Pathway of bichromatic high-order harmonic generation

We present a theoretical study of the pathway of harmonic generation in bichromatic linearly polarized laser fields. A "harmonic-collapse" phenomenon is observed in the power spectrum for a particular value of the amplitude ratio of two components of the external field. We employ an offset frequency shift to the additional field to distinguish the harmonic pathway and then to explain the harmonic-collapse phenomenon. The harmonic intensity as a function of the relative amplitude ratio and, furthermore, the fine structure of the harmonic spectrum can be well understood on the basis of a pathway analysis.