Pulses Of Different Shapes Research Articles

Manifestations of Rabi Dynamics in the Photoelectron Energy Spectra at Resonant Two-Photon Ionization of Atom by Intense Short Laser Pulses

We study the Rabi flopping of the population between the ground and excited 2p states of the hydrogen atom, induced by intense short laser pulses of different shapes and of carrier frequency ω=0.375 a.u. which resonantly couples the two states, and manifestations of this dynamics in the energy spectra of photoelectrons produced in the subsequent ionization of the atom from the excited state. It is found that, for Gaussian, half-Gaussian and rectangular pulses, characterized by the same pulse area, the final populations take the same values and the spectra consist of similar patterns having the same number of peaks and approximately the same separation between the prominent edge (Autler–Townes) peaks. The additional analysis in terms of dressed states showed that the mechanism of formation of multiple-peak structures during the photoionization process is the same regardless of the pulse shape. These facts disprove the hypothesis proposed in earlier studies with Gaussian pulse, that the multiple-peak pattern appears due to dynamic interference of the photoelectrons emitted with a time delay at the rising and falling sides of the pulse, since the hypothesis is not applicable to either a half-Gaussian pulse that has no rising part or a rectangular pulse whose intensity is constant.

Light emission from microwave discharges in liquid hydrocarbons at the initial stages of their development

AbstractTime‐resolved optical techniques (spectrograph, high‐speed camera, photodiode) were used for scrutinizing the initial stages of the microwave discharge (2.45 GHz, 200–500 W) achieved in liquid alkanes (CnH2n + 2). Petroleum solvent Nefras S2 80/120 (a mixture of light hydrocarbons with boiling temperatures ranging between 33°C and 205°C) was considered to be representative of alkanes. The discharge was ignited in the liquid, at the end of a molybdenum antenna located under ambient conditions above the liquid. The discharge displaced sequences of light pulses of different shapes and amplitudes, randomly distributed in time. The minimum pulse duration is 0.5 ms. At times close to 1 ms, the spectrum comprises emission bands of the C2 molecule, lines of hydrogen, and the C+ ion, but there is no continuum associated with the emission of the formed solid carbon‐containing particles. In a number of cases, an anomalous spectrum of the C2 molecule was recorded, with no such sequences having been observed before.

Analysis and prediction of the stability of train control systems to ultra-wideband electromagnetic pulses of intentional action

Problem of ensuring stability of railway automation systems to ultra-wideband pulses of an electromagnetic field is considered. These pulses can be used to intentionally affect the equipment of these systems. It is shown that modern complex systems of control and safety of train traffic are vulnerable to electromagnetic impulses of intentional action. The features that determine the difference between the problem of ensuring the stability of railway automation systems from information systems are highlighted. In particular, microprocessor-based automation systems are geographically distributed and accessible for action from a short distance. The test generators used to prove immunity to electromagnetic impulses from intentional exposure are unique installations. Therefore, to reduce the number of tests design estimates and comprehensive tests for immunity to electromagnetic interference are required. Electrostatic discharges have the widest frequency spectrum. They act on the same apertures in the housings of technical means of railway automation systems as impulses of intentional action. The level of energy absorbed in electronic nodes can be calculated using the Rayleigh theorem. The electromagnetic field inside the housing is determined by the known relations for the radiation of the antennas and depends on the spectrum of the electric component of the field in the antenna aperture. This spectrum, in turn, depends on the spectrum of the electrostatic discharge pulse and the geometric parameters of the antenna. When an electromagnetic wave of an intentional impulse is incident, the aperture releases this impulse and transmits it into the housing. Therefore, the intentional impulse emitted into the housing and the electrostatic discharge impulse can be compared in shape and amplitude. Pulses of different shapes, in turn, can be compared using the spectral-energy equivalence condition. Intentional impulse equivalent to an electrostatic discharge impulse and, accordingly, having the same energy with it, causes exactly the same failures or setback of the element base. The amplitude of the pulse received by the aperture and the amplitude of the transmitted pulse are related by the utilization of the antenna. Thus, authors have obtained a technique for indirectly assessing the influence of an electromagnetic pulse of intentional action based on the calculated prediction of the resistance of railway automation systems to electrostatic discharges. An analogue of the equation of power suppression of radioelectronic means is obtained, which allows finding the parameters of an electromagnetic pulse generator that creates dangerous pulses for railway automation systems. Also, this equation allows calculating the size of the suppression zone for a given generator.

A numerical method for pulse propagation in nonlinear fiber Bragg grating with ternary stability nature

Abstract Optical bistability and multistability are often produced in nonlinear media. Although continuous wave (CW) bistability and multistability have widely studied, solution of the nonlinear pulse propagation equations is rarely studied and practiced in bistable and ternary stable systems. In such systems, the history of the pulse should be considered for equation solving. In the present paper, the pulse propagation through the ternary nonlinear chalcogenide fiber Bragg grating is numerically investigated. A novel numerical method is proposed for solving the above mentioned problem using Fourier and inverse Fourier transform as well as dividing the Gaussian input pulse into two intervals which leads to the simplicity of hysteresis and time domain problems. As a result, an algorithm is suggested for solving nonlinear pulse propagation equation in bistable and ternary stable systems and the output pulse is obtained. This method can also be applied on various pulses of different shapes travelling through the other types of Fiber Bragg grating like chirped, phase-shifted, etc. Optical multistability systems are the main building blocks for designing all-optical pulse processing, all-optical memory and all-optical switching.

Antiferromagnetic nano-oscillator in external magnetic fields

We describe the dynamics of an antiferromagnetic nano-oscillator in an external magnetic field of any given time distribution. The oscillator is powered by a spin current originating from spin-orbit effects in a neighboring heavy metal layer, and is capable of emitting a THz signal in the presence of an additional easy-plane anisotropy. We derive an analytical formula describing the interaction between such a system and an external field, which can affect the output signal character. Interactions with magnetic pulses of different shapes, with a sinusoidal magnetic field and with a sequence of rapidly changing magnetic fields are discussed. We also perform numerical simulations based on the Landau-Lifshitz-Gilbert equation with spin-transfer torque effects to verify the obtained results and find a very good quantitative agreement between analytical and numerical predictions.

Experimental optimization of power-function-shaped drive pulse for stick-slip piezo actuators

Motion of a stick-slip piezo actuator is generally controlled by the parameters related to its mechanical design and characteristics of the driving pulses applied to piezoceramic shear plates. The goal of the proposed optimization method is to find the driving pulse parameters leading to the fastest and the most reliable actuator operation. In the paper the method is tested on a rotary stick-slip piezo actuating system utilized in an atomic force microscope.The optimization is based on the measurement of the actuator response to driving pulses of different shapes and repetition frequencies at various load forces. To provide it, a computer controlled testing system generating the driving pulses, and detecting and recording the corresponding angular motion response of the actuator by a position sensitive photo detector (PSPD) in real time has been developed. To better understand and interpret the experimental results, supportive methods based on a simple analytical model and numerical simulations were used as well.In this way the shapes of the single driving pulses and values of the load force providing the biggest actuator steps were determined. Generally, the maximal steps were achieved for such a combination of the pulse shapes and load forces providing high velocities at the end of the sticking mode of the actuator motion and, at the same time, lower decelerations during the slipping mode.As for the multiple driving pulses, the pulse shapes and values of repetition frequency ensuring the sticking mode of the actuator motion during the pulse rise time together with the maximum average angular rotor velocity were specified. In this way the effective and stable operation conditions of the actuator were provided.In principle, the presented method can be applied for the testing and optimization of any linear or angular stick-slip actuator.

FORMING CAPABILITIES OF A PULSE MAGNETIC FIELD GENERATOR

Purpose. Determination of areas ratio of the parameters of the discharge circuit elements of the generator, which ensure the formation of magnetic field pulses of different shapes. Methodology. Numerical simulation using dimensionless variables that determine the nature of the transition process in the discharge circuit of the generator, and use the procedure for determining the pulse points of meeting the conditions of extremum and the transition through zero. Results. Obtained a description of the formation of the three specific areas of waveforms: oscillatory weakly damped oscillatory strongly damped and unipolar pulse with a monotonic rise and fall values. A relation to the choice of parameters of elements of the discharge circuit of the generator, which formed unipolar pulses with a monotonic rise and fall values. Originality. A completed and extended database that implements the mapping of the formal description of the pulse shape with a description of areas ratio parameters for high-voltage pulse discharge circuit test units, with respect to the pulses of current flowing in the formation of the magnetic field. Practical value. The relations obtained allow to select the parameters of the discharge circuit elements of the generator designed to generate test pulses of magnetic field.

Two-electron quantum ring in short pulses

The response of a two-electron quantum ring system to the short laser pulses of different shapes in the presence of external static electric field is studied. The variation of transition probabilities of the two-electron quantum ring from ground state to excited states with a number of parameters is shown and explained. The energy levels and wavefunctions of the system in the presence of static electric field are found by solving the time-independent Schrödinger equation numerically by the finite difference method. The shape of the pulse plays a dominant role on the dynamics.

Errors of oscillometric blood pressure measurement as predicted by simulation

To study factors affecting the accuracy of oscillometric measurement. By means of a computer-based simulator, a variety of arterial pressure pulses of different shapes and amplitudes were composed as an input signal of the model, whereas cuff volume oscillations were obtained as an output signal. The shape of the artery-cuff pressure/volume relationship was modified. Thereafter, oscillation envelopes were drawn and an estimation of systolic and diastolic pressures was performed by implementing fixed characteristic ratios. The mean arterial pressure was estimated using the maximum oscillation criterion. Altogether, 32 combinations of four affecting factors were studied. For the studied range of affecting factors, the induced errors in systolic pressure, diastolic pressure, and mean arterial pressures were 15, 14, and 27 mmHg, respectively. Systolic readings moved toward underestimation and diastolic readings moved toward overestimation if pulse pressure increased. Arterial stiffening induced systematic overestimation of the systolic pressure and overestimation or underestimation of the diastolic pressure compared with a normal artery, with errors depending on the interaction of the symmetry and steepness indices of the artery-cuff pressure/volume curve. Errors of mean arterial pressure were proportional to pulse pressure, showing overestimation if stiffness increased and/or arterial pressure pulses became steeper. Oscillometric readings of systolic and diastolic pressures are strongly influenced by pulse pressure and the shape of the artery-cuff pressure/volume curve, whereas those of the mean arterial pressure are affected by pulse pressure and both the shape of the artery-cuff pressure/volume curve and the shape of the arterial pulse.

Feynman-Alpha and Rossi-Alpha Formulas with Delayed Neutrons for Subcritical Reactors Driven by Pulsed Non-Poisson Sources with Correlation between Different Pulses

Through our earlier papers, we have shown that reactor noise in accelerator-driven systems (ADS) is different from that in critical or radioactive source-driven subcritical systems due to periodically pulsed source and its non-Poisson character. We have developed a theory of reactor noise for ADS, taking into account the non-Poisson character of the source. Various noise descriptors, such as Rossi-alpha, Feynman-alpha (or variance to mean), power spectral density, and cross power spectral density, have been derived for a periodically pulsed source, including correlation between different pulses and finite pulses of different shapes. For mathematical simplicity, the theory was restricted to the case of prompt neutrons only. Recently, we extended the theory to the delayed neutron case and derived Feynman-alpha and Rossi-alpha formulae by considering the source to be a periodically pulsed non-Poisson source, without correlations between different pulses. The present paper extends the treatment to account for the possibility of correlations between pulses. Feynman-alpha and Rossi-alpha formulas are derived by considering the source to be a periodic sequence of delta function non-Poisson pulses, with exponential correlations.

Laser ablation plume dynamics in nanoparticle synthesis

The dynamics of the plume ejected from the surface of solid targets (YSZ, Nd:YAG and graphite) by a CO2 laser pulse with a duration of ∼500 μs (at the 0.03 level), energy of 1.0–1.3 J and peak power of 6–7 kW have been studied using high-speed photography of the plume luminescence and shadow. The targets were used to produce nanopowders by laser evaporation. About 200 μs after termination of the pulse, shadowgraph images of the plumes above the YSZ and Nd:YAG targets showed dark straight tracks produced by large particles. The formation of large (∼10 μm) particles is tentatively attributed to cracking of the solidified melt at the bottom of the ablation crater. This is supported by the fact that no large particles are ejected from graphite, which sublimes without melting. Further support to this hypothesis is provided by numerical 3D modelling of melt cooling in craters produced by laser pulses of different shapes.

Feynman-Alpha and Rossi-Alpha Formulas with Delayed Neutrons for Subcritical Reactors Driven by Pulsed Non-Poisson Sources

In our earlier papers, we developed a theory of reactor noise for accelerator-driven systems (ADSs). It was shown that reactor noise in ADSs is different from that in critical or radioactive source-driven subcritical systems because of the periodically pulsed source and its non-Poisson character. Various noise descriptors, such as Rossi alpha, Feynman alpha (or variance to mean), power spectral density, and cross-power spectral density, were derived, for a periodically pulsed source, including correlation between different pulses and finite pulses of different shapes. Throughout the work we restricted ourselves to the case of prompt neutrons only. In the present paper, we extend the theory to the delayed neutron case. Feynman-alpha and Rossi-alpha formulas are derived by considering the source to be a periodically pulsed non-Poisson source, without correlations between different pulses. Each pulse is assumed to be a delta function. The calculations are carried out in the time domain that leads to closed-form expressions for these descriptors.

Dual control of slow light in reciprocal electromagnetically-induced-transparency conditions

Electromagnetically-induced transparency (EIT) of atomic medium interacting with two laser beams can exhibit effects of reciprocal influence of both beams on optical properties of the medium. If the probe beam reaches instantaneous intensity comparable to that of the control beam, considerable energy is transferred to the latter. Resulting increase in the control-beam intensity temporarily modifies EIT conditions and consequently affects propagation of probe-beam pulses. The effect is particularly strong in atomic media exhibiting significant dissipation of radiation energy due to relaxation processes efficiently counteracting the optical pumping. Coherent interaction of atoms with both beams causes reduction of dissipation of the control beam (reciprocal EIT conditions). Our experiments with hot rubidium vapor demonstrate the possibility of dual control over slow propagation of light pulses in such media. Besides the well-known dependence on input control-beam intensity, the delays of output probe pulses depended significantly also on the energy of input probe pulses. Moreover, for relatively short input pulses, very specific shape of output pulses was observed, invariant in wide ranges of input probe pulse energies and control-beam intensities. The shapes of the output pulses were also the same for short input pulses of different shapes as well as for pulses temporarily stored in the medium by switching off the control beam during their propagation.

Pulsed Electric Strength of Vacuum Gaps

To plot unified dependences of breakdown voltage and electric strength on gap spacing, known experimental data for breakdown delay time in vacuum are analyzed and generalized. Analysis and generalization are realized from positions of the cathode- initiated breakdown. The data, which correspond to optimal modes of conditioning and are received at different experimental conditions for pulses of different shapes and durations in electric fields with a different degree of heterogeneity, are resulted in unified conditions for uniform field and rectangular pulse with given duration. Dependences of breakdown voltage U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> (d) and electric strength E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0br</sub> (d) on gap spacing d for copper electrodes in range 3times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> les d les 2times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> mm are plotted. At plotting curves U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">br</sub> (d) and E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0br</sub> (d), the total voltage effect is considered. At pulses duration of t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> = 0.5 mus, the breakdown voltage makes up 2.5 kV for d = 3 mum and rises with interelectrode distance by three orders of magnitude up to 5 MV for d = 20 cm. The electric strength increases ~ 30 times from 2.5times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> V/m for d = 20 cm up to 7.5 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> V/m for d = 3 mum with decrease in gap spacing.

Fine tuning of quantum operations performed via Raman transitions

A scheme for fine tuning of quantum operations to improve their performance is proposed. A quantum system in $\Lambda$ configuration with two-photon Raman transitions is considered without adiabatic elimination of the excited (intermediate) state. Conditional dynamics of the system is studied with focus on improving fidelity of quantum operations. In particular, the $\pi$ pulse and $\pi/2$ pulse quantum operations are considered. The dressed states for the atom-field system, with an atom driven on one transition by a classical field and on the other by a quantum cavity field, are found. A discrete set of detunings is given for which high fidelity of desired states is achieved. Analytical solutions for the quantum state amplitudes are found in the first order perturbation theory with respect to the cavity damping rate $\kappa$ and the spontaneous emission rate $\gamma$. Numerical solutions for higher values of $\kappa$ and $\gamma$ indicate a stabilizing role of spontaneous emission in the $\pi$ and $\pi/2$ pulse quantum operations. The idea can also be applied for excitation pulses of different shapes.

Propagation of arbitrarily shaped femtosecond laser pulses through a photonic crystal fiber

We produce femtosecond optical pulses of different shapes to investigate the linear pulse propagation through a photonic crystal fiber in a regime where the conventional group velocity has no meaning and show that the temporal positions of centroid of energy are the same for both arbitrary pulses and transform-limited coherent pulses. By the concept of net group delay we explain the propagation delay for both the cases despite the fact that transmitted pulses suffer a severe distortion in the fiber.

Effect of defibrillation pulses of different shapes on biomembranes: experimental study.

We studied the effects of high-voltage single, double unipolar, and double bipolar electric pulses of exponential or sine shape on erythrocyte membranes. Either single or double (mono- or bipolar) pulses were used. All pulses electroporated the membranes, and the electroporation threshold did not depend on the pulse shape. Two successive pulses decreased erythrocyte number in a nonadditive way. Similar to defibrillation of the whole heart, the effect of two bipolar pulses on erythrocytes was more pronounced than the effect of two unipolar pulses.

Control of (1+1′)-photondissociation in NaH

We have studied -photon dissociation of the NaH molecule (ab initio) from thev = 0 level of the groundelectronic state (X 1Σ+) to therepulsive B 1Π state via thebound intermediate A 1Σ+ state. By solving the one-dimensional time-dependent Schrödinger equation for nuclearmotion using the Fourier grid technique we have shown that the maximum of the-photon dissociation cross section and the shape of the dissociation spectrum can becontrolled by controlling the time delay between the two photoexcitation processes,i.e. bound–bound excitation by the first photon and bound–continuum excitation by thesecond photon respectively. The oscillation of the maximum of the dissociation crosssection with time delay between the two pulses has been shown to be due to theexcitation of different oscillating wavepackets at different delays from the intermediateA 1Σ+ state. It has also been shown that the dissociation spectrum depends on the duration andtemporal profile of the femtosecond pulses used for excitation and hence dissociation can becontrolled by choosing pulses of different shapes and duration. At a particularfrequency of the second pulse, the dissociation cross section oscillates with timedelay and this oscillation in the cross section can be used as a time-dependentquantum gate.

On the propagation of magnetohydrodynamic perturbations in a Harris‐type current sheet: 2. Propagation on continuous modes and resonant absorption

We study the propagation of magnetohydrodynamic perturbations in a one‐dimensional (1‐D) model of the magnetotail current sheet. Starting from a general analysis of the MHD linear response of a Harris current sheet to external pressure pulse, we reconstruct the magnetic, velocity, and pressure perturbations for successive times and at different positions. The spatial‐temporal evolution of the signal propagating on the discrete modes has been studied by Fruit et al. [2002]. We here describe the propagation in the continuous spectrum domain that results from the existence of a spatial singularity in the differential equation defining the modes of propagation. At t = 0, the current sheet is supposed to be excited by an external pressure pulse. We estimate the energy that propagates in the continuum domain and will thus be absorbed by the resonant absorption process. Initial pulses of different shapes and characteristic spatial and temporal scales are used. Since we perform the complete inverse Laplace and Fourier transform of the continuous modes, we get the full description of the perturbations in real space, hence, the typical temporal and spatial scales of the absorption process. For example, starting with a pulse of total amplitude of 1 Pme (magnetic pressure in the lobe) with a spatial scale of 4 Earth radii and a temporal scale of 20 s, we show that 2% of the initial plasma thermal energy are transferred to the sheet over a distance of 20 Earth radii and in less than 10 min. Pulses of larger scales have no energetic consequences.

Coherent control of photon-assisted tunneling between quantum dots: A theoretical approach using genetic algorithms

We analyze theoretically the electron tunneling induced by an ultrashort pulse of electric field between two metallic reservoirs coupled through a double quantum dot. We solve the equations of motion for the reduced density matrix to determine the transferred charge, which is a functional of the external field. Then, we use genetic algorithms to determine the optimal shape of the electric field that maximizes the transferred charge. Results show that, due to the presence of Rabi oscillations, a sequence of pulses of different shapes is needed. Such pulse sequence leads to a remarkable enhancement of the current with respect to a single (Gaussian or square) pulse. We analyze the cases of interdot Coulomb repulsion $U=0$ and $\stackrel{\ensuremath{\rightarrow}}{U}\ensuremath{\infty}.$