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 moment. We show that this phase exhibits a piecewise-linear dependence on the laser intensity. This dependence can be interpreted in quasiclassical terms, and is related to the action acquired by the electron during its motion in the laser field. The value of the phase, however, is also affected by the quantum effects of tunneling, diffusion, and interference. The phase of the dipole moment considerably influences the conversion efficiency, as well as the coherence properties, of the harmonics generated in macroscopic media.