Shim B, Hays G, Zgadzaj R, Ditmire T, Downer M C
FOCUS Center, Department of Physics, University of Texas at Austin, Austin, Texas 78712-1081, USA.
Phys Rev Lett. 2007 Mar 23;98(12):123902. doi: 10.1103/PhysRevLett.98.123902. Epub 2007 Mar 20.
We report controlled enhancement of optical third harmonic generation (THG) from hydrodynamically expanding clusters of approximately 6x10(5) noble-gas atoms several hundred femtoseconds following ionization and heating by ultrashort pump pulses. This resonant enhancement is more pronounced for orthogonal than for parallel pump-probe polarizations, a consequence of faster cluster expansion along the pump polarization. Simulations show that the nonlinear susceptibility chi(3) of the individual clusters and the coherence length of the clustered plasma medium are optimized nearly simultaneously as the clusters expand, and both contribute to the observed THG enhancement. This dual enhancement mechanism may be scalable to relativistic probe intensity and to generation of high-order harmonics in the soft-x-ray regime.
我们报告了在通过超短泵浦脉冲进行电离和加热后的几百飞秒内，对由约6×10⁵个惰性气体原子组成的流体动力学膨胀团簇产生的光学三次谐波（THG）的可控增强。这种共振增强对于正交泵浦 - 探测偏振比平行泵浦 - 探测偏振更为明显，这是团簇沿泵浦偏振方向更快膨胀的结果。模拟表明，随着团簇膨胀，单个团簇的非线性极化率χ(3)和团簇等离子体介质的相干长度几乎同时得到优化，并且两者都有助于观察到的THG增强。这种双重增强机制可能可扩展到相对论探测强度以及软X射线区域的高次谐波产生。
