  We study the influence of the pulse duration on high harmonic generation
(HHG) with exploring a wide laser-parameter region theoretically. Previous
studies have showed that for high laser intensities near to the saturation
ionization intensity, the HHG inversion efficiency is higher for shorter pulses
since the ground-state depletion is weaker in the latter. Surprisingly, our
simulations show this high efficiency also appears even for a moderate laser
intensity at which the ionization is not strong. A classical effect relating to
shorter travel distances of the rescattering electron in shorter pulses, is
found to contribute importantly to this high efficiency. The effect can be
amplified significantly as a two-color laser field is used, suggesting an
effective approach for increasing the HHG yield.
