Summary form only given. The high-order harmonic generation (HHG) has attracted much attention because of its potential not only for a coherent femtosecond X-ray source but for an attosecond pulse generation. The existence of a wide plateau region in the high harmonic spectrum is well suited for the formation of attosecond pulses if a proper phase relation is met between different orders. The strong-field approximation (SFA) model of HHG showed that the phase of dipole moment induced on an atom by a driving laser depended on the laser intensity, and the harmonic phase directly followed this intrinsic atomic phase. It also showed that several different kinds of quantum paths existed in the ionized electron motion driven by the laser field. If the contributions from several quantum paths were added together to generate high-order harmonic emission, different harmonic orders would not be in phase and have randomly-distributed harmonic phase values. Consequently, the selection of high harmonics from a single quantum path will be required to obtain high-order harmonics with well-defined phase relation and to generate attosecond pulses.
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