Aspects of harmonic emission in the attoclock

We study the emission of below-threshold harmonics from atoms in strong near-circular laser fields numerically and analytically. The calculated harmonic spectrum shows a double-plateau structure with two cutoffs. When the position of the second cutoff with higher energy holds at the ionization threshold, the first one with lower energy depends on the laser parameters. Around the first cutoff, the spectral characteristics of the first and the second plateaus differ remarkably from each other. The position of the first cutoff can be well described using a semiclassical Coulomb-included strong-field model. This model predicts that due to the near-nucleus Coulomb effect, the tunneling electron will be located at a transition state around the tunnel exit before ionization. The energy of the transition state determines the first-cutoff position. Our work provides deeper insights into the roles of Coulomb potential in strong-field processes.