四能级原子系统中量子相干增强的Kerr非线性效应的研究

应用密度矩阵方程计算了四能级原子系统中三阶非线性极化率随信号光和探针光频率失谐的变化关系。结果表明，由于量子干涉对信号光强度的敏感性，使四能级原子介质的交叉Kerr非线性作用大大增强，与三能级系统相比，四能级原子介质的Kerr非线性系数可增强两个数量级。     

弱非线性耦合二维各向异性谐振子的一阶近似Lie对称性与近似守恒量

用近似Lie对称性理论研究弱非线性耦合二维各向异性 谐振子的一阶近似Lie对称性与近似守恒量, 并以频率比为2:1的弱非线性耦合二维各向异性谐振子为例, 得到其6个一阶近似Lie对称性和一阶近似守恒量, 其中1个一阶近似守恒量实为系统的精确守恒量, 4个一阶近似守恒量为平凡的一阶近似守恒量, 只有1 个一阶近似守恒量为稳定的一阶近似守恒量.     

分子一阶超极化率溶剂效应的理论研究

基于一维谐振子模型建立了极性溶剂对光学二阶非线性分子一阶超极化率产生影响的简化理论模型,推导出了非线性分子的线性极化率α和一阶超极化率β的表达式. 以对硝基苯胺为例,所建立的理论模型计算结果较好地解释了实验获得的溶剂极性对其分子线性吸收峰波长（λ_p）和β值的影响. 关键词： 二阶非线性 溶剂效应 谐振子     

含温有界原子模型下电子与离子碰撞激发和电离截面的理论研究

采用扭曲波玻恩交换近似方法，在自由原子模型下计算了电子与离子碰撞激发、电离截面，计算值与实验一致；在含温有界平均原子模型下，系统研究了不同温度、密度等离子体中离子的电子碰撞直接电离截面，发现由于温度、密度效应导致离子的能级漂移，引起等离子体中离子的碰撞电离截面比自由原子情形发生较大变化. 关键词： 平均原子模型 扭曲波波恩交换近似 电离截面     

原子相干引起的五阶非线性增强

采用腔透射谱法测量了Λ型三能级系统的五阶克尔非线性系数。对二能级和Λ型三能级系统的五阶非线性系数进行数值模拟,比较发现,原子相干对非线性系数具有增强作用。同时将三阶和五阶极化率的模拟结果与测量结果进行对比,发现两者具有很好的一致性。     

倒V形四能级亚飞秒极化拍的三光子吸收和色散

基于孪生色锁噪声场的互相关效应，从理论上研究了倒V形(RV)四能级飞秒量级差频极化拍与极化拍中五阶非线性极化率，并提出了用一种相位敏感的方法来研究多能级原子相干造成的三光子六波混频过程.在抽泵光束为窄带末端情况下，场关联对信号和极化率的影响都很弱，在非窄带情况下，场关联会使极化强度自相干信号和极化拍信号产生关于零延时的不对称性和时域扫描上的辐射与物质失谐阻尼振荡(RDO)，而极化率会产生频域扫描上的RDO振荡.参考光信号是一路传播方向和六波混频信号稍有区别的双光于非简并四波混频信号.五阶非线性响应则由享生     

Tavis-Cummings模型中三体纠缠态纠缠量的演化特性

研究两个全同二能级原子与单模场相互作用的Tavis-Cummings模型中的量子纠缠.在场和两原子初始分别为真空场和纠缠或非纠缠状态下，得出体系状态将演化为三体近似W纠缠态.通过对纠缠量的计算得出：该三体近似W纠缠态纠缠量的演化特性将随三体所处的初始状态、原子间及原子与场间的耦合系数、失谐程度、原子反转粒子数的变化而变化.值得一提的是可得出原子间耦合作用强度对纠缠量的非线性效应，并且纠缠量展现出与原子反转粒子数一致的周期振荡现象. 关键词： W纠缠态 纠缠量 反转粒子数     

第二玻恩近似理论研究激光辅助e-Ar散射

在电子入射方向平行于激光场的极化方向这种特殊的散射模式下，应用第二玻恩近似(SBA)理论，分别利用含有极化势的静电屏蔽势和单纯的静电屏蔽势这两种原子势模型对激光场中电子-氩原子散射进行了研究，并与低频近似和实验数据进行了比较.结果表明，第二玻恩近似理论给出的结果与实验符合较好.另外，极化势在激光辅助电子-原子散射中起着重要作用. 关键词： 极化势 第二玻恩近似 微分截面 激光场     

倒V形四能级亚飞秒极化拍的三光子吸收和色散

基于孪生色锁噪声场的互相关效应,从理论上研究了倒V形(RV)四能级飞秒量级差频极化拍与极化拍中五阶非线性极化率,并提出了用一种相位敏感的方法来研究多能级原子相干造成的三光子六波混频过程.在抽泵光束为窄带末端情况下,场关联对信号和极化率的影响都很弱,在非窄带情况下,场关联会使极化强度自相干信号和极化拍信号产生关于零延时的不对称性和时域扫描上的辐射与物质失谐阻尼振荡(RDO),而极化率会产生频域扫描上的RDO振荡.参考光信号是一路传播方向和六波混频信号稍有区别的双光于非简并四波混频信号.五阶非线性响应则由享生 关键词： 外差法 吸收 色散 非线性极化     

闭壳层原子相关效应的多体理论计算：Ne原子基态相关能

对coupled-pair many-electron theory(CPMET)展开中的非线性四体效应采取了近似处理,用角动量图技术和两体函数方法得到了适用于一般闭壳层原子的径向微分方程。然后以X_a模型作为零级近似,通过迭代求解成功地计算了Ne原子基态的任意阶相相关能,从而说明至少对于闭壳层原子体系而言,用X_a模型作零级近似同样可以进行成功的多体理论计算。     

Stochastic heating and stochastic outer ionization of an atomic cluster in a laser field

The dynamics of cluster electrons in a laser field is considered in the framework of the nonlinear oscillator model. The point map describing the motion of a cluster electron in the laser field is constructed. The critical value of the laser field, at which a transition to stochastic motion takes place, is determined. As a result of random walk in the energy space, an electron may accumulate an energy sufficient for overcoming the cluster potential barrier and for passing to the continuum. In this case, outer ionization of the cluster takes place. Estimates are obtained for the heating rate and the time of stochastic outer ionization of the cluster.     

Theory of resonant relativistic oscillator with nonuniform focusing field

The two-dimensional model of millimeter wave resonant O-type oscillator (as orotron, ledatron, resonant BWO, etc.) with a relativistic electron beam is analyzed. The selfconsistent nonlinear simultaneous equations have been obtained for the arbitrary space distribution of the magnetic guide field. The start generation characteristics are analyzed under small-signal conditions with an analytical solution taken for the case of inclined focusing magnetic field. It is found that the efficiency of electron-wave interaction appreciably depends on the focusing field strength and the relativistic mass factor. The results of numerical optimization of the guide field structure are presented to show possibility of improvement of the start characteristics of the oscillator.     

Coherence and saturation properties of second-harmonic generation with a nonlinear crystal inside the laser cavity

The basic equations for second-harmonic generation including noise are derived for the case that the nonlinear crystal is put inside the laser cavity. A realistic model of a (detuned) laser with two-level atoms in single-mode operation is taken using the nonlinear theory of laser noise which describes the laser saturation effects, the phase diffusion and the intensity fluctuations. The reaction of the second-harmonic field on the fundamental field is taken into account as well as the reaction of the fundamental field on the laser. The nonlinear crystal is described by microscopic anharmonic oscillator equations (without introducing nonlinear susceptibilities by perturbation theory). The saturation of the polarization of the nonlinear medium is taken into account exactly with the only assumption that the influence of third and higher harmonics should be small. The electromagnetic field is described semiclassically by stochastic equations. In all equations, the damping is introduced simultaneously with Markoffian fluctuating forces by coupling to heatbaths. The equations are solved exactly in the stationary state without noise (the time dependent solution including noise will be presented in a subsequent paper). The most important saturation effect is a frequency shift which depends on the laser intensity.     

Mode Competition and Self-Consistent Simulation of a Second Harmonic Gyrotron Oscillator

A nonlinear numerical simulation of a second harmonic gyrotron oscillator is presented in this paper. Mode competition from a competing fundamental mode is investigated. In addition, a self-consistent nonlinear theoretical model, including the effect of velocity spread, is applied to analyze the second harmonic gyrotron. A series of numerical calculations is carried out for different electron beam parameters. The results are compared with that of calculations using the cold-cavity, fixed field approximation and with experiments. Good agreement is found between our calculations and an experiment at Fukui University. The azimuthal polarization of the field and the choice of the electron beam radius are also discussed.     

导引场下电磁泵浦自由电子激光的非线性理论

本文以Vlasov-Maxwell方程组为基础,用非线性动力学理论研究了存在导引场情况下电磁波泵浦的自由电子激光器的作用机制,导出了线性及非线性色散关系式,求出了非线性不稳定性增长率和自由电子激光的能量转换效率.结果表明,只要合理地选择参量条件,附加导引场可以大大提高自由电子激光的增长率和能量转换效率.     

Quantum Nonlinear Oscillator with Two Degrees of Freedom in a Laser Field

The semiclassical dynamics of a quantum nonlinear oscillator with two degrees of freedom and anharmonicity of the fourth order in a periodic laser field is studied both analytically and numerically. In the absence of external excitation and dissipation, the equations of motion for the mean values of the coordinate and momentum operators of both degrees of freedom reduce to the equation of a onedimensional nonlinear pendulum. The general solution of this equation is written in terms of the Jacobian elliptic functions. As can be expected, the energy of the free oscillator is redistributed periodically between degrees of freedom. The periodic excitation of the nonlinear oscillator may substantially change its motion pattern. Using as an example an oscillator with two coupled vibrational degrees of freedom, it is numerically shown that the amount of laser photons absorbed depending on the parameter values and initial conditions may vary with time in a rather complex manner, including chaotic oscillations. A nonlinear oscillator is capable of manifesting bistable behavior with allowance for dissipation. The analytical condition for the origination of bistability is found. Examples of the bistable dependence of the number of quanta in the oscillator vibrational mode on the level of laser excitation are presented.     

Chaos Generation in Opto-Mechanical Coupling System Assisted by Two-Level Atoms

Chaos is important in nonlinear science and promotes the development of fundamental studies, such as neural networks, extreme event statistics, and electron transport. In this paper, a theoretical scheme for generating dynamical chaos in a Fabry–Perot cavity with two-level atoms is investigated. Through the injection of atoms, controllable chaos of the cavity and mechanical oscillator is achieved simultaneously by the external laser field. The trajectory and the maximal Lyapunov exponent that characterize the properties of the chaos could be optimized by adjusting the coupling constant, driving field, injection of atoms, the frequency of the cavity, and the frequency of the mechanical oscillator. This study provides a new idea to manipulate the cavity and mechanical chaos assisted by two level atoms. It is hoped that the results presented can benefit controllable chaos generation and its applications.     

`Spiking' radiation in the Columbia free electron laser

A report is presented on the observation of spikes of high-intensity radiation emitted from a 2-mm wavelength Raman free electron laser oscillator. The spikes are correlated with a well-developed sideband spectrum, including several sideband harmonics, together with the carrier. A pulse width ~150 ps is obtained from analysis of data obtained with a two-slit Young's experiment. An even shorter pulse width results from a numerical model. Using an elementary model of the spike, it is estimated that the peak spike pulse power is ~100 MW     

Exact solutions for nonlinear Hamiltonians

We find the eigenvalues and eigenvectors of two nonlinear Hamiltonians describing the interaction between a two-level system and a quantized linear harmonic oscillator. In the first case we obtain exact isolated solutions for the Hamiltonian used as a model of an ion in a harmonic trap and interacting with a laser field, not restricted to the Lamb-Dicke limit. After projecting these eigenstates onto one of the levels of the two-level system the oscillator state is described by a finite superposition of Fock states. In the second case we consider a Hamiltonian, with a squeeze operator in the interaction part. We give perturbation results in the weak-coupling limit and results obtained by numerical diagonalization for the strong coupling limit. Non-classical results are pointed out also in this case.