As a promising terahertz (THz) source, two-color laser filamentation in gases attracts an exponential growth of interest because of the scalability of output THz pulse energy and bandwidth with input parameters. However, nonlinear effects within the laser filament on the THz pulse generation process, such as intensity-dependent refractive index change induced additional phase, have never been studied in depth. In this paper, we experimentally investigate the THz waveform evolution with input infrared pump energy from 1.75 ~ 2.75 mJ were recorded by an air biased coherent detection (ABCD) sampling setup. We observe that the magnitude ratio of the positive and negative peaks increases with the pump pulse energy, and the smooth peak at the tail gradually disappears. Based on the photocurrent model, our calculation shows that the modulation is caused by the plasma-induced additional phase that varies with incident laser intensity. Additionally, the smooth peak at the tail is considered to be caused by the fading of the residual current after the negative peak.

作为一种很有前途的太赫兹 (THz) 源，由于输出太赫兹脉冲能量和带宽与输入参数的可扩展性，气体中的双色激光灯丝引起了人们的兴趣指数增长。然而，激光灯丝内的非线性效应对太赫兹脉冲产生过程的影响，例如强度相关的折射率变化引起的附加相位，从未被深入研究过。在本文中，我们通过空气偏置相干检测 (ABCD) 采样设置记录了 1.75 ~ 2.75 mJ 的输入红外泵浦能量的太赫兹波形演变。我们观察到正负峰的幅度比随着泵浦脉冲能量的增加而增加，尾部的平滑峰逐渐消失。基于光电流模型，我们的计算表明调制是由等离子体引起的附加相位引起的，该相位随入射激光强度而变化。另外，尾部的平滑峰值被认为是由负峰值后剩余电流衰减引起的。