS (TOol for PArticle Simulation), designed to make Monte Carlo simulations with Geant4 more accessible to both clinical and research physicists. To demonstrate the method, three clinical situations were simulated: a variable water column used to verify constancy of the Bragg peak of the Crocker Lab eye treatment facility of the University of California, the double-scattering treatment mode of the passive beam scattering system at Massachusetts General Hospital (MGH), where a spinning range modulator wheel accompanied by beam current modulation produces a spread-out Bragg peak, and the scanning mode at MGH, where time-dependent pulse shape, energy distribution and magnetic fields control Bragg peak positions. Results confirm the clinical applicability of the method.

Ramsey Fringes and Time-Domain Multiple-Slit Interference from Vacuum

Sequences of alternating-sign time-dependent electric field pulses lead to coherent interference effects in Schwinger vacuum pair production, producing a Ramsey interferometer, an all-optical time-domain realization of the multiple-slit interference effect, directly from the quantum vacuum. The interference, obeying fermionic quantum statistics, is manifest in the momentum dependence of the number of produced electrons and positrons along the linearly polarized electric field. The central value grows like N(2) for N pulses [i.e., N "slits"], and the functional form is well described by a coherent multiple-slit expression. This behavior is generic for many driven quantum systems.

An analytical approach for free vibration and transient response of functionally graded piezoelectric cylindrical panels subjected to impulsive loads

In this article, the free vibration and dynamic response of simply supported functionally graded piezoelectric cylindrical panel impacted by time-dependent blast pulses are analytically investigated. Using Hamilton’s principle, the equations of motion based on the first-order shear deformation theory are derived. Also, Maxwell’s electricity equation is taken as one of the governing equations. Three sets of electric surface conditions including closed circuit and two mixtures of closed and open circuit surface conditions are considered. By introducing an analytical approach and using the Fourier series expansions, the Laplace transform and Laplace inverse method, the solution of unknown variables are obtained in the real time domain based on a combination of system frequencies. Finally, the effects of various electric surface conditions, geometric parameters and the material power law index on the free vibration and transient response of functionally graded piezoelectric cylindrical panels subjected to various impulsive loads are examined in detail.

Particle Self-Bunching in the Schwinger Effect in Spacetime-Dependent Electric Fields

Nonperturbative electron-positron pair creation (the Schwinger effect) is studied based on the Dirac-Heisenberg-Wigner formalism in 1+1 dimensions. An ab initio calculation of the Schwinger effect in the presence of a simple space- and time-dependent electric field pulse is performed for the first time, allowing for the calculation of the time evolution of observable quantities such as the charge density, the particle number density or the total number of created particles. We predict a new self-bunching effect of charges in phase space due to the spatial and temporal structure of the pulse.

Response of solid Ne upon photoexcitation of a NO impurity: A quantum dynamics study

The ultrafast geometrical rearrangement dynamics of NO doped cryogenic Ne matrices after femtosecond laser pulse excitation is studied using a quantum dynamical approach based on a multi-dimensional shell model, with the shell radii being the dynamical variables. The Ne-NO interaction being only weakly anisotropic allows the model to account for the main dynamical features of the rare gas solid. Employing quantum wave packet propagation within the time dependent Hartree approximation, both, the static deformation of the solid due to the impurity and the dynamical response after femtosecond excitation, are analysed. The photoinduced dynamics of the surrounding rare gas atoms is found to be a complex high-dimensional process. The approach allows to consider realistic time-dependent femtosecond pulses and the effect of the pulse duration is clearly shown. Finally, using the pulse parameters of previous experiments, pump-probe signals are calculated and found to be in good agreement with experimental results, allowing for a clear analysis of the ultrafast mechanism of the energy transfer into the solid.