Role of the dressed and bound states on below-threshold harmonic generation of He atom**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674268, 11764038, and 11765018) and the Scientific Research Foundation of Physics of CPEE–NWNU and NWNU-LKQN-17-1, China.

Abstract

High-order harmonic generation below ionization threshold of He atom in the laser field is investigated by solving the three-dimensional time-dependent Schrödinger equation. An angular momentum-dependent model potential of He atom was used for getting the accurate energy levels of singlet states. The satellite-peak structures of the below-threshold harmonic generation (BTHG) of He are observed. We analyze the emission properties of the BTHG by employing a synchrosqueezing transform technique. We find that the satellite-peak structures have two types related to two kinds of transitions. One is the transition of the dressed states of the excited states, the other is the transition between the excited states and the ground state in the field-free case. Furthermore, our results show that the maximum Stark shift of the 2p state is about (penderomotive energy), and that of the 4p state is about . It indicates that the energy difference between some satellite- and main-peaks of the BTHG can be used to measure the maximum Stark shift of the excited states of He atom in the laser field.