Perturbative optical nonlinearities induced by static electric fields have proven useful in visualizing dynamical function in systems including operating circuits^,, electric and magnetic domain walls, and biological matter, and in controlling light for applications in silicon photonics. Here, we extend field-induced second-harmonic generation to the non-perturbative regime. We demonstrate that static or transient fields up to terahertz (THz) frequencies applied to silicon and ZnO crystals generate even-order high harmonics. Images of the even harmonics confirm that static fields delivered with conventional electronics control the spatial properties of the high-harmonic emission. Extending our methodology to higher-harmonic photon energies^, paves the way for realizing active optics in the extreme ultraviolet and will allow imaging of operating electronic circuits, of Si-photonic devices and of other functional materials^,, with higher spatio-temporal resolution than perturbative methods. For THz spectroscopy, our method has the bandwidth to allow measurement of attosecond transients imprinted on THz waveforms.