Electron-positron pair production by electromagnetic pulses

Electron-positron pair production in vacuum by a single focused laser pulse and by two counter-propagating colliding focused pulses is analyzed. A focused laser pulse is described using a realistic three-dimensional model based on an exact solution of Maxwell’s equations. In particular, this model reproduces an important property of focused beams, namely, the existence of two types of waves with a transverse electric or magnetic vector (e-or h-polarized wave, respectively). The dependence of the number of produced pairs on the radiation intensity and focusing parameter is studied. It has been shown that the number of pairs produced in the field of a single e-polarized pulse is many orders of magnitude larger than that for an h-polarized pulse. The pulse-intensity dependence of the number of pairs produced by a single pulse is so sharp that the total energy of pairs produced by the e-polarized pulse with intensity near the intensity I _S = 4.65 × 10²⁹ W/cm² characteristic of QED is comparable with the energy of the pulse itself. This circumstance imposes a natural physical bound on the maximum attainable intensity of a laser pulse. For the case of two colliding circularly polarized pulses, it is shown that pair production becomes experimentally observable when the intensity of each beam is I ～ 10²⁶ W/cm², which is one to two orders of magnitude lower than that for a single pulse.     

Effect of acoustic waves on the repetition frequency of high repetition rate TEA-CO2 lasers and calculating optimization condition

The gas channel of a pulse periodic TEA-CO₂ laser is considered as an acoustic resonator. In this paper, a three-dimensional mathematical modeling has been considered for describe of laser action. By calculating of the equations obtained from this model, the effects of cavity dimensions, Mach number and repetition frequency of laser on the acoustic wave spectrum have been investigated. At last optimum conditions for performance of laser operation has been arrived.     

On <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> pair production by colliding electromagnetic pulses

Electron-positron pair production from vacuum in an electromagnetic field created by two counterpropagating focused laser pulses interacting with each other is analyzed. The dependence of the number of produced pairs on the intensity of a laser pulse and the focusing parameter is studied with a realistic three-dimensional model of the electromagnetic field of the focused wave, which is an exact solution of the Maxwell equations. It has been shown that e⁺e^? pair production can be experimentally observed when the intensity of each beam is I～10²⁶ W/cm², which is two orders of magnitude lower than that for a single pulse.     

Exact solution of the AF coherent pulse propagation equations

The exact solution of the adiabatic following pulse propagation equations describing coherent propagation of a single-photon quasi-resonant short pulse of arbitrary shape and phase is reported.     

The Bloch Equations for an AB System and the Design of Spin-State-Selective RF Pulses for Coupled Spin Systems

Optimized AM/FM pulses are described which excite an arbitrary coherence of a coupled spin system with optimal intensity. These pulses may become important for creating maximum signal intensity in MQC-filtered and polarization-transfer experiments in HRNMR and in vivo spectroscopy. For an AB system, for example, a pure DQC or ZQC state is obtained (100%) compared to maximal 50% with a conventional π/2-π/2 MQC pulse sequence. The design and numerical optimization of these pulses, with pulse times of about 1/2J (J is the coupling constant), are described. Examples of three of these spin-state-selective pulses (SSSP) for an AB system are given; simulations and experiments confirm their expected performance. To get some insight in the relations between the expectation values of the several coherences under the influence of Zeeman, J-coupling, and RF terms, Bloch-like equations for a proton AB system are derived, with neglection of relaxation. A system of 15 first-order coupled differential equations is found. A solution to these equations is given for the evolution of the zero-quantum coherence and longitudinal magnetization.     

Modeling the pulse shape of Q-switched lasers to account for terminal-level relaxation

To account for the effect of lower-level relaxation,we have derived a characteristic equation for describing the laser pulse from the modified rate equations for Q-switched lasers.The pulse temporal profile is related to the ratio of the lower-level lifetime to the cavity lifetime and the number of times the population inversion density is above the threshold.By solving the coupled rate equations numerically,the effect of terminal-level lifetime on pulse temporal behaviour is analysed.The mode is applied to the case of a diode-pumped Nd:YAG laser that is passively Q-switched by a Cr 4+:YAG absorber.Theoretical results show good agreement with the experiments.     

Four possible types of pulses for self-induced transparency

We have found four types of possible steady-state solutions to the coupled Maxwell-Bloch equations which describe extremely intense pulse propagation in a resonant medium. This is possible due to the replacement of the usual slowly varying envelope approximation by the assumption that (c/V-1)/ωτ « 1, where V is the pulse speed and τ is the pulse duration. We ignore possible non-resonant losses and set T₁ = T₂ = ∞.     

Numerical simulation of air-breathing nanosecond laser propulsion considering subsonic inflow and multi-pulse

The model of paraboloid point focusing laser thruster is adopted to investigate numerically the air-breathing nanosecond laser propulsion under subsonic inflow and multi-pulse condition. The influence of pulse number and subsonic inflow on propulsive performance is analyzed. The simulation results indicate that the average impulse coupling coefficient C_mn decreases significantly with the increasing of pulse number because the air in the nozzle cannot recover to the initial state, but the trend becomes smooth due to the similar flow field before subsequent pulse. When the Mach number increases, the multi-pulse C_mn falls down at the same number of laser pulse. It shows that the faster subsonic inflow, the higher air drag and the shorter time of interaction between the shock wave and the thruster.     

XPM-induced pulse compression in birefringent dispersion decreasing fiber

The pulse compression induced by cross-phase modulation in birefringent dispersion decreasing fiber is discussed theoretically by solving the coupled Schrödinger equations which include the contribution of the high-order non-linear effects, and third-order dispersion. In particular, it is found that a high quality compressed signal pulse can be obtained by a pump pulse of low intense through the technique. The dependence of optimum compression on the non-linear factor N, time delay τ_d and the dispersive ratio f is also discussed in detail.     

All-optical modulation based on electromagnetically induced transparency

We numerically investigate the implementation of all-optical absorption modulation of electromagnetic pulses by a medium that exhibits electromagnetically induced transparency. The quantum system is modelled as a three-level Λ-type system that interacts with two electromagnetic pulses, a probe pulse and a coupling pulse. The dynamics of the system is described by the coupled Maxwell-density matrix equations, and we explore the dependence of the optical modulation efficiency on the parameters of the system.     

强损耗介质中相干光脉冲的传输效应

本文利用密度矩阵形式的Bloch方程和Maxwell方程耦合,讨论了相干光脉冲在共振强损耗介质中的传输过程,同时考查了脉冲演化和对介质的影响,得到了脉冲面积的阶跃式下降和空间损耗粒子数的周期振荡等强损耗条件下特有的瞬态相干现象。 关键词：     

Large angle spin-echo imaging

A study was undertaken to assess the use of excitation flip angles greater than 90° for T₁ weighted spin-echo (SE) imaging with a single 180° refocusing pulse and short TR values. Theoretical predictions of signal intensity for SE images with excitation pulse angles of 90–180° were calculated based on the Bloch equations and then measured experimentally from MR images of MnCl₂ phantoms of various concentrations. Liver signal-to-noise ratios (SNR) and liver-spleen contrast-to-noise ratios (CNR) were measured from breathhold MR images of the upper abdomen in 16 patients using 90 and 110° excitation flip angles. The theoretical predictions showed significant improvements in SNR with excitation flip angles >90°, which were more pronounced at small TR values. The phantom studies showed reasonably good agreement with the theoretical predictions in correlating the excitation pulse angle with signal intensity. In the human imaging studies, the 110° excitation pulse angle resulted in a 7.4% (p < .01) increase in liver SNR and an 8.2% (p = .2) increase in liver-spleen CNR compared to the 90° pulse angle at TR = 275 ms. Increased signal intensity resulting from the use of large flip angle excitation pulses with a single echo SE pulse sequence was predicted and confirmed experimentally in phantoms and humans.     

Electron–positron pair production from vacuum in the field of high-intensity laser radiation

The works dealing with the theory of e⁺e^– pair production from vacuum under the action of highintensity laser radiation are reviewed. The following problems are discussed: pair production in a constant electric field E and time-variable homogeneous field E(t); the dependence of the number of produced pairs \({N_{{e^ + }{e^ - }}}\) on the shape of a laser pulse (dynamic Schwinger effect); and a realistic three-dimensional model of a focused laser pulse, which is based on exact solution of Maxwell’s equations and contains parameters such as focal spot radius R, diffraction length L, focusing parameter Δ, pulse duration τ, and pulse shape. This model is used to calculate \({N_{{e^ + }{e^ - }}}\) for both a single laser pulse (n = 1) and several (n ≥ 2) coherent pulses with a fixed total energy that simultaneously “collide” in a laser focus. It is shown that, at n ? 1, the number of pairs increases by several orders of magnitude as compared to the case of a single pulse. The screening of a laser field by the vapors that are generated in vacuum, its “depletion,” and the limiting fields to be achieved in laser experiments are considered. The relation between pair production, the problem of a quantum frequency-variable oscillator, and the theory of groups SU(1, 1) and SU(2) is discussed. The relativistic version of the imaginary time method is used in calculations. In terms of this version, a relativistic theory of tunneling is developed and the Keldysh theory is generalized to the case of ionization of relativistic bound systems, namely, atoms and ions. The ionization rate of a hydrogen-like ion with a charge 1 ≤ Z ≤ 92 is calculated as a function of laser radiation intensity (F and ellipticity ρ.     

Coherence levels and coherence pathways in NMR. A simple way to design phase cycling procedures

The paper presents a simple way of calculating what a phase cycling procedure in a pulse NMR experiment does. This is done by exploiting the fact that if a pulse is phase shifted by angle φ and if it changes the coherence level by Δm, then the effect on the coherence is a multiplication by e^iΔmφ The coherence level is a fundamental quantum number of the system-the z component of the Liouville space angular momentum—and is a generalization of the number of quanta in a multiple-quantum transition. The sequence of coherence levels that a signal passes through between the first pulse and the time it reaches the receiver is the coherence pathway. This pathway, combined with the above formula, provides a powerful method for analyzing pulse NMR experiments.     

Quasi-microscopic theory of propagation of extremely short pulses in media with induced anisotropy

The nonlinear polarization response of a medium with induced anisotropy to the action of a high-intensity pulse containing an arbitrary number (down to one) optical oscillations is studied using quantum-mechanical concepts. Based on the proposed model, a set of nonlinear wave equations is obtained for the ordinary and extraordinary components of the pulse. The previously known equations for quasi-monochromatic pulses are shown to follow from this set as a special case.     

Scattering of relativistic electrons by a focused laser pulse

The problem of the motion of a classical relativistic electron in a focused high-intensity laser pulse is solved. A new three-dimensional model of the electromagnetic field, which is an exact solution of Maxwell’s equations, is proposed to describe a stationary laser beam. An extension of the model is proposed. This extension describes a laser pulse of finite duration and is an approximate solution of Maxwell’s equations. The equations for the average motion of an electron in the field of a laser pulse, described by our model, are derived assuming weak spatial and temporal nonuniformities of the field. It is shown that, to a first approximation in the parameters of the nonuniformities, the average (ponderomotive) force acting on a particle is described by the gradient of the ponderomotive potential, but it loses its potential character even in second order. It is found that the three-dimensional ponderomotive potential is asymmetric. The trajectories of relativistic electrons moving in a laser field are obtained and the cross sections for scattering of electrons by a stationary laser beam are calculated. It is shown that reflection of electrons from the laser pulse and the surfing effect are present in the model studied. It is found that for certain impact parameters of the incident electrons the asymmetic ponderomotive potential can manifest itself effectively as an attractive potential. It is also shown that even in the case of a symmetric potential the scattering cross section contains singularities, known as rainbow scattering. The results are applicable for fields characterized by large (compared to 1) values of the dimensionless parameter η² = e ²〈E ²〉/m ²ω² and arbitrary electron energies.     

Retardation and nonstationarity effects on the generation of terahertz radiation by the e-h plasma in a semiconductor

Radiative processes in a nonequilibrium e-h plasma are theoretically studied using a self-consistent solution of the kinetic equation and Maxwell’s equations. The terahertz emission from a finite-thickness semiconductor sample is due to the retardation and nonstationarity of the electromagnetic interaction of the photocurrent in the e-h plasma and the radiation field. The duplex waveform of the terahertz electromagnetic pulse for an arbitrary ratio of the radiation formation length and the plate thickness originates due to coherent radiative processes accompanying the generation of the e-h plasma at the input boundary and its extinction at the output boundary of a semiconductor plate through which a weakly absorbed ultrashort laser pulse propagates. The theoretical conclusions show analogies with the radiative phenomena accompanying the start-stop motion of external currents (Tamm problem) and the nonlinear interaction of optical waves in a finite-thickness medium.     

Improvement of stability and pulse energy in a diode-pumped dual-loss-modulated QML Nd:Lu0.2Y0.8VO4 laser with EO modulator and GaAs

The Q-switched and mode-locked (QML) performance in a diode-pumped Nd:Lu_0.2Y_0.8VO₄ laser with electro-optic (EO) modulator and GaAs saturaber absorber is investigated. In comparison with the solely passively QML laser with GaAs, the dual-loss-modulated QML laser with EO and GaAs can generate pulses with higher stability and shorter pulse width of Q-switched envelope, as well as higher pulse energy. At the repetition rate 1 kHz of EO, the pulse width of Q-switched pulse envelope has a compression of 89% and the pulse energy has an improvement of 24 times. The QML laser characteristics such as the pulse width, pulse peak power etc. have been measured for different small-signal transmittance (T₀) of GaAs, different reflectivity (R) of output coupler and modulation frequencies of the EO modulator (f_e). The highest peak power and the shortest pulse width of mode-locked pulses are obtained at f_e = 1 kHz, R = 90% and T₀ = 92.6%. By considering the influences of EO modulator, a developed rate equation model for the dual-loss-modulated QML laser with EO modulator and GaAs is proposed. The numerical solutions of the equations are in good agreement with the experimental results.     

Optical solitons in dense resonant media

Exact single-soliton solutions of the modified system of Maxwell-Bloch equations, in which the dipole-dipole interactions of the atoms of a dense resonant medium are taken into account, are obtained. Two propagation regimes are analyzed: “coherent,” where the pulse duration is much shorter than both relaxation times (T _p ? T ₁, T ₂), and “incoherent,” where the pulse duration falls between the relaxation times (T ₂ ? T _p ? T ₁). It is predicted, for the first time, that soliton propagation of an ultrashort pulse is possible in a dense resonant absorbing medium in an incoherent interaction regime. The differences between the amplitude and phase characteristics of the solitons considered and the corresponding characteristics of the solitons for McCall-Hahn self-induced transparency are noted.     

The role of particular collision and radiative processes in recombination

Rate equations for level populations of atomic hydrogen are solved to obtain collisional-radiative recombination coefficients. Several particular collision and radiative processes entering the rate equations have been accounted for or have been suppressed in order to obtain information on how strongly the solution depends on the different assumptions. Under certain conditions the results differ by orders of magnitude. The results also vary strongly with the number of quantum levelsp entering the rate equations. The critical quantum numbern ^* which defines the limit for partial L.T.E. cannot be considered as the critical number of rate equations above which the population coefficients become independent ofp.