  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. 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. Remarkably, the nonlinear response emerging from
the associated sub-cycle injection dynamics of electrons into the conduction
band has been almost completely overlooked in solids and only partially
considered in the gas phase. Here we reveal this strong-field-induced
nonlinearity in amorphous wide-bandgap dielectrics 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 sub-cycle
ionization dynamics offers an original approach to characterize the evolution
of laser-induced plasma formation in optical microprocessing.
