双色激光诱导空气等离子体产生的太赫兹辐射波形
Waveshape of THz radiation produced by two-color laser-induced air plasmas
AI总结：
本研究提出增强型锥形发射模型，理论分析双色激光空气等离子体产生太赫兹辐射的空间光谱分布，揭示有效等离子体长度与退相长度差异，并发现光学像差降低转换效率并致光谱不对称。
摘要：
理论上研究了聚焦激光脉冲和成丝状态下双色空气等离子体发射的太赫兹（THz）脉冲的空间和光谱分布。我们推导了一个所谓的“增强型”锥形发射模型，该模型与You等人最初提出的模型（Phys. Rev. Lett. 109, 183902 (2012)）类似，将激光谐波沿等离子体通道的相位匹配、等离子体密度和长度以及发射频率作为关键参数。然而，我们的增强模型处理了包络效应，并考虑了等离子体驱动和克尔驱动的潜在太赫兹发射体的横向变化。我们强调了这两种转换机制的特征空间-光谱分布在对角辐射功率表达式中的重要性。我们的模型结果与一个全面的、全空间和时间分辨的单向求解器提供的数据成功进行了比较。重要的是，这些数值模拟阐明了太赫兹发射发展的有效等离子体长度，与激光基波和二次谐波变得异相的退相长度相比。还研究了常见光学像差（如球差、像散和彗差）对太赫兹产生的影响。结果表明，像差通常会降低激光到太赫兹的转换效率，并可能导致太赫兹光谱的空间不对称性和变窄。
英文摘要：
The spatial and spectral distributions of terahertz (THz) pulses emitted by two-color air plasmas are theoretically investigated for focused laser pulses and in the filamentation regime. We derive a so-called ''augmented'' conical emission model, which, similarly to the one originally proposed by You et al.\ [Phys.\ Rev.\ Lett.\ {\bf 109}, 183902 (2012)], involves phase matching between laser harmonics along the plasma channel, the plasma density and length, and the emitted frequency as key parameters. Our augmented model, however, treats envelope effects and accounts for transverse variations of both plasma- and Kerr-driven potential THz emitters. We highlight the importance of the characteristic spatio-spectral distributions of these two conversion mechanisms in the expression of the angular radiated power. The results of our model are successfully compared with data provided by a comprehensive, fully space and time-resolved unidirectional solver. Importantly, these numerical simulations clear up the effective plasma length along which THz emission develops, compared with the dephasing length along which the laser fundamental and second harmonic become out-of-phase. The impact of common optical aberrations, such as sphericity, astigmatism, and coma, on the THz generation is also investigated. Aberrations are shown to generally decrease the laser-to-THz conversion efficiency and potentially induce spatial asymmetries and narrowing in the THz spectra.
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