Quasi-phase-matching high-harmonic radiation using chirped THz pulses

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 (�170 eV) using a THz pulse of constant wavelength, and a further factor of 3 enhancement when a chirped THz pulse is used. High-order harmonic generation (HHG) in a noble gas medium is currently the widely used method to produce coherent radiation in the XUV and soft X-ray regime. The elementary laser-atom interaction leading to HHG is understood via the three-step model [1], but the complete description of the HHG process requires considering the elementary laser-atom interaction together with the macroscopic aspects of laser and harmonic field propagation in the ionized gaseous medium. The temporal, spectral and spatial distortions of the fundamental pulse during propagation result in a varying intensity and phase which strongly influence the produced harmonic radiation on both the single-atom and the macroscopic level [2]. The ultimate goal in an experiment is to obtain intense, coherent XUV or soft X-ray radiation which in turn are useful in the designed applications like molecular imaging or pump-probe experiments [3]. In Fig. 1 we show a typical case of phase mismatch (PMM) of the harmonic radiation along the HHG cell