arXiv 0802.01482008-02-01physics.atom-phphysics.chem-ph

通过向IR辐射添加高频XUV辐射产生高次谐波

Generation of Higher-Order Harmonics By Addition of a High Frequency XUV Radiation to the IR One

Avner Fleischer, Nimrod Moiseyev

备注：: 15 pages, 6 figures

摘要：

频率为$ω$的强红外激光场照射原子会导致发射高达某个最大截止频率的$ω$奇次谐波（“红外谐波”）。将频率$\tilde{q}ω$大于IR截止频率的XUV场添加到IR驱动场导致出现新的高阶谐波（“XUV谐波”）$\tilde{q} \pm 2K, 2\tilde{q} \pm (2K-1), 3\tilde{q} \pm 2K,...$（$K$整数），其在仅存在IR场的情况下不存在于光谱中。负责的XUV谐波的外观的机制进行了分析，使用的半经典再碰撞（三步）模型的高次谐波产生的概括。结果表明，发射的高次谐波辐射场可以写成一系列项，而高次谐波辐射场是由最熟悉的三步模型[P. B. Corkum，\textit{Phys. Rev. Lett.} { \bf 71}，1994（1993）]中的方法。高阶项的起源被证明是由XUV场引起的剩余基态电子态的交流斯塔克振荡。这些项是HGS中新XUV谐波出现的原因。XUV谐波由形成IR谐波的相同电子轨迹形成，并且具有相同的发射时间，但是由于ac-Stark振荡的小振幅，其强度比IR谐波低得多。然而，这种机制允许在HGS中的截止的扩展，而不需要增加IR场强度，如通过数值的时间相关的薛定谔方程模拟的IR和XUV场的组合发光的HGS原子所验证的。

英文摘要：

The irradiation of atoms by a strong IR laser field of frequency $ω$ results in the emission of odd-harmonics of $ω$ ("IR harmonics") up to some maximal cut-off frequency. The addition of an XUV field of frequency $\tilde{q}ω$ larger than the IR cut-off frequency to the IR driver field leads to the appearance of new higher-order harmonics ("XUV harmonics") $\tilde{q} \pm 2K, 2\tilde{q} \pm (2K-1), 3\tilde{q} \pm 2K,...$ ($K$ integer) which were absent in the spectra in the presence of the IR field alone. The mechanism responsible for the appearance of the XUV harmonics is analyzed analytically using a generalization of the semiclassical re-collision (three-step) model of high harmonic generation. It is shown that the emitted HHG radiation field can be written as a serie of terms, with the HHG field obtained from the three-step model in its most familiar context [P. B. Corkum, \textit{Phys. Rev. Lett.} {\bf 71}, 1994 (1993)] resulting from the zeroth-order term. The origin of the higher-order terms is shown to be the ac-Stark oscillations of the remaining ground electronic state which are induced by the XUV field. These terms are responsible for the appearance of the new XUV harmonics in the HGS. The XUV harmonics are formed by the same electron trajectories which form the IR harmonics and have the same emission times, but a much lower intensity than the IR harmonics, due to the small quiver amplitude of the ac-Stark oscillation. Nevertheless, this mechanism allows the extension of the cut-off in the HGS without the necessity of increasing the IR field intensity, as is verified by numerical time-dependent Schrödinger equation simulation of a Xe atom shined by a combination of IR and XUV field.

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