Phase-dependent effects in bichromatic high-order harmonic generation

We address high-order harmonic generation with linearly polarized bichromatic fields, concentrating on a modulation in the harmonic yield as a function of the relative phase between the two field components, and on an offset phase shift of this modulation for neighboring cutoff harmonics. These effects have been recently observed in experiments where the relative phase between the two driving fields was controlled. Using the three-step model and the fully numerical solution of the time-dependent Schrödinger equation, we discuss the phase-dependent modulation and show that the offset phase is inherent to a particular set of semiclassical trajectories for the returning electron. These trajectories are identified using classical arguments and isolated by means of the saddle-point method, which allows a detailed investigation of their interference. Thus, by adding a second driving field whose amplitude lies within an adequate parameter range, one is able to single out a set of trajectories according to its behavior with respect to the relative phase. This effect is already present at the the single-atom-response level.