Abstract
<title>Abstract</title> <p>Low-order harmonics, normally valued for their efficiency but often viewed as perturbative, are shown here to arise from a strongly non-perturbative intraband response, enabling halfcycle switching. In wide-bandgap dielectrics near the damage threshold, a gas-phase-inspired injection current has gained broad attention as a putative third radiation channel beyond the intra- and interband responses. Combining phase-locked ω–3ω spectroscopy in crystalline α-quartz and fused silica with orientation-resolved measurements in α-quartz, we determine the contributing microscopic channels of low-order harmonics. A polarization-consistent decomposition demonstrates that this gas-phase analogy cannot be directly transferred to solids: the injection contribution cancels from the total current and does not survive as an independent radiation channel. Experimentally, the third and fifth harmonics follow the optical waveform and switch every 3.3 fs between ON and OFF states, exhibiting nearly 100-fold signal contrast. A field-driven evolution from crystallographic anisotropy to near-isotropic emission is observed in α-quartz. These measurements, together with the large nonlinear indicator previously reported for wave mixing1, are all accounted for by the intraband current. Our results reveal a distinctively band-structure-driven route to strongly non-perturbative nonlinearity at low harmonic orders, extending subcycle lightwave control from transient conductivity to bright, petahertz-bandwidth subcycle harmonic switching.</p>