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首页> 外文会议>NATO Advanced Research Workshop on Super-Intense Laser-Atom Physics Sep 24-30, 2000 Han-sur-Lesse, Belgium >INTENSE LASER-ATOM INTERACTIONS: BEYOND THE DIPOLE APPROXIMATION
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INTENSE LASER-ATOM INTERACTIONS: BEYOND THE DIPOLE APPROXIMATION

机译：强烈的激光-原子相互作用：超越偶极近似

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摘要

High harmonic generation by atoms and the stabilization of atoms are two intense-field phenomena that have been studied extensively in recent years. Harmonic generation can be understood qualitatively in terms of a semi-classical, three-step model. In this so-called simpleman's model", the active electron of the atom or ion is first detached by tunneling through an effective potential barrier formed by the instanta leous electric field of the laser pulse and the potential that binds the electron. The ejected electron is then assumed to move like a free particle in the electric field of the incident laser pulse. As the field reverses, the electron can re-collide with the core and recombine radiatively. A fully quantum mechanical description of harmonic generation that incorporates the essential ideas of the semi-classical model has been developed by Lewenstein and co-workers. Stabilization in the high-intensity, high-frequency regime can, on the other hand, be understood with the help of the high-frequency Floquet theory (HFFT) developed by Gavrila and co-workers. First, a transformation is made to the rest frame of a classical electron in the laser field. In this Kramers-Henneberger (K-H) frame, the electron experiences an effective time-dependent Coulomb potential that oscillates in space. Gavrila and co-workers have shown that in a stationary field, to first approximation, the dynamics of the atom in the field are governed by the time-average of this potential. This theory can be modified to accommodate the non-adiabatic evolution of the atom in a short laser pulse and, if the turn-on of the laser pulse is short enough, the electron wavepacket dynamics is essentially those of a free electron in the pulse. At high laser intensities, these relatively simple descriptions of harmonic generation and stabilization, respectively, can break down sinee non-dipole and relativistic effects must be accounted for. Indeed, for only moderately high intensities, the magnetic field component of the laser pulse cannot be a priori neglected. This magnetic field induces a drift in the laser propagation direction that, for a free classical electron, can be quite large. If taken literally, the semi-classical model of harmonic generation predicts no harmonic generation beyond the dipole approximation since an ejected electron would never return to the core. Similarly, in the high-frequency regime, a K-H transformation to the rest frame of an oscillating and drifting electron in a laser pulse would imply that the Coulomb potential moves away from the electron during the pulse. This, in turn, would affect the stabilization of the atom in the laser pulse. Obviously, these observations are oversimplified. However, we will show that the magnetic f eld component of an intense laser pulse can indeed strongly influence both high harmonic generation and stabilization.
机译：由原子产生的高次谐波和原子的稳定化是近年来已广泛研究的两个强场现象。谐波产生可以用半经典的三步模型定性地理解。在这种所谓的“简单人模型”中，原子或离子的活性电子首先通过隧穿穿过激光脉冲的瞬时电场和结合电子的电势形成的有效势垒而脱离。然后假设它在入射激光脉冲的电场中像自由粒子一样运动，当电场反转时，电子可以与核重新碰撞并以辐射方式复合。由Lewenstein和他的同事开发的半经典模型，另一方面，在高强度，高频状态下的稳定性可以借助由他开发的高频浮球理论（HFFT）来理解。 Gavrila和他的同事们：首先，在激光场中对经典电子的其余框架进行了转换，在这种Kramers-Henneberger（KH）框架中，电子经历了一个效应与时间有关的库仑电势在空间中振荡。 Gavrila和他的同事们已经证明，在一个稳定的场中，首先近似地，该场中原子的动力学受该势的时间平均支配。可以对该理论进行修改，以适应短激光脉冲中原子的非绝热演化，如果激光脉冲的开启足够短，则电子波包的动力学本质上就是脉冲中自由电子的动力学。在高激光强度下，谐波产生和稳定的这些相对简单的描述分别会破坏单极非偶极子效应，因此必须考虑相对论效应。实际上，仅对于适度的高强度，激光脉冲的磁场分量不能被先验地忽略。该磁场在激光传播方向上引起漂移，对于自由的经典电子来说，漂移可能很大。如果从字面上看，谐波产生的半经典模型预测不会超出偶极近似，因为射出的电子将永远不会返回铁芯。类似地，在高频状态下，K-H变换到激光脉冲中振荡和漂移的电子的其余帧将意味着在脉冲过程中库仑电势从电子移开。反过来，这将影响激光脉冲中原子的稳定性。显然，这些观察过于简单。但是，我们将证明强激光脉冲的磁场分量确实可以强烈影响高次谐波的产生和稳定性。

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