GENERALIZED PHASE-MATCHING CONDITIONS FOR HIGH HARMONICS - THE ROLE OF FIELD-GRADIENT FORCES

We present an approach to describe the phase matching of high harmonic s emitted by laser driven atoms in a nonperturbative regime, for which the atomic response displays an intrinsic intensity-dependent phase. We show that the traditional phase-matching conditions involving conse rvation of wave vectors should be modified by taking into account the gradient of this atomic phase. We investigate various focusing geometr ies and interpret the numerical results of Salieres et al. [Phys. Rev. Lett. 74, 3776 (1995)]. Within the framework of the two-step model, w e demonstrate that the gradient of the intensity-dependent phase can b e considered as the canonical momentum gained by the electron in the c ontinuum due to acceleration by field-gradient forces, including in pa rticular the ponderomotive force.