We establish and analytically derive the cutoff law for high harmonics generated with two interfering electron wave packets in the continuum. The phenomenon occurs for laser intensities high enough to spark substantial ionization, yet low enough that depletion of the ground state remains moderate to ensure a large interference yield. The continuum-continuum (CC) cutoff emerges from the maximal action difference of the dominant classical trajectories in the phase of the time-dependent dipole response. This action difference translates into an interference of a maximally time-delayed, so called stranded continuum trajectory with the trajectory of a high energy continuum electron. The time delay renders the CC harmonics more sensitive to electron wave-packet dispersion than standard high harmonics. Hence, CC harmonics should be most prominent in dimensionally reduced systems.
|Physical Review A
|Published - May 2022