Hidden coherence along space-time trajectories in parametric wave mixing.

We show theoretically that the waves generated through the generic parametric three- or four-wave-mixing processes exhibit, as a general rule, a hidden coherence characterized by skewed coherence lines along specific space-time trajectories. Our study generalizes the concept of coherence in the sense that these previously unrecognized coherent states cannot be described through the standard definitions of spatial and temporal coherence.