Femtosecond double optical pulse experiments with nanosecond time delay were conducted in air. Terahertz pulses were generated from laser plasmas produced by the irradiation with the main pulses on shockwave excited by the pre-pulse illumination. Intensity of the terahertz pulses depends on the time difference Δt and relative position Δy between the pre- and main pulses. We found a characteristic dependence of the intensity of the terahertz pulses on Δy at a longer time delay, Δt = 14.7 ns, which has two maxima while only one maximum was observed when delay times were at and shorter than 9.7 ns. The results are discussed taking into account air density distribution of the shockwave. The terahertz emission spectroscopy shown here can be a useful technique for investigating density structure and its time variation of shockwave with high temporal resolution.

中文翻译：

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飞秒光脉冲照射的冲击波前的太赫兹辐射

在空气中进行了具有纳秒级延时的飞秒双光脉冲实验。太赫兹脉冲是由预脉冲照明激发的冲击波上的主脉冲照射产生的激光等离子体产生的。太赫兹脉冲的强度取决于预脉冲和主脉冲之间的时间差 Δt 和相对位置 Δy。我们发现，在较长时间延迟（Δt = 14.7 ns）下，太赫兹脉冲强度对 Δy 存在一种特征依赖性，它有两个最大值，而当延迟时间为 9.7 ns 或短于 9.7 ns 时，仅观察到一个最大值。讨论结果时考虑了冲击波的空气密度分布。这里显示的太赫兹发射光谱可以成为研究高时间分辨率冲击波的密度结构及其时间变化的有用技术。