We study the influence of the polarization states of femtosecond two-color pulses ionizing gases on the emitted terahertz radiation. A local-current model and plane-wave evaluations justify the previously-reported impact on the THz energy yield and an (almost) linearly-polarized THz field when using circularly-polarized laser harmonics. For such pump pulses, the THz yield is independent on the relative phase between the two colors. When the pump pulses have same helicity, the increase in the THz yield is associated to longer ionization sequences and higher electron transverse momenta acquired in the driving field. Reversely, for two color pulses with opposite helicity, the dramatic loss of THz power comes from destructive interferences driven by the highly symmetric response of the photocurrents lined up on the third harmonic of the fundamental pulse. While our experiments confirm an increased THz yield for circularly polarized pumps of same helicity, surprisingly, the emitted THz radiation is not linearly-polarized. This effect is explained by means of comprehensive 3D numerical simulations highlighting the role of the spatial alignment and non-collinear propagation of the two colors.

中文翻译：

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圆偏振双色激光场产生太赫兹脉冲

我们研究了飞秒双色脉冲电离气体的极化状态对发射的太赫兹辐射的影响。当使用圆偏振激光谐波时，局部电流模型和平面波评估证明先前报告的对太赫兹能量产额和（几乎）线性偏振太赫兹场的影响是合理的。对于此类泵浦脉冲，太赫兹产量与两种颜色之间的相对相位无关。当泵浦脉冲具有相同的螺旋度时，THz 产率的增加与更长的电离序列和在驱动场中获得的更高的电子横向动量有关。相反，对于两个具有相反螺旋度的颜色脉冲，太赫兹功率的显着损失来自由排列在基波脉冲三次谐波上的光电流的高度对称响应驱动的相消干涉。虽然我们的实验证实了相同螺旋度的圆极化泵的太赫兹产量增加，但令人惊讶的是，发射的太赫兹辐射不是线性极化的。这种效果是通过全面的 3D 数值模拟来解释的，强调了两种颜色的空间排列和非共线传播的作用。