PHASE OF THE ATOMIC POLARIZATION IN HIGH-ORDER HARMONIC-GENERATION

A recently formulated theory of high-order harmonic generation by low- frequency laser fields [Anne L'Huillier et al., Phys. Rev. A 48, R3433 (1993)] allows us to study the phase of the induced atomic dipole mom ent. We show that this phase exhibits a piecewise-linear dependence on the laser intensity. This dependence can be interpreted in quasiclass ical terms, and is related to the action acquired by the electron duri ng its motion in the laser held. The value of the phase, however, is a lso affected by the quantum effects of tunneling, diffusion, and inter ference. The phase of the dipole moment considerably influences the co nversion efficiency, as well as the coherence properties, of the harmo nics generated in macroscopic media.