Kerr-type nonlinearities form the basis for our physical understanding of nonlinear optical phenomena in condensed matter, such as self-focusing, solitary waves and wave mixing^1,2,3. In strong fields, they are complemented by higher-order nonlinearities that enable high-harmonic generation, which is currently understood as the interplay of light-driven intraband charge dynamics and interband recombination^4,5,6. Remarkably, the nonlinear response emerging from the subcycle injection dynamics of electrons into the conduction band, that is from ionization, has been almost completely overlooked in solids and only partially considered in the gas phase^7,8,9,10. Here, we reveal this strong-field-induced nonlinearity in a-SiO₂ as a typical wide-bandgap dielectric by means of time-resolved, low-order wave-mixing experiments, and show that, close to the material damage threshold, the so far unexplored injection current provides the leading contribution. The sensitivity of the harmonic emission to the subcycle ionization dynamics offers an original approach to characterize the evolution of laser-induced plasma formation in optical microprocessing.

Kerr型非线性构成了我们对凝聚态非线性光学现象的物理理解的基础，这些凝聚态包括自聚焦，孤波和混波^1,2,3。在强场中，它们被高阶非线性所补充，使得能够产生高谐波，这目前被理解为光驱动带内电荷动力学与带间重组^4,5,6的相互作用。值得注意的是，从子循环注入电子到导带的动态动力学所产生的非线性响应，即来自电离，在固体中几乎被完全忽略，而在气相中仅被部分考虑^7,8,9,10。在这里，我们揭示了强磁场在a-SiO _2中引起的非线性作为一种典型的宽带隙电介质，通过时间分辨的低阶波混频实验，表明在接近材料损伤阈值的情况下，迄今未探索的注入电流提供了主导作用。谐波发射对子循环电离动力学的敏感性提供了一种新颖的方法来表征光学微加工中激光诱导的等离子体形成的演变。