  High-order harmonic generation in the presence of a chirped THz pulse is
investigated numerically with a complete 3D non-adiabatic model. The assisting
THz pulse illuminates the HHG gas cell laterally inducing quasi-phase-matching.
We demonstrate that it is possible to compensate the phase mismatch during
propagation and extend the macroscopic cutoff of a propagated strong IR pulse
to the single-dipole cutoff. We obtain two orders of magnitude increase in the
harmonic efficiency of cutoff harmonics ($\approx$170 eV) using a THz pulse of
constant wavelength, and a further factor of 3 enhancement when a chirped THz
pulse is used.
