Absorption of gamma-ray photons in a vacuum neutron star magnetosphere: I. Electron-positron pair production

The production of electron-positron pairs in a vacuum neutron star magnetosphere is investigated for both low (compared to the Schwinger one) and high magnetic fields. The case of a strong longitudinal electric field where the produced electrons and positrons acquire a stationary Lorentz factor in a short time is considered. The source of electron-positron pairs has been calculated with allowance made for the pair production by curvature and synchrotron photons. Synchrotron photons are shown to make a major contribution to the total pair production rate in a weak magnetic field. At the same time, the contribution from bremsstrahlung photons may be neglected. The existence of a time delay due to the finiteness of the electron and positron acceleration time leads to a great reduction in the electron-positron plasma generation rate compared to the case of a zero time delay. The effective local source of electron-positron pairs has been constructed. It can be used in the hydrodynamic equations that describe the development of a cascade after the absorption of a photon from the cosmic gamma-ray background in a neutron star magnetosphere.     

强场下真空中粒子对产生的研究进展

随着激光技术的飞快发展,激光强度不断提高,超强外场下真空中正负电子对产生的过程,即能量向质量转化过程,已经成为一个研究热点。主要综述了近几年量子Vlasov方程方法和计算量子场论(数值求解Dirac方程)方法在研究强场下真空中正负电子对产生方面的进展,分别介绍了空间均匀场和空间不均匀场下的粒子对产生的情况。第一种情况主要介绍双脉冲结构振荡电场中电子-正电子对的产生、强双频振荡电场中非微扰电子-正电子对的产生、频率调制的激光场中电子-正电子对的产生和Dirac真空对啁啾外场的快速分辨。第二种情况主要介绍优化空间局域电场提高粒子对的产生率、多个势阱-垒结构的振荡场对粒子对产生的增强、振荡 Sauter 电势中正负电子对产生的问题、操纵Dirac真空以控制其在场诱导下的衰变、作为信息传输介质的Dirac真空还有正负电子对产生中的相干和非相干啁啾机制的转变。     

Modulation instability of intense laser beam in an electron-positron plasma

Modulation instability of an intense right-hand elliptically polarized laser beam propagating through an electron-positron plasma is investigated by a new method. The nonlinear dispersion relation, in which the relativistic and ponderomotive nonlinearities are taken into account, is obtained for the laser radiation in electron-positron plasma by the Lorentz transformation. The Karpman equation is generalized to the case of three dimensions with three field components. When the nonlinear frequency shift of the electromagnetic field in plasma is involved, the nonlinear evolution equation for the slowly varying envelope of the laser field is obtained. Thus, modulation instability of the intense laser beam in electron-positron plasma is studied and the temporal growth rate of the instability is derived. The analysis shows that the growth rate of modulation instability is increased significantly near the critical surface in a laser-plasma.     

Enhanced electron–positron pair production by frequency chirping in one-and two-color laser pulse fields

Enhanced electron–positron pair production by frequency chirping in one- and two-color laser pulse fields is investigated by solving the quantum Vlasov equation. A small frequency chirp shifts the momentum spectrum along the momentum axis. The positive and negative frequency chirp parameters play the same role in increasing the pair number density. The sign change of the frequency chirp parameter at the moment t = 0 leads the pulse shape and momentum spectrum to be symmetric, and the number density to be increased. The number density of produced pairs in the two-color pulse field is much higher than that in the one-color pulse field and the larger frequency chirp pulse field dominates more strongly. In the two-color pulse fields, the relation between the frequency ratio of two colors and the number density is not sensitive to the parameters of small frequency chirp added in either a low frequency strong field or a high frequency weak field but sensitive to the parameters of large frequency chirp added in a high frequency weak field.     

Dynamically assisted Schwinger mechanism

We study electron-positron pair creation from the Dirac vacuum induced by a strong and slowly varying electric field (Schwinger effect) which is superimposed by a weak and rapidly changing electromagnetic field (dynamical pair creation). In the subcritical regime where both mechanisms separately are strongly suppressed, their combined impact yields a pair creation rate which is dramatically enhanced. Intuitively speaking, the strong electric field lowers the threshold for dynamical particle creation--or, alternatively, the fast electromagnetic field generates additional seeds for the Schwinger mechanism. These findings could be relevant for planned ultrahigh intensity lasers.     

Monopole pair production in a magnetic field

The rate of monopole pair production in a weak external magnetic field is computed by a semiclassical method. The result is analogous to that for electron-positron pair production in an electric field.     

The Breit-Wheeler process in the laser field

The electron-positron pair production in a two-photon collision in the field of a moderately strong circularly polarized laser wave was theoretically investigated. The nonresonant kinematics of the pair production has been studied and the cross section for the Breit-Wheeler process was obtained.     

Pair production at strong coupling in weak external fields

The rate of electron-positron pair production in a weak external electric field at large values of e is computed. The method is extended to compute the rate of monopole-antimonopole pair production in a weak external magnetic field at large magnetic charge.     

Electron-Positron Pair Production in an Arbitrary Polarized Ultrastrong Laser Field

Electron-positron pair production in an arbitrary polarized ultrastrong laser field is investigated in the first order perturbation approximation in which the Volkov states are used for convenient calculation of scattering amplitude and cross section. It is found surprisingly that the optimal pair production depends strongly on the polarization. For some cases of field parameters, the optimal field is elliptically polarized or evenly circularly polarized one, rather than the usual linear polarization as indicated by previous works. Some insights into pair generation are given and some interesting unexpected features are also discussed briefly.     

Production of a pair by a polarized photon in a uniform constant electromagnetic field

The total probability of production of an electron-positron pair by a polarized photon in a constant uniform electromagnetic field of an arbitrary configuration is determined using the imaginary part of the diagonalized polarization operator. Approximate expressions are derived for this probability in four ranges of photon energy. In the high-energy range, the corrections to the standard semiclassical approximation are calculated. In the range of intermediate energies, in which this approximation is inapplicable, the probability of the process is calculated using the steepest descent method. It is shown that in the range of photon energies higher than the pair production threshold in a magnetic field, a weak electric field removes root divergences in the probability of production of the particles at the Landau levels. For relatively low photon energies, a low-energy approximation is developed. At such energies, the effect of the electric field on the process is decisive, while the effect of the magnetic field is associated with its interaction with the magnetic moment of the particles being produced. Such an interaction is manifested, in particular, in the difference in the probabilities of production of a pair by an external field for scalar and spinor particles.     

Electron-Positron Pair Production in an Arbitrary Polarized Ultrastrong Laser Field

Electron-positron pair production in an arbitrary polarized ultrastrong laser field is investigated in the first order perturbation approximation in which the Volkov states are used for convenient calculation of scattering amplitude and cross section. It is found surprisingly that the optimal pair production depends strongly on the polarization. For some cases of field parameters, the optimal field is elliptically polarized or evenly circularly polarized one, rather than the usual linear polarization as indicated by previous works. Some insights into pair generation are given and some interesting unexpected features are also discussed briefly.     

Electron-Positron Pair Production in a Strong Laser Field Enhanced by an Assisted High Frequency Weak Field

Electron-positron pair production in a strong laser field enhanced by an assisted high frequency weak field is investigated by solving the quantum Vlasov equation.The average and residual pair number densities are obtained for sinusoid electric field and it is found that the high frequency assisted weak field will enhance pair production significantly.There exists an optimal frequency of assisted field that makes the pair production number density get a maximum one,which is a few orders of higher than that without assisted field.We also discuss the other possible assisted fields.     

Interference effect in the photoproduction of electron-positron pairs on a nucleus in the field of two light waves

In the general relativistic case, the nonresonance photoproduction of electron-positron pairs on a nucleus in the field of two circularly polarized light waves propagating in one direction is theoretically investigated. It is shown that there are two essentially different kinematical regions: the noninterference region, where the Bunkin-Fedorov quantum parameters play the role of multiphoton parameters, and the interference region, where interference effects are important and where quantum interference parameters are treated as multiphoton parameters. The interference effect is found in the photoproduction of an electron-positron pair on a nucleus in the field of two light waves. This effect occurs in the interference region and is due to the production of an electron-positron pair in the plane spanned by the light wave vector and the incident-photon momentum and to stimulated correlated emission and absorption of photons of the two waves. The cross sections for pair photoproduction on a nucleus in the above kinematical regions are determined for moderately strong fields. It is shown that, in the interference region, the partial cross sections for the case where there is a correlated emission (absorption) of the same number of photons of the two waves can significantly exceed the corresponding cross sections in any other geometry.     

Electron-positron pair production by photons in nonuniform strong fields

A probability of electron-positron pair production by photons in strong nonuniform fields is derived by applying a model trajectory method in the frame of a semiclassical approach. In addition to the well known invariant field parameter chi, a new invariant parameter nu is introduced to characterize the nonuniformity of the field. For nu>1, the obtained expression is reduced to the uniform-field approximation while it approaches the Bethe-Heitler formula for nu<1. The pair production is predicted for relatively weak external fields where the uniform-field approximation gives no effect. The theory agrees well with the experimental results of crystal-assisted pair production.     

Confined states of a positronium in a spherical quantum dot

Confined states of a positronium (Ps) in the spherical quantum dot (QD) are theoretically investigated in three size-quantization (SQ) regimes: strong, weak and intermediate. In the strong SQ regime, analytical expressions for the wave functions (WFs) and energy of the electron-positron pair are obtained. In the weak SQ regime, the Ps energy and binding energy are analytically calculated. To calculate the Ps energy in the intermediate SQ regime, variational and numerical methods are used. It is shown that, in the corresponding limits, the results obtained by variational method agree with those obtained in the strong and weak SQ regimes.     

Effect of symmetrical frequency chirp on pair production

By using Dirac–Heisenberg–Wigner formalism we study electron–positron pair production for linear, elliptic, nearly circular, and circular polarizations of electric fields with symmetrical frequency chirp, and we obtain momentum spectra and pair yield. The difference of results among polarized fields is obvious for the small chirp. When the chirp parameter increases, the momentum spectra tend to exhibit the multiphoton pair generation that is characterized by the multi-concentric ring structure. The increase of the number density is also remarkable compared to the case of asymmetrical frequency chirp. Note that the dynamically assisted Schwinger mechanism plays an important role for the enhanced pair production in the symmetrical frequency chirp.     

Theoretical study on phase conjugation in weakly injected vertical-cavity surface-emitting lasers

This paper gives a detailed theoretical investigation on phase conjugation induced by nearly degenerate fourwave mixing in single mode vertical-cavity surface-emitting lasers （VCSELs） with weak optical injection. Considering VCSELs that can work in linearly polarized or elliptically polarized states, it derives the theoretical expression of the conjugated field by small signal analysis based on the vectoral rate equations-the spin-flip model. For linearly polarized state, VCSELs show similar conjugate spectra to edge-emitting semiconductor lasers. For the elliptically polarized state, dichroism and birefringence parameters as well as the spin-flip rate can change the conjugate spectra. Especially, when frequency detuning of the probe and pump waves is between the positive and negative relaxation oscillation frequency, changes are evident. For specific values of parameters, conjugate efficiency between 20 dB to 40 dB are obtained.     

Intermediate regime for the diffraction of light on ultrasound in gyrotropic anisotropic crystals

An intermediate regime for the diffraction of light on ultrasound in gyrotropic anisotropic and cubic crystals in an external electric field is considered. A system of equations of the coupled waves, which describes acoustooptic diffraction in gyrotropic anisotropic crystals with consideration of the electrically induced optical anisotropy for a strong interaction between light and ultrasound, is presented. An intermediate regime for the diffraction of light on ultrasound in gyrotropic anisotropic crystals which is close to the Bragg regime for a weak acoustooptic interaction is studied. It is shown that the diffracted light is elliptically polarized and that the ellipticity and polarization azimuth of the diffracted wave depend on the anisotropy of the photoelasticity, the electrically induced anisotropy of the crystal in the external electric field, the gyrotropy, and the asymmetry of the diffraction structure. Zh. Tekh. Fiz. 67, 80–82 (September 1997)     

Change in polarization of a weak signal induced by the optical pumping of atoms at an adjacent transition

A change in polarization of a weak wave under optical pumping of atoms at an adjacent atomic transition is studied. It is shown that an elliptically polarized probe signal undergoes substantial polarization changes in the field of a circularly polarized pump wave. For the probe wave component with the same circular polarization as that of a strong signal, the medium becomes transparent.     

Seed and vacuum pair production in strong laser field

Researches on the electron-positron pair production in the presence of the intense laser field are reviewed, motivated by the theoretical importance of the nonperturbative QED problem and the worldwide development of high power laser facilities. According to the distinct experimental requirements and theoretical methods, two types of pair production are elaborated, which are, respectively, pair production in the combination of a seed particle and the strong laser, and vacuum pair production without a seed particle. The origin of the nonperturbative problem caused by the strong field is analyzed. The main ideas, realisation, achievements, validity, challenges and bottleneck problems of the nonperturbative methods developed for each type of the pair production problem are discussed.