Photodissociation dynamics of triatomic molecule in presence of pulsed and bichromatic laser field

The photodissociation dynamics of a triatomic molecule in the presence of pulsed and bichromatic electric field have been investigated. We have considered MgH₂ molecule in its ground state potential energy surface. The time-dependent Fourier Grid Hamiltonian method is invoked to follow the dissociation dynamics as a function of field strength, frequency, phase difference, etc.     

Mode competition of bichromatic laser emission in two-dimensional strong scattering random medium

The mode competition of the bichromatic laser emission in two-dimensional (2D) random medium was studied. Based on the time-dependent theory, a model for dual-wavelength random laser was established. The Maxwell equations and rate equations were combined and solved by using the finite different time domain (FDTD) method. Results show that the emission intensity of both wavelength ranges increases simultaneously with the increasing of the surface-filling fraction. The mode dominance of the bichromatic emission can be switched by changing the scatterer radius. The emission intensity of the longer wavelength increases when enlarging the excitation area. The controllability of these lasing modes may provide a potential application of optical switches.     

Phase relations between focused bichromatic laser pulses in terahertz wave generation from gas plasma

Terahertz emission from air excited by the tightly focused fundamental and second harmonic pulses of femtosecond Ti:Sapphire laser has been analyzed. It has been found that the curved phase fronts of the pump waves cancel each other resulting in the flat phase front of the nonlinear polarization at terahertz frequency. Also, in addition to the phase terms obtained using plane-wave approximation, we have found the yet unreported phase term, which in most cases can be as large as π/2.     

Positron-impact ionisation of atomic hydrogen in the presence of a bichromatic laser field

The positron impact-ionisation of atomic hydrogen in the presence of a linearly polarised bichromatic field is investigated in the first Born approximation.The field is composed of a fundamental frequency and its second harmonic.The state of positron in the field is described by the Volkov wavefunction,and the continuum state of the ejected electron is described by the Coulomb-Volkov wavefunction.The dressed ground state of target is a first order time-dependent perturbative wavefunction.The triple differential cross sections and their dependencies on laser field parameters are discussed and compared with the results modified by a monochromatic field.Numerical results show that the coherent phase control is significant and the laser-assisted ionisation cross sections caused by positron and electron are different.     

Coherent phase control in electron argon scattering in a bichromatic laser field

This paper theoretically investigates the coherent phase control in electron--argon scattering assisted by a bichromatic laser field. The laser field is composed of a fundamental component and its second harmonic. The incoming and out going states of electron are described by the Volkov wave functions, and the electron--target interaction is treated as a screening potential. Numerical results for differential cross section of multiphoton processes vs the phase difference between the two components of laser field are discussed for several scattering angles and impact energies.     

Coherent hole-burnings induced by a bichromatic laser field

In a Doppler-broadened three-level Λ-type system driven simultaneously by a coupling laser and two equal-amplitude saturating laser fields with a frequency difference 2δ, the absorption spectrum of a weak probe laser exhibits multiple deep spectral holes through coherent hole-burning CHB with controllable numbers, widths, depths, and positions. More significant, changing δ or lasers directions, CHBs can degenerate into narrower and deeper spectral holes where the slope of the refractive index is very steep. The multiple narrow spectral holes in a single absorption profile are expected to have potential applications in high density storage, optical information processes, and slow-light.     

Kroll-Watson-type theorem for potential scattering in a bichromatic laser field

Summary We show that for potential scattering of electrons in a bichromatic laser field a Kroll-Watson type of theorem can be derived in which the scatteringT-matrix element is composed of a renormalized on-shell matrix element for scattering without a field times a generalized Bessel function factor. The radiation field has two components of frequency ω₁ and sω₁ (s=2,3,…) and both components are out of phase by an angle ϕ. Our paper is a generalization of earlier investigations which were performed in the first-order Born approximation.     

飞秒激光与固体靶相互作用中自生磁场的测量

 利用OMA光学多道分析谱仪，分别在激光镜反方向和接近靶面法线方向测量了2倍频谐波的精细结构。在激光功率密度为5×10¹⁷ W/cm²的条件下，通过散射光2倍频谐波谱的精细结构，回推出激光与等离子体相互作用中产生的自生磁场达100T量级。     

Atom trap trace analysis

A new method of ultrasensitive isotope trace analysis has been developed. This method, based on the technique of laser manipulation of neutral atoms, has been used to count individual ⁸⁵Kr and ⁸¹Kr atoms present in a natural krypton gas sample with isotopic abundances in the range of 10⁻¹¹ and 10⁻¹³, respectively. The method is free of contamination from other isotopes and elements, and can be applied to many different isotope tracers for a wide range of applications. This revised version was published online in August 2006 with corrections to the Cover Date.     

飞秒激光与固体靶相互作用中自生磁场的测量

利用OMA光学多道分析谱仪,分别在激光镜反方向和接近靶面法线方向测量了2倍频谐波的精细结构。在激光功率密度为5×1017 W/cm2的条件下,通过散射光2倍频谐波谱的精细结构,回推出激光与等离子体相互作用中产生的自生磁场达100T量级。     

Susceptibility of a three-level V-type quantum system in a bichromatic field

A theory of interaction of a three-level V-type quantum system with a bichromatic field of two waves, one of which is weak, has been developed. It is suggested that the upper two levels of this system are closely spaced and, therefore, both waves interact simultaneously with both allowed transitions. An expression for nonlinear macroscopic polarization of the V-system has been obtained by the density-matrix method.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 67–71, January–February, 2005.     

飞秒强激光场中氢原子团簇的各向异性膨胀

利用LAMMPS程序研究了氢原子团簇在飞秒强激光场下的动力学行为, 讨论了引起小氢原子团簇各向异性膨胀的原因.通过对外电离过程中团簇内部电子的行为以及团簇各个方向上最外层质子距离团簇中心的距离随时间的变化情况的分析, 发现团簇的膨胀呈现各向异性,且引起这种各向异性的根源在于团簇内部电子的抖动以及逃逸.对氢原子团簇与强激光场相互作用过程中质子各能量分量以及各向异性程度随时间变化情况进行了研究,发现各向异性程度是随时间变化的, 这种各向异性程度首先随着激光电场的增强而增加,随后又逐渐减小,直到最后趋于某一大于1的稳定值.分析了激光脉冲结束后质子的平均能量与观测角之间的关系, 并将分析结果与Ditmire小组的实验结果进行了比较,发现我们的模拟结果在定性上与实验相符合.     

Spin orbit coupled Bose Einstein condensate in a two dimensional bichromatic optical lattice

We numerically investigate the localization of Bose Einstein condensate (BEC) with spin orbit coupling in a two dimensional bichromatic optical lattice. We study localization in weakly interacting and non-interacting regimes. The existence of stationary localized states in the presence of spin–orbit and Rabi couplings has been confirmed. We find that spin orbit coupling favors localization, whereas Rabi coupling has a slight delocalization effect.     

The dissociative ionization and Coulomb explosion of ethane by a femtosecond laser field

Femtosecond laser-induced dissociation and Coulomb explosion of polyatomic molecule C_2H_6 were systematically investigated using a time-of-flight mass spectrometer and a chirped pulse amplifier laser. With the laser intensity varying from 2.4×10^{15}W/cm^{2} to 1.2×10^{16}W/cm^2, strong molecular ions C_2H_n^+ (n=0－6) and atomic ions C^{m+} (m=1－3) signals were observed. The double-peak structure of atomic ions indicated the occurrence of Coulomb explosion. Compared with the nearly isotropic distribution of C^{+}, highly charged ions C^{m+} (m=2－3) exhibited a sharply anisotropic angular distribution, which was attributed to the geometric alignment.     

Dispersion of a three-level quantum system in a bichromatic field

We consider aspects of the interaction between a V-type three-level quantum system and the bichromatic field of two waves, one of which is assumed to be weak. We trace the dynamics of splitting into individual components for the spectral contours of the absorption coefficient (gain) for the signal wave. We discuss the effect of amplification of the signal wave for its frequency-degenerate two-wave interaction with pumping, when the field frequency is tuned to the “zero dispersion” point. We determine the conditions with respect to the concentration of resonant atoms and the spectral regions of the frequencies of the interacting waves for which, in order to correctly describe the effects under consideration, we need to take into account the effect of pumping and the signal in both dipole-allowed transitions of an ensemble of V type three-level quantum systems. We present the results of numerical calculations of the imaginary and real parts of the susceptibility of the resonant medium at the frequency of the signal wave, as a function of the parameters for the high-power radiation and resonant transitions of the quantum V system. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 631–636, September–October, 2007.     

飞秒激光场中原子所受光学偶极力研究

通过求解全波矢布洛赫方程研究了两能级原子与飞秒超快激光脉冲的相互作用过程,计算了不同拉比频率取值下原子所受光学偶极力和粒子数布居随时间的演化情况,分析了光场失谐量对光学势分布情况的影响.研究发现:由飞秒激光场产生的横向光力的时间平均值并不等于零,而是随着拉比频率的增加呈现振荡的增大趋势;纵向光力的时间平均作用也并非是拉比频率的单调函数,而是随着拉比频率的增加呈现周期性的振荡分布特性;光学势的分布对光场的失谐量具有明显的依赖性,随着失谐量的变化,光学势的性质也随之发生了改变.     

Polarization control of multi-photon absorption under intermediate femtosecond laser field

It has been shown that the femtosecond laser polarization modulation is a very simple and well-established method to control the multi-photon absorption process by the light–matter interaction. Previous studies mainly focused on the multiphoton absorption control in the weak field. In this paper, we further explore the polarization control behavior of multiphoton absorption process in the intermediate femtosecond laser field. In the weak femtosecond laser field, the secondorder perturbation theory can well describe the non-resonant two-photon absorption process. However, the higher order nonlinear effect(e.g., four-photon absorption) can occur in the intermediate femtosecond laser field, and thus it is necessary to establish new theoretical model to describe the multi-photon absorption process, which includes the two-photon and four-photon transitions. Here, we construct a fourth-order perturbation theory to study the polarization control behavior of this multi-photon absorption under the intermediate femtosecond laser field excitation, and our theoretical results show that the two-photon and four-photon excitation pathways can induce a coherent interference, while the coherent interference is constructive or destructive that depends on the femtosecond laser center frequency. Moreover, the two-photon and fourphoton transitions have the different polarization control efficiency, and the four-photon absorption can obtain the higher polarization control efficiency. Thus, the polarization control efficiency of the whole excitation process can be increased or decreased by properly designing the femtosecond laser field intensity and laser center frequency. These studies can provide a clear physical picture for understanding and controlling the multi-photon absorption process in the intermediate femtosecond laser field, and also can provide a theoretical guidance for the future experimental realization.     

TheB (3) field as a link between gravitation and electromagnetism in the vacuum

The emergence of theB ⁽³⁾ field in vacuo has shown that electromagnetism is non-Abelian and similar in structure to gravitation. In this paper the Christoffel symbol used in general relativity is developed for electromagnetism in curvilinear coordinates: The former becomes describable as the antisymmetric part of the gravitational Ricci tensor. Therefore gravitation and electromagnetism are respectively the symmetric and antisymmetric parts of thesame Ricci tensor within a proportionality factor. Both fields are obtained from the Riemann curvature tensor, both are expressions of curvature in spacetime.     

An alternative approach to determine the spot-size of a multi-mode laser beam and its application to diode laser beams

An alternative approach is suggested to determine the spot-size of a multi-mode laser beam. It has been shown by simulations that the suggested approach can give the beam quality factor and characteristic radius with less than 5% error. Unlike the power content method, the proposed method is applicable to the beams even with diameter one tenth of the CCD size. The new approach has been applied to a multi-mode diode laser output and it is shown that the ABCD matrix analysis can be used for beam propagation, with the measured parameters of the laser.