双色场诱导气体产生相干可控的四次谐波

Coherently controlled fourth harmonic generation in gases induced by a two-color field

利用钛宝石飞秒激光器输出的基频脉冲ω及其倍频脉冲2ω所构成双色场作用空气,实验中检测到了中心波长处于真空紫外波段的四阶谐波.在气体未发生电离的情况下,四次谐波强度对双色场的能力依赖关系显示其产生是参量过程2ω+ω+ω→4ω的贡献.当气体发生电离,四次谐波强度与双色场相对相位有关,可通过双色场相干控制.实验研究了四次谐波对双色场相位的依赖性以及与太赫兹波的关联性,其结果与数值模拟结果相符,分析发现当气体发生电离时四次谐波的产生过程存在太赫兹辐射Ω_(THz)的参与,是参量过程2ω+2ω+Ω_(THz)→4ω和2ω+ω+ω→4ω的共同贡献.

Induced by the harmonically related two-color pulse, the fourth harmonic wave in the vacuum ultraviolet range has been observed in gases. The process of the fourth harmonic generation is studied. In the case of non-ionized gas, the intensity dependence on the pump pulse reveals that the fourth harmonic generation is a third-order parametric process of 2ω＋ω＋ω → 4ω. In the case of ionized gas, the intensity of the fourth harmonic wave can be coherently controlled by the relative phase of the pump. Employing synchronous measurements of the fourth-order harmonic and terahertz emissions, the optimized relative phases of the two emissions have been studied and compared with each other experimentally. Results agree well with those of the time-dependent Schr？dinger equation （TDSE）. According to the reported optimized phase for the terahertz generation, the fourth harmonic generation involves two parametric processes：2ω＋2ω±ΩTHz →4ω and 2ω＋ω＋ω →4ω. The modulation of the fourth harmonic wave can be understood as the interference of the two channels.