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

## TL;DR

This paper uncovers a dominant strong-field-induced nonlinearity in amorphous wide-bandgap dielectrics, driven by sub-cycle electron injection dynamics, which influences low-order harmonic generation near damage thresholds.

## Contribution

It reveals the previously overlooked role of sub-cycle electron injection currents in strong-field nonlinear responses of amorphous solids, expanding understanding of high harmonic generation mechanisms.

## Key findings

- Injection current dominates near damage threshold
- Harmonic emission is sensitive to sub-cycle ionization dynamics
- Provides a new method to characterize laser-induced plasma formation

## Abstract

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.