Strong terahertz (THz) electric and magnetic transients open up new horizons in science and applications. We review the most promising way of achieving sub-cycle THz pulses with extreme field strengths. During the nonlinear propagation of two-color mid-infrared and far-infrared ultrashort laser pulses, long, and thick plasma strings are produced, where strong photocurrents result in intense THz transients. The corresponding THz electric and magnetic field strengths can potentially reach the gigavolt per centimeter and kilotesla levels, respectively. The intensities of these THz fields enable extreme nonlinear optics and relativistic physics. We offer a comprehensive review, starting from the microscopic physical processes of light-matter interactions with mid-infrared and far-infrared ultrashort laser pulses, the theoretical and numerical advances in the nonlinear propagation of these laser fields, and the most important experimental demonstrations to date.

太赫兹（THz）的电磁瞬变为科学和应用开辟了新的视野。我们回顾了实现具有极强场强的亚周期太赫兹脉冲的最有前途的方法。在双色中红外和远红外超短激光脉冲的非线性传播过程中，会产生长而粗的等离子线，强光电流会导致强烈的THz瞬变。相应的太赫兹电场和磁场强度有可能分别达到千兆伏每厘米和千特的水平。这些太赫兹场的强度使极端的非线性光学和相对论物理学成为可能。我们提供全面的综述，从光物质与中红外和远红外超短激光脉冲相互作用的微观物理过程开始，