Abstract Here we use the two-dimensional photocurrent model to analyze the terahertz (THz) wave generation from the air plasma excited by the harmonic three-color femtosecond lasers with wavelengths of 1200 nm, 600 nm, and 400 nm, respectively. This three-color harmonic frequency combination is readily-accessible under laboratory condition by using the suitable optical parametric amplifier and frequency doubling crystals, without the need of complex optical components for third harmonic wave. Through theoretical simulations, we find that the addition of 400 nm ultrafast laser with extremely low energy to the two-color laser fields of 1200 nm and 600 nm could effectively enhance the THz output. The THz wave energy emitted from the plasma point source could be easily enhanced up to 5.8 times. By either replacing the 400 nm laser with 800 nm laser or changing the near-infrared fundamental wavelength from 1200 nm to 1600 nm, the enhancement effect all noticeably reduces. These results reveal that the three-color laser fields which satisfy the harmonic frequency relation could achieve the utmost THz yield compared with the common two-color fields or non-harmonic three-color fields. This is of great significance for obtaining more powerful THz sources. The optimal conditions for the strongest THz wave generation are also calculated as a guidance for application.

中文翻译：

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谐波三色激光场激发空气等离子体的太赫兹波发射增强

摘要 在这里，我们使用二维光电流模型分析了由波长分别为 1200 nm、600 nm 和 400 nm 的谐波三色飞秒激光器激发的空气等离子体产生的太赫兹 (THz) 波。通过使用合适的光学参量放大器和倍频晶体，这种三色谐波频率组合在实验室条件下很容易获得，而不需要复杂的三次谐波光学元件。通过理论模拟，我们发现在1200nm和600nm两色激光场中加入能量极低的400nm超快激光可以有效提高太赫兹输出。从等离子体点源发射的太赫兹波能量可以轻松增强至 5.8 倍。通过用800nm激光代替400nm激光或将近红外基波从1200nm改变为1600nm，增强效果都明显降低。这些结果表明，与普通的二色场或非谐波三色场相比，满足谐波频率关系的三色激光场可以实现最大的太赫兹输出。这对于获得更强大的太赫兹源具有重要意义。还计算了产生最强太赫兹波的最佳条件，作为应用指南。这些结果表明，与普通的二色场或非谐波三色场相比，满足谐波频率关系的三色激光场可以实现最大的太赫兹输出。这对于获得更强大的太赫兹源具有重要意义。还计算了产生最强太赫兹波的最佳条件，作为应用指南。这些结果表明，与普通的二色场或非谐波三色场相比，满足谐波频率关系的三色激光场可以实现最大的太赫兹输出。这对于获得更强大的太赫兹源具有重要意义。还计算了产生最强太赫兹波的最佳条件，作为应用指南。