激光在大气中驱动的强太赫兹辐射的理论和实验研究

两束双色激光脉冲能在大气中产生MV/cm的强太赫兹波.本文主要介绍了我们最近的三项理论和实验工作,澄清了双色激光方案的物理机制这个长期存在的问题,并对该方案进行了推广.为了在气体中有效地产生太赫兹波,在广泛研究的双色激光方案中两束激光的频率比ω_2/ω_1总是被取为1:2.首先从理论上预测采用其他频率比时,此方案仍能有效地工作,并通过实验进行证实.实验上观察到在新的频率比ω_2/ω_1=1:4,2:3下,也能有效地产生太赫兹波;观察到通过旋转较长波长的激光脉冲的偏振方向,能够有效地调节太赫兹波的偏振,但是旋转波长较短的激光脉冲的偏振方向,太赫兹波的偏振几乎没有变化,这违背了多波混频理论中极化率张量对称性的要求;采用不同的频率比时,太赫兹能量定标率并没有显示出明显的区别,这与多波混频理论预测的能量定标率不符.这些实验结果与等离子体电流模型及粒子模拟结果符合得很好.因此,该研究不仅对双色激光方案进行了推广,而且证实了其物理机制应该归结为等离子体电流模型.

Theoretical and experimental studies on terahertz radiation from laser-driven air plasma

Strong terahertz (THz) radiation of MV/cm can be generated from air via two-color laser scheme. In this paper, we introduce three recent theoretical and experimental researches conducted by Wang et al., in which they explored the long-standing problem of THz generation mechanism and extended the scheme with uncommon frequency ratio. In the widely-studied two-color laser scheme, the frequency ratio of the two lasers is usually fixed at ω₂/ω₁=1:2. In 2013 they predicted according to the plasma current model, for the first time, that the two-color scheme can be extended to a new frequency ratio 1:2n, where n is an positive integer. In 2017 they found that the frequency ratio can be further extended to much broader values. In that year, their experiments showed, for the first time, efficient THz generation with new ratios of ω₂/ω₁=1:4 and 2:3. They observed that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization, but the polarization adjustment becomes inefficient by rotating the other laser polarization, which is inconsistent with the symmetric nature in the susceptibility tensor required by the multi-wave mixing theory; the THz energy shows similar scaling laws with different frequency ratios, which is inconsistent with the scaling predicted according to the multi-wave mixing theory. These experimental results are in agreement with the plasma current model and particle-in-cell simulations. Therefore, their studies not only push the development of the two-color scheme, but also show that the THz generation mechanism should be mainly attributed to the plasma current model.