Trajectory Selection in High Harmonic Generation by Controlling the Phase between Orthogonal Two-Color Fields

We demonstrate control of short and long quantum trajectories in high harmonic emission through the use of an orthogonally polarized two-color field. By controlling the relative phase phi between the two fields we show via classical and quantum calculations that we can steer the two-dimensional trajectories to return, or not, to the core and so control the relative strength of the short or long quantum trajectory contribution. In experiments, we demonstrate that this leads to robust control over the trajectory contributions using a drive field from a femtosecond laser composed of the fundamental omega at 800 nm (intensity similar to 1.2 x 10(14) W cm(-2)) and its weaker orthogonally polarized second harmonic 2 omega ( intensity similar to 0.3 x 10(14) W cm(-2)) with the relative phase between the omega and 2 omega fields varied simply by tilting a fused silica plate. This is the first demonstration of short and long quantum trajectory control at the single-atom level.