Tracking origins of below-threshold harmonics with a trajectory-resolved fully quantum approach

The emission of below-threshold harmonics (BTHs) is a promising tool for producing vacuum-ultraviolet frequency combs. Nevertheless, the generation mechanism of BTHs is still an open question and a lot of efforts have been devoted to this issue. Here, with a developed trajectory-resolved fully quantum numerical scheme which allows us to "coherently" trace the motion of the rescattering electron, the complex roles of different rescattering trajectories, such as the first return or multiple returns in the origin of BTHs are identified unambiguously. We show that the emission of BTHs is influenced dramatically by laser intensity. At strong laser fields, multiple returns (especially the third return) play an important role in BTHs. At weak fields, effects related to multiphoton processes dominate. The interference of the different contributions to BTHs is also addressed.National Basic Research Program of China (973 Program) [2013CB834100]; National Natural Science Foundation of China [11374040, 11274051, 11274090, 11475027, 11575027]SCI(E)ARTICLEchenyjhb@gmail.com; lbfu@iapcm.ac.cn69