Origin of strong-field-induced low-order harmonic generation in amorphous quartz

Strong-field-induced nonlinearities from the injection of electrons into the conduction band contribute to harmonic generation in amorphous quartz. Close to the damage threshold, they dominate over intraband and interband contributions. 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(1-3). 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(4-6). 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(7-10). Here, we reveal this strong-field-induced nonlinearity in a-SiO(2)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.