As a femtosecond terawatt laser pulse propagates along a positive density gradient within a stratified plasma column, it drives a low-frequency electromagnetic wake wave, the period of electron fluid oscillations in the wake gradually shrinking. Phase velocity of the wake promptly exceeds the vacuum speed of light, setting in near-forward emission of terahertz Cherenkov radiation. As the wake accelerates further, the Cherenkov emission ray rotates by 180^∘. Emission from a given plasma locality is sustained for a finite interval of time, in the middle of which the wake experiences a ‘reversal,’ its phase velocity becoming singular and changing sign (Zhang C J et al 2017 Phys. Rev. Lett. 119 064801) At this instant, the electromagnetic energy flows at 90^∘, the emission power reaching its peak. After the reversal, the wake keeps radiating into the rear hemisphere until its phase velocity becomes subluminal. Experimentally capturing evolution of the Cherenkov signal may thus shed light onto the plasma wake dynamics. Far away from the plasma, the radiation fills an expanding, almost spherical shell, the shell thickness increasing with an increase in the observation angle. The length of the terahertz signal sampled in the wave zone thus ranges from zero (forward emission) to a few tens of picoseconds (backward emission). The signal is positively chirped, its frequency increasing from the Langmuir frequency at the foot of the column to the Langmuir frequency at the top. Theoretical estimates for the regimes involving 10 TW-class drive pulses promise a few-kW emission power; the energy conversion efficiency, from optical to terahertz, of order 10⁻⁷; and an MV m⁻¹-scale electric field strength centimeters away from the plasma.

当飞秒太瓦激光脉冲在分层等离子体柱内沿正密度梯度传播时，它驱动低频电磁尾波，尾流中电子流体振荡的周期逐渐缩短。尾流的相速度迅速超过真空光速，设置为太赫兹切伦科夫辐射的近前发射。随着尾流进一步加速，切伦科夫发射射线旋转 180 ^∘。来自给定等离子体位置的发射持续有限的时间间隔，在此期间尾流经历“反转”，其相速度变得奇异且符号变化（Zhang CJ等人2017 Phys. Rev. Lett . 119064801）在该时刻，所述电磁能量在90流动^∘，发射功率达到峰值。反转后，尾流继续辐射到后半球，直到其相速度变为亚光速。因此，通过实验捕获切伦科夫信号的演变可以揭示等离子体尾流动力学。在远离等离子体的地方，辐射填充了一个膨胀的、几乎是球形的壳层，壳层的厚度随着观察角的增加而增加。因此，在波区采样的太赫兹信号的长度范围从零（前向发射）到几十皮秒（后向发射）。信号为正啁啾，其频率从列底部的朗缪尔频率增加到顶部的朗缪尔频率。对涉及 10 个 TW 级驱动脉冲的机制的理论估计有望实现几千瓦的发射功率；⁻⁷ ; 和距离等离子体数厘米的 MV m ^-1尺度电场强度。