Frequency shift in high-harmonic generation from two-color laser fields driven water

By solving the semiconductor Bloch equations in length gauge, we studied the high-harmonic generation from water driven by two-color laser fields that consist of a fundamental field and its second harmonic. We show that through changing the relative phase of the two-color laser fields, the even- and odd-harmonics are periodically modulated. As the relative phase is tunned from 0-π, the redshift of odd-harmonics increases, and the odd-harmonics yield increases first and then decreases. By separating the contributions from positive and negative half-cycles in time domain, these phenomena can be attributed to inter-half-cycle interference between positive and negative half-cycles.