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.     

Analytical Study of Pair Production Rate from Vacuum in an Elliptic Polarized Field by a Two-Level Transition Technique

Electron-positron pair production rate created from vacuum in the presence of an elliptically polarized laser field is investigated.By applying the technique of two level transition amplitude,a routine for obtaining pair production rate is presented,and approximate analytical expressions are given both for the low frequency strong field regime and the high frequency weak field regime.We found that for an elliptically polarized field,the electron-positron pair production rate decrease when the elliptic eccentricity increase in the high frequency weak field regime,however,in the low frequency strong field regime,there is almost the same electron-positron pair production rate as in the constant electric field case.     

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.     

Manipulation of Spontaneous Emission via Quantum Interference in an Elliptically Polarized Laser Field

Manipulation of spontaneous emission from an atom confined in three kinds of modified reservoirs has been investigated by means of an elliptically polarized laser field. Some interesting phenomena such as the multi-peak structure, extreme spectral narrowing, and cancellation of spontaneous emission can be observed by adjusting controllable system parameters. Moreover, these phenomena depend on the constructive or destructive quantum interference between multiple decay channels and which can be changed appreciably by varying the phase difference between the two circularly polarized components of the probe field. These results demonstrate the importance of an elliptically polarized laser field in controlling the spontaneous emission and its potential applications in high-precision spectroscopy.     

Pair Production in a Uniform Electric Field

We study the Klein-Gordon field coupled with an external uniform vector potential. We compute pair production in a finite time t using the semiclassical approximation, and show that, after the interaction of the Klein-Gordon field with the external potential, when 0 the average number of produced pairs is zero. There is agreement with the classical limit because the classical limit involves no production of pairs. We compared our results with those of Schwinger. Finally we saw that the random variable N(t)= net number of pairs produced at time t is in the semiclassical limit a stochastic Poisson process.     

Control of Spontaneous Emission via a Single Elliptically Polarized Light in a Five-Level Atomic System

We investigate the features of the spontaneous emission spectra in a cold five-level atomic system coupled by a single elliptically polarized control field. We use wave function approach to derive the explicit and analytical expressions of atomic spontaneous emission spectra. It is shown that some interesting phenomena such as spectralline enhancement, spectral-line suppression, spectral-line narrowing, spectral-line splitting and dark fluorescence can be observed in the spectra by appropriately modulating the phase difference between the right-hand circularly (LHC) and left-hand circularly (RHC) polarized components of the elliptically polarized control field and the intensity of external magnetic field. The number of emission peaks, the positions of fluorescence-quenching points can be also controlled. Furthermore, we propose an ultracold 87Rb atomic system for experimental observation. These investigations may find applications in high-precision spectroscopy.     

Electron-Helium Atom Collisions in the Presence of a Bichromatic Laser Field

The differential cross section （DCS） for electron-helium atom collisions in the presence of a bichromatic CO2 laser field is investigated as a function of the scattering angle θ by employing first-Born approximation （FBA） with a simple screening electric potential. We discuss in detail the influence of the scattering geometry, the photon energy and the number of photons exchanged on the DCSs. These illustrate that the three factors have important effects on the elastic scattering and the screening electric potential is effective.     

Electron-positron pair production in a strong asymmetric laser electric field

By solving the quantum Vlasov equation, electron positron pair production in a strong electric field with asymmetric laser pulses has been investigated. We consider three different situations of subcycle, cycle and supercycle laser pulses. It is found that in asymmetric laser pulse field, i.e.. when the pulse length of one rising or falling side is fixed while the pulse length of the other side is changed, the pair production rate and mnnber density can be significantly modified comparable to symmetric situation. For each ca,se of these three different cycle pulses, when one side pulse length is constant and the other side pulse length becomes shorter, i.e., the whole pulse is compressed, the more pairs can be produced than that in tile vice versa case, i.e., the whole pulse is elongated. In compressed pulse case there exists an optimum pulse length ratio of asylnmetric pulse lengths which makes the pair number density maximunn. Moreover, the created maximum pair number density by subcycle pulse is larger than that by cycle or/and supercycle pulse. In elongated pulse case, however, only for supercycle laser pulse the created pairs is enhanced and there exists also an optimum asymmetric pulse length ratio that maximizes the pair number density. On the other hand. surprisingly, in both cases of subcycle and cycle elongated laser pulses, the pair number density is monotonically decreasing as the asymmetry of pulse increases.     

Dynamics of an Orientational-Kink Pair in a Hydrogen-Bonded Chain in an External Field

We discuss dynamic properties of an orientational-kink pair in hydrogen-bonded chains in the presence of an external force and damping, based on a new two-component soliton model. We study the scattering of an electromagnetic wave by an orientational-kink pair. Finally, we find the expressions of the scattering cross-section of an orientational-kink pair for an electromagnetic wave and the mobility of the orientational-kink pair.     

高能e+e-对撞机上top介子辅助的b夸克对产生

在顶色辅助的人工色模型下计算了高能e^ e^-对撞机上top介子辅助的b夸克对产生过程e^ e^-→bb^-π^0t的产生截面．发现在一定的参数范围内，这个过程的产生截面大于lOfb，比标准模型下标量Higgs粒子辅助的b夸克对产生过程大许多，与最小超对称模型下标量和赝标量Higgs粒子h^0，H^0，A^0的相应产生截面大致相当．人工色理论的信息可能在下一代正负电子对撞实验中探测到．     

Neutral Higgs Boson Pair Production in Standard Model with the Fourth Generation Quarks at LHC

We investigated the neutral Higgs boson pairproduction at the CERN Large Hadron Collider (LHC) in the SM withfour families. We found that the gluon-gluon fusion mode is themost dominant one in producing neutral Higgs boson pair at theLHC, and it can be used to probe the trilinear Higgs coupling. Ifthe heavy quarks of the fourth generation really exist within theSM, they can manifest their effect on the cross section of theHiggs pair production process at the LHC. Our numerical resultsshow that there will be 2×10⁴ neutral Higgs boson pairproduction events per year if the next generation heavy quarksreally exist, while there will be only 2×10³ eventsproduced per year if there are only three families in the SM.     

The Photoionization of Atomic Hydrogen in an Intense Circularly Polarized Laser Field

We calculate nonperturbatively the irfluence of a strong circularly polarized laser beam on the hydrogen atom energy levels by making use of the time-independent formalism proposed by one of us. The photoionization cross section of the hydrogen atom irradiated by this laser beam and the angular distribution of photoelectrons are also calculated. From the numerical results we clearly see the intensity dependence of the whole photoionization process,including the intensity dependence of the photoelectron energies.     

丰质子核反应中轻带电粒子发射的同位旋效应

采用同位旋相关的Boltzmann—Langevin方程计算了核素^12-15N和^17-20Ne反应中轻带电粒子发射的同位旋效应。^12-15N与^28Si靶的反应结果显示轻带电粒子的产生截面有明显的同位旋效应,^12N的轻带电粒子产生截面突然增大,与实验得出的结论相同,由此检验了所采用的计算方法的可行性。同时还计算了^17-20Ne与^9Be靶的反应,发现^17Ne的轻带电粒子产生截面也是突然增大,并且其质子分布有较大的弥散,据此认为^17Ne可能具有晕结构。 Within the Boltzmann-Langevin equation, the isospin effects on the production cross sections of light charged particles （LCP） in the reaction of proton rich nuclei were studied. We have calculated the LCP productio n cross sections of the reaction ^12-15N ＋ ^28Si at 55 MeV/u and ^17-20Ne ＋ ^9Be at 60 MeV/u. The LCP production cross sections of ^12N increase abnormally in comparison with those of ^13-15N. The result accorded with experimental one. It also happened to ^17-20Ne. It suggests that ^17Ne is probably a proton halo nucleus.     

超短脉冲强激光场中电子剩余能计算

为了控制光场感应电离等离子体的温度以使得基于光场感应电离机制的X射线激光辐射获得更高的增益值,利用准静态隧道电离模型,定量计算了电子剩余能量与激光偏振参量、波长(频率)、激光强度以及电离介质电离能的变化关系。计算结果表明线偏振短波长激光以及高电离能介质产牛的等离子体中电子剩余能比较低,因此适合用作复合机制X射线激光;对于低价离子存偏振度α介于0～1之间的某一处最高,因此,可以通过调整偏振度α,获得某一椭圆偏振光,会更有利于基于光场感应电离电子碰撞机制X射线激光增益的提高。     

一维多原子分子离子与超短激光场作用产生的高次谐波

从经典方程出发,采用克兰克-尼科尔森中心差分法对一维多原子分子离子与一维超短强激光场相互作用产生的高次谐波进行了数值模拟计算,发现激光场的初始相位对高次谐波辐射谱有明显影响;通过进一步研究,给出了初步解释。从而,可以通过调节适当的激光初始相位和选取适当的系统来改善高次谐波的相位匹配,产生清晰的高次谐波辐射谱。     

Extending the High-Order Harmonics Plateau and Isolated Attosecond Pulse Generation in an Intense Three-Color Laser Field

We propose a method to extend the high-order harmonics plateau and generate an isolated sub-10-as pulse by adding a weak control pulse (10 fs, 1600 nm) to modify a two-color laser field (5 fs, 800 nm; 10 fs, 400 nm). The numerical results show that the plateau is extended obviously in the three-color laser field regime. Additionally, the ionization rate and classical returning kinetic-energy maps are calculated to better understand the physical origin of the high-order harmonics generation (HHG). By means of the ionization probability and the time-frequency distributions, more features of the HHG are revealed. Furthermore, our simulations show that the width of the plateau and the relative conversion efficiency of the harmonic spectra are highly dependent on the relative phase. Finally, by adjusting the relative phase and superposing a properly selected range of the HHG spectrum, an isolated attosecond pulse with a duration of 7 as is obtained.     

强激光场高次谐波的两能级原子模型

利用两能级原子模型，通过直接求解含时薛定谔方程得到了强激光场中的高次谐波谱，和实验中观察到的谐波谱相似，这种模型可用于研究激光强度，激光频率以及介质的电离能等因素对高次谐波的影响。     

Self—Trapping State and Atomic Tunnelling Current of an Atomic Bose—Einstein Condensate Interacting with a Laser Field in a Double—Well Potential

We present a theoretical treatment of dynamics of an atomic Bose-Einstein condensation interacting with a single-mode quantized travelling-wave laser field in a double-well potential.When the atom-field system is initially in a coherent state,expressions for the energy exchange between atoms and photons are derived.It is revealed that atoms in the two wells can be in a self-trapping state when the tunnelling frequency satisfies two specific conditions,in which the resonant and far off-resonant cases are included.It is found that there is an alternating current with two different sinusoidal oscillations between the two wells,but no dc characteristic of the atomic tunnelling current occurs.It should be emphasized that when without the laser field,both the population difference and the atomic tunnelling current are only a single oscillation.But they will respectively become a superposition of two oscillations with different oscillatory frequencies in the presence of the laser field.For the two oscillations of the population difference,one always has an increment in the oscillatory frequency,the other can have an increment or a decrease under different cases.These conclusions are also suitable to those of the atomic tunnelling current.As a possible application,by measurement of the atomic tunnelling current between the two wells,the number of Bose-condensed atoms can be evaluated.By poperly selecting the laser field,the expected atomic tunnelling current can be obtained too.