INTENSITY DEPENDENCE OF THE PHASE OF HARMONIES IN ONE-FREQUENCY AND 2-FREQUENCY LASER FIELDS

The phase relationship between an induced atomic dipole moment and a d riving laser field is an essential aspect of high harmonic generation (HHG) in a dense medium. A ''phase-matching'' criterion must be fulfil led to obtain coherent amplification of harmonic radiation, which is c ritical for practical HHG. We have performed numerical calculations on model atoms to survey the intensity dependence of the phase in the ca se of strong one- or two-frequency laser fields. The calculations show that multiphoton resonances significantly affect the yield of harmoni c generation and that in the ''plateau'' regime, the phases exhibit on ly weak dependence upon the intensity of the driving field.