We study the influence of the polarization state of multi-color femtosecond laser pulses ionizing air or noble gases on the emitted terahertz radiation. A local-current model and plane wave evaluations predict a cross-over in the THz energy yields with increasing number of pump harmonics, for which circular laser polarization is more efficient for a few harmonics, and linear polarization is favorable for more than six pump colors. Comprehensive 3D numerical simulations of gas jet experiments confirm this property for singly and multiply ionized gases. Rotation of the THz polarization ellipse in the case of circular laser polarization is explained by phase shifts that may alter the phase angle between the harmonics.

中文翻译：

---

线偏振与圆偏振的多色激光场产生太赫兹脉冲

我们研究了电离空气或惰性气体的多色飞秒激光脉冲的偏振态对发射的太赫兹辐射的影响。局部电流模型和平面波评估预测，随着泵浦谐波数量的增加，太赫兹能量产额会发生交叉，其中圆形激光偏振对少数谐波更有效，而线性偏振对超过六种泵浦颜色有利. 气体射流实验的综合 3D 数值模拟证实了单电离气体和多电离气体的这种特性。在圆形激光偏振的情况下，太赫兹偏振椭圆的旋转可以通过相移来解释，相移可能会改变谐波之间的相位角。