采用附加共振高频场提高强场高次谐波的产生效率

提高高次谐波效率一直是强场物理研究中的一个重要课题,对此人们采取了很多方法,如采用双色场,实现相位匹配,附加一个强电场或强磁场等。本文在研究双色场高次谐波的过程中,发现原子的能级结构对于高次谐波的产生效率有极大的影响,据此提出利用附加共振高频场来大幅度提高高次谐波的产生效率。     

一维多原子分子离子与超短激光场作用产生的高次谐波

从经典方程出发，采用克兰克-尼科尔森中心差分法对一维多原子分子离子与一维超短强激光场相互作用产生的高次谐波进行了数值模拟计算，发现激光场的初始相位对高次谐波辐射谱有明显影响；通过进一步研究，给出了初步解释。从而，可以通过调节适当的激光初始相位和选取适当的系统来改善高次谐波的相位匹配，产生清晰的高次谐波辐射谱。     

Plasmon‐Enhanced Second‐ and Third‐Order Harmonic Generation in Spherical Free‐Electron Nanoclusters with Diffuse Surface

The nonlinear scattering of a laser pulse off spherical nanoclusters with free electrons and with a diffuse surface is examined in the collisionless hydrodynamics approximation in the framework of perturbation theory with respect to the laser pulse intensity, as well as of the steady‐state approximation. In a previous publication [S.V. Fomichev and W. Becker, Phys. Rev. A 81 , 063201 (2010)] we reported the full nonlinear hydrodynamic model of forced collective electron motion confined to a cluster with diffuse surface and introduced two different perturbation theories corresponding to different laser intensity regimes. In the current paper, in the framework of this hydrodynamic model we focus on the properties of plasmon resonance‐enhanced third‐harmonic generation in a spherical cluster and its dependence on the shape of its diffuse surface whose role increases for nonlinear processes. At the same time, the quadrupole second‐harmonic generation in a spherical cluster is also inspected as a necessary intermediate step. Both cold metal clusters in vacuum or in a dielectric surrounding and hot laser‐heated and laser‐ionized clusters are considered within the same approach for a wide range of the fundamental laser frequency. Nonlinear laser excitation of the dipole plasmon Mie resonance in spherical clusters, as well as of other respective multipole plasmon resonances is investigated analytically and numerically in detail (position, width, and strength) versus the cluster‐surface diffuseness, the outer ionization degree in charged clusters, the electron‐density diffuseness, and their interplay. Under certain conditions, depending on the various cluster parameters, different secondary nonlinear resonances are found. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)