Аннотация:We show that in the presence of an extreme terahertz (THz) field an additional weak field in the optical frequency range can modulate ionization probability and thereby generate high-order harmonics. The extreme THz pump suppresses recollisions, thereby also fully suppressing the recollision-based harmonics. Nevertheless, high-order harmonics are effectively generated via an alternative mechanism, the so-called Brunel mechanism, based on ionization and subsequent acceleration in the driving field (but not on reabsorption). Using ab initio simulations for the hydrogen atom and short few-cycle optical driver, we show the appearance of a coherent carrier-envelope-phase-insensitive supercontinuum formed by such harmonics, compressible into an isolated pulse with 100-attosecond-scale duration