We study the conditions for optimal coherence of harmonics emitted by atoms in strong laser fields. The variation of the phase of the atomic dipole moment induces a dramatic dependence of the spatial and spectral coherence on the focusing geometry. However, these coherence properties can be controlled and optimized by moving the laser focus position relative to the nonlinear medium.

• Received 15 November 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.3776