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High-harmonic generation with excitation pulses shorter than 25 fs is studied theoretically using a 3D model. For very short excitation pulses, a new regime of harmonic generation by a ``single-cycle'' of the driver pulse can be reached. In this regime, the temporal coherence of the adjacent harmonic orders is dramatically improved compared with longer excitation pulses, even though the discrete harmonic structure in the emission disappears. X-ray pulses as short as 100 attoseconds can be emitted, with increased conversion efficiency of laser-to-harmonic radiation.
Christov et al. (Mon,) studied this question.