定义
气体中的强烈的输入激光束产生高次谐波的现象。
当一个非常强的激光脉冲在气体中聚焦时（通常是减压的情况），由于强的非线性相互作用会导致非常高的奇次谐波的产生。这通常会发生在光强大于1014W/cm2或更高的情况下。虽然只有一小部分激光功率被转换成高次谐波，但其对于很多波长短至紫外光甚至X射线区域的测量方面的应用依然十分有用。这可以用来代替同步辐射的方法。这种方法也可在极端紫外的光谱区域产生阿秒脉冲[2，7，8，10，9，19]。这种阿秒脉冲现在用于各种基础研究中，如各种材料中的电子运动。甚至亚阿秒的脉冲（如zeptosecond脉冲，其脉宽远低于1阿秒）也是可能的[21]。
在大多数情况下，高次谐波产生的的泵源由一个被动锁模激光器和增益介质为钛：蓝宝石晶体的再生放大器组成。重复率则为几赫兹到几千赫兹之间。谐振腔（增强谐振腔）也被用来代替放大器从而来增加脉冲能量至所需的高次谐波产生的水平[12]。这种方法可以得到甚至大于100MHz重复频率。
虽然高次谐波产生背后的物理过程是十分复杂的（通常依赖于计算密集的数值量子模拟），一些“简单的模型”[1]，也可以用来描述高次谐波产生过程：电子在强大的电磁场下会脱离原子的束缚并加速，然后与原子发生碰撞，从而发射谐波辐射。
参考文献
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[14] F. Krausz et al., “Attosecond pulse generation and detection”, http://www.mpq.mpg.de/lpg/research/attoseconds/attosecond.html
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[19] F. Krausz and M. Ivanov, “Attosecond physics”, Rev. Mod. Phys. 81, 163 (2009)
[20] S. Hädrich et al., “Generation of μW level plateau harmonics at high repetition rate”, Opt. Express 19 (20), 19374 (2011)
[21] C. Hernández-García et al., “Zeptosecond high harmonic keV X-ray waveforms driven by midinfrared laser pulses”, Phys. Rev. Lett. 111 (3), 033002 (2013)
[22] M. Chini, K. Zhao and Z. Chang, “The generation, characterization and applications of broadband isolated attosecond pulses”, Nature Photon. 8, 178 (2014)
参阅：超连续谱产生、非线性频率变换
